text: /* SPDX-License-Identifier: Apache-2.0 */
  /**
   * \file ssl.h
   *
   * \brief SSL/TLS functions.
   */
  /*
   *  Copyright (C) 2006-2015, ARM Limited, All Rights Reserved
   *
   *  Licensed under the Apache License, Version 2.0 (the "License"); you may
   *  not use this file except in compliance with the License.
   *  You may obtain a copy of the License at
   *
   *  http://www.apache.org/licenses/LICENSE-2.0
   *
   *  Unless required by applicable law or agreed to in writing, software
   *  distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
   *  WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
   *  See the License for the specific language governing permissions and
   *  limitations under the License.
   *
   *  This file is part of mbed TLS (https://tls.mbed.org)
   */
  #ifndef MBEDTLS_SSL_H
  #define MBEDTLS_SSL_H
  
  #if !defined(MBEDTLS_CONFIG_FILE)
  #include "mbedtls/config.h"
  #else
  #include MBEDTLS_CONFIG_FILE
  #endif
  
  #include "mbedtls/bignum.h"
  #include "mbedtls/ecp.h"
  
  #include "mbedtls/ssl_ciphersuites.h"
  
  #if defined(MBEDTLS_X509_CRT_PARSE_C)
  #include "mbedtls/x509_crt.h"
  #include "mbedtls/x509_crl.h"
  #endif
  
  #if defined(MBEDTLS_DHM_C)
  #include "mbedtls/dhm.h"
  #endif
  
  #if defined(MBEDTLS_ECDH_C)
  #include "mbedtls/ecdh.h"
  #endif
  
  #if defined(MBEDTLS_ZLIB_SUPPORT)
  
  #if defined(MBEDTLS_DEPRECATED_WARNING)
  #warning "Record compression support via MBEDTLS_ZLIB_SUPPORT is deprecated and will be removed in the next major revision of the library"
  #endif
  
  #if defined(MBEDTLS_DEPRECATED_REMOVED)
  #error "Record compression support via MBEDTLS_ZLIB_SUPPORT is deprecated and cannot be used if MBEDTLS_DEPRECATED_REMOVED is set"
  #endif
  
  #include "zlib.h"
  #endif
  
  #if defined(MBEDTLS_HAVE_TIME)
  #include "mbedtls/platform_time.h"
  #endif
  
  #if defined(MBEDTLS_USE_PSA_CRYPTO)
  #include "psa/crypto.h"
  #endif /* MBEDTLS_USE_PSA_CRYPTO */
  
  /*
   * SSL Error codes
   */
  #define MBEDTLS_ERR_SSL_FEATURE_UNAVAILABLE               -0x7080  /**< The requested feature is not available. */
  #define MBEDTLS_ERR_SSL_BAD_INPUT_DATA                    -0x7100  /**< Bad input parameters to function. */
  #define MBEDTLS_ERR_SSL_INVALID_MAC                       -0x7180  /**< Verification of the message MAC failed. */
  #define MBEDTLS_ERR_SSL_INVALID_RECORD                    -0x7200  /**< An invalid SSL record was received. */
  #define MBEDTLS_ERR_SSL_CONN_EOF                          -0x7280  /**< The connection indicated an EOF. */
  #define MBEDTLS_ERR_SSL_UNKNOWN_CIPHER                    -0x7300  /**< An unknown cipher was received. */
  #define MBEDTLS_ERR_SSL_NO_CIPHER_CHOSEN                  -0x7380  /**< The server has no ciphersuites in common with the client. */
  #define MBEDTLS_ERR_SSL_NO_RNG                            -0x7400  /**< No RNG was provided to the SSL module. */
  #define MBEDTLS_ERR_SSL_NO_CLIENT_CERTIFICATE             -0x7480  /**< No client certification received from the client, but required by the authentication mode. */
  #define MBEDTLS_ERR_SSL_CERTIFICATE_TOO_LARGE             -0x7500  /**< Our own certificate(s) is/are too large to send in an SSL message. */
  #define MBEDTLS_ERR_SSL_CERTIFICATE_REQUIRED              -0x7580  /**< The own certificate is not set, but needed by the server. */
  #define MBEDTLS_ERR_SSL_PRIVATE_KEY_REQUIRED              -0x7600  /**< The own private key or pre-shared key is not set, but needed. */
  #define MBEDTLS_ERR_SSL_CA_CHAIN_REQUIRED                 -0x7680  /**< No CA Chain is set, but required to operate. */
  #define MBEDTLS_ERR_SSL_UNEXPECTED_MESSAGE                -0x7700  /**< An unexpected message was received from our peer. */
  #define MBEDTLS_ERR_SSL_FATAL_ALERT_MESSAGE               -0x7780  /**< A fatal alert message was received from our peer. */
  #define MBEDTLS_ERR_SSL_PEER_VERIFY_FAILED                -0x7800  /**< Verification of our peer failed. */
  #define MBEDTLS_ERR_SSL_PEER_CLOSE_NOTIFY                 -0x7880  /**< The peer notified us that the connection is going to be closed. */
  #define MBEDTLS_ERR_SSL_BAD_HS_CLIENT_HELLO               -0x7900  /**< Processing of the ClientHello handshake message failed. */
  #define MBEDTLS_ERR_SSL_BAD_HS_SERVER_HELLO               -0x7980  /**< Processing of the ServerHello handshake message failed. */
  #define MBEDTLS_ERR_SSL_BAD_HS_CERTIFICATE                -0x7A00  /**< Processing of the Certificate handshake message failed. */
  #define MBEDTLS_ERR_SSL_BAD_HS_CERTIFICATE_REQUEST        -0x7A80  /**< Processing of the CertificateRequest handshake message failed. */
  #define MBEDTLS_ERR_SSL_BAD_HS_SERVER_KEY_EXCHANGE        -0x7B00  /**< Processing of the ServerKeyExchange handshake message failed. */
  #define MBEDTLS_ERR_SSL_BAD_HS_SERVER_HELLO_DONE          -0x7B80  /**< Processing of the ServerHelloDone handshake message failed. */
  #define MBEDTLS_ERR_SSL_BAD_HS_CLIENT_KEY_EXCHANGE        -0x7C00  /**< Processing of the ClientKeyExchange handshake message failed. */
  #define MBEDTLS_ERR_SSL_BAD_HS_CLIENT_KEY_EXCHANGE_RP     -0x7C80  /**< Processing of the ClientKeyExchange handshake message failed in DHM / ECDH Read Public. */
  #define MBEDTLS_ERR_SSL_BAD_HS_CLIENT_KEY_EXCHANGE_CS     -0x7D00  /**< Processing of the ClientKeyExchange handshake message failed in DHM / ECDH Calculate Secret. */
  #define MBEDTLS_ERR_SSL_BAD_HS_CERTIFICATE_VERIFY         -0x7D80  /**< Processing of the CertificateVerify handshake message failed. */
  #define MBEDTLS_ERR_SSL_BAD_HS_CHANGE_CIPHER_SPEC         -0x7E00  /**< Processing of the ChangeCipherSpec handshake message failed. */
  #define MBEDTLS_ERR_SSL_BAD_HS_FINISHED                   -0x7E80  /**< Processing of the Finished handshake message failed. */
  #define MBEDTLS_ERR_SSL_ALLOC_FAILED                      -0x7F00  /**< Memory allocation failed */
  #define MBEDTLS_ERR_SSL_HW_ACCEL_FAILED                   -0x7F80  /**< Hardware acceleration function returned with error */
  #define MBEDTLS_ERR_SSL_HW_ACCEL_FALLTHROUGH              -0x6F80  /**< Hardware acceleration function skipped / left alone data */
  #define MBEDTLS_ERR_SSL_COMPRESSION_FAILED                -0x6F00  /**< Processing of the compression / decompression failed */
  #define MBEDTLS_ERR_SSL_BAD_HS_PROTOCOL_VERSION           -0x6E80  /**< Handshake protocol not within min/max boundaries */
  #define MBEDTLS_ERR_SSL_BAD_HS_NEW_SESSION_TICKET         -0x6E00  /**< Processing of the NewSessionTicket handshake message failed. */
  #define MBEDTLS_ERR_SSL_SESSION_TICKET_EXPIRED            -0x6D80  /**< Session ticket has expired. */
  #define MBEDTLS_ERR_SSL_PK_TYPE_MISMATCH                  -0x6D00  /**< Public key type mismatch (eg, asked for RSA key exchange and presented EC key) */
  #define MBEDTLS_ERR_SSL_UNKNOWN_IDENTITY                  -0x6C80  /**< Unknown identity received (eg, PSK identity) */
  #define MBEDTLS_ERR_SSL_INTERNAL_ERROR                    -0x6C00  /**< Internal error (eg, unexpected failure in lower-level module) */
  #define MBEDTLS_ERR_SSL_COUNTER_WRAPPING                  -0x6B80  /**< A counter would wrap (eg, too many messages exchanged). */
  #define MBEDTLS_ERR_SSL_WAITING_SERVER_HELLO_RENEGO       -0x6B00  /**< Unexpected message at ServerHello in renegotiation. */
  #define MBEDTLS_ERR_SSL_HELLO_VERIFY_REQUIRED             -0x6A80  /**< DTLS client must retry for hello verification */
  #define MBEDTLS_ERR_SSL_BUFFER_TOO_SMALL                  -0x6A00  /**< A buffer is too small to receive or write a message */
  #define MBEDTLS_ERR_SSL_NO_USABLE_CIPHERSUITE             -0x6980  /**< None of the common ciphersuites is usable (eg, no suitable certificate, see debug messages). */
  #define MBEDTLS_ERR_SSL_WANT_READ                         -0x6900  /**< No data of requested type currently available on underlying transport. */
  #define MBEDTLS_ERR_SSL_WANT_WRITE                        -0x6880  /**< Connection requires a write call. */
  #define MBEDTLS_ERR_SSL_TIMEOUT                           -0x6800  /**< The operation timed out. */
  #define MBEDTLS_ERR_SSL_CLIENT_RECONNECT                  -0x6780  /**< The client initiated a reconnect from the same port. */
  #define MBEDTLS_ERR_SSL_UNEXPECTED_RECORD                 -0x6700  /**< Record header looks valid but is not expected. */
  #define MBEDTLS_ERR_SSL_NON_FATAL                         -0x6680  /**< The alert message received indicates a non-fatal error. */
  #define MBEDTLS_ERR_SSL_INVALID_VERIFY_HASH               -0x6600  /**< Couldn't set the hash for verifying CertificateVerify */
  #define MBEDTLS_ERR_SSL_CONTINUE_PROCESSING               -0x6580  /**< Internal-only message signaling that further message-processing should be done */
  #define MBEDTLS_ERR_SSL_ASYNC_IN_PROGRESS                 -0x6500  /**< The asynchronous operation is not completed yet. */
  #define MBEDTLS_ERR_SSL_EARLY_MESSAGE                     -0x6480  /**< Internal-only message signaling that a message arrived early. */
  #define MBEDTLS_ERR_SSL_UNEXPECTED_CID                    -0x6000  /**< An encrypted DTLS-frame with an unexpected CID was received. */
  #define MBEDTLS_ERR_SSL_VERSION_MISMATCH                  -0x5F00  /**< An operation failed due to an unexpected version or configuration. */
  #define MBEDTLS_ERR_SSL_CRYPTO_IN_PROGRESS                -0x7000  /**< A cryptographic operation is in progress. Try again later. */
  
  /*
   * Various constants
   */
  #define MBEDTLS_SSL_MAJOR_VERSION_3             3
  #define MBEDTLS_SSL_MINOR_VERSION_0             0   /*!< SSL v3.0 */
  #define MBEDTLS_SSL_MINOR_VERSION_1             1   /*!< TLS v1.0 */
  #define MBEDTLS_SSL_MINOR_VERSION_2             2   /*!< TLS v1.1 */
  #define MBEDTLS_SSL_MINOR_VERSION_3             3   /*!< TLS v1.2 */
  
  #define MBEDTLS_SSL_TRANSPORT_STREAM            0   /*!< TLS      */
  #define MBEDTLS_SSL_TRANSPORT_DATAGRAM          1   /*!< DTLS     */
  
  #define MBEDTLS_SSL_MAX_HOST_NAME_LEN           255 /*!< Maximum host name defined in RFC 1035 */
  
  /* RFC 6066 section 4, see also mfl_code_to_length in ssl_tls.c
   * NONE must be zero so that memset()ing structure to zero works */
  #define MBEDTLS_SSL_MAX_FRAG_LEN_NONE           0   /*!< don't use this extension   */
  #define MBEDTLS_SSL_MAX_FRAG_LEN_512            1   /*!< MaxFragmentLength 2^9      */
  #define MBEDTLS_SSL_MAX_FRAG_LEN_1024           2   /*!< MaxFragmentLength 2^10     */
  #define MBEDTLS_SSL_MAX_FRAG_LEN_2048           3   /*!< MaxFragmentLength 2^11     */
  #define MBEDTLS_SSL_MAX_FRAG_LEN_4096           4   /*!< MaxFragmentLength 2^12     */
  #define MBEDTLS_SSL_MAX_FRAG_LEN_INVALID        5   /*!< first invalid value        */
  
  #define MBEDTLS_SSL_IS_CLIENT                   0
  #define MBEDTLS_SSL_IS_SERVER                   1
  
  #define MBEDTLS_SSL_IS_NOT_FALLBACK             0
  #define MBEDTLS_SSL_IS_FALLBACK                 1
  
  #define MBEDTLS_SSL_EXTENDED_MS_DISABLED        0
  #define MBEDTLS_SSL_EXTENDED_MS_ENABLED         1
  
  #define MBEDTLS_SSL_CID_DISABLED                0
  #define MBEDTLS_SSL_CID_ENABLED                 1
  
  #define MBEDTLS_SSL_ETM_DISABLED                0
  #define MBEDTLS_SSL_ETM_ENABLED                 1
  
  #define MBEDTLS_SSL_COMPRESS_NULL               0
  #define MBEDTLS_SSL_COMPRESS_DEFLATE            1
  
  #define MBEDTLS_SSL_VERIFY_NONE                 0
  #define MBEDTLS_SSL_VERIFY_OPTIONAL             1
  #define MBEDTLS_SSL_VERIFY_REQUIRED             2
  #define MBEDTLS_SSL_VERIFY_UNSET                3 /* Used only for sni_authmode */
  
  #define MBEDTLS_SSL_LEGACY_RENEGOTIATION        0
  #define MBEDTLS_SSL_SECURE_RENEGOTIATION        1
  
  #define MBEDTLS_SSL_RENEGOTIATION_DISABLED      0
  #define MBEDTLS_SSL_RENEGOTIATION_ENABLED       1
  
  #define MBEDTLS_SSL_ANTI_REPLAY_DISABLED        0
  #define MBEDTLS_SSL_ANTI_REPLAY_ENABLED         1
  
  #define MBEDTLS_SSL_RENEGOTIATION_NOT_ENFORCED  -1
  #define MBEDTLS_SSL_RENEGO_MAX_RECORDS_DEFAULT  16
  
  #define MBEDTLS_SSL_LEGACY_NO_RENEGOTIATION     0
  #define MBEDTLS_SSL_LEGACY_ALLOW_RENEGOTIATION  1
  #define MBEDTLS_SSL_LEGACY_BREAK_HANDSHAKE      2
  
  #define MBEDTLS_SSL_TRUNC_HMAC_DISABLED         0
  #define MBEDTLS_SSL_TRUNC_HMAC_ENABLED          1
  #define MBEDTLS_SSL_TRUNCATED_HMAC_LEN          10  /* 80 bits, rfc 6066 section 7 */
  
  #define MBEDTLS_SSL_SESSION_TICKETS_DISABLED     0
  #define MBEDTLS_SSL_SESSION_TICKETS_ENABLED      1
  
  #define MBEDTLS_SSL_CBC_RECORD_SPLITTING_DISABLED    0
  #define MBEDTLS_SSL_CBC_RECORD_SPLITTING_ENABLED     1
  
  #define MBEDTLS_SSL_ARC4_ENABLED                0
  #define MBEDTLS_SSL_ARC4_DISABLED               1
  
  #define MBEDTLS_SSL_PRESET_DEFAULT              0
  #define MBEDTLS_SSL_PRESET_SUITEB               2
  
  #define MBEDTLS_SSL_CERT_REQ_CA_LIST_ENABLED       1
  #define MBEDTLS_SSL_CERT_REQ_CA_LIST_DISABLED      0
  
  /*
   * Default range for DTLS retransmission timer value, in milliseconds.
   * RFC 6347 4.2.4.1 says from 1 second to 60 seconds.
   */
  #define MBEDTLS_SSL_DTLS_TIMEOUT_DFL_MIN    1000
  #define MBEDTLS_SSL_DTLS_TIMEOUT_DFL_MAX   60000
  
  /**
   * \name SECTION: Module settings
   *
   * The configuration options you can set for this module are in this section.
   * Either change them in config.h or define them on the compiler command line.
   * \{
   */
  
  #if !defined(MBEDTLS_SSL_DEFAULT_TICKET_LIFETIME)
  #define MBEDTLS_SSL_DEFAULT_TICKET_LIFETIME     86400 /**< Lifetime of session tickets (if enabled) */
  #endif
  
  /*
   * Maximum fragment length in bytes,
   * determines the size of each of the two internal I/O buffers.
   *
   * Note: the RFC defines the default size of SSL / TLS messages. If you
   * change the value here, other clients / servers may not be able to
   * communicate with you anymore. Only change this value if you control
   * both sides of the connection and have it reduced at both sides, or
   * if you're using the Max Fragment Length extension and you know all your
   * peers are using it too!
   */
  #if !defined(MBEDTLS_SSL_MAX_CONTENT_LEN)
  #define MBEDTLS_SSL_MAX_CONTENT_LEN         16384   /**< Size of the input / output buffer */
  #endif
  
  #if !defined(MBEDTLS_SSL_IN_CONTENT_LEN)
  #define MBEDTLS_SSL_IN_CONTENT_LEN MBEDTLS_SSL_MAX_CONTENT_LEN
  #endif
  
  #if !defined(MBEDTLS_SSL_OUT_CONTENT_LEN)
  #define MBEDTLS_SSL_OUT_CONTENT_LEN MBEDTLS_SSL_MAX_CONTENT_LEN
  #endif
  
  /*
   * Maximum number of heap-allocated bytes for the purpose of
   * DTLS handshake message reassembly and future message buffering.
   */
  #if !defined(MBEDTLS_SSL_DTLS_MAX_BUFFERING)
  #define MBEDTLS_SSL_DTLS_MAX_BUFFERING 32768
  #endif
  
  /*
   * Maximum length of CIDs for incoming and outgoing messages.
   */
  #if !defined(MBEDTLS_SSL_CID_IN_LEN_MAX)
  #define MBEDTLS_SSL_CID_IN_LEN_MAX          32
  #endif
  
  #if !defined(MBEDTLS_SSL_CID_OUT_LEN_MAX)
  #define MBEDTLS_SSL_CID_OUT_LEN_MAX         32
  #endif
  
  #if !defined(MBEDTLS_SSL_CID_PADDING_GRANULARITY)
  #define MBEDTLS_SSL_CID_PADDING_GRANULARITY 16
  #endif
  
  /* \} name SECTION: Module settings */
  
  /*
   * Length of the verify data for secure renegotiation
   */
  #if defined(MBEDTLS_SSL_PROTO_SSL3)
  #define MBEDTLS_SSL_VERIFY_DATA_MAX_LEN 36
  #else
  #define MBEDTLS_SSL_VERIFY_DATA_MAX_LEN 12
  #endif
  
  /*
   * Signaling ciphersuite values (SCSV)
   */
  #define MBEDTLS_SSL_EMPTY_RENEGOTIATION_INFO    0xFF   /**< renegotiation info ext */
  #define MBEDTLS_SSL_FALLBACK_SCSV_VALUE         0x5600 /**< RFC 7507 section 2 */
  
  /*
   * Supported Signature and Hash algorithms (For TLS 1.2)
   * RFC 5246 section 7.4.1.4.1
   */
  #define MBEDTLS_SSL_HASH_NONE                0
  #define MBEDTLS_SSL_HASH_MD5                 1
  #define MBEDTLS_SSL_HASH_SHA1                2
  #define MBEDTLS_SSL_HASH_SHA224              3
  #define MBEDTLS_SSL_HASH_SHA256              4
  #define MBEDTLS_SSL_HASH_SHA384              5
  #define MBEDTLS_SSL_HASH_SHA512              6
  
  #define MBEDTLS_SSL_SIG_ANON                 0
  #define MBEDTLS_SSL_SIG_RSA                  1
  #define MBEDTLS_SSL_SIG_ECDSA                3
  
  /*
   * Client Certificate Types
   * RFC 5246 section 7.4.4 plus RFC 4492 section 5.5
   */
  #define MBEDTLS_SSL_CERT_TYPE_RSA_SIGN       1
  #define MBEDTLS_SSL_CERT_TYPE_ECDSA_SIGN    64
  
  /*
   * Message, alert and handshake types
   */
  #define MBEDTLS_SSL_MSG_CHANGE_CIPHER_SPEC     20
  #define MBEDTLS_SSL_MSG_ALERT                  21
  #define MBEDTLS_SSL_MSG_HANDSHAKE              22
  #define MBEDTLS_SSL_MSG_APPLICATION_DATA       23
  #define MBEDTLS_SSL_MSG_CID                    25
  
  #define MBEDTLS_SSL_ALERT_LEVEL_WARNING         1
  #define MBEDTLS_SSL_ALERT_LEVEL_FATAL           2
  
  #define MBEDTLS_SSL_ALERT_MSG_CLOSE_NOTIFY           0  /* 0x00 */
  #define MBEDTLS_SSL_ALERT_MSG_UNEXPECTED_MESSAGE    10  /* 0x0A */
  #define MBEDTLS_SSL_ALERT_MSG_BAD_RECORD_MAC        20  /* 0x14 */
  #define MBEDTLS_SSL_ALERT_MSG_DECRYPTION_FAILED     21  /* 0x15 */
  #define MBEDTLS_SSL_ALERT_MSG_RECORD_OVERFLOW       22  /* 0x16 */
  #define MBEDTLS_SSL_ALERT_MSG_DECOMPRESSION_FAILURE 30  /* 0x1E */
  #define MBEDTLS_SSL_ALERT_MSG_HANDSHAKE_FAILURE     40  /* 0x28 */
  #define MBEDTLS_SSL_ALERT_MSG_NO_CERT               41  /* 0x29 */
  #define MBEDTLS_SSL_ALERT_MSG_BAD_CERT              42  /* 0x2A */
  #define MBEDTLS_SSL_ALERT_MSG_UNSUPPORTED_CERT      43  /* 0x2B */
  #define MBEDTLS_SSL_ALERT_MSG_CERT_REVOKED          44  /* 0x2C */
  #define MBEDTLS_SSL_ALERT_MSG_CERT_EXPIRED          45  /* 0x2D */
  #define MBEDTLS_SSL_ALERT_MSG_CERT_UNKNOWN          46  /* 0x2E */
  #define MBEDTLS_SSL_ALERT_MSG_ILLEGAL_PARAMETER     47  /* 0x2F */
  #define MBEDTLS_SSL_ALERT_MSG_UNKNOWN_CA            48  /* 0x30 */
  #define MBEDTLS_SSL_ALERT_MSG_ACCESS_DENIED         49  /* 0x31 */
  #define MBEDTLS_SSL_ALERT_MSG_DECODE_ERROR          50  /* 0x32 */
  #define MBEDTLS_SSL_ALERT_MSG_DECRYPT_ERROR         51  /* 0x33 */
  #define MBEDTLS_SSL_ALERT_MSG_EXPORT_RESTRICTION    60  /* 0x3C */
  #define MBEDTLS_SSL_ALERT_MSG_PROTOCOL_VERSION      70  /* 0x46 */
  #define MBEDTLS_SSL_ALERT_MSG_INSUFFICIENT_SECURITY 71  /* 0x47 */
  #define MBEDTLS_SSL_ALERT_MSG_INTERNAL_ERROR        80  /* 0x50 */
  #define MBEDTLS_SSL_ALERT_MSG_INAPROPRIATE_FALLBACK 86  /* 0x56 */
  #define MBEDTLS_SSL_ALERT_MSG_USER_CANCELED         90  /* 0x5A */
  #define MBEDTLS_SSL_ALERT_MSG_NO_RENEGOTIATION     100  /* 0x64 */
  #define MBEDTLS_SSL_ALERT_MSG_UNSUPPORTED_EXT      110  /* 0x6E */
  #define MBEDTLS_SSL_ALERT_MSG_UNRECOGNIZED_NAME    112  /* 0x70 */
  #define MBEDTLS_SSL_ALERT_MSG_UNKNOWN_PSK_IDENTITY 115  /* 0x73 */
  #define MBEDTLS_SSL_ALERT_MSG_NO_APPLICATION_PROTOCOL 120 /* 0x78 */
  
  #define MBEDTLS_SSL_HS_HELLO_REQUEST            0
  #define MBEDTLS_SSL_HS_CLIENT_HELLO             1
  #define MBEDTLS_SSL_HS_SERVER_HELLO             2
  #define MBEDTLS_SSL_HS_HELLO_VERIFY_REQUEST     3
  #define MBEDTLS_SSL_HS_NEW_SESSION_TICKET       4
  #define MBEDTLS_SSL_HS_CERTIFICATE             11
  #define MBEDTLS_SSL_HS_SERVER_KEY_EXCHANGE     12
  #define MBEDTLS_SSL_HS_CERTIFICATE_REQUEST     13
  #define MBEDTLS_SSL_HS_SERVER_HELLO_DONE       14
  #define MBEDTLS_SSL_HS_CERTIFICATE_VERIFY      15
  #define MBEDTLS_SSL_HS_CLIENT_KEY_EXCHANGE     16
  #define MBEDTLS_SSL_HS_FINISHED                20
  
  /*
   * TLS extensions
   */
  #define MBEDTLS_TLS_EXT_SERVERNAME                   0
  #define MBEDTLS_TLS_EXT_SERVERNAME_HOSTNAME          0
  
  #define MBEDTLS_TLS_EXT_MAX_FRAGMENT_LENGTH          1
  
  #define MBEDTLS_TLS_EXT_TRUNCATED_HMAC               4
  
  #define MBEDTLS_TLS_EXT_SUPPORTED_ELLIPTIC_CURVES   10
  #define MBEDTLS_TLS_EXT_SUPPORTED_POINT_FORMATS     11
  
  #define MBEDTLS_TLS_EXT_SIG_ALG                     13
  
  #define MBEDTLS_TLS_EXT_ALPN                        16
  
  #define MBEDTLS_TLS_EXT_ENCRYPT_THEN_MAC            22 /* 0x16 */
  #define MBEDTLS_TLS_EXT_EXTENDED_MASTER_SECRET  0x0017 /* 23 */
  
  #define MBEDTLS_TLS_EXT_SESSION_TICKET              35
  
  /* The value of the CID extension is still TBD as of
   * draft-ietf-tls-dtls-connection-id-05
   * (https://tools.ietf.org/html/draft-ietf-tls-dtls-connection-id-05) */
  #define MBEDTLS_TLS_EXT_CID                        254 /* TBD */
  
  #define MBEDTLS_TLS_EXT_ECJPAKE_KKPP               256 /* experimental */
  
  #define MBEDTLS_TLS_EXT_RENEGOTIATION_INFO      0xFF01
  
  /*
   * Size defines
   */
  #if !defined(MBEDTLS_PSK_MAX_LEN)
  #define MBEDTLS_PSK_MAX_LEN            32 /* 256 bits */
  #endif
  
  /* Dummy type used only for its size */
  union mbedtls_ssl_premaster_secret
  {
  #if defined(MBEDTLS_KEY_EXCHANGE_RSA_ENABLED)
      unsigned char _pms_rsa[48];                         /* RFC 5246 8.1.1 */
  #endif
  #if defined(MBEDTLS_KEY_EXCHANGE_DHE_RSA_ENABLED)
      unsigned char _pms_dhm[MBEDTLS_MPI_MAX_SIZE];      /* RFC 5246 8.1.2 */
  #endif
  #if defined(MBEDTLS_KEY_EXCHANGE_ECDHE_RSA_ENABLED)    || \
      defined(MBEDTLS_KEY_EXCHANGE_ECDHE_ECDSA_ENABLED)  || \
      defined(MBEDTLS_KEY_EXCHANGE_ECDH_RSA_ENABLED)     || \
      defined(MBEDTLS_KEY_EXCHANGE_ECDH_ECDSA_ENABLED)
      unsigned char _pms_ecdh[MBEDTLS_ECP_MAX_BYTES];    /* RFC 4492 5.10 */
  #endif
  #if defined(MBEDTLS_KEY_EXCHANGE_PSK_ENABLED)
      unsigned char _pms_psk[4 + 2 * MBEDTLS_PSK_MAX_LEN];       /* RFC 4279 2 */
  #endif
  #if defined(MBEDTLS_KEY_EXCHANGE_DHE_PSK_ENABLED)
      unsigned char _pms_dhe_psk[4 + MBEDTLS_MPI_MAX_SIZE
                                   + MBEDTLS_PSK_MAX_LEN];       /* RFC 4279 3 */
  #endif
  #if defined(MBEDTLS_KEY_EXCHANGE_RSA_PSK_ENABLED)
      unsigned char _pms_rsa_psk[52 + MBEDTLS_PSK_MAX_LEN];      /* RFC 4279 4 */
  #endif
  #if defined(MBEDTLS_KEY_EXCHANGE_ECDHE_PSK_ENABLED)
      unsigned char _pms_ecdhe_psk[4 + MBEDTLS_ECP_MAX_BYTES
                                     + MBEDTLS_PSK_MAX_LEN];     /* RFC 5489 2 */
  #endif
  #if defined(MBEDTLS_KEY_EXCHANGE_ECJPAKE_ENABLED)
      unsigned char _pms_ecjpake[32];     /* Thread spec: SHA-256 output */
  #endif
  };
  
  #define MBEDTLS_PREMASTER_SIZE     sizeof( union mbedtls_ssl_premaster_secret )
  
  #ifdef __cplusplus
  extern "C" {
  #endif
  
  /*
   * SSL state machine
   */
  typedef enum
  {
      MBEDTLS_SSL_HELLO_REQUEST,
      MBEDTLS_SSL_CLIENT_HELLO,
      MBEDTLS_SSL_SERVER_HELLO,
      MBEDTLS_SSL_SERVER_CERTIFICATE,
      MBEDTLS_SSL_SERVER_KEY_EXCHANGE,
      MBEDTLS_SSL_CERTIFICATE_REQUEST,
      MBEDTLS_SSL_SERVER_HELLO_DONE,
      MBEDTLS_SSL_CLIENT_CERTIFICATE,
      MBEDTLS_SSL_CLIENT_KEY_EXCHANGE,
      MBEDTLS_SSL_CERTIFICATE_VERIFY,
      MBEDTLS_SSL_CLIENT_CHANGE_CIPHER_SPEC,
      MBEDTLS_SSL_CLIENT_FINISHED,
      MBEDTLS_SSL_SERVER_CHANGE_CIPHER_SPEC,
      MBEDTLS_SSL_SERVER_FINISHED,
      MBEDTLS_SSL_FLUSH_BUFFERS,
      MBEDTLS_SSL_HANDSHAKE_WRAPUP,
      MBEDTLS_SSL_HANDSHAKE_OVER,
      MBEDTLS_SSL_SERVER_NEW_SESSION_TICKET,
      MBEDTLS_SSL_SERVER_HELLO_VERIFY_REQUEST_SENT,
  }
  mbedtls_ssl_states;
  
  /*
   * The tls_prf function types.
   */
  typedef enum
  {
     MBEDTLS_SSL_TLS_PRF_NONE,
     MBEDTLS_SSL_TLS_PRF_SSL3,
     MBEDTLS_SSL_TLS_PRF_TLS1,
     MBEDTLS_SSL_TLS_PRF_SHA384,
     MBEDTLS_SSL_TLS_PRF_SHA256
  }
  mbedtls_tls_prf_types;
  /**
   * \brief          Callback type: send data on the network.
   *
   * \note           That callback may be either blocking or non-blocking.
   *
   * \param ctx      Context for the send callback (typically a file descriptor)
   * \param buf      Buffer holding the data to send
   * \param len      Length of the data to send
   *
   * \return         The callback must return the number of bytes sent if any,
   *                 or a non-zero error code.
   *                 If performing non-blocking I/O, \c MBEDTLS_ERR_SSL_WANT_WRITE
   *                 must be returned when the operation would block.
   *
   * \note           The callback is allowed to send fewer bytes than requested.
   *                 It must always return the number of bytes actually sent.
   */
  typedef int mbedtls_ssl_send_t( void *ctx,
                                  const unsigned char *buf,
                                  size_t len );
  
  /**
   * \brief          Callback type: receive data from the network.
   *
   * \note           That callback may be either blocking or non-blocking.
   *
   * \param ctx      Context for the receive callback (typically a file
   *                 descriptor)
   * \param buf      Buffer to write the received data to
   * \param len      Length of the receive buffer
   *
   * \return         The callback must return the number of bytes received,
   *                 or a non-zero error code.
   *                 If performing non-blocking I/O, \c MBEDTLS_ERR_SSL_WANT_READ
   *                 must be returned when the operation would block.
   *
   * \note           The callback may receive fewer bytes than the length of the
   *                 buffer. It must always return the number of bytes actually
   *                 received and written to the buffer.
   */
  typedef int mbedtls_ssl_recv_t( void *ctx,
                                  unsigned char *buf,
                                  size_t len );
  
  /**
   * \brief          Callback type: receive data from the network, with timeout
   *
   * \note           That callback must block until data is received, or the
   *                 timeout delay expires, or the operation is interrupted by a
   *                 signal.
   *
   * \param ctx      Context for the receive callback (typically a file descriptor)
   * \param buf      Buffer to write the received data to
   * \param len      Length of the receive buffer
   * \param timeout  Maximum nomber of millisecondes to wait for data
   *                 0 means no timeout (potentially waiting forever)
   *
   * \return         The callback must return the number of bytes received,
   *                 or a non-zero error code:
   *                 \c MBEDTLS_ERR_SSL_TIMEOUT if the operation timed out,
   *                 \c MBEDTLS_ERR_SSL_WANT_READ if interrupted by a signal.
   *
   * \note           The callback may receive fewer bytes than the length of the
   *                 buffer. It must always return the number of bytes actually
   *                 received and written to the buffer.
   */
  typedef int mbedtls_ssl_recv_timeout_t( void *ctx,
                                          unsigned char *buf,
                                          size_t len,
                                          uint32_t timeout );
  /**
   * \brief          Callback type: set a pair of timers/delays to watch
   *
   * \param ctx      Context pointer
   * \param int_ms   Intermediate delay in milliseconds
   * \param fin_ms   Final delay in milliseconds
   *                 0 cancels the current timer.
   *
   * \note           This callback must at least store the necessary information
   *                 for the associated \c mbedtls_ssl_get_timer_t callback to
   *                 return correct information.
   *
   * \note           If using a event-driven style of programming, an event must
   *                 be generated when the final delay is passed. The event must
   *                 cause a call to \c mbedtls_ssl_handshake() with the proper
   *                 SSL context to be scheduled. Care must be taken to ensure
   *                 that at most one such call happens at a time.
   *
   * \note           Only one timer at a time must be running. Calling this
   *                 function while a timer is running must cancel it. Cancelled
   *                 timers must not generate any event.
   */
  typedef void mbedtls_ssl_set_timer_t( void * ctx,
                                        uint32_t int_ms,
                                        uint32_t fin_ms );
  
  /**
   * \brief          Callback type: get status of timers/delays
   *
   * \param ctx      Context pointer
   *
   * \return         This callback must return:
   *                 -1 if cancelled (fin_ms == 0),
   *                  0 if none of the delays have passed,
   *                  1 if only the intermediate delay has passed,
   *                  2 if the final delay has passed.
   */
  typedef int mbedtls_ssl_get_timer_t( void * ctx );
  
  /* Defined below */
  typedef struct mbedtls_ssl_session mbedtls_ssl_session;
  typedef struct mbedtls_ssl_context mbedtls_ssl_context;
  typedef struct mbedtls_ssl_config  mbedtls_ssl_config;
  
  /* Defined in ssl_internal.h */
  typedef struct mbedtls_ssl_transform mbedtls_ssl_transform;
  typedef struct mbedtls_ssl_handshake_params mbedtls_ssl_handshake_params;
  typedef struct mbedtls_ssl_sig_hash_set_t mbedtls_ssl_sig_hash_set_t;
  #if defined(MBEDTLS_X509_CRT_PARSE_C)
  typedef struct mbedtls_ssl_key_cert mbedtls_ssl_key_cert;
  #endif
  #if defined(MBEDTLS_SSL_PROTO_DTLS)
  typedef struct mbedtls_ssl_flight_item mbedtls_ssl_flight_item;
  #endif
  
  #if defined(MBEDTLS_SSL_ASYNC_PRIVATE)
  #if defined(MBEDTLS_X509_CRT_PARSE_C)
  /**
   * \brief           Callback type: start external signature operation.
   *
   *                  This callback is called during an SSL handshake to start
   *                  a signature decryption operation using an
   *                  external processor. The parameter \p cert contains
   *                  the public key; it is up to the callback function to
   *                  determine how to access the associated private key.
   *
   *                  This function typically sends or enqueues a request, and
   *                  does not wait for the operation to complete. This allows
   *                  the handshake step to be non-blocking.
   *
   *                  The parameters \p ssl and \p cert are guaranteed to remain
   *                  valid throughout the handshake. On the other hand, this
   *                  function must save the contents of \p hash if the value
   *                  is needed for later processing, because the \p hash buffer
   *                  is no longer valid after this function returns.
   *
   *                  This function may call mbedtls_ssl_set_async_operation_data()
   *                  to store an operation context for later retrieval
   *                  by the resume or cancel callback.
   *
   * \note            For RSA signatures, this function must produce output
   *                  that is consistent with PKCS#1 v1.5 in the same way as
   *                  mbedtls_rsa_pkcs1_sign(). Before the private key operation,
   *                  apply the padding steps described in RFC 8017, section 9.2
   *                  "EMSA-PKCS1-v1_5" as follows.
   *                  - If \p md_alg is #MBEDTLS_MD_NONE, apply the PKCS#1 v1.5
   *                    encoding, treating \p hash as the DigestInfo to be
   *                    padded. In other words, apply EMSA-PKCS1-v1_5 starting
   *                    from step 3, with `T = hash` and `tLen = hash_len`.
   *                  - If `md_alg != MBEDTLS_MD_NONE`, apply the PKCS#1 v1.5
   *                    encoding, treating \p hash as the hash to be encoded and
   *                    padded. In other words, apply EMSA-PKCS1-v1_5 starting
   *                    from step 2, with `digestAlgorithm` obtained by calling
   *                    mbedtls_oid_get_oid_by_md() on \p md_alg.
   *
   * \note            For ECDSA signatures, the output format is the DER encoding
   *                  `Ecdsa-Sig-Value` defined in
   *                  [RFC 4492 section 5.4](https://tools.ietf.org/html/rfc4492#section-5.4).
   *
   * \param ssl             The SSL connection instance. It should not be
   *                        modified other than via
   *                        mbedtls_ssl_set_async_operation_data().
   * \param cert            Certificate containing the public key.
   *                        In simple cases, this is one of the pointers passed to
   *                        mbedtls_ssl_conf_own_cert() when configuring the SSL
   *                        connection. However, if other callbacks are used, this
   *                        property may not hold. For example, if an SNI callback
   *                        is registered with mbedtls_ssl_conf_sni(), then
   *                        this callback determines what certificate is used.
   * \param md_alg          Hash algorithm.
   * \param hash            Buffer containing the hash. This buffer is
   *                        no longer valid when the function returns.
   * \param hash_len        Size of the \c hash buffer in bytes.
   *
   * \return          0 if the operation was started successfully and the SSL
   *                  stack should call the resume callback immediately.
   * \return          #MBEDTLS_ERR_SSL_ASYNC_IN_PROGRESS if the operation
   *                  was started successfully and the SSL stack should return
   *                  immediately without calling the resume callback yet.
   * \return          #MBEDTLS_ERR_SSL_HW_ACCEL_FALLTHROUGH if the external
   *                  processor does not support this key. The SSL stack will
   *                  use the private key object instead.
   * \return          Any other error indicates a fatal failure and is
   *                  propagated up the call chain. The callback should
   *                  use \c MBEDTLS_ERR_PK_xxx error codes, and <b>must not</b>
   *                  use \c MBEDTLS_ERR_SSL_xxx error codes except as
   *                  directed in the documentation of this callback.
   */
  typedef int mbedtls_ssl_async_sign_t( mbedtls_ssl_context *ssl,
                                        mbedtls_x509_crt *cert,
                                        mbedtls_md_type_t md_alg,
                                        const unsigned char *hash,
                                        size_t hash_len );
  
  /**
   * \brief           Callback type: start external decryption operation.
   *
   *                  This callback is called during an SSL handshake to start
   *                  an RSA decryption operation using an
   *                  external processor. The parameter \p cert contains
   *                  the public key; it is up to the callback function to
   *                  determine how to access the associated private key.
   *
   *                  This function typically sends or enqueues a request, and
   *                  does not wait for the operation to complete. This allows
   *                  the handshake step to be non-blocking.
   *
   *                  The parameters \p ssl and \p cert are guaranteed to remain
   *                  valid throughout the handshake. On the other hand, this
   *                  function must save the contents of \p input if the value
   *                  is needed for later processing, because the \p input buffer
   *                  is no longer valid after this function returns.
   *
   *                  This function may call mbedtls_ssl_set_async_operation_data()
   *                  to store an operation context for later retrieval
   *                  by the resume or cancel callback.
   *
   * \warning         RSA decryption as used in TLS is subject to a potential
   *                  timing side channel attack first discovered by Bleichenbacher
   *                  in 1998. This attack can be remotely exploitable
   *                  in practice. To avoid this attack, you must ensure that
   *                  if the callback performs an RSA decryption, the time it
   *                  takes to execute and return the result does not depend
   *                  on whether the RSA decryption succeeded or reported
   *                  invalid padding.
   *
   * \param ssl             The SSL connection instance. It should not be
   *                        modified other than via
   *                        mbedtls_ssl_set_async_operation_data().
   * \param cert            Certificate containing the public key.
   *                        In simple cases, this is one of the pointers passed to
   *                        mbedtls_ssl_conf_own_cert() when configuring the SSL
   *                        connection. However, if other callbacks are used, this
   *                        property may not hold. For example, if an SNI callback
   *                        is registered with mbedtls_ssl_conf_sni(), then
   *                        this callback determines what certificate is used.
   * \param input           Buffer containing the input ciphertext. This buffer
   *                        is no longer valid when the function returns.
   * \param input_len       Size of the \p input buffer in bytes.
   *
   * \return          0 if the operation was started successfully and the SSL
   *                  stack should call the resume callback immediately.
   * \return          #MBEDTLS_ERR_SSL_ASYNC_IN_PROGRESS if the operation
   *                  was started successfully and the SSL stack should return
   *                  immediately without calling the resume callback yet.
   * \return          #MBEDTLS_ERR_SSL_HW_ACCEL_FALLTHROUGH if the external
   *                  processor does not support this key. The SSL stack will
   *                  use the private key object instead.
   * \return          Any other error indicates a fatal failure and is
   *                  propagated up the call chain. The callback should
   *                  use \c MBEDTLS_ERR_PK_xxx error codes, and <b>must not</b>
   *                  use \c MBEDTLS_ERR_SSL_xxx error codes except as
   *                  directed in the documentation of this callback.
   */
  typedef int mbedtls_ssl_async_decrypt_t( mbedtls_ssl_context *ssl,
                                           mbedtls_x509_crt *cert,
                                           const unsigned char *input,
                                           size_t input_len );
  #endif /* MBEDTLS_X509_CRT_PARSE_C */
  
  /**
   * \brief           Callback type: resume external operation.
   *
   *                  This callback is called during an SSL handshake to resume
   *                  an external operation started by the
   *                  ::mbedtls_ssl_async_sign_t or
   *                  ::mbedtls_ssl_async_decrypt_t callback.
   *
   *                  This function typically checks the status of a pending
   *                  request or causes the request queue to make progress, and
   *                  does not wait for the operation to complete. This allows
   *                  the handshake step to be non-blocking.
   *
   *                  This function may call mbedtls_ssl_get_async_operation_data()
   *                  to retrieve an operation context set by the start callback.
   *                  It may call mbedtls_ssl_set_async_operation_data() to modify
   *                  this context.
   *
   *                  Note that when this function returns a status other than
   *                  #MBEDTLS_ERR_SSL_ASYNC_IN_PROGRESS, it must free any
   *                  resources associated with the operation.
   *
   * \param ssl             The SSL connection instance. It should not be
   *                        modified other than via
   *                        mbedtls_ssl_set_async_operation_data().
   * \param output          Buffer containing the output (signature or decrypted
   *                        data) on success.
   * \param output_len      On success, number of bytes written to \p output.
   * \param output_size     Size of the \p output buffer in bytes.
   *
   * \return          0 if output of the operation is available in the
   *                  \p output buffer.
   * \return          #MBEDTLS_ERR_SSL_ASYNC_IN_PROGRESS if the operation
   *                  is still in progress. Subsequent requests for progress
   *                  on the SSL connection will call the resume callback
   *                  again.
   * \return          Any other error means that the operation is aborted.
   *                  The SSL handshake is aborted. The callback should
   *                  use \c MBEDTLS_ERR_PK_xxx error codes, and <b>must not</b>
   *                  use \c MBEDTLS_ERR_SSL_xxx error codes except as
   *                  directed in the documentation of this callback.
   */
  typedef int mbedtls_ssl_async_resume_t( mbedtls_ssl_context *ssl,
                                          unsigned char *output,
                                          size_t *output_len,
                                          size_t output_size );
  
  /**
   * \brief           Callback type: cancel external operation.
   *
   *                  This callback is called if an SSL connection is closed
   *                  while an asynchronous operation is in progress. Note that
   *                  this callback is not called if the
   *                  ::mbedtls_ssl_async_resume_t callback has run and has
   *                  returned a value other than
   *                  #MBEDTLS_ERR_SSL_ASYNC_IN_PROGRESS, since in that case
   *                  the asynchronous operation has already completed.
   *
   *                  This function may call mbedtls_ssl_get_async_operation_data()
   *                  to retrieve an operation context set by the start callback.
   *
   * \param ssl             The SSL connection instance. It should not be
   *                        modified.
   */
  typedef void mbedtls_ssl_async_cancel_t( mbedtls_ssl_context *ssl );
  #endif /* MBEDTLS_SSL_ASYNC_PRIVATE */
  
  #if defined(MBEDTLS_KEY_EXCHANGE_WITH_CERT_ENABLED) &&        \
      !defined(MBEDTLS_SSL_KEEP_PEER_CERTIFICATE)
  #define MBEDTLS_SSL_PEER_CERT_DIGEST_MAX_LEN  48
  #if defined(MBEDTLS_SHA256_C)
  #define MBEDTLS_SSL_PEER_CERT_DIGEST_DFL_TYPE MBEDTLS_MD_SHA256
  #define MBEDTLS_SSL_PEER_CERT_DIGEST_DFL_LEN  32
  #elif defined(MBEDTLS_SHA512_C)
  #define MBEDTLS_SSL_PEER_CERT_DIGEST_DFL_TYPE MBEDTLS_MD_SHA384
  #define MBEDTLS_SSL_PEER_CERT_DIGEST_DFL_LEN  48
  #elif defined(MBEDTLS_SHA1_C)
  #define MBEDTLS_SSL_PEER_CERT_DIGEST_DFL_TYPE MBEDTLS_MD_SHA1
  #define MBEDTLS_SSL_PEER_CERT_DIGEST_DFL_LEN  20
  #else
  /* This is already checked in check_config.h, but be sure. */
  #error "Bad configuration - need SHA-1, SHA-256 or SHA-512 enabled to compute digest of peer CRT."
  #endif
  #endif /* MBEDTLS_KEY_EXCHANGE_WITH_CERT_ENABLED &&
            !MBEDTLS_SSL_KEEP_PEER_CERTIFICATE */
  
  /*
   * This structure is used for storing current session data.
   *
   * Note: when changing this definition, we need to check and update:
   *  - in tests/suites/test_suite_ssl.function:
   *      ssl_populate_session() and ssl_serialize_session_save_load()
   *  - in library/ssl_tls.c:
   *      mbedtls_ssl_session_init() and mbedtls_ssl_session_free()
   *      mbedtls_ssl_session_save() and ssl_session_load()
   *      ssl_session_copy()
   */
  struct mbedtls_ssl_session
  {
  #if defined(MBEDTLS_HAVE_TIME)
      mbedtls_time_t start;       /*!< starting time      */
  #endif
      int ciphersuite;            /*!< chosen ciphersuite */
      int compression;            /*!< chosen compression */
      size_t id_len;              /*!< session id length  */
      unsigned char id[32];       /*!< session identifier */
      unsigned char master[48];   /*!< the master secret  */
  
  #if defined(MBEDTLS_X509_CRT_PARSE_C)
  #if defined(MBEDTLS_SSL_KEEP_PEER_CERTIFICATE)
      mbedtls_x509_crt *peer_cert;       /*!< peer X.509 cert chain */
  #else /* MBEDTLS_SSL_KEEP_PEER_CERTIFICATE */
      /*! The digest of the peer's end-CRT. This must be kept to detect CRT
       *  changes during renegotiation, mitigating the triple handshake attack. */
      unsigned char *peer_cert_digest;
      size_t peer_cert_digest_len;
      mbedtls_md_type_t peer_cert_digest_type;
  #endif /* !MBEDTLS_SSL_KEEP_PEER_CERTIFICATE */
  #endif /* MBEDTLS_X509_CRT_PARSE_C */
      uint32_t verify_result;          /*!<  verification result     */
  
  #if defined(MBEDTLS_SSL_SESSION_TICKETS) && defined(MBEDTLS_SSL_CLI_C)
      unsigned char *ticket;      /*!< RFC 5077 session ticket */
      size_t ticket_len;          /*!< session ticket length   */
      uint32_t ticket_lifetime;   /*!< ticket lifetime hint    */
  #endif /* MBEDTLS_SSL_SESSION_TICKETS && MBEDTLS_SSL_CLI_C */
  
  #if defined(MBEDTLS_SSL_MAX_FRAGMENT_LENGTH)
      unsigned char mfl_code;     /*!< MaxFragmentLength negotiated by peer */
  #endif /* MBEDTLS_SSL_MAX_FRAGMENT_LENGTH */
  
  #if defined(MBEDTLS_SSL_TRUNCATED_HMAC)
      int trunc_hmac;             /*!< flag for truncated hmac activation   */
  #endif /* MBEDTLS_SSL_TRUNCATED_HMAC */
  
  #if defined(MBEDTLS_SSL_ENCRYPT_THEN_MAC)
      int encrypt_then_mac;       /*!< flag for EtM activation                */
  #endif
  };
  
  /**
   * SSL/TLS configuration to be shared between mbedtls_ssl_context structures.
   */
  struct mbedtls_ssl_config
  {
      /* Group items by size (largest first) to minimize padding overhead */
  
      /*
       * Pointers
       */
  
      const int *ciphersuite_list[4]; /*!< allowed ciphersuites per version   */
  
      /** Callback for printing debug output                                  */
      void (*f_dbg)(void *, int, const char *, int, const char *);
      void *p_dbg;                    /*!< context for the debug function     */
  
      /** Callback for getting (pseudo-)random numbers                        */
      int  (*f_rng)(void *, unsigned char *, size_t);
      void *p_rng;                    /*!< context for the RNG function       */
  
      /** Callback to retrieve a session from the cache                       */
      int (*f_get_cache)(void *, mbedtls_ssl_session *);
      /** Callback to store a session into the cache                          */
      int (*f_set_cache)(void *, const mbedtls_ssl_session *);
      void *p_cache;                  /*!< context for cache callbacks        */
  
  #if defined(MBEDTLS_SSL_SERVER_NAME_INDICATION)
      /** Callback for setting cert according to SNI extension                */
      int (*f_sni)(void *, mbedtls_ssl_context *, const unsigned char *, size_t);
      void *p_sni;                    /*!< context for SNI callback           */
  #endif
  
  #if defined(MBEDTLS_X509_CRT_PARSE_C)
      /** Callback to customize X.509 certificate chain verification          */
      int (*f_vrfy)(void *, mbedtls_x509_crt *, int, uint32_t *);
      void *p_vrfy;                   /*!< context for X.509 verify calllback */
  #endif
  
  #if defined(MBEDTLS_KEY_EXCHANGE_SOME_PSK_ENABLED)
      /** Callback to retrieve PSK key from identity                          */
      int (*f_psk)(void *, mbedtls_ssl_context *, const unsigned char *, size_t);
      void *p_psk;                    /*!< context for PSK callback           */
  #endif
  
  #if defined(MBEDTLS_SSL_DTLS_HELLO_VERIFY) && defined(MBEDTLS_SSL_SRV_C)
      /** Callback to create & write a cookie for ClientHello veirifcation    */
      int (*f_cookie_write)( void *, unsigned char **, unsigned char *,
                             const unsigned char *, size_t );
      /** Callback to verify validity of a ClientHello cookie                 */
      int (*f_cookie_check)( void *, const unsigned char *, size_t,
                             const unsigned char *, size_t );
      void *p_cookie;                 /*!< context for the cookie callbacks   */
  #endif
  
  #if defined(MBEDTLS_SSL_SESSION_TICKETS) && defined(MBEDTLS_SSL_SRV_C)
      /** Callback to create & write a session ticket                         */
      int (*f_ticket_write)( void *, const mbedtls_ssl_session *,
              unsigned char *, const unsigned char *, size_t *, uint32_t * );
      /** Callback to parse a session ticket into a session structure         */
      int (*f_ticket_parse)( void *, mbedtls_ssl_session *, unsigned char *, size_t);
      void *p_ticket;                 /*!< context for the ticket callbacks   */
  #endif /* MBEDTLS_SSL_SESSION_TICKETS && MBEDTLS_SSL_SRV_C */
  
  #if defined(MBEDTLS_SSL_EXPORT_KEYS)
      /** Callback to export key block and master secret                      */
      int (*f_export_keys)( void *, const unsigned char *,
              const unsigned char *, size_t, size_t, size_t );
      /** Callback to export key block, master secret,
       *  tls_prf and random bytes. Should replace f_export_keys    */
      int (*f_export_keys_ext)( void *, const unsigned char *,
                  const unsigned char *, size_t, size_t, size_t,
                  const unsigned char[32], const unsigned char[32],
                  mbedtls_tls_prf_types );
      void *p_export_keys;            /*!< context for key export callback    */
  #endif
  
  #if defined(MBEDTLS_SSL_DTLS_CONNECTION_ID)
      size_t cid_len; /*!< The length of CIDs for incoming DTLS records.      */
  #endif /* MBEDTLS_SSL_DTLS_CONNECTION_ID */
  
  #if defined(MBEDTLS_X509_CRT_PARSE_C)
      const mbedtls_x509_crt_profile *cert_profile; /*!< verification profile */
      mbedtls_ssl_key_cert *key_cert; /*!< own certificate/key pair(s)        */
      mbedtls_x509_crt *ca_chain;     /*!< trusted CAs                        */
      mbedtls_x509_crl *ca_crl;       /*!< trusted CAs CRLs                   */
  #if defined(MBEDTLS_X509_TRUSTED_CERTIFICATE_CALLBACK)
      mbedtls_x509_crt_ca_cb_t f_ca_cb;
      void *p_ca_cb;
  #endif /* MBEDTLS_X509_TRUSTED_CERTIFICATE_CALLBACK */
  #endif /* MBEDTLS_X509_CRT_PARSE_C */
  
  #if defined(MBEDTLS_SSL_ASYNC_PRIVATE)
  #if defined(MBEDTLS_X509_CRT_PARSE_C)
      mbedtls_ssl_async_sign_t *f_async_sign_start; /*!< start asynchronous signature operation */
      mbedtls_ssl_async_decrypt_t *f_async_decrypt_start; /*!< start asynchronous decryption operation */
  #endif /* MBEDTLS_X509_CRT_PARSE_C */
      mbedtls_ssl_async_resume_t *f_async_resume; /*!< resume asynchronous operation */
      mbedtls_ssl_async_cancel_t *f_async_cancel; /*!< cancel asynchronous operation */
      void *p_async_config_data; /*!< Configuration data set by mbedtls_ssl_conf_async_private_cb(). */
  #endif /* MBEDTLS_SSL_ASYNC_PRIVATE */
  
  #if defined(MBEDTLS_KEY_EXCHANGE_WITH_CERT_ENABLED)
      const int *sig_hashes;          /*!< allowed signature hashes           */
  #endif
  
  #if defined(MBEDTLS_ECP_C)
      const mbedtls_ecp_group_id *curve_list; /*!< allowed curves             */
  #endif
  
  #if defined(MBEDTLS_DHM_C)
      mbedtls_mpi dhm_P;              /*!< prime modulus for DHM              */
      mbedtls_mpi dhm_G;              /*!< generator for DHM                  */
  #endif
  
  #if defined(MBEDTLS_KEY_EXCHANGE_SOME_PSK_ENABLED)
  
  #if defined(MBEDTLS_USE_PSA_CRYPTO)
      psa_key_handle_t psk_opaque; /*!< PSA key slot holding opaque PSK.
                                    *   This field should only be set via
                                    *   mbedtls_ssl_conf_psk_opaque().
                                    *   If either no PSK or a raw PSK have
                                    *   been configured, this has value \c 0. */
  #endif /* MBEDTLS_USE_PSA_CRYPTO */
  
      unsigned char *psk;      /*!< The raw pre-shared key. This field should
                                *   only be set via mbedtls_ssl_conf_psk().
                                *   If either no PSK or an opaque PSK
                                *   have been configured, this has value NULL. */
      size_t         psk_len;  /*!< The length of the raw pre-shared key.
                                *   This field should only be set via
                                *   mbedtls_ssl_conf_psk().
                                *   Its value is non-zero if and only if
                                *   \c psk is not \c NULL. */
  
      unsigned char *psk_identity;    /*!< The PSK identity for PSK negotiation.
                                       *   This field should only be set via
                                       *   mbedtls_ssl_conf_psk().
                                       *   This is set if and only if either
                                       *   \c psk or \c psk_opaque are set. */
      size_t         psk_identity_len;/*!< The length of PSK identity.
                                       *   This field should only be set via
                                       *   mbedtls_ssl_conf_psk().
                                       *   Its value is non-zero if and only if
                                       *   \c psk is not \c NULL or \c psk_opaque
                                       *   is not \c 0. */
  #endif /* MBEDTLS_KEY_EXCHANGE_SOME_PSK_ENABLED */
  
  #if defined(MBEDTLS_SSL_ALPN)
      const char **alpn_list;         /*!< ordered list of protocols          */
  #endif
  
      /*
       * Numerical settings (int then char)
       */
  
      uint32_t read_timeout;          /*!< timeout for mbedtls_ssl_read (ms)  */
  
  #if defined(MBEDTLS_SSL_PROTO_DTLS)
      uint32_t hs_timeout_min;        /*!< initial value of the handshake
                                           retransmission timeout (ms)        */
      uint32_t hs_timeout_max;        /*!< maximum value of the handshake
                                           retransmission timeout (ms)        */
  #endif
  
  #if defined(MBEDTLS_SSL_RENEGOTIATION)
      int renego_max_records;         /*!< grace period for renegotiation     */
      unsigned char renego_period[8]; /*!< value of the record counters
                                           that triggers renegotiation        */
  #endif
  
  #if defined(MBEDTLS_SSL_DTLS_BADMAC_LIMIT)
      unsigned int badmac_limit;      /*!< limit of records with a bad MAC    */
  #endif
  
  #if defined(MBEDTLS_DHM_C) && defined(MBEDTLS_SSL_CLI_C)
      unsigned int dhm_min_bitlen;    /*!< min. bit length of the DHM prime   */
  #endif
  
      unsigned char max_major_ver;    /*!< max. major version used            */
      unsigned char max_minor_ver;    /*!< max. minor version used            */
      unsigned char min_major_ver;    /*!< min. major version used            */
      unsigned char min_minor_ver;    /*!< min. minor version used            */
  
      /*
       * Flags (bitfields)
       */
  
      unsigned int endpoint : 1;      /*!< 0: client, 1: server               */
      unsigned int transport : 1;     /*!< stream (TLS) or datagram (DTLS)    */
      unsigned int authmode : 2;      /*!< MBEDTLS_SSL_VERIFY_XXX             */
      /* needed even with renego disabled for LEGACY_BREAK_HANDSHAKE          */
      unsigned int allow_legacy_renegotiation : 2 ; /*!< MBEDTLS_LEGACY_XXX   */
  #if defined(MBEDTLS_ARC4_C)
      unsigned int arc4_disabled : 1; /*!< blacklist RC4 ciphersuites?        */
  #endif
  #if defined(MBEDTLS_SSL_MAX_FRAGMENT_LENGTH)
      unsigned int mfl_code : 3;      /*!< desired fragment length            */
  #endif
  #if defined(MBEDTLS_SSL_ENCRYPT_THEN_MAC)
      unsigned int encrypt_then_mac : 1 ; /*!< negotiate encrypt-then-mac?    */
  #endif
  #if defined(MBEDTLS_SSL_EXTENDED_MASTER_SECRET)
      unsigned int extended_ms : 1;   /*!< negotiate extended master secret?  */
  #endif
  #if defined(MBEDTLS_SSL_DTLS_ANTI_REPLAY)
      unsigned int anti_replay : 1;   /*!< detect and prevent replay?         */
  #endif
  #if defined(MBEDTLS_SSL_CBC_RECORD_SPLITTING)
      unsigned int cbc_record_splitting : 1;  /*!< do cbc record splitting    */
  #endif
  #if defined(MBEDTLS_SSL_RENEGOTIATION)
      unsigned int disable_renegotiation : 1; /*!< disable renegotiation?     */
  #endif
  #if defined(MBEDTLS_SSL_TRUNCATED_HMAC)
      unsigned int trunc_hmac : 1;    /*!< negotiate truncated hmac?          */
  #endif
  #if defined(MBEDTLS_SSL_SESSION_TICKETS)
      unsigned int session_tickets : 1;   /*!< use session tickets?           */
  #endif
  #if defined(MBEDTLS_SSL_FALLBACK_SCSV) && defined(MBEDTLS_SSL_CLI_C)
      unsigned int fallback : 1;      /*!< is this a fallback?                */
  #endif
  #if defined(MBEDTLS_SSL_SRV_C)
      unsigned int cert_req_ca_list : 1;  /*!< enable sending CA list in
                                            Certificate Request messages?     */
  #endif
  #if defined(MBEDTLS_SSL_DTLS_CONNECTION_ID)
      unsigned int ignore_unexpected_cid : 1; /*!< Determines whether DTLS
                                               *   record with unexpected CID
                                               *   should lead to failure.    */
  #endif /* MBEDTLS_SSL_DTLS_CONNECTION_ID */
  };
  
  
  struct mbedtls_ssl_context
  {
      const mbedtls_ssl_config *conf; /*!< configuration information          */
  
      /*
       * Miscellaneous
       */
      int state;                  /*!< SSL handshake: current state     */
  #if defined(MBEDTLS_SSL_RENEGOTIATION)
      int renego_status;          /*!< Initial, in progress, pending?   */
      int renego_records_seen;    /*!< Records since renego request, or with DTLS,
                                    number of retransmissions of request if
                                    renego_max_records is < 0           */
  #endif /* MBEDTLS_SSL_RENEGOTIATION */
  
      int major_ver;              /*!< equal to  MBEDTLS_SSL_MAJOR_VERSION_3    */
      int minor_ver;              /*!< either 0 (SSL3) or 1 (TLS1.0)    */
  
  #if defined(MBEDTLS_SSL_DTLS_BADMAC_LIMIT)
      unsigned badmac_seen;       /*!< records with a bad MAC received    */
  #endif /* MBEDTLS_SSL_DTLS_BADMAC_LIMIT */
  
  #if defined(MBEDTLS_X509_CRT_PARSE_C)
      /** Callback to customize X.509 certificate chain verification          */
      int (*f_vrfy)(void *, mbedtls_x509_crt *, int, uint32_t *);
      void *p_vrfy;                   /*!< context for X.509 verify callback */
  #endif
  
      mbedtls_ssl_send_t *f_send; /*!< Callback for network send */
      mbedtls_ssl_recv_t *f_recv; /*!< Callback for network receive */
      mbedtls_ssl_recv_timeout_t *f_recv_timeout;
                                  /*!< Callback for network receive with timeout */
  
      void *p_bio;                /*!< context for I/O operations   */
  
      /*
       * Session layer
       */
      mbedtls_ssl_session *session_in;            /*!<  current session data (in)   */
      mbedtls_ssl_session *session_out;           /*!<  current session data (out)  */
      mbedtls_ssl_session *session;               /*!<  negotiated session data     */
      mbedtls_ssl_session *session_negotiate;     /*!<  session data in negotiation */
  
      mbedtls_ssl_handshake_params *handshake;    /*!<  params required only during
                                                the handshake process        */
  
      /*
       * Record layer transformations
       */
      mbedtls_ssl_transform *transform_in;        /*!<  current transform params (in)   */
      mbedtls_ssl_transform *transform_out;       /*!<  current transform params (in)   */
      mbedtls_ssl_transform *transform;           /*!<  negotiated transform params     */
      mbedtls_ssl_transform *transform_negotiate; /*!<  transform params in negotiation */
  
      /*
       * Timers
       */
      void *p_timer;              /*!< context for the timer callbacks */
  
      mbedtls_ssl_set_timer_t *f_set_timer;       /*!< set timer callback */
      mbedtls_ssl_get_timer_t *f_get_timer;       /*!< get timer callback */
  
      /*
       * Record layer (incoming data)
       */
      unsigned char *in_buf;      /*!< input buffer                     */
      unsigned char *in_ctr;      /*!< 64-bit incoming message counter
                                       TLS: maintained by us
                                       DTLS: read from peer             */
      unsigned char *in_hdr;      /*!< start of record header           */
  #if defined(MBEDTLS_SSL_DTLS_CONNECTION_ID)
      unsigned char *in_cid;      /*!< The start of the CID;
                                   *   (the end is marked by in_len).   */
  #endif /* MBEDTLS_SSL_DTLS_CONNECTION_ID */
      unsigned char *in_len;      /*!< two-bytes message length field   */
      unsigned char *in_iv;       /*!< ivlen-byte IV                    */
      unsigned char *in_msg;      /*!< message contents (in_iv+ivlen)   */
      unsigned char *in_offt;     /*!< read offset in application data  */
  
      int in_msgtype;             /*!< record header: message type      */
      size_t in_msglen;           /*!< record header: message length    */
      size_t in_left;             /*!< amount of data read so far       */
  #if defined(MBEDTLS_SSL_VARIABLE_BUFFER_LENGTH)
      size_t in_buf_len;          /*!< length of input buffer           */
  #endif
  #if defined(MBEDTLS_SSL_PROTO_DTLS)
      uint16_t in_epoch;          /*!< DTLS epoch for incoming records  */
      size_t next_record_offset;  /*!< offset of the next record in datagram
                                       (equal to in_left if none)       */
  #endif /* MBEDTLS_SSL_PROTO_DTLS */
  #if defined(MBEDTLS_SSL_DTLS_ANTI_REPLAY)
      uint64_t in_window_top;     /*!< last validated record seq_num    */
      uint64_t in_window;         /*!< bitmask for replay detection     */
  #endif /* MBEDTLS_SSL_DTLS_ANTI_REPLAY */
  
      size_t in_hslen;            /*!< current handshake message length,
                                       including the handshake header   */
      int nb_zero;                /*!< # of 0-length encrypted messages */
  
      int keep_current_message;   /*!< drop or reuse current message
                                       on next call to record layer? */
  
  #if defined(MBEDTLS_SSL_PROTO_DTLS)
      uint8_t disable_datagram_packing;  /*!< Disable packing multiple records
                                          *   within a single datagram.  */
  #endif /* MBEDTLS_SSL_PROTO_DTLS */
  
      /*
       * Record layer (outgoing data)
       */
      unsigned char *out_buf;     /*!< output buffer                    */
      unsigned char *out_ctr;     /*!< 64-bit outgoing message counter  */
      unsigned char *out_hdr;     /*!< start of record header           */
  #if defined(MBEDTLS_SSL_DTLS_CONNECTION_ID)
      unsigned char *out_cid;     /*!< The start of the CID;
                                   *   (the end is marked by in_len).   */
  #endif /* MBEDTLS_SSL_DTLS_CONNECTION_ID */
      unsigned char *out_len;     /*!< two-bytes message length field   */
      unsigned char *out_iv;      /*!< ivlen-byte IV                    */
      unsigned char *out_msg;     /*!< message contents (out_iv+ivlen)  */
  
      int out_msgtype;            /*!< record header: message type      */
      size_t out_msglen;          /*!< record header: message length    */
      size_t out_left;            /*!< amount of data not yet written   */
  #if defined(MBEDTLS_SSL_VARIABLE_BUFFER_LENGTH)
      size_t out_buf_len;         /*!< length of output buffer          */
  #endif
  
      unsigned char cur_out_ctr[8]; /*!<  Outgoing record sequence  number. */
  
  #if defined(MBEDTLS_SSL_PROTO_DTLS)
      uint16_t mtu;               /*!< path mtu, used to fragment outgoing messages */
  #endif /* MBEDTLS_SSL_PROTO_DTLS */
  
  #if defined(MBEDTLS_ZLIB_SUPPORT)
      unsigned char *compress_buf;        /*!<  zlib data buffer        */
  #endif /* MBEDTLS_ZLIB_SUPPORT */
  #if defined(MBEDTLS_SSL_CBC_RECORD_SPLITTING)
      signed char split_done;     /*!< current record already splitted? */
  #endif /* MBEDTLS_SSL_CBC_RECORD_SPLITTING */
  
      /*
       * PKI layer
       */
      int client_auth;                    /*!<  flag for client auth.   */
  
      /*
       * User settings
       */
  #if defined(MBEDTLS_X509_CRT_PARSE_C)
      char *hostname;             /*!< expected peer CN for verification
                                       (and SNI if available)                 */
  #endif /* MBEDTLS_X509_CRT_PARSE_C */
  
  #if defined(MBEDTLS_SSL_ALPN)
      const char *alpn_chosen;    /*!<  negotiated protocol                   */
  #endif /* MBEDTLS_SSL_ALPN */
  
      /*
       * Information for DTLS hello verify
       */
  #if defined(MBEDTLS_SSL_DTLS_HELLO_VERIFY) && defined(MBEDTLS_SSL_SRV_C)
      unsigned char  *cli_id;         /*!<  transport-level ID of the client  */
      size_t          cli_id_len;     /*!<  length of cli_id                  */
  #endif /* MBEDTLS_SSL_DTLS_HELLO_VERIFY && MBEDTLS_SSL_SRV_C */
  
      /*
       * Secure renegotiation
       */
      /* needed to know when to send extension on server */
      int secure_renegotiation;           /*!<  does peer support legacy or
                                                secure renegotiation           */
  #if defined(MBEDTLS_SSL_RENEGOTIATION)
      size_t verify_data_len;             /*!<  length of verify data stored   */
      char own_verify_data[MBEDTLS_SSL_VERIFY_DATA_MAX_LEN]; /*!<  previous handshake verify data */
      char peer_verify_data[MBEDTLS_SSL_VERIFY_DATA_MAX_LEN]; /*!<  previous handshake verify data */
  #endif /* MBEDTLS_SSL_RENEGOTIATION */
  
  #if defined(MBEDTLS_SSL_DTLS_CONNECTION_ID)
      /* CID configuration to use in subsequent handshakes. */
  
      /*! The next incoming CID, chosen by the user and applying to
       *  all subsequent handshakes. This may be different from the
       *  CID currently used in case the user has re-configured the CID
       *  after an initial handshake. */
      unsigned char own_cid[ MBEDTLS_SSL_CID_IN_LEN_MAX ];
      uint8_t own_cid_len;   /*!< The length of \c own_cid. */
      uint8_t negotiate_cid; /*!< This indicates whether the CID extension should
                              *   be negotiated in the next handshake or not.
                              *   Possible values are #MBEDTLS_SSL_CID_ENABLED
                              *   and #MBEDTLS_SSL_CID_DISABLED. */
  #endif /* MBEDTLS_SSL_DTLS_CONNECTION_ID */
  };
  
  #if defined(MBEDTLS_SSL_HW_RECORD_ACCEL)
  
  #if !defined(MBEDTLS_DEPRECATED_REMOVED)
  
  #define MBEDTLS_SSL_CHANNEL_OUTBOUND   MBEDTLS_DEPRECATED_NUMERIC_CONSTANT( 0 )
  #define MBEDTLS_SSL_CHANNEL_INBOUND    MBEDTLS_DEPRECATED_NUMERIC_CONSTANT( 1 )
  
  #if defined(MBEDTLS_DEPRECATED_WARNING)
  #define MBEDTLS_DEPRECATED      __attribute__((deprecated))
  #else
  #define MBEDTLS_DEPRECATED
  #endif /* MBEDTLS_DEPRECATED_WARNING */
  
  MBEDTLS_DEPRECATED extern int (*mbedtls_ssl_hw_record_init)(
                      mbedtls_ssl_context *ssl,
                      const unsigned char *key_enc, const unsigned char *key_dec,
                      size_t keylen,
                      const unsigned char *iv_enc,  const unsigned char *iv_dec,
                      size_t ivlen,
                      const unsigned char *mac_enc, const unsigned char *mac_dec,
                      size_t maclen);
  MBEDTLS_DEPRECATED extern int (*mbedtls_ssl_hw_record_activate)(
                                                      mbedtls_ssl_context *ssl,
                                                      int direction );
  MBEDTLS_DEPRECATED extern int (*mbedtls_ssl_hw_record_reset)(
                                                      mbedtls_ssl_context *ssl );
  MBEDTLS_DEPRECATED extern int (*mbedtls_ssl_hw_record_write)(
                                                      mbedtls_ssl_context *ssl );
  MBEDTLS_DEPRECATED extern int (*mbedtls_ssl_hw_record_read)(
                                                      mbedtls_ssl_context *ssl );
  MBEDTLS_DEPRECATED extern int (*mbedtls_ssl_hw_record_finish)(
                                                      mbedtls_ssl_context *ssl );
  
  #undef MBEDTLS_DEPRECATED
  #endif /* !MBEDTLS_DEPRECATED_REMOVED */
  
  #endif /* MBEDTLS_SSL_HW_RECORD_ACCEL */
  
  /**
   * \brief               Return the name of the ciphersuite associated with the
   *                      given ID
   *
   * \param ciphersuite_id SSL ciphersuite ID
   *
   * \return              a string containing the ciphersuite name
   */
  const char *mbedtls_ssl_get_ciphersuite_name( const int ciphersuite_id );
  
  /**
   * \brief               Return the ID of the ciphersuite associated with the
   *                      given name
   *
   * \param ciphersuite_name SSL ciphersuite name
   *
   * \return              the ID with the ciphersuite or 0 if not found
   */
  int mbedtls_ssl_get_ciphersuite_id( const char *ciphersuite_name );
  
  /**
   * \brief          Initialize an SSL context
   *                 Just makes the context ready for mbedtls_ssl_setup() or
   *                 mbedtls_ssl_free()
   *
   * \param ssl      SSL context
   */
  void mbedtls_ssl_init( mbedtls_ssl_context *ssl );
  
  /**
   * \brief          Set up an SSL context for use
   *
   * \note           No copy of the configuration context is made, it can be
   *                 shared by many mbedtls_ssl_context structures.
   *
   * \warning        The conf structure will be accessed during the session.
   *                 It must not be modified or freed as long as the session
   *                 is active.
   *
   * \warning        This function must be called exactly once per context.
   *                 Calling mbedtls_ssl_setup again is not supported, even
   *                 if no session is active.
   *
   * \param ssl      SSL context
   * \param conf     SSL configuration to use
   *
   * \return         0 if successful, or MBEDTLS_ERR_SSL_ALLOC_FAILED if
   *                 memory allocation failed
   */
  int mbedtls_ssl_setup( mbedtls_ssl_context *ssl,
                         const mbedtls_ssl_config *conf );
  
  /**
   * \brief          Reset an already initialized SSL context for re-use
   *                 while retaining application-set variables, function
   *                 pointers and data.
   *
   * \param ssl      SSL context
   * \return         0 if successful, or MBEDTLS_ERR_SSL_ALLOC_FAILED,
                     MBEDTLS_ERR_SSL_HW_ACCEL_FAILED or
   *                 MBEDTLS_ERR_SSL_COMPRESSION_FAILED
   */
  int mbedtls_ssl_session_reset( mbedtls_ssl_context *ssl );
  
  /**
   * \brief          Set the current endpoint type
   *
   * \param conf     SSL configuration
   * \param endpoint must be MBEDTLS_SSL_IS_CLIENT or MBEDTLS_SSL_IS_SERVER
   */
  void mbedtls_ssl_conf_endpoint( mbedtls_ssl_config *conf, int endpoint );
  
  /**
   * \brief           Set the transport type (TLS or DTLS).
   *                  Default: TLS
   *
   * \note            For DTLS, you must either provide a recv callback that
   *                  doesn't block, or one that handles timeouts, see
   *                  \c mbedtls_ssl_set_bio(). You also need to provide timer
   *                  callbacks with \c mbedtls_ssl_set_timer_cb().
   *
   * \param conf      SSL configuration
   * \param transport transport type:
   *                  MBEDTLS_SSL_TRANSPORT_STREAM for TLS,
   *                  MBEDTLS_SSL_TRANSPORT_DATAGRAM for DTLS.
   */
  void mbedtls_ssl_conf_transport( mbedtls_ssl_config *conf, int transport );
  
  /**
   * \brief          Set the certificate verification mode
   *                 Default: NONE on server, REQUIRED on client
   *
   * \param conf     SSL configuration
   * \param authmode can be:
   *
   *  MBEDTLS_SSL_VERIFY_NONE:      peer certificate is not checked
   *                        (default on server)
   *                        (insecure on client)
   *
   *  MBEDTLS_SSL_VERIFY_OPTIONAL:  peer certificate is checked, however the
   *                        handshake continues even if verification failed;
   *                        mbedtls_ssl_get_verify_result() can be called after the
   *                        handshake is complete.
   *
   *  MBEDTLS_SSL_VERIFY_REQUIRED:  peer *must* present a valid certificate,
   *                        handshake is aborted if verification failed.
   *                        (default on client)
   *
   * \note On client, MBEDTLS_SSL_VERIFY_REQUIRED is the recommended mode.
   * With MBEDTLS_SSL_VERIFY_OPTIONAL, the user needs to call mbedtls_ssl_get_verify_result() at
   * the right time(s), which may not be obvious, while REQUIRED always perform
   * the verification as soon as possible. For example, REQUIRED was protecting
   * against the "triple handshake" attack even before it was found.
   */
  void mbedtls_ssl_conf_authmode( mbedtls_ssl_config *conf, int authmode );
  
  #if defined(MBEDTLS_X509_CRT_PARSE_C)
  /**
   * \brief          Set the verification callback (Optional).
   *
   *                 If set, the provided verify callback is called for each
   *                 certificate in the peer's CRT chain, including the trusted
   *                 root. For more information, please see the documentation of
   *                 \c mbedtls_x509_crt_verify().
   *
   * \note           For per context callbacks and contexts, please use
   *                 mbedtls_ssl_set_verify() instead.
   *
   * \param conf     The SSL configuration to use.
   * \param f_vrfy   The verification callback to use during CRT verification.
   * \param p_vrfy   The opaque context to be passed to the callback.
   */
  void mbedtls_ssl_conf_verify( mbedtls_ssl_config *conf,
                       int (*f_vrfy)(void *, mbedtls_x509_crt *, int, uint32_t *),
                       void *p_vrfy );
  #endif /* MBEDTLS_X509_CRT_PARSE_C */
  
  /**
   * \brief          Set the random number generator callback
   *
   * \param conf     SSL configuration
   * \param f_rng    RNG function
   * \param p_rng    RNG parameter
   */
  void mbedtls_ssl_conf_rng( mbedtls_ssl_config *conf,
                    int (*f_rng)(void *, unsigned char *, size_t),
                    void *p_rng );
  
  /**
   * \brief          Set the debug callback
   *
   *                 The callback has the following argument:
   *                 void *           opaque context for the callback
   *                 int              debug level
   *                 const char *     file name
   *                 int              line number
   *                 const char *     message
   *
   * \param conf     SSL configuration
   * \param f_dbg    debug function
   * \param p_dbg    debug parameter
   */
  void mbedtls_ssl_conf_dbg( mbedtls_ssl_config *conf,
                    void (*f_dbg)(void *, int, const char *, int, const char *),
                    void  *p_dbg );
  
  /**
   * \brief          Set the underlying BIO callbacks for write, read and
   *                 read-with-timeout.
   *
   * \param ssl      SSL context
   * \param p_bio    parameter (context) shared by BIO callbacks
   * \param f_send   write callback
   * \param f_recv   read callback
   * \param f_recv_timeout blocking read callback with timeout.
   *
   * \note           One of f_recv or f_recv_timeout can be NULL, in which case
   *                 the other is used. If both are non-NULL, f_recv_timeout is
   *                 used and f_recv is ignored (as if it were NULL).
   *
   * \note           The two most common use cases are:
   *                 - non-blocking I/O, f_recv != NULL, f_recv_timeout == NULL
   *                 - blocking I/O, f_recv == NULL, f_recv_timout != NULL
   *
   * \note           For DTLS, you need to provide either a non-NULL
   *                 f_recv_timeout callback, or a f_recv that doesn't block.
   *
   * \note           See the documentations of \c mbedtls_ssl_sent_t,
   *                 \c mbedtls_ssl_recv_t and \c mbedtls_ssl_recv_timeout_t for
   *                 the conventions those callbacks must follow.
   *
   * \note           On some platforms, net_sockets.c provides
   *                 \c mbedtls_net_send(), \c mbedtls_net_recv() and
   *                 \c mbedtls_net_recv_timeout() that are suitable to be used
   *                 here.
   */
  void mbedtls_ssl_set_bio( mbedtls_ssl_context *ssl,
                            void *p_bio,
                            mbedtls_ssl_send_t *f_send,
                            mbedtls_ssl_recv_t *f_recv,
                            mbedtls_ssl_recv_timeout_t *f_recv_timeout );
  
  #if defined(MBEDTLS_SSL_PROTO_DTLS)
  
  #if defined(MBEDTLS_SSL_DTLS_CONNECTION_ID)
  
  
  /**
   * \brief             Configure the use of the Connection ID (CID)
   *                    extension in the next handshake.
   *
   *                    Reference: draft-ietf-tls-dtls-connection-id-05
   *                    https://tools.ietf.org/html/draft-ietf-tls-dtls-connection-id-05
   *
   *                    The DTLS CID extension allows the reliable association of
   *                    DTLS records to DTLS connections across changes in the
   *                    underlying transport (changed IP and Port metadata) by
   *                    adding explicit connection identifiers (CIDs) to the
   *                    headers of encrypted DTLS records. The desired CIDs are
   *                    configured by the application layer and are exchanged in
   *                    new `ClientHello` / `ServerHello` extensions during the
   *                    handshake, where each side indicates the CID it wants the
   *                    peer to use when writing encrypted messages. The CIDs are
   *                    put to use once records get encrypted: the stack discards
   *                    any incoming records that don't include the configured CID
   *                    in their header, and adds the peer's requested CID to the
   *                    headers of outgoing messages.
   *
   *                    This API enables or disables the use of the CID extension
   *                    in the next handshake and sets the value of the CID to
   *                    be used for incoming messages.
   *
   * \param ssl         The SSL context to configure. This must be initialized.
   * \param enable      This value determines whether the CID extension should
   *                    be used or not. Possible values are:
   *                    - MBEDTLS_SSL_CID_ENABLED to enable the use of the CID.
   *                    - MBEDTLS_SSL_CID_DISABLED (default) to disable the use
   *                      of the CID.
   * \param own_cid     The address of the readable buffer holding the CID we want
   *                    the peer to use when sending encrypted messages to us.
   *                    This may be \c NULL if \p own_cid_len is \c 0.
   *                    This parameter is unused if \p enabled is set to
   *                    MBEDTLS_SSL_CID_DISABLED.
   * \param own_cid_len The length of \p own_cid.
   *                    This parameter is unused if \p enabled is set to
   *                    MBEDTLS_SSL_CID_DISABLED.
   *
   * \note              The value of \p own_cid_len must match the value of the
   *                    \c len parameter passed to mbedtls_ssl_conf_cid()
   *                    when configuring the ::mbedtls_ssl_config that \p ssl
   *                    is bound to.
   *
   * \note              This CID configuration applies to subsequent handshakes
   *                    performed on the SSL context \p ssl, but does not trigger
   *                    one. You still have to call `mbedtls_ssl_handshake()`
   *                    (for the initial handshake) or `mbedtls_ssl_renegotiate()`
   *                    (for a renegotiation handshake) explicitly after a
   *                    successful call to this function to run the handshake.
   *
   * \note              This call cannot guarantee that the use of the CID
   *                    will be successfully negotiated in the next handshake,
   *                    because the peer might not support it. Specifically:
   *                    - On the Client, enabling the use of the CID through
   *                      this call implies that the `ClientHello` in the next
   *                      handshake will include the CID extension, thereby
   *                      offering the use of the CID to the server. Only if
   *                      the `ServerHello` contains the CID extension, too,
   *                      the CID extension will actually be put to use.
   *                    - On the Server, enabling the use of the CID through
   *                      this call implies that that the server will look for
   *                      the CID extension in a `ClientHello` from the client,
   *                      and, if present, reply with a CID extension in its
   *                      `ServerHello`.
   *
   * \note              To check whether the use of the CID was negotiated
   *                    after the subsequent handshake has completed, please
   *                    use the API mbedtls_ssl_get_peer_cid().
   *
   * \warning           If the use of the CID extension is enabled in this call
   *                    and the subsequent handshake negotiates its use, Mbed TLS
   *                    will silently drop every packet whose CID does not match
   *                    the CID configured in \p own_cid. It is the responsibility
   *                    of the user to adapt the underlying transport to take care
   *                    of CID-based demultiplexing before handing datagrams to
   *                    Mbed TLS.
   *
   * \return            \c 0 on success. In this case, the CID configuration
   *                    applies to the next handshake.
   * \return            A negative error code on failure.
   */
  int mbedtls_ssl_set_cid( mbedtls_ssl_context *ssl,
                           int enable,
                           unsigned char const *own_cid,
                           size_t own_cid_len );
  
  /**
   * \brief              Get information about the use of the CID extension
   *                     in the current connection.
   *
   * \param ssl          The SSL context to query.
   * \param enabled      The address at which to store whether the CID extension
   *                     is currently in use or not. If the CID is in use,
   *                     `*enabled` is set to MBEDTLS_SSL_CID_ENABLED;
   *                     otherwise, it is set to MBEDTLS_SSL_CID_DISABLED.
   * \param peer_cid     The address of the buffer in which to store the CID
   *                     chosen by the peer (if the CID extension is used).
   *                     This may be \c NULL in case the value of peer CID
   *                     isn't needed. If it is not \c NULL, \p peer_cid_len
   *                     must not be \c NULL.
   * \param peer_cid_len The address at which to store the size of the CID
   *                     chosen by the peer (if the CID extension is used).
   *                     This is also the number of Bytes in \p peer_cid that
   *                     have been written.
   *                     This may be \c NULL in case the length of the peer CID
   *                     isn't needed. If it is \c NULL, \p peer_cid must be
   *                     \c NULL, too.
   *
   * \note               This applies to the state of the CID negotiated in
   *                     the last complete handshake. If a handshake is in
   *                     progress, this function will attempt to complete
   *                     the handshake first.
   *
   * \note               If CID extensions have been exchanged but both client
   *                     and server chose to use an empty CID, this function
   *                     sets `*enabled` to #MBEDTLS_SSL_CID_DISABLED
   *                     (the rationale for this is that the resulting
   *                     communication is the same as if the CID extensions
   *                     hadn't been used).
   *
   * \return            \c 0 on success.
   * \return            A negative error code on failure.
   */
  int mbedtls_ssl_get_peer_cid( mbedtls_ssl_context *ssl,
                       int *enabled,
                       unsigned char peer_cid[ MBEDTLS_SSL_CID_OUT_LEN_MAX ],
                       size_t *peer_cid_len );
  
  #endif /* MBEDTLS_SSL_DTLS_CONNECTION_ID */
  
  /**
   * \brief          Set the Maximum Tranport Unit (MTU).
   *                 Special value: 0 means unset (no limit).
   *                 This represents the maximum size of a datagram payload
   *                 handled by the transport layer (usually UDP) as determined
   *                 by the network link and stack. In practice, this controls
   *                 the maximum size datagram the DTLS layer will pass to the
   *                 \c f_send() callback set using \c mbedtls_ssl_set_bio().
   *
   * \note           The limit on datagram size is converted to a limit on
   *                 record payload by subtracting the current overhead of
   *                 encapsulation and encryption/authentication if any.
   *
   * \note           This can be called at any point during the connection, for
   *                 example when a Path Maximum Transfer Unit (PMTU)
   *                 estimate becomes available from other sources,
   *                 such as lower (or higher) protocol layers.
   *
   * \note           This setting only controls the size of the packets we send,
   *                 and does not restrict the size of the datagrams we're
   *                 willing to receive. Client-side, you can request the
   *                 server to use smaller records with \c
   *                 mbedtls_ssl_conf_max_frag_len().
   *
   * \note           If both a MTU and a maximum fragment length have been
   *                 configured (or negotiated with the peer), the resulting
   *                 lower limit on record payload (see first note) is used.
   *
   * \note           This can only be used to decrease the maximum size
   *                 of datagrams (hence records, see first note) sent. It
   *                 cannot be used to increase the maximum size of records over
   *                 the limit set by #MBEDTLS_SSL_OUT_CONTENT_LEN.
   *
   * \note           Values lower than the current record layer expansion will
   *                 result in an error when trying to send data.
   *
   * \note           Using record compression together with a non-zero MTU value
   *                 will result in an error when trying to send data.
   *
   * \param ssl      SSL context
   * \param mtu      Value of the path MTU in bytes
   */
  void mbedtls_ssl_set_mtu( mbedtls_ssl_context *ssl, uint16_t mtu );
  #endif /* MBEDTLS_SSL_PROTO_DTLS */
  
  #if defined(MBEDTLS_X509_CRT_PARSE_C)
  /**
   * \brief          Set a connection-specific verification callback (optional).
   *
   *                 If set, the provided verify callback is called for each
   *                 certificate in the peer's CRT chain, including the trusted
   *                 root. For more information, please see the documentation of
   *                 \c mbedtls_x509_crt_verify().
   *
   * \note           This call is analogous to mbedtls_ssl_conf_verify() but
   *                 binds the verification callback and context to an SSL context
   *                 as opposed to an SSL configuration.
   *                 If mbedtls_ssl_conf_verify() and mbedtls_ssl_set_verify()
   *                 are both used, mbedtls_ssl_set_verify() takes precedence.
   *
   * \param ssl      The SSL context to use.
   * \param f_vrfy   The verification callback to use during CRT verification.
   * \param p_vrfy   The opaque context to be passed to the callback.
   */
  void mbedtls_ssl_set_verify( mbedtls_ssl_context *ssl,
                       int (*f_vrfy)(void *, mbedtls_x509_crt *, int, uint32_t *),
                       void *p_vrfy );
  #endif /* MBEDTLS_X509_CRT_PARSE_C */
  
  /**
   * \brief          Set the timeout period for mbedtls_ssl_read()
   *                 (Default: no timeout.)
   *
   * \param conf     SSL configuration context
   * \param timeout  Timeout value in milliseconds.
   *                 Use 0 for no timeout (default).
   *
   * \note           With blocking I/O, this will only work if a non-NULL
   *                 \c f_recv_timeout was set with \c mbedtls_ssl_set_bio().
   *                 With non-blocking I/O, this will only work if timer
   *                 callbacks were set with \c mbedtls_ssl_set_timer_cb().
   *
   * \note           With non-blocking I/O, you may also skip this function
   *                 altogether and handle timeouts at the application layer.
   */
  void mbedtls_ssl_conf_read_timeout( mbedtls_ssl_config *conf, uint32_t timeout );
  
  #if defined(MBEDTLS_SSL_RECORD_CHECKING)
  /**
   * \brief          Check whether a buffer contains a valid and authentic record
   *                 that has not been seen before. (DTLS only).
   *
   *                 This function does not change the user-visible state
   *                 of the SSL context. Its sole purpose is to provide
   *                 an indication of the legitimacy of an incoming record.
   *
   *                 This can be useful e.g. in distributed server environments
   *                 using the DTLS Connection ID feature, in which connections
   *                 might need to be passed between service instances on a change
   *                 of peer address, but where such disruptive operations should
   *                 only happen after the validity of incoming records has been
   *                 confirmed.
   *
   * \param ssl      The SSL context to use.
   * \param buf      The address of the buffer holding the record to be checked.
   *                 This must be a read/write buffer of length \p buflen Bytes.
   * \param buflen   The length of \p buf in Bytes.
   *
   * \note           This routine only checks whether the provided buffer begins
   *                 with a valid and authentic record that has not been seen
   *                 before, but does not check potential data following the
   *                 initial record. In particular, it is possible to pass DTLS
   *                 datagrams containing multiple records, in which case only
   *                 the first record is checked.
   *
   * \note           This function modifies the input buffer \p buf. If you need
   *                 to preserve the original record, you have to maintain a copy.
   *
   * \return         \c 0 if the record is valid and authentic and has not been
   *                 seen before.
   * \return         MBEDTLS_ERR_SSL_INVALID_MAC if the check completed
   *                 successfully but the record was found to be not authentic.
   * \return         MBEDTLS_ERR_SSL_INVALID_RECORD if the check completed
   *                 successfully but the record was found to be invalid for
   *                 a reason different from authenticity checking.
   * \return         MBEDTLS_ERR_SSL_UNEXPECTED_RECORD if the check completed
   *                 successfully but the record was found to be unexpected
   *                 in the state of the SSL context, including replayed records.
   * \return         Another negative error code on different kinds of failure.
   *                 In this case, the SSL context becomes unusable and needs
   *                 to be freed or reset before reuse.
   */
  int mbedtls_ssl_check_record( mbedtls_ssl_context const *ssl,
                                unsigned char *buf,
                                size_t buflen );
  #endif /* MBEDTLS_SSL_RECORD_CHECKING */
  
  /**
   * \brief          Set the timer callbacks (Mandatory for DTLS.)
   *
   * \param ssl      SSL context
   * \param p_timer  parameter (context) shared by timer callbacks
   * \param f_set_timer   set timer callback
   * \param f_get_timer   get timer callback. Must return:
   *
   * \note           See the documentation of \c mbedtls_ssl_set_timer_t and
   *                 \c mbedtls_ssl_get_timer_t for the conventions this pair of
   *                 callbacks must follow.
   *
   * \note           On some platforms, timing.c provides
   *                 \c mbedtls_timing_set_delay() and
   *                 \c mbedtls_timing_get_delay() that are suitable for using
   *                 here, except if using an event-driven style.
   *
   * \note           See also the "DTLS tutorial" article in our knowledge base.
   *                 https://tls.mbed.org/kb/how-to/dtls-tutorial
   */
  void mbedtls_ssl_set_timer_cb( mbedtls_ssl_context *ssl,
                                 void *p_timer,
                                 mbedtls_ssl_set_timer_t *f_set_timer,
                                 mbedtls_ssl_get_timer_t *f_get_timer );
  
  /**
   * \brief           Callback type: generate and write session ticket
   *
   * \note            This describes what a callback implementation should do.
   *                  This callback should generate an encrypted and
   *                  authenticated ticket for the session and write it to the
   *                  output buffer. Here, ticket means the opaque ticket part
   *                  of the NewSessionTicket structure of RFC 5077.
   *
   * \param p_ticket  Context for the callback
   * \param session   SSL session to be written in the ticket
   * \param start     Start of the output buffer
   * \param end       End of the output buffer
   * \param tlen      On exit, holds the length written
   * \param lifetime  On exit, holds the lifetime of the ticket in seconds
   *
   * \return          0 if successful, or
   *                  a specific MBEDTLS_ERR_XXX code.
   */
  typedef int mbedtls_ssl_ticket_write_t( void *p_ticket,
                                          const mbedtls_ssl_session *session,
                                          unsigned char *start,
                                          const unsigned char *end,
                                          size_t *tlen,
                                          uint32_t *lifetime );
  
  #if defined(MBEDTLS_SSL_EXPORT_KEYS)
  /**
   * \brief           Callback type: Export key block and master secret
   *
   * \note            This is required for certain uses of TLS, e.g. EAP-TLS
   *                  (RFC 5216) and Thread. The key pointers are ephemeral and
   *                  therefore must not be stored. The master secret and keys
   *                  should not be used directly except as an input to a key
   *                  derivation function.
   *
   * \param p_expkey  Context for the callback
   * \param ms        Pointer to master secret (fixed length: 48 bytes)
   * \param kb        Pointer to key block, see RFC 5246 section 6.3
   *                  (variable length: 2 * maclen + 2 * keylen + 2 * ivlen).
   * \param maclen    MAC length
   * \param keylen    Key length
   * \param ivlen     IV length
   *
   * \return          0 if successful, or
   *                  a specific MBEDTLS_ERR_XXX code.
   */
  typedef int mbedtls_ssl_export_keys_t( void *p_expkey,
                                  const unsigned char *ms,
                                  const unsigned char *kb,
                                  size_t maclen,
                                  size_t keylen,
                                  size_t ivlen );
  
  /**
   * \brief           Callback type: Export key block, master secret,
   *                                 handshake randbytes and the tls_prf function
   *                                 used to derive keys.
   *
   * \note            This is required for certain uses of TLS, e.g. EAP-TLS
   *                  (RFC 5216) and Thread. The key pointers are ephemeral and
   *                  therefore must not be stored. The master secret and keys
   *                  should not be used directly except as an input to a key
   *                  derivation function.
   *
   * \param p_expkey  Context for the callback.
   * \param ms        Pointer to master secret (fixed length: 48 bytes).
   * \param kb            Pointer to key block, see RFC 5246 section 6.3.
   *                      (variable length: 2 * maclen + 2 * keylen + 2 * ivlen).
   * \param maclen        MAC length.
   * \param keylen        Key length.
   * \param ivlen         IV length.
   * \param client_random The client random bytes.
   * \param server_random The server random bytes.
   * \param tls_prf_type The tls_prf enum type.
   *
   * \return          0 if successful, or
   *                  a specific MBEDTLS_ERR_XXX code.
   */
  typedef int mbedtls_ssl_export_keys_ext_t( void *p_expkey,
                                             const unsigned char *ms,
                                             const unsigned char *kb,
                                             size_t maclen,
                                             size_t keylen,
                                             size_t ivlen,
                                             const unsigned char client_random[32],
                                             const unsigned char server_random[32],
                                             mbedtls_tls_prf_types tls_prf_type );
  #endif /* MBEDTLS_SSL_EXPORT_KEYS */
  
  /**
   * \brief           Callback type: parse and load session ticket
   *
   * \note            This describes what a callback implementation should do.
   *                  This callback should parse a session ticket as generated
   *                  by the corresponding mbedtls_ssl_ticket_write_t function,
   *                  and, if the ticket is authentic and valid, load the
   *                  session.
   *
   * \note            The implementation is allowed to modify the first len
   *                  bytes of the input buffer, eg to use it as a temporary
   *                  area for the decrypted ticket contents.
   *
   * \param p_ticket  Context for the callback
   * \param session   SSL session to be loaded
   * \param buf       Start of the buffer containing the ticket
   * \param len       Length of the ticket.
   *
   * \return          0 if successful, or
   *                  MBEDTLS_ERR_SSL_INVALID_MAC if not authentic, or
   *                  MBEDTLS_ERR_SSL_SESSION_TICKET_EXPIRED if expired, or
   *                  any other non-zero code for other failures.
   */
  typedef int mbedtls_ssl_ticket_parse_t( void *p_ticket,
                                          mbedtls_ssl_session *session,
                                          unsigned char *buf,
                                          size_t len );
  
  #if defined(MBEDTLS_SSL_SESSION_TICKETS) && defined(MBEDTLS_SSL_SRV_C)
  /**
   * \brief           Configure SSL session ticket callbacks (server only).
   *                  (Default: none.)
   *
   * \note            On server, session tickets are enabled by providing
   *                  non-NULL callbacks.
   *
   * \note            On client, use \c mbedtls_ssl_conf_session_tickets().
   *
   * \param conf      SSL configuration context
   * \param f_ticket_write    Callback for writing a ticket
   * \param f_ticket_parse    Callback for parsing a ticket
   * \param p_ticket          Context shared by the two callbacks
   */
  void mbedtls_ssl_conf_session_tickets_cb( mbedtls_ssl_config *conf,
          mbedtls_ssl_ticket_write_t *f_ticket_write,
          mbedtls_ssl_ticket_parse_t *f_ticket_parse,
          void *p_ticket );
  #endif /* MBEDTLS_SSL_SESSION_TICKETS && MBEDTLS_SSL_SRV_C */
  
  #if defined(MBEDTLS_SSL_EXPORT_KEYS)
  /**
   * \brief           Configure key export callback.
   *                  (Default: none.)
   *
   * \note            See \c mbedtls_ssl_export_keys_t.
   *
   * \param conf      SSL configuration context
   * \param f_export_keys     Callback for exporting keys
   * \param p_export_keys     Context for the callback
   */
  void mbedtls_ssl_conf_export_keys_cb( mbedtls_ssl_config *conf,
          mbedtls_ssl_export_keys_t *f_export_keys,
          void *p_export_keys );
  
  /**
   * \brief           Configure extended key export callback.
   *                  (Default: none.)
   *
   * \note            See \c mbedtls_ssl_export_keys_ext_t.
   *
   * \param conf      SSL configuration context
   * \param f_export_keys_ext Callback for exporting keys
   * \param p_export_keys     Context for the callback
   */
  void mbedtls_ssl_conf_export_keys_ext_cb( mbedtls_ssl_config *conf,
          mbedtls_ssl_export_keys_ext_t *f_export_keys_ext,
          void *p_export_keys );
  #endif /* MBEDTLS_SSL_EXPORT_KEYS */
  
  #if defined(MBEDTLS_SSL_ASYNC_PRIVATE)
  /**
   * \brief           Configure asynchronous private key operation callbacks.
   *
   * \param conf              SSL configuration context
   * \param f_async_sign      Callback to start a signature operation. See
   *                          the description of ::mbedtls_ssl_async_sign_t
   *                          for more information. This may be \c NULL if the
   *                          external processor does not support any signature
   *                          operation; in this case the private key object
   *                          associated with the certificate will be used.
   * \param f_async_decrypt   Callback to start a decryption operation. See
   *                          the description of ::mbedtls_ssl_async_decrypt_t
   *                          for more information. This may be \c NULL if the
   *                          external processor does not support any decryption
   *                          operation; in this case the private key object
   *                          associated with the certificate will be used.
   * \param f_async_resume    Callback to resume an asynchronous operation. See
   *                          the description of ::mbedtls_ssl_async_resume_t
   *                          for more information. This may not be \c NULL unless
   *                          \p f_async_sign and \p f_async_decrypt are both
   *                          \c NULL.
   * \param f_async_cancel    Callback to cancel an asynchronous operation. See
   *                          the description of ::mbedtls_ssl_async_cancel_t
   *                          for more information. This may be \c NULL if
   *                          no cleanup is needed.
   * \param config_data       A pointer to configuration data which can be
   *                          retrieved with
   *                          mbedtls_ssl_conf_get_async_config_data(). The
   *                          library stores this value without dereferencing it.
   */
  void mbedtls_ssl_conf_async_private_cb( mbedtls_ssl_config *conf,
                                          mbedtls_ssl_async_sign_t *f_async_sign,
                                          mbedtls_ssl_async_decrypt_t *f_async_decrypt,
                                          mbedtls_ssl_async_resume_t *f_async_resume,
                                          mbedtls_ssl_async_cancel_t *f_async_cancel,
                                          void *config_data );
  
  /**
   * \brief           Retrieve the configuration data set by
   *                  mbedtls_ssl_conf_async_private_cb().
   *
   * \param conf      SSL configuration context
   * \return          The configuration data set by
   *                  mbedtls_ssl_conf_async_private_cb().
   */
  void *mbedtls_ssl_conf_get_async_config_data( const mbedtls_ssl_config *conf );
  
  /**
   * \brief           Retrieve the asynchronous operation user context.
   *
   * \note            This function may only be called while a handshake
   *                  is in progress.
   *
   * \param ssl       The SSL context to access.
   *
   * \return          The asynchronous operation user context that was last
   *                  set during the current handshake. If
   *                  mbedtls_ssl_set_async_operation_data() has not yet been
   *                  called during the current handshake, this function returns
   *                  \c NULL.
   */
  void *mbedtls_ssl_get_async_operation_data( const mbedtls_ssl_context *ssl );
  
  /**
   * \brief           Retrieve the asynchronous operation user context.
   *
   * \note            This function may only be called while a handshake
   *                  is in progress.
   *
   * \param ssl       The SSL context to access.
   * \param ctx       The new value of the asynchronous operation user context.
   *                  Call mbedtls_ssl_get_async_operation_data() later during the
   *                  same handshake to retrieve this value.
   */
  void mbedtls_ssl_set_async_operation_data( mbedtls_ssl_context *ssl,
                                   void *ctx );
  #endif /* MBEDTLS_SSL_ASYNC_PRIVATE */
  
  /**
   * \brief          Callback type: generate a cookie
   *
   * \param ctx      Context for the callback
   * \param p        Buffer to write to,
   *                 must be updated to point right after the cookie
   * \param end      Pointer to one past the end of the output buffer
   * \param info     Client ID info that was passed to
   *                 \c mbedtls_ssl_set_client_transport_id()
   * \param ilen     Length of info in bytes
   *
   * \return         The callback must return 0 on success,
   *                 or a negative error code.
   */
  typedef int mbedtls_ssl_cookie_write_t( void *ctx,
                                  unsigned char **p, unsigned char *end,
                                  const unsigned char *info, size_t ilen );
  
  /**
   * \brief          Callback type: verify a cookie
   *
   * \param ctx      Context for the callback
   * \param cookie   Cookie to verify
   * \param clen     Length of cookie
   * \param info     Client ID info that was passed to
   *                 \c mbedtls_ssl_set_client_transport_id()
   * \param ilen     Length of info in bytes
   *
   * \return         The callback must return 0 if cookie is valid,
   *                 or a negative error code.
   */
  typedef int mbedtls_ssl_cookie_check_t( void *ctx,
                                  const unsigned char *cookie, size_t clen,
                                  const unsigned char *info, size_t ilen );
  
  #if defined(MBEDTLS_SSL_DTLS_HELLO_VERIFY) && defined(MBEDTLS_SSL_SRV_C)
  /**
   * \brief           Register callbacks for DTLS cookies
   *                  (Server only. DTLS only.)
   *
   *                  Default: dummy callbacks that fail, in order to force you to
   *                  register working callbacks (and initialize their context).
   *
   *                  To disable HelloVerifyRequest, register NULL callbacks.
   *
   * \warning         Disabling hello verification allows your server to be used
   *                  for amplification in DoS attacks against other hosts.
   *                  Only disable if you known this can't happen in your
   *                  particular environment.
   *
   * \note            See comments on \c mbedtls_ssl_handshake() about handling
   *                  the MBEDTLS_ERR_SSL_HELLO_VERIFY_REQUIRED that is expected
   *                  on the first handshake attempt when this is enabled.
   *
   * \note            This is also necessary to handle client reconnection from
   *                  the same port as described in RFC 6347 section 4.2.8 (only
   *                  the variant with cookies is supported currently). See
   *                  comments on \c mbedtls_ssl_read() for details.
   *
   * \param conf              SSL configuration
   * \param f_cookie_write    Cookie write callback
   * \param f_cookie_check    Cookie check callback
   * \param p_cookie          Context for both callbacks
   */
  void mbedtls_ssl_conf_dtls_cookies( mbedtls_ssl_config *conf,
                             mbedtls_ssl_cookie_write_t *f_cookie_write,
                             mbedtls_ssl_cookie_check_t *f_cookie_check,
                             void *p_cookie );
  
  /**
   * \brief          Set client's transport-level identification info.
   *                 (Server only. DTLS only.)
   *
   *                 This is usually the IP address (and port), but could be
   *                 anything identify the client depending on the underlying
   *                 network stack. Used for HelloVerifyRequest with DTLS.
   *                 This is *not* used to route the actual packets.
   *
   * \param ssl      SSL context
   * \param info     Transport-level info identifying the client (eg IP + port)
   * \param ilen     Length of info in bytes
   *
   * \note           An internal copy is made, so the info buffer can be reused.
   *
   * \return         0 on success,
   *                 MBEDTLS_ERR_SSL_BAD_INPUT_DATA if used on client,
   *                 MBEDTLS_ERR_SSL_ALLOC_FAILED if out of memory.
   */
  int mbedtls_ssl_set_client_transport_id( mbedtls_ssl_context *ssl,
                                   const unsigned char *info,
                                   size_t ilen );
  
  #endif /* MBEDTLS_SSL_DTLS_HELLO_VERIFY && MBEDTLS_SSL_SRV_C */
  
  #if defined(MBEDTLS_SSL_DTLS_ANTI_REPLAY)
  /**
   * \brief          Enable or disable anti-replay protection for DTLS.
   *                 (DTLS only, no effect on TLS.)
   *                 Default: enabled.
   *
   * \param conf     SSL configuration
   * \param mode     MBEDTLS_SSL_ANTI_REPLAY_ENABLED or MBEDTLS_SSL_ANTI_REPLAY_DISABLED.
   *
   * \warning        Disabling this is a security risk unless the application
   *                 protocol handles duplicated packets in a safe way. You
   *                 should not disable this without careful consideration.
   *                 However, if your application already detects duplicated
   *                 packets and needs information about them to adjust its
   *                 transmission strategy, then you'll want to disable this.
   */
  void mbedtls_ssl_conf_dtls_anti_replay( mbedtls_ssl_config *conf, char mode );
  #endif /* MBEDTLS_SSL_DTLS_ANTI_REPLAY */
  
  #if defined(MBEDTLS_SSL_DTLS_BADMAC_LIMIT)
  /**
   * \brief          Set a limit on the number of records with a bad MAC
   *                 before terminating the connection.
   *                 (DTLS only, no effect on TLS.)
   *                 Default: 0 (disabled).
   *
   * \param conf     SSL configuration
   * \param limit    Limit, or 0 to disable.
   *
   * \note           If the limit is N, then the connection is terminated when
   *                 the Nth non-authentic record is seen.
   *
   * \note           Records with an invalid header are not counted, only the
   *                 ones going through the authentication-decryption phase.
   *
   * \note           This is a security trade-off related to the fact that it's
   *                 often relatively easy for an active attacker ot inject UDP
   *                 datagrams. On one hand, setting a low limit here makes it
   *                 easier for such an attacker to forcibly terminated a
   *                 connection. On the other hand, a high limit or no limit
   *                 might make us waste resources checking authentication on
   *                 many bogus packets.
   */
  void mbedtls_ssl_conf_dtls_badmac_limit( mbedtls_ssl_config *conf, unsigned limit );
  #endif /* MBEDTLS_SSL_DTLS_BADMAC_LIMIT */
  
  #if defined(MBEDTLS_SSL_PROTO_DTLS)
  
  /**
   * \brief          Allow or disallow packing of multiple handshake records
   *                 within a single datagram.
   *
   * \param ssl           The SSL context to configure.
   * \param allow_packing This determines whether datagram packing may
   *                      be used or not. A value of \c 0 means that every
   *                      record will be sent in a separate datagram; a
   *                      value of \c 1 means that, if space permits,
   *                      multiple handshake messages (including CCS) belonging to
   *                      a single flight may be packed within a single datagram.
   *
   * \note           This is enabled by default and should only be disabled
   *                 for test purposes, or if datagram packing causes
   *                 interoperability issues with peers that don't support it.
   *
   * \note           Allowing datagram packing reduces the network load since
   *                 there's less overhead if multiple messages share the same
   *                 datagram. Also, it increases the handshake efficiency
   *                 since messages belonging to a single datagram will not
   *                 be reordered in transit, and so future message buffering
   *                 or flight retransmission (if no buffering is used) as
   *                 means to deal with reordering are needed less frequently.
   *
   * \note           Application records are not affected by this option and
   *                 are currently always sent in separate datagrams.
   *
   */
  void mbedtls_ssl_set_datagram_packing( mbedtls_ssl_context *ssl,
                                         unsigned allow_packing );
  
  /**
   * \brief          Set retransmit timeout values for the DTLS handshake.
   *                 (DTLS only, no effect on TLS.)
   *
   * \param conf     SSL configuration
   * \param min      Initial timeout value in milliseconds.
   *                 Default: 1000 (1 second).
   * \param max      Maximum timeout value in milliseconds.
   *                 Default: 60000 (60 seconds).
   *
   * \note           Default values are from RFC 6347 section 4.2.4.1.
   *
   * \note           The 'min' value should typically be slightly above the
   *                 expected round-trip time to your peer, plus whatever time
   *                 it takes for the peer to process the message. For example,
   *                 if your RTT is about 600ms and you peer needs up to 1s to
   *                 do the cryptographic operations in the handshake, then you
   *                 should set 'min' slightly above 1600. Lower values of 'min'
   *                 might cause spurious resends which waste network resources,
   *                 while larger value of 'min' will increase overall latency
   *                 on unreliable network links.
   *
   * \note           The more unreliable your network connection is, the larger
   *                 your max / min ratio needs to be in order to achieve
   *                 reliable handshakes.
   *
   * \note           Messages are retransmitted up to log2(ceil(max/min)) times.
   *                 For example, if min = 1s and max = 5s, the retransmit plan
   *                 goes: send ... 1s -> resend ... 2s -> resend ... 4s ->
   *                 resend ... 5s -> give up and return a timeout error.
   */
  void mbedtls_ssl_conf_handshake_timeout( mbedtls_ssl_config *conf, uint32_t min, uint32_t max );
  #endif /* MBEDTLS_SSL_PROTO_DTLS */
  
  #if defined(MBEDTLS_SSL_SRV_C)
  /**
   * \brief          Set the session cache callbacks (server-side only)
   *                 If not set, no session resuming is done (except if session
   *                 tickets are enabled too).
   *
   *                 The session cache has the responsibility to check for stale
   *                 entries based on timeout. See RFC 5246 for recommendations.
   *
   *                 Warning: session.peer_cert is cleared by the SSL/TLS layer on
   *                 connection shutdown, so do not cache the pointer! Either set
   *                 it to NULL or make a full copy of the certificate.
   *
   *                 The get callback is called once during the initial handshake
   *                 to enable session resuming. The get function has the
   *                 following parameters: (void *parameter, mbedtls_ssl_session *session)
   *                 If a valid entry is found, it should fill the master of
   *                 the session object with the cached values and return 0,
   *                 return 1 otherwise. Optionally peer_cert can be set as well
   *                 if it is properly present in cache entry.
   *
   *                 The set callback is called once during the initial handshake
   *                 to enable session resuming after the entire handshake has
   *                 been finished. The set function has the following parameters:
   *                 (void *parameter, const mbedtls_ssl_session *session). The function
   *                 should create a cache entry for future retrieval based on
   *                 the data in the session structure and should keep in mind
   *                 that the mbedtls_ssl_session object presented (and all its referenced
   *                 data) is cleared by the SSL/TLS layer when the connection is
   *                 terminated. It is recommended to add metadata to determine if
   *                 an entry is still valid in the future. Return 0 if
   *                 successfully cached, return 1 otherwise.
   *
   * \param conf           SSL configuration
   * \param p_cache        parmater (context) for both callbacks
   * \param f_get_cache    session get callback
   * \param f_set_cache    session set callback
   */
  void mbedtls_ssl_conf_session_cache( mbedtls_ssl_config *conf,
          void *p_cache,
          int (*f_get_cache)(void *, mbedtls_ssl_session *),
          int (*f_set_cache)(void *, const mbedtls_ssl_session *) );
  #endif /* MBEDTLS_SSL_SRV_C */
  
  #if defined(MBEDTLS_SSL_CLI_C)
  /**
   * \brief          Request resumption of session (client-side only)
   *                 Session data is copied from presented session structure.
   *
   * \param ssl      SSL context
   * \param session  session context
   *
   * \return         0 if successful,
   *                 MBEDTLS_ERR_SSL_ALLOC_FAILED if memory allocation failed,
   *                 MBEDTLS_ERR_SSL_BAD_INPUT_DATA if used server-side or
   *                 arguments are otherwise invalid
   *
   * \sa             mbedtls_ssl_get_session()
   */
  int mbedtls_ssl_set_session( mbedtls_ssl_context *ssl, const mbedtls_ssl_session *session );
  #endif /* MBEDTLS_SSL_CLI_C */
  
  /**
   * \brief          Load serialized session data into a session structure.
   *                 On client, this can be used for loading saved sessions
   *                 before resuming them with mbedstls_ssl_set_session().
   *                 On server, this can be used for alternative implementations
   *                 of session cache or session tickets.
   *
   * \warning        If a peer certificate chain is associated with the session,
   *                 the serialized state will only contain the peer's
   *                 end-entity certificate and the result of the chain
   *                 verification (unless verification was disabled), but not
   *                 the rest of the chain.
   *
   * \see            mbedtls_ssl_session_save()
   * \see            mbedtls_ssl_set_session()
   *
   * \param session  The session structure to be populated. It must have been
   *                 initialised with mbedtls_ssl_session_init() but not
   *                 populated yet.
   * \param buf      The buffer holding the serialized session data. It must be a
   *                 readable buffer of at least \p len bytes.
   * \param len      The size of the serialized data in bytes.
   *
   * \return         \c 0 if successful.
   * \return         #MBEDTLS_ERR_SSL_ALLOC_FAILED if memory allocation failed.
   * \return         #MBEDTLS_ERR_SSL_BAD_INPUT_DATA if input data is invalid.
   * \return         #MBEDTLS_ERR_SSL_VERSION_MISMATCH if the serialized data
   *                 was generated in a different version or configuration of
   *                 Mbed TLS.
   * \return         Another negative value for other kinds of errors (for
   *                 example, unsupported features in the embedded certificate).
   */
  int mbedtls_ssl_session_load( mbedtls_ssl_session *session,
                                const unsigned char *buf,
                                size_t len );
  
  /**
   * \brief          Save session structure as serialized data in a buffer.
   *                 On client, this can be used for saving session data,
   *                 potentially in non-volatile storage, for resuming later.
   *                 On server, this can be used for alternative implementations
   *                 of session cache or session tickets.
   *
   * \see            mbedtls_ssl_session_load()
   * \see            mbedtls_ssl_get_session_pointer()
   *
   * \param session  The session structure to be saved.
   * \param buf      The buffer to write the serialized data to. It must be a
   *                 writeable buffer of at least \p len bytes, or may be \c
   *                 NULL if \p len is \c 0.
   * \param buf_len  The number of bytes available for writing in \p buf.
   * \param olen     The size in bytes of the data that has been or would have
   *                 been written. It must point to a valid \c size_t.
   *
   * \note           \p olen is updated to the correct value regardless of
   *                 whether \p buf_len was large enough. This makes it possible
   *                 to determine the necessary size by calling this function
   *                 with \p buf set to \c NULL and \p buf_len to \c 0.
   *
   * \return         \c 0 if successful.
   * \return         #MBEDTLS_ERR_SSL_BUFFER_TOO_SMALL if \p buf is too small.
   */
  int mbedtls_ssl_session_save( const mbedtls_ssl_session *session,
                                unsigned char *buf,
                                size_t buf_len,
                                size_t *olen );
  
  /**
   * \brief          Get a pointer to the current session structure, for example
   *                 to serialize it.
   *
   * \warning        Ownership of the session remains with the SSL context, and
   *                 the returned pointer is only guaranteed to be valid until
   *                 the next API call operating on the same \p ssl context.
   *
   * \see            mbedtls_ssl_session_save()
   *
   * \param ssl      The SSL context.
   *
   * \return         A pointer to the current session if successful.
   * \return         \c NULL if no session is active.
   */
  const mbedtls_ssl_session *mbedtls_ssl_get_session_pointer( const mbedtls_ssl_context *ssl );
  
  /**
   * \brief               Set the list of allowed ciphersuites and the preference
   *                      order. First in the list has the highest preference.
   *                      (Overrides all version-specific lists)
   *
   *                      The ciphersuites array is not copied, and must remain
   *                      valid for the lifetime of the ssl_config.
   *
   *                      Note: The server uses its own preferences
   *                      over the preference of the client unless
   *                      MBEDTLS_SSL_SRV_RESPECT_CLIENT_PREFERENCE is defined!
   *
   * \param conf          SSL configuration
   * \param ciphersuites  0-terminated list of allowed ciphersuites
   */
  void mbedtls_ssl_conf_ciphersuites( mbedtls_ssl_config *conf,
                                     const int *ciphersuites );
  
  #if defined(MBEDTLS_SSL_DTLS_CONNECTION_ID)
  #define MBEDTLS_SSL_UNEXPECTED_CID_IGNORE 0
  #define MBEDTLS_SSL_UNEXPECTED_CID_FAIL   1
  /**
   * \brief               Specify the length of Connection IDs for incoming
   *                      encrypted DTLS records, as well as the behaviour
   *                      on unexpected CIDs.
   *
   *                      By default, the CID length is set to \c 0,
   *                      and unexpected CIDs are silently ignored.
   *
   * \param conf          The SSL configuration to modify.
   * \param len           The length in Bytes of the CID fields in encrypted
   *                      DTLS records using the CID mechanism. This must
   *                      not be larger than #MBEDTLS_SSL_CID_OUT_LEN_MAX.
   * \param ignore_other_cids This determines the stack's behaviour when
   *                          receiving a record with an unexpected CID.
   *                          Possible values are:
   *                          - #MBEDTLS_SSL_UNEXPECTED_CID_IGNORE
   *                            In this case, the record is silently ignored.
   *                          - #MBEDTLS_SSL_UNEXPECTED_CID_FAIL
   *                            In this case, the stack fails with the specific
   *                            error code #MBEDTLS_ERR_SSL_UNEXPECTED_CID.
   *
   * \note                The CID specification allows implementations to either
   *                      use a common length for all incoming connection IDs or
   *                      allow variable-length incoming IDs. Mbed TLS currently
   *                      requires a common length for all connections sharing the
   *                      same SSL configuration; this allows simpler parsing of
   *                      record headers.
   *
   * \return              \c 0 on success.
   * \return              #MBEDTLS_ERR_SSL_BAD_INPUT_DATA if \p own_cid_len
   *                      is too large.
   */
  int mbedtls_ssl_conf_cid( mbedtls_ssl_config *conf, size_t len,
                            int ignore_other_cids );
  #endif /* MBEDTLS_SSL_DTLS_CONNECTION_ID */
  
  /**
   * \brief               Set the list of allowed ciphersuites and the
   *                      preference order for a specific version of the protocol.
   *                      (Only useful on the server side)
   *
   *                      The ciphersuites array is not copied, and must remain
   *                      valid for the lifetime of the ssl_config.
   *
   * \param conf          SSL configuration
   * \param ciphersuites  0-terminated list of allowed ciphersuites
   * \param major         Major version number (only MBEDTLS_SSL_MAJOR_VERSION_3
   *                      supported)
   * \param minor         Minor version number (MBEDTLS_SSL_MINOR_VERSION_0,
   *                      MBEDTLS_SSL_MINOR_VERSION_1 and MBEDTLS_SSL_MINOR_VERSION_2,
   *                      MBEDTLS_SSL_MINOR_VERSION_3 supported)
   *
   * \note                With DTLS, use MBEDTLS_SSL_MINOR_VERSION_2 for DTLS 1.0
   *                      and MBEDTLS_SSL_MINOR_VERSION_3 for DTLS 1.2
   */
  void mbedtls_ssl_conf_ciphersuites_for_version( mbedtls_ssl_config *conf,
                                         const int *ciphersuites,
                                         int major, int minor );
  
  #if defined(MBEDTLS_X509_CRT_PARSE_C)
  /**
   * \brief          Set the X.509 security profile used for verification
   *
   * \note           The restrictions are enforced for all certificates in the
   *                 chain. However, signatures in the handshake are not covered
   *                 by this setting but by \b mbedtls_ssl_conf_sig_hashes().
   *
   * \param conf     SSL configuration
   * \param profile  Profile to use
   */
  void mbedtls_ssl_conf_cert_profile( mbedtls_ssl_config *conf,
                                      const mbedtls_x509_crt_profile *profile );
  
  /**
   * \brief          Set the data required to verify peer certificate
   *
   * \note           See \c mbedtls_x509_crt_verify() for notes regarding the
   *                 parameters ca_chain (maps to trust_ca for that function)
   *                 and ca_crl.
   *
   * \param conf     SSL configuration
   * \param ca_chain trusted CA chain (meaning all fully trusted top-level CAs)
   * \param ca_crl   trusted CA CRLs
   */
  void mbedtls_ssl_conf_ca_chain( mbedtls_ssl_config *conf,
                                 mbedtls_x509_crt *ca_chain,
                                 mbedtls_x509_crl *ca_crl );
  
  #if defined(MBEDTLS_X509_TRUSTED_CERTIFICATE_CALLBACK)
  /**
   * \brief          Set the trusted certificate callback.
   *
   *                 This API allows to register the set of trusted certificates
   *                 through a callback, instead of a linked list as configured
   *                 by mbedtls_ssl_conf_ca_chain().
   *
   *                 This is useful for example in contexts where a large number
   *                 of CAs are used, and the inefficiency of maintaining them
   *                 in a linked list cannot be tolerated. It is also useful when
   *                 the set of trusted CAs needs to be modified frequently.
   *
   *                 See the documentation of `mbedtls_x509_crt_ca_cb_t` for
   *                 more information.
   *
   * \param conf     The SSL configuration to register the callback with.
   * \param f_ca_cb  The trusted certificate callback to use when verifying
   *                 certificate chains.
   * \param p_ca_cb  The context to be passed to \p f_ca_cb (for example,
   *                 a reference to a trusted CA database).
   *
   * \note           This API is incompatible with mbedtls_ssl_conf_ca_chain():
   *                 Any call to this function overwrites the values set through
   *                 earlier calls to mbedtls_ssl_conf_ca_chain() or
   *                 mbedtls_ssl_conf_ca_cb().
   *
   * \note           This API is incompatible with CA indication in
   *                 CertificateRequest messages: A server-side SSL context which
   *                 is bound to an SSL configuration that uses a CA callback
   *                 configured via mbedtls_ssl_conf_ca_cb(), and which requires
   *                 client authentication, will send an empty CA list in the
   *                 corresponding CertificateRequest message.
   *
   * \note           This API is incompatible with mbedtls_ssl_set_hs_ca_chain():
   *                 If an SSL context is bound to an SSL configuration which uses
   *                 CA callbacks configured via mbedtls_ssl_conf_ca_cb(), then
   *                 calls to mbedtls_ssl_set_hs_ca_chain() have no effect.
   *
   * \note           The use of this API disables the use of restartable ECC
   *                 during X.509 CRT signature verification (but doesn't affect
   *                 other uses).
   *
   * \warning        This API is incompatible with the use of CRLs. Any call to
   *                 mbedtls_ssl_conf_ca_cb() unsets CRLs configured through
   *                 earlier calls to mbedtls_ssl_conf_ca_chain().
   *
   * \warning        In multi-threaded environments, the callback \p f_ca_cb
   *                 must be thread-safe, and it is the user's responsibility
   *                 to guarantee this (for example through a mutex
   *                 contained in the callback context pointed to by \p p_ca_cb).
   */
  void mbedtls_ssl_conf_ca_cb( mbedtls_ssl_config *conf,
                               mbedtls_x509_crt_ca_cb_t f_ca_cb,
                               void *p_ca_cb );
  #endif /* MBEDTLS_X509_TRUSTED_CERTIFICATE_CALLBACK */
  
  /**
   * \brief          Set own certificate chain and private key
   *
   * \note           own_cert should contain in order from the bottom up your
   *                 certificate chain. The top certificate (self-signed)
   *                 can be omitted.
   *
   * \note           On server, this function can be called multiple times to
   *                 provision more than one cert/key pair (eg one ECDSA, one
   *                 RSA with SHA-256, one RSA with SHA-1). An adequate
   *                 certificate will be selected according to the client's
   *                 advertised capabilities. In case multiple certificates are
   *                 adequate, preference is given to the one set by the first
   *                 call to this function, then second, etc.
   *
   * \note           On client, only the first call has any effect. That is,
   *                 only one client certificate can be provisioned. The
   *                 server's preferences in its CertficateRequest message will
   *                 be ignored and our only cert will be sent regardless of
   *                 whether it matches those preferences - the server can then
   *                 decide what it wants to do with it.
   *
   * \note           The provided \p pk_key needs to match the public key in the
   *                 first certificate in \p own_cert, or all handshakes using
   *                 that certificate will fail. It is your responsibility
   *                 to ensure that; this function will not perform any check.
   *                 You may use mbedtls_pk_check_pair() in order to perform
   *                 this check yourself, but be aware that this function can
   *                 be computationally expensive on some key types.
   *
   * \param conf     SSL configuration
   * \param own_cert own public certificate chain
   * \param pk_key   own private key
   *
   * \return         0 on success or MBEDTLS_ERR_SSL_ALLOC_FAILED
   */
  int mbedtls_ssl_conf_own_cert( mbedtls_ssl_config *conf,
                                mbedtls_x509_crt *own_cert,
                                mbedtls_pk_context *pk_key );
  #endif /* MBEDTLS_X509_CRT_PARSE_C */
  
  #if defined(MBEDTLS_KEY_EXCHANGE_SOME_PSK_ENABLED)
  /**
   * \brief          Configure a pre-shared key (PSK) and identity
   *                 to be used in PSK-based ciphersuites.
   *
   * \note           This is mainly useful for clients. Servers will usually
   *                 want to use \c mbedtls_ssl_conf_psk_cb() instead.
   *
   * \warning        Currently, clients can only register a single pre-shared key.
   *                 Calling this function or mbedtls_ssl_conf_psk_opaque() more
   *                 than once will overwrite values configured in previous calls.
   *                 Support for setting multiple PSKs on clients and selecting
   *                 one based on the identity hint is not a planned feature,
   *                 but feedback is welcomed.
   *
   * \param conf     The SSL configuration to register the PSK with.
   * \param psk      The pointer to the pre-shared key to use.
   * \param psk_len  The length of the pre-shared key in bytes.
   * \param psk_identity      The pointer to the pre-shared key identity.
   * \param psk_identity_len  The length of the pre-shared key identity
   *                          in bytes.
   *
   * \note           The PSK and its identity are copied internally and
   *                 hence need not be preserved by the caller for the lifetime
   *                 of the SSL configuration.
   *
   * \return         \c 0 if successful.
   * \return         An \c MBEDTLS_ERR_SSL_XXX error code on failure.
   */
  int mbedtls_ssl_conf_psk( mbedtls_ssl_config *conf,
                  const unsigned char *psk, size_t psk_len,
                  const unsigned char *psk_identity, size_t psk_identity_len );
  
  #if defined(MBEDTLS_USE_PSA_CRYPTO)
  /**
   * \brief          Configure an opaque pre-shared key (PSK) and identity
   *                 to be used in PSK-based ciphersuites.
   *
   * \note           This is mainly useful for clients. Servers will usually
   *                 want to use \c mbedtls_ssl_conf_psk_cb() instead.
   *
   * \warning        Currently, clients can only register a single pre-shared key.
   *                 Calling this function or mbedtls_ssl_conf_psk() more than
   *                 once will overwrite values configured in previous calls.
   *                 Support for setting multiple PSKs on clients and selecting
   *                 one based on the identity hint is not a planned feature,
   *                 but feedback is welcomed.
   *
   * \param conf     The SSL configuration to register the PSK with.
   * \param psk      The identifier of the key slot holding the PSK.
   *                 Until \p conf is destroyed or this function is successfully
   *                 called again, the key slot \p psk must be populated with a
   *                 key of type PSA_ALG_CATEGORY_KEY_DERIVATION whose policy
   *                 allows its use for the key derivation algorithm applied
   *                 in the handshake.
   * \param psk_identity      The pointer to the pre-shared key identity.
   * \param psk_identity_len  The length of the pre-shared key identity
   *                          in bytes.
   *
   * \note           The PSK identity hint is copied internally and hence need
   *                 not be preserved by the caller for the lifetime of the
   *                 SSL configuration.
   *
   * \return         \c 0 if successful.
   * \return         An \c MBEDTLS_ERR_SSL_XXX error code on failure.
   */
  int mbedtls_ssl_conf_psk_opaque( mbedtls_ssl_config *conf,
                                   psa_key_handle_t psk,
                                   const unsigned char *psk_identity,
                                   size_t psk_identity_len );
  #endif /* MBEDTLS_USE_PSA_CRYPTO */
  
  /**
   * \brief          Set the pre-shared Key (PSK) for the current handshake.
   *
   * \note           This should only be called inside the PSK callback,
   *                 i.e. the function passed to \c mbedtls_ssl_conf_psk_cb().
   *
   * \param ssl      The SSL context to configure a PSK for.
   * \param psk      The pointer to the pre-shared key.
   * \param psk_len  The length of the pre-shared key in bytes.
   *
   * \return         \c 0 if successful.
   * \return         An \c MBEDTLS_ERR_SSL_XXX error code on failure.
   */
  int mbedtls_ssl_set_hs_psk( mbedtls_ssl_context *ssl,
                              const unsigned char *psk, size_t psk_len );
  
  #if defined(MBEDTLS_USE_PSA_CRYPTO)
  /**
   * \brief          Set an opaque pre-shared Key (PSK) for the current handshake.
   *
   * \note           This should only be called inside the PSK callback,
   *                 i.e. the function passed to \c mbedtls_ssl_conf_psk_cb().
   *
   * \param ssl      The SSL context to configure a PSK for.
   * \param psk      The identifier of the key slot holding the PSK.
   *                 For the duration of the current handshake, the key slot
   *                 must be populated with a key of type
   *                 PSA_ALG_CATEGORY_KEY_DERIVATION whose policy allows its
   *                 use for the key derivation algorithm
   *                 applied in the handshake.
    *
   * \return         \c 0 if successful.
   * \return         An \c MBEDTLS_ERR_SSL_XXX error code on failure.
   */
  int mbedtls_ssl_set_hs_psk_opaque( mbedtls_ssl_context *ssl,
                                     psa_key_handle_t psk );
  #endif /* MBEDTLS_USE_PSA_CRYPTO */
  
  /**
   * \brief          Set the PSK callback (server-side only).
   *
   *                 If set, the PSK callback is called for each
   *                 handshake where a PSK-based ciphersuite was negotiated.
   *                 The caller provides the identity received and wants to
   *                 receive the actual PSK data and length.
   *
   *                 The callback has the following parameters:
   *                 - \c void*: The opaque pointer \p p_psk.
   *                 - \c mbedtls_ssl_context*: The SSL context to which
   *                                            the operation applies.
   *                 - \c const unsigned char*: The PSK identity
   *                                            selected by the client.
   *                 - \c size_t: The length of the PSK identity
   *                              selected by the client.
   *
   *                 If a valid PSK identity is found, the callback should use
   *                 \c mbedtls_ssl_set_hs_psk() or
   *                 \c mbedtls_ssl_set_hs_psk_opaque()
   *                 on the SSL context to set the correct PSK and return \c 0.
   *                 Any other return value will result in a denied PSK identity.
   *
   * \note           If you set a PSK callback using this function, then you
   *                 don't need to set a PSK key and identity using
   *                 \c mbedtls_ssl_conf_psk().
   *
   * \param conf     The SSL configuration to register the callback with.
   * \param f_psk    The callback for selecting and setting the PSK based
   *                 in the PSK identity chosen by the client.
   * \param p_psk    A pointer to an opaque structure to be passed to
   *                 the callback, for example a PSK store.
   */
  void mbedtls_ssl_conf_psk_cb( mbedtls_ssl_config *conf,
                       int (*f_psk)(void *, mbedtls_ssl_context *, const unsigned char *,
                                    size_t),
                       void *p_psk );
  #endif /* MBEDTLS_KEY_EXCHANGE_SOME_PSK_ENABLED */
  
  #if defined(MBEDTLS_DHM_C) && defined(MBEDTLS_SSL_SRV_C)
  
  #if !defined(MBEDTLS_DEPRECATED_REMOVED)
  
  #if defined(MBEDTLS_DEPRECATED_WARNING)
  #define MBEDTLS_DEPRECATED    __attribute__((deprecated))
  #else
  #define MBEDTLS_DEPRECATED
  #endif
  
  /**
   * \brief          Set the Diffie-Hellman public P and G values,
   *                 read as hexadecimal strings (server-side only)
   *                 (Default values: MBEDTLS_DHM_RFC3526_MODP_2048_[PG])
   *
   * \param conf     SSL configuration
   * \param dhm_P    Diffie-Hellman-Merkle modulus
   * \param dhm_G    Diffie-Hellman-Merkle generator
   *
   * \deprecated     Superseded by \c mbedtls_ssl_conf_dh_param_bin.
   *
   * \return         0 if successful
   */
  MBEDTLS_DEPRECATED int mbedtls_ssl_conf_dh_param( mbedtls_ssl_config *conf,
                                                    const char *dhm_P,
                                                    const char *dhm_G );
  
  #endif /* MBEDTLS_DEPRECATED_REMOVED */
  
  /**
   * \brief          Set the Diffie-Hellman public P and G values
   *                 from big-endian binary presentations.
   *                 (Default values: MBEDTLS_DHM_RFC3526_MODP_2048_[PG]_BIN)
   *
   * \param conf     SSL configuration
   * \param dhm_P    Diffie-Hellman-Merkle modulus in big-endian binary form
   * \param P_len    Length of DHM modulus
   * \param dhm_G    Diffie-Hellman-Merkle generator in big-endian binary form
   * \param G_len    Length of DHM generator
   *
   * \return         0 if successful
   */
  int mbedtls_ssl_conf_dh_param_bin( mbedtls_ssl_config *conf,
                                     const unsigned char *dhm_P, size_t P_len,
                                     const unsigned char *dhm_G,  size_t G_len );
  
  /**
   * \brief          Set the Diffie-Hellman public P and G values,
   *                 read from existing context (server-side only)
   *
   * \param conf     SSL configuration
   * \param dhm_ctx  Diffie-Hellman-Merkle context
   *
   * \return         0 if successful
   */
  int mbedtls_ssl_conf_dh_param_ctx( mbedtls_ssl_config *conf, mbedtls_dhm_context *dhm_ctx );
  #endif /* MBEDTLS_DHM_C && defined(MBEDTLS_SSL_SRV_C) */
  
  #if defined(MBEDTLS_DHM_C) && defined(MBEDTLS_SSL_CLI_C)
  /**
   * \brief          Set the minimum length for Diffie-Hellman parameters.
   *                 (Client-side only.)
   *                 (Default: 1024 bits.)
   *
   * \param conf     SSL configuration
   * \param bitlen   Minimum bit length of the DHM prime
   */
  void mbedtls_ssl_conf_dhm_min_bitlen( mbedtls_ssl_config *conf,
                                        unsigned int bitlen );
  #endif /* MBEDTLS_DHM_C && MBEDTLS_SSL_CLI_C */
  
  #if defined(MBEDTLS_ECP_C)
  /**
   * \brief          Set the allowed curves in order of preference.
   *                 (Default: all defined curves.)
   *
   *                 On server: this only affects selection of the ECDHE curve;
   *                 the curves used for ECDH and ECDSA are determined by the
   *                 list of available certificates instead.
   *
   *                 On client: this affects the list of curves offered for any
   *                 use. The server can override our preference order.
   *
   *                 Both sides: limits the set of curves accepted for use in
   *                 ECDHE and in the peer's end-entity certificate.
   *
   * \note           This has no influence on which curves are allowed inside the
   *                 certificate chains, see \c mbedtls_ssl_conf_cert_profile()
   *                 for that. For the end-entity certificate however, the key
   *                 will be accepted only if it is allowed both by this list
   *                 and by the cert profile.
   *
   * \note           This list should be ordered by decreasing preference
   *                 (preferred curve first).
   *
   * \param conf     SSL configuration
   * \param curves   Ordered list of allowed curves,
   *                 terminated by MBEDTLS_ECP_DP_NONE.
   */
  void mbedtls_ssl_conf_curves( mbedtls_ssl_config *conf,
                                const mbedtls_ecp_group_id *curves );
  #endif /* MBEDTLS_ECP_C */
  
  #if defined(MBEDTLS_KEY_EXCHANGE_WITH_CERT_ENABLED)
  /**
   * \brief          Set the allowed hashes for signatures during the handshake.
   *                 (Default: all available hashes except MD5.)
   *
   * \note           This only affects which hashes are offered and can be used
   *                 for signatures during the handshake. Hashes for message
   *                 authentication and the TLS PRF are controlled by the
   *                 ciphersuite, see \c mbedtls_ssl_conf_ciphersuites(). Hashes
   *                 used for certificate signature are controlled by the
   *                 verification profile, see \c mbedtls_ssl_conf_cert_profile().
   *
   * \note           This list should be ordered by decreasing preference
   *                 (preferred hash first).
   *
   * \param conf     SSL configuration
   * \param hashes   Ordered list of allowed signature hashes,
   *                 terminated by \c MBEDTLS_MD_NONE.
   */
  void mbedtls_ssl_conf_sig_hashes( mbedtls_ssl_config *conf,
                                    const int *hashes );
  #endif /* MBEDTLS_KEY_EXCHANGE_WITH_CERT_ENABLED */
  
  #if defined(MBEDTLS_X509_CRT_PARSE_C)
  /**
   * \brief          Set or reset the hostname to check against the received
   *                 server certificate. It sets the ServerName TLS extension,
   *                 too, if that extension is enabled. (client-side only)
   *
   * \param ssl      SSL context
   * \param hostname the server hostname, may be NULL to clear hostname
  
   * \note           Maximum hostname length MBEDTLS_SSL_MAX_HOST_NAME_LEN.
   *
   * \return         0 if successful, MBEDTLS_ERR_SSL_ALLOC_FAILED on
   *                 allocation failure, MBEDTLS_ERR_SSL_BAD_INPUT_DATA on
   *                 too long input hostname.
   *
   *                 Hostname set to the one provided on success (cleared
   *                 when NULL). On allocation failure hostname is cleared.
   *                 On too long input failure, old hostname is unchanged.
   */
  int mbedtls_ssl_set_hostname( mbedtls_ssl_context *ssl, const char *hostname );
  #endif /* MBEDTLS_X509_CRT_PARSE_C */
  
  #if defined(MBEDTLS_SSL_SERVER_NAME_INDICATION)
  /**
   * \brief          Set own certificate and key for the current handshake
   *
   * \note           Same as \c mbedtls_ssl_conf_own_cert() but for use within
   *                 the SNI callback.
   *
   * \param ssl      SSL context
   * \param own_cert own public certificate chain
   * \param pk_key   own private key
   *
   * \return         0 on success or MBEDTLS_ERR_SSL_ALLOC_FAILED
   */
  int mbedtls_ssl_set_hs_own_cert( mbedtls_ssl_context *ssl,
                                   mbedtls_x509_crt *own_cert,
                                   mbedtls_pk_context *pk_key );
  
  /**
   * \brief          Set the data required to verify peer certificate for the
   *                 current handshake
   *
   * \note           Same as \c mbedtls_ssl_conf_ca_chain() but for use within
   *                 the SNI callback.
   *
   * \param ssl      SSL context
   * \param ca_chain trusted CA chain (meaning all fully trusted top-level CAs)
   * \param ca_crl   trusted CA CRLs
   */
  void mbedtls_ssl_set_hs_ca_chain( mbedtls_ssl_context *ssl,
                                    mbedtls_x509_crt *ca_chain,
                                    mbedtls_x509_crl *ca_crl );
  
  /**
   * \brief          Set authmode for the current handshake.
   *
   * \note           Same as \c mbedtls_ssl_conf_authmode() but for use within
   *                 the SNI callback.
   *
   * \param ssl      SSL context
   * \param authmode MBEDTLS_SSL_VERIFY_NONE, MBEDTLS_SSL_VERIFY_OPTIONAL or
   *                 MBEDTLS_SSL_VERIFY_REQUIRED
   */
  void mbedtls_ssl_set_hs_authmode( mbedtls_ssl_context *ssl,
                                    int authmode );
  
  /**
   * \brief          Set server side ServerName TLS extension callback
   *                 (optional, server-side only).
   *
   *                 If set, the ServerName callback is called whenever the
   *                 server receives a ServerName TLS extension from the client
   *                 during a handshake. The ServerName callback has the
   *                 following parameters: (void *parameter, mbedtls_ssl_context *ssl,
   *                 const unsigned char *hostname, size_t len). If a suitable
   *                 certificate is found, the callback must set the
   *                 certificate(s) and key(s) to use with \c
   *                 mbedtls_ssl_set_hs_own_cert() (can be called repeatedly),
   *                 and may optionally adjust the CA and associated CRL with \c
   *                 mbedtls_ssl_set_hs_ca_chain() as well as the client
   *                 authentication mode with \c mbedtls_ssl_set_hs_authmode(),
   *                 then must return 0. If no matching name is found, the
   *                 callback must either set a default cert, or
   *                 return non-zero to abort the handshake at this point.
   *
   * \param conf     SSL configuration
   * \param f_sni    verification function
   * \param p_sni    verification parameter
   */
  void mbedtls_ssl_conf_sni( mbedtls_ssl_config *conf,
                    int (*f_sni)(void *, mbedtls_ssl_context *, const unsigned char *,
                                 size_t),
                    void *p_sni );
  #endif /* MBEDTLS_SSL_SERVER_NAME_INDICATION */
  
  #if defined(MBEDTLS_KEY_EXCHANGE_ECJPAKE_ENABLED)
  /**
   * \brief          Set the EC J-PAKE password for current handshake.
   *
   * \note           An internal copy is made, and destroyed as soon as the
   *                 handshake is completed, or when the SSL context is reset or
   *                 freed.
   *
   * \note           The SSL context needs to be already set up. The right place
   *                 to call this function is between \c mbedtls_ssl_setup() or
   *                 \c mbedtls_ssl_reset() and \c mbedtls_ssl_handshake().
   *
   * \param ssl      SSL context
   * \param pw       EC J-PAKE password (pre-shared secret)
   * \param pw_len   length of pw in bytes
   *
   * \return         0 on success, or a negative error code.
   */
  int mbedtls_ssl_set_hs_ecjpake_password( mbedtls_ssl_context *ssl,
                                           const unsigned char *pw,
                                           size_t pw_len );
  #endif /*MBEDTLS_KEY_EXCHANGE_ECJPAKE_ENABLED */
  
  #if defined(MBEDTLS_SSL_ALPN)
  /**
   * \brief          Set the supported Application Layer Protocols.
   *
   * \param conf     SSL configuration
   * \param protos   Pointer to a NULL-terminated list of supported protocols,
   *                 in decreasing preference order. The pointer to the list is
   *                 recorded by the library for later reference as required, so
   *                 the lifetime of the table must be atleast as long as the
   *                 lifetime of the SSL configuration structure.
   *
   * \return         0 on success, or MBEDTLS_ERR_SSL_BAD_INPUT_DATA.
   */
  int mbedtls_ssl_conf_alpn_protocols( mbedtls_ssl_config *conf, const char **protos );
  
  /**
   * \brief          Get the name of the negotiated Application Layer Protocol.
   *                 This function should be called after the handshake is
   *                 completed.
   *
   * \param ssl      SSL context
   *
   * \return         Protcol name, or NULL if no protocol was negotiated.
   */
  const char *mbedtls_ssl_get_alpn_protocol( const mbedtls_ssl_context *ssl );
  #endif /* MBEDTLS_SSL_ALPN */
  
  /**
   * \brief          Set the maximum supported version sent from the client side
   *                 and/or accepted at the server side
   *                 (Default: MBEDTLS_SSL_MAX_MAJOR_VERSION, MBEDTLS_SSL_MAX_MINOR_VERSION)
   *
   * \note           This ignores ciphersuites from higher versions.
   *
   * \note           With DTLS, use MBEDTLS_SSL_MINOR_VERSION_2 for DTLS 1.0 and
   *                 MBEDTLS_SSL_MINOR_VERSION_3 for DTLS 1.2
   *
   * \param conf     SSL configuration
   * \param major    Major version number (only MBEDTLS_SSL_MAJOR_VERSION_3 supported)
   * \param minor    Minor version number (MBEDTLS_SSL_MINOR_VERSION_0,
   *                 MBEDTLS_SSL_MINOR_VERSION_1 and MBEDTLS_SSL_MINOR_VERSION_2,
   *                 MBEDTLS_SSL_MINOR_VERSION_3 supported)
   */
  void mbedtls_ssl_conf_max_version( mbedtls_ssl_config *conf, int major, int minor );
  
  /**
   * \brief          Set the minimum accepted SSL/TLS protocol version
   *                 (Default: TLS 1.0)
   *
   * \note           Input outside of the SSL_MAX_XXXXX_VERSION and
   *                 SSL_MIN_XXXXX_VERSION range is ignored.
   *
   * \note           MBEDTLS_SSL_MINOR_VERSION_0 (SSL v3) should be avoided.
   *
   * \note           With DTLS, use MBEDTLS_SSL_MINOR_VERSION_2 for DTLS 1.0 and
   *                 MBEDTLS_SSL_MINOR_VERSION_3 for DTLS 1.2
   *
   * \param conf     SSL configuration
   * \param major    Major version number (only MBEDTLS_SSL_MAJOR_VERSION_3 supported)
   * \param minor    Minor version number (MBEDTLS_SSL_MINOR_VERSION_0,
   *                 MBEDTLS_SSL_MINOR_VERSION_1 and MBEDTLS_SSL_MINOR_VERSION_2,
   *                 MBEDTLS_SSL_MINOR_VERSION_3 supported)
   */
  void mbedtls_ssl_conf_min_version( mbedtls_ssl_config *conf, int major, int minor );
  
  #if defined(MBEDTLS_SSL_FALLBACK_SCSV) && defined(MBEDTLS_SSL_CLI_C)
  /**
   * \brief          Set the fallback flag (client-side only).
   *                 (Default: MBEDTLS_SSL_IS_NOT_FALLBACK).
   *
   * \note           Set to MBEDTLS_SSL_IS_FALLBACK when preparing a fallback
   *                 connection, that is a connection with max_version set to a
   *                 lower value than the value you're willing to use. Such
   *                 fallback connections are not recommended but are sometimes
   *                 necessary to interoperate with buggy (version-intolerant)
   *                 servers.
   *
   * \warning        You should NOT set this to MBEDTLS_SSL_IS_FALLBACK for
   *                 non-fallback connections! This would appear to work for a
   *                 while, then cause failures when the server is upgraded to
   *                 support a newer TLS version.
   *
   * \param conf     SSL configuration
   * \param fallback MBEDTLS_SSL_IS_NOT_FALLBACK or MBEDTLS_SSL_IS_FALLBACK
   */
  void mbedtls_ssl_conf_fallback( mbedtls_ssl_config *conf, char fallback );
  #endif /* MBEDTLS_SSL_FALLBACK_SCSV && MBEDTLS_SSL_CLI_C */
  
  #if defined(MBEDTLS_SSL_ENCRYPT_THEN_MAC)
  /**
   * \brief           Enable or disable Encrypt-then-MAC
   *                  (Default: MBEDTLS_SSL_ETM_ENABLED)
   *
   * \note            This should always be enabled, it is a security
   *                  improvement, and should not cause any interoperability
   *                  issue (used only if the peer supports it too).
   *
   * \param conf      SSL configuration
   * \param etm       MBEDTLS_SSL_ETM_ENABLED or MBEDTLS_SSL_ETM_DISABLED
   */
  void mbedtls_ssl_conf_encrypt_then_mac( mbedtls_ssl_config *conf, char etm );
  #endif /* MBEDTLS_SSL_ENCRYPT_THEN_MAC */
  
  #if defined(MBEDTLS_SSL_EXTENDED_MASTER_SECRET)
  /**
   * \brief           Enable or disable Extended Master Secret negotiation.
   *                  (Default: MBEDTLS_SSL_EXTENDED_MS_ENABLED)
   *
   * \note            This should always be enabled, it is a security fix to the
   *                  protocol, and should not cause any interoperability issue
   *                  (used only if the peer supports it too).
   *
   * \param conf      SSL configuration
   * \param ems       MBEDTLS_SSL_EXTENDED_MS_ENABLED or MBEDTLS_SSL_EXTENDED_MS_DISABLED
   */
  void mbedtls_ssl_conf_extended_master_secret( mbedtls_ssl_config *conf, char ems );
  #endif /* MBEDTLS_SSL_EXTENDED_MASTER_SECRET */
  
  #if defined(MBEDTLS_ARC4_C)
  /**
   * \brief          Disable or enable support for RC4
   *                 (Default: MBEDTLS_SSL_ARC4_DISABLED)
   *
   * \warning        Use of RC4 in DTLS/TLS has been prohibited by RFC 7465
   *                 for security reasons. Use at your own risk.
   *
   * \note           This function is deprecated and will be removed in
   *                 a future version of the library.
   *                 RC4 is disabled by default at compile time and needs to be
   *                 actively enabled for use with legacy systems.
   *
   * \param conf     SSL configuration
   * \param arc4     MBEDTLS_SSL_ARC4_ENABLED or MBEDTLS_SSL_ARC4_DISABLED
   */
  void mbedtls_ssl_conf_arc4_support( mbedtls_ssl_config *conf, char arc4 );
  #endif /* MBEDTLS_ARC4_C */
  
  #if defined(MBEDTLS_SSL_SRV_C)
  /**
   * \brief          Whether to send a list of acceptable CAs in
   *                 CertificateRequest messages.
   *                 (Default: do send)
   *
   * \param conf     SSL configuration
   * \param cert_req_ca_list   MBEDTLS_SSL_CERT_REQ_CA_LIST_ENABLED or
   *                          MBEDTLS_SSL_CERT_REQ_CA_LIST_DISABLED
   */
  void mbedtls_ssl_conf_cert_req_ca_list( mbedtls_ssl_config *conf,
                                            char cert_req_ca_list );
  #endif /* MBEDTLS_SSL_SRV_C */
  
  #if defined(MBEDTLS_SSL_MAX_FRAGMENT_LENGTH)
  /**
   * \brief          Set the maximum fragment length to emit and/or negotiate.
   *                 (Typical: the smaller of #MBEDTLS_SSL_IN_CONTENT_LEN and
   *                 #MBEDTLS_SSL_OUT_CONTENT_LEN, usually `2^14` bytes)
   *                 (Server: set maximum fragment length to emit,
   *                 usually negotiated by the client during handshake)
   *                 (Client: set maximum fragment length to emit *and*
   *                 negotiate with the server during handshake)
   *                 (Default: #MBEDTLS_SSL_MAX_FRAG_LEN_NONE)
   *
   * \note           On the client side, the maximum fragment length extension
   *                 *will not* be used, unless the maximum fragment length has
   *                 been set via this function to a value different than
   *                 #MBEDTLS_SSL_MAX_FRAG_LEN_NONE.
   *
   * \note           With TLS, this currently only affects ApplicationData (sent
   *                 with \c mbedtls_ssl_read()), not handshake messages.
   *                 With DTLS, this affects both ApplicationData and handshake.
   *
   * \note           This sets the maximum length for a record's payload,
   *                 excluding record overhead that will be added to it, see
   *                 \c mbedtls_ssl_get_record_expansion().
   *
   * \note           For DTLS, it is also possible to set a limit for the total
   *                 size of daragrams passed to the transport layer, including
   *                 record overhead, see \c mbedtls_ssl_set_mtu().
   *
   * \param conf     SSL configuration
   * \param mfl_code Code for maximum fragment length (allowed values:
   *                 MBEDTLS_SSL_MAX_FRAG_LEN_512,  MBEDTLS_SSL_MAX_FRAG_LEN_1024,
   *                 MBEDTLS_SSL_MAX_FRAG_LEN_2048, MBEDTLS_SSL_MAX_FRAG_LEN_4096)
   *
   * \return         0 if successful or MBEDTLS_ERR_SSL_BAD_INPUT_DATA
   */
  int mbedtls_ssl_conf_max_frag_len( mbedtls_ssl_config *conf, unsigned char mfl_code );
  #endif /* MBEDTLS_SSL_MAX_FRAGMENT_LENGTH */
  
  #if defined(MBEDTLS_SSL_TRUNCATED_HMAC)
  /**
   * \brief          Activate negotiation of truncated HMAC
   *                 (Default: MBEDTLS_SSL_TRUNC_HMAC_DISABLED)
   *
   * \param conf     SSL configuration
   * \param truncate Enable or disable (MBEDTLS_SSL_TRUNC_HMAC_ENABLED or
   *                                    MBEDTLS_SSL_TRUNC_HMAC_DISABLED)
   */
  void mbedtls_ssl_conf_truncated_hmac( mbedtls_ssl_config *conf, int truncate );
  #endif /* MBEDTLS_SSL_TRUNCATED_HMAC */
  
  #if defined(MBEDTLS_SSL_CBC_RECORD_SPLITTING)
  /**
   * \brief          Enable / Disable 1/n-1 record splitting
   *                 (Default: MBEDTLS_SSL_CBC_RECORD_SPLITTING_ENABLED)
   *
   * \note           Only affects SSLv3 and TLS 1.0, not higher versions.
   *                 Does not affect non-CBC ciphersuites in any version.
   *
   * \param conf     SSL configuration
   * \param split    MBEDTLS_SSL_CBC_RECORD_SPLITTING_ENABLED or
   *                 MBEDTLS_SSL_CBC_RECORD_SPLITTING_DISABLED
   */
  void mbedtls_ssl_conf_cbc_record_splitting( mbedtls_ssl_config *conf, char split );
  #endif /* MBEDTLS_SSL_CBC_RECORD_SPLITTING */
  
  #if defined(MBEDTLS_SSL_SESSION_TICKETS) && defined(MBEDTLS_SSL_CLI_C)
  /**
   * \brief          Enable / Disable session tickets (client only).
   *                 (Default: MBEDTLS_SSL_SESSION_TICKETS_ENABLED.)
   *
   * \note           On server, use \c mbedtls_ssl_conf_session_tickets_cb().
   *
   * \param conf     SSL configuration
   * \param use_tickets   Enable or disable (MBEDTLS_SSL_SESSION_TICKETS_ENABLED or
   *                                         MBEDTLS_SSL_SESSION_TICKETS_DISABLED)
   */
  void mbedtls_ssl_conf_session_tickets( mbedtls_ssl_config *conf, int use_tickets );
  #endif /* MBEDTLS_SSL_SESSION_TICKETS && MBEDTLS_SSL_CLI_C */
  
  #if defined(MBEDTLS_SSL_RENEGOTIATION)
  /**
   * \brief          Enable / Disable renegotiation support for connection when
   *                 initiated by peer
   *                 (Default: MBEDTLS_SSL_RENEGOTIATION_DISABLED)
   *
   * \warning        It is recommended to always disable renegotation unless you
   *                 know you need it and you know what you're doing. In the
   *                 past, there have been several issues associated with
   *                 renegotiation or a poor understanding of its properties.
   *
   * \note           Server-side, enabling renegotiation also makes the server
   *                 susceptible to a resource DoS by a malicious client.
   *
   * \param conf    SSL configuration
   * \param renegotiation     Enable or disable (MBEDTLS_SSL_RENEGOTIATION_ENABLED or
   *                                             MBEDTLS_SSL_RENEGOTIATION_DISABLED)
   */
  void mbedtls_ssl_conf_renegotiation( mbedtls_ssl_config *conf, int renegotiation );
  #endif /* MBEDTLS_SSL_RENEGOTIATION */
  
  /**
   * \brief          Prevent or allow legacy renegotiation.
   *                 (Default: MBEDTLS_SSL_LEGACY_NO_RENEGOTIATION)
   *
   *                 MBEDTLS_SSL_LEGACY_NO_RENEGOTIATION allows connections to
   *                 be established even if the peer does not support
   *                 secure renegotiation, but does not allow renegotiation
   *                 to take place if not secure.
   *                 (Interoperable and secure option)
   *
   *                 MBEDTLS_SSL_LEGACY_ALLOW_RENEGOTIATION allows renegotiations
   *                 with non-upgraded peers. Allowing legacy renegotiation
   *                 makes the connection vulnerable to specific man in the
   *                 middle attacks. (See RFC 5746)
   *                 (Most interoperable and least secure option)
   *
   *                 MBEDTLS_SSL_LEGACY_BREAK_HANDSHAKE breaks off connections
   *                 if peer does not support secure renegotiation. Results
   *                 in interoperability issues with non-upgraded peers
   *                 that do not support renegotiation altogether.
   *                 (Most secure option, interoperability issues)
   *
   * \param conf     SSL configuration
   * \param allow_legacy  Prevent or allow (SSL_NO_LEGACY_RENEGOTIATION,
   *                                        SSL_ALLOW_LEGACY_RENEGOTIATION or
   *                                        MBEDTLS_SSL_LEGACY_BREAK_HANDSHAKE)
   */
  void mbedtls_ssl_conf_legacy_renegotiation( mbedtls_ssl_config *conf, int allow_legacy );
  
  #if defined(MBEDTLS_SSL_RENEGOTIATION)
  /**
   * \brief          Enforce renegotiation requests.
   *                 (Default: enforced, max_records = 16)
   *
   *                 When we request a renegotiation, the peer can comply or
   *                 ignore the request. This function allows us to decide
   *                 whether to enforce our renegotiation requests by closing
   *                 the connection if the peer doesn't comply.
   *
   *                 However, records could already be in transit from the peer
   *                 when the request is emitted. In order to increase
   *                 reliability, we can accept a number of records before the
   *                 expected handshake records.
   *
   *                 The optimal value is highly dependent on the specific usage
   *                 scenario.
   *
   * \note           With DTLS and server-initiated renegotiation, the
   *                 HelloRequest is retransmited every time mbedtls_ssl_read() times
   *                 out or receives Application Data, until:
   *                 - max_records records have beens seen, if it is >= 0, or
   *                 - the number of retransmits that would happen during an
   *                 actual handshake has been reached.
   *                 Please remember the request might be lost a few times
   *                 if you consider setting max_records to a really low value.
   *
   * \warning        On client, the grace period can only happen during
   *                 mbedtls_ssl_read(), as opposed to mbedtls_ssl_write() and mbedtls_ssl_renegotiate()
   *                 which always behave as if max_record was 0. The reason is,
   *                 if we receive application data from the server, we need a
   *                 place to write it, which only happens during mbedtls_ssl_read().
   *
   * \param conf     SSL configuration
   * \param max_records Use MBEDTLS_SSL_RENEGOTIATION_NOT_ENFORCED if you don't want to
   *                 enforce renegotiation, or a non-negative value to enforce
   *                 it but allow for a grace period of max_records records.
   */
  void mbedtls_ssl_conf_renegotiation_enforced( mbedtls_ssl_config *conf, int max_records );
  
  /**
   * \brief          Set record counter threshold for periodic renegotiation.
   *                 (Default: 2^48 - 1)
   *
   *                 Renegotiation is automatically triggered when a record
   *                 counter (outgoing or incoming) crosses the defined
   *                 threshold. The default value is meant to prevent the
   *                 connection from being closed when the counter is about to
   *                 reached its maximal value (it is not allowed to wrap).
   *
   *                 Lower values can be used to enforce policies such as "keys
   *                 must be refreshed every N packets with cipher X".
   *
   *                 The renegotiation period can be disabled by setting
   *                 conf->disable_renegotiation to
   *                 MBEDTLS_SSL_RENEGOTIATION_DISABLED.
   *
   * \note           When the configured transport is
   *                 MBEDTLS_SSL_TRANSPORT_DATAGRAM the maximum renegotiation
   *                 period is 2^48 - 1, and for MBEDTLS_SSL_TRANSPORT_STREAM,
   *                 the maximum renegotiation period is 2^64 - 1.
   *
   * \param conf     SSL configuration
   * \param period   The threshold value: a big-endian 64-bit number.
   */
  void mbedtls_ssl_conf_renegotiation_period( mbedtls_ssl_config *conf,
                                     const unsigned char period[8] );
  #endif /* MBEDTLS_SSL_RENEGOTIATION */
  
  /**
   * \brief          Check if there is data already read from the
   *                 underlying transport but not yet processed.
   *
   * \param ssl      SSL context
   *
   * \return         0 if nothing's pending, 1 otherwise.
   *
   * \note           This is different in purpose and behaviour from
   *                 \c mbedtls_ssl_get_bytes_avail in that it considers
   *                 any kind of unprocessed data, not only unread
   *                 application data. If \c mbedtls_ssl_get_bytes
   *                 returns a non-zero value, this function will
   *                 also signal pending data, but the converse does
   *                 not hold. For example, in DTLS there might be
   *                 further records waiting to be processed from
   *                 the current underlying transport's datagram.
   *
   * \note           If this function returns 1 (data pending), this
   *                 does not imply that a subsequent call to
   *                 \c mbedtls_ssl_read will provide any data;
   *                 e.g., the unprocessed data might turn out
   *                 to be an alert or a handshake message.
   *
   * \note           This function is useful in the following situation:
   *                 If the SSL/TLS module successfully returns from an
   *                 operation - e.g. a handshake or an application record
   *                 read - and you're awaiting incoming data next, you
   *                 must not immediately idle on the underlying transport
   *                 to have data ready, but you need to check the value
   *                 of this function first. The reason is that the desired
   *                 data might already be read but not yet processed.
   *                 If, in contrast, a previous call to the SSL/TLS module
   *                 returned MBEDTLS_ERR_SSL_WANT_READ, it is not necessary
   *                 to call this function, as the latter error code entails
   *                 that all internal data has been processed.
   *
   */
  int mbedtls_ssl_check_pending( const mbedtls_ssl_context *ssl );
  
  /**
   * \brief          Return the number of application data bytes
   *                 remaining to be read from the current record.
   *
   * \param ssl      SSL context
   *
   * \return         How many bytes are available in the application
   *                 data record read buffer.
   *
   * \note           When working over a datagram transport, this is
   *                 useful to detect the current datagram's boundary
   *                 in case \c mbedtls_ssl_read has written the maximal
   *                 amount of data fitting into the input buffer.
   *
   */
  size_t mbedtls_ssl_get_bytes_avail( const mbedtls_ssl_context *ssl );
  
  /**
   * \brief          Return the result of the certificate verification
   *
   * \param ssl      The SSL context to use.
   *
   * \return         \c 0 if the certificate verification was successful.
   * \return         \c -1u if the result is not available. This may happen
   *                 e.g. if the handshake aborts early, or a verification
   *                 callback returned a fatal error.
   * \return         A bitwise combination of \c MBEDTLS_X509_BADCERT_XXX
   *                 and \c MBEDTLS_X509_BADCRL_XXX failure flags; see x509.h.
   */
  uint32_t mbedtls_ssl_get_verify_result( const mbedtls_ssl_context *ssl );
  
  /**
   * \brief          Return the name of the current ciphersuite
   *
   * \param ssl      SSL context
   *
   * \return         a string containing the ciphersuite name
   */
  const char *mbedtls_ssl_get_ciphersuite( const mbedtls_ssl_context *ssl );
  
  /**
   * \brief          Return the current SSL version (SSLv3/TLSv1/etc)
   *
   * \param ssl      SSL context
   *
   * \return         a string containing the SSL version
   */
  const char *mbedtls_ssl_get_version( const mbedtls_ssl_context *ssl );
  
  /**
   * \brief          Return the (maximum) number of bytes added by the record
   *                 layer: header + encryption/MAC overhead (inc. padding)
   *
   * \note           This function is not available (always returns an error)
   *                 when record compression is enabled.
   *
   * \param ssl      SSL context
   *
   * \return         Current maximum record expansion in bytes, or
   *                 MBEDTLS_ERR_SSL_FEATURE_UNAVAILABLE if compression is
   *                 enabled, which makes expansion much less predictable
   */
  int mbedtls_ssl_get_record_expansion( const mbedtls_ssl_context *ssl );
  
  #if defined(MBEDTLS_SSL_MAX_FRAGMENT_LENGTH)
  /**
   * \brief          Return the maximum fragment length (payload, in bytes) for
   *                 the output buffer. For the client, this is the configured
   *                 value. For the server, it is the minimum of two - the
   *                 configured value and the negotiated one.
   *
   * \sa             mbedtls_ssl_conf_max_frag_len()
   * \sa             mbedtls_ssl_get_max_record_payload()
   *
   * \param ssl      SSL context
   *
   * \return         Current maximum fragment length for the output buffer.
   */
  size_t mbedtls_ssl_get_output_max_frag_len( const mbedtls_ssl_context *ssl );
  
  /**
   * \brief          Return the maximum fragment length (payload, in bytes) for
   *                 the input buffer. This is the negotiated maximum fragment
   *                 length, or, if there is none, MBEDTLS_SSL_MAX_CONTENT_LEN.
   *                 If it is not defined either, the value is 2^14. This function
   *                 works as its predecessor, \c mbedtls_ssl_get_max_frag_len().
   *
   * \sa             mbedtls_ssl_conf_max_frag_len()
   * \sa             mbedtls_ssl_get_max_record_payload()
   *
   * \param ssl      SSL context
   *
   * \return         Current maximum fragment length for the output buffer.
   */
  size_t mbedtls_ssl_get_input_max_frag_len( const mbedtls_ssl_context *ssl );
  
  #if !defined(MBEDTLS_DEPRECATED_REMOVED)
  
  #if defined(MBEDTLS_DEPRECATED_WARNING)
  #define MBEDTLS_DEPRECATED    __attribute__((deprecated))
  #else
  #define MBEDTLS_DEPRECATED
  #endif
  
  /**
   * \brief          This function is a deprecated approach to getting the max
   *                 fragment length. Its an alias for
   *                 \c mbedtls_ssl_get_output_max_frag_len(), as the behaviour
   *                 is the same. See \c mbedtls_ssl_get_output_max_frag_len() for
   *                 more detail.
   *
   * \sa             mbedtls_ssl_get_input_max_frag_len()
   * \sa             mbedtls_ssl_get_output_max_frag_len()
   *
   * \param ssl      SSL context
   *
   * \return         Current maximum fragment length for the output buffer.
   */
  MBEDTLS_DEPRECATED size_t mbedtls_ssl_get_max_frag_len(
                                          const mbedtls_ssl_context *ssl );
  #endif /* MBEDTLS_DEPRECATED_REMOVED */
  #endif /* MBEDTLS_SSL_MAX_FRAGMENT_LENGTH */
  
  /**
   * \brief          Return the current maximum outgoing record payload in bytes.
   *                 This takes into account the config.h setting \c
   *                 MBEDTLS_SSL_OUT_CONTENT_LEN, the configured and negotiated
   *                 max fragment length extension if used, and for DTLS the
   *                 path MTU as configured and current record expansion.
   *
   * \note           With DTLS, \c mbedtls_ssl_write() will return an error if
   *                 called with a larger length value.
   *                 With TLS, \c mbedtls_ssl_write() will fragment the input if
   *                 necessary and return the number of bytes written; it is up
   *                 to the caller to call \c mbedtls_ssl_write() again in
   *                 order to send the remaining bytes if any.
   *
   * \note           This function is not available (always returns an error)
   *                 when record compression is enabled.
   *
   * \sa             mbedtls_ssl_set_mtu()
   * \sa             mbedtls_ssl_get_output_max_frag_len()
   * \sa             mbedtls_ssl_get_input_max_frag_len()
   * \sa             mbedtls_ssl_get_record_expansion()
   *
   * \param ssl      SSL context
   *
   * \return         Current maximum payload for an outgoing record,
   *                 or a negative error code.
   */
  int mbedtls_ssl_get_max_out_record_payload( const mbedtls_ssl_context *ssl );
  
  #if defined(MBEDTLS_X509_CRT_PARSE_C)
  /**
   * \brief          Return the peer certificate from the current connection.
   *
   * \param  ssl     The SSL context to use. This must be initialized and setup.
   *
   * \return         The current peer certificate, if available.
   *                 The returned certificate is owned by the SSL context and
   *                 is valid only until the next call to the SSL API.
   * \return         \c NULL if no peer certificate is available. This might
   *                 be because the chosen ciphersuite doesn't use CRTs
   *                 (PSK-based ciphersuites, for example), or because
   *                 #MBEDTLS_SSL_KEEP_PEER_CERTIFICATE has been disabled,
   *                 allowing the stack to free the peer's CRT to save memory.
   *
   * \note           For one-time inspection of the peer's certificate during
   *                 the handshake, consider registering an X.509 CRT verification
   *                 callback through mbedtls_ssl_conf_verify() instead of calling
   *                 this function. Using mbedtls_ssl_conf_verify() also comes at
   *                 the benefit of allowing you to influence the verification
   *                 process, for example by masking expected and tolerated
   *                 verification failures.
   *
   * \warning        You must not use the pointer returned by this function
   *                 after any further call to the SSL API, including
   *                 mbedtls_ssl_read() and mbedtls_ssl_write(); this is
   *                 because the pointer might change during renegotiation,
   *                 which happens transparently to the user.
   *                 If you want to use the certificate across API calls,
   *                 you must make a copy.
   */
  const mbedtls_x509_crt *mbedtls_ssl_get_peer_cert( const mbedtls_ssl_context *ssl );
  #endif /* MBEDTLS_X509_CRT_PARSE_C */
  
  #if defined(MBEDTLS_SSL_CLI_C)
  /**
   * \brief          Save session in order to resume it later (client-side only)
   *                 Session data is copied to presented session structure.
   *
   *
   * \param ssl      SSL context
   * \param session  session context
   *
   * \return         0 if successful,
   *                 MBEDTLS_ERR_SSL_ALLOC_FAILED if memory allocation failed,
   *                 MBEDTLS_ERR_SSL_BAD_INPUT_DATA if used server-side or
   *                 arguments are otherwise invalid.
   *
   * \note           Only the server certificate is copied, and not the full chain,
   *                 so you should not attempt to validate the certificate again
   *                 by calling \c mbedtls_x509_crt_verify() on it.
   *                 Instead, you should use the results from the verification
   *                 in the original handshake by calling \c mbedtls_ssl_get_verify_result()
   *                 after loading the session again into a new SSL context
   *                 using \c mbedtls_ssl_set_session().
   *
   * \note           Once the session object is not needed anymore, you should
   *                 free it by calling \c mbedtls_ssl_session_free().
   *
   * \sa             mbedtls_ssl_set_session()
   */
  int mbedtls_ssl_get_session( const mbedtls_ssl_context *ssl, mbedtls_ssl_session *session );
  #endif /* MBEDTLS_SSL_CLI_C */
  
  /**
   * \brief          Perform the SSL handshake
   *
   * \param ssl      SSL context
   *
   * \return         \c 0 if successful.
   * \return         #MBEDTLS_ERR_SSL_WANT_READ or #MBEDTLS_ERR_SSL_WANT_WRITE
   *                 if the handshake is incomplete and waiting for data to
   *                 be available for reading from or writing to the underlying
   *                 transport - in this case you must call this function again
   *                 when the underlying transport is ready for the operation.
   * \return         #MBEDTLS_ERR_SSL_ASYNC_IN_PROGRESS if an asynchronous
   *                 operation is in progress (see
   *                 mbedtls_ssl_conf_async_private_cb()) - in this case you
   *                 must call this function again when the operation is ready.
   * \return         #MBEDTLS_ERR_SSL_CRYPTO_IN_PROGRESS if a cryptographic
   *                 operation is in progress (see mbedtls_ecp_set_max_ops()) -
   *                 in this case you must call this function again to complete
   *                 the handshake when you're done attending other tasks.
   * \return         #MBEDTLS_ERR_SSL_HELLO_VERIFY_REQUIRED if DTLS is in use
   *                 and the client did not demonstrate reachability yet - in
   *                 this case you must stop using the context (see below).
   * \return         Another SSL error code - in this case you must stop using
   *                 the context (see below).
   *
   * \warning        If this function returns something other than
   *                 \c 0,
   *                 #MBEDTLS_ERR_SSL_WANT_READ,
   *                 #MBEDTLS_ERR_SSL_WANT_WRITE,
   *                 #MBEDTLS_ERR_SSL_ASYNC_IN_PROGRESS or
   *                 #MBEDTLS_ERR_SSL_CRYPTO_IN_PROGRESS,
   *                 you must stop using the SSL context for reading or writing,
   *                 and either free it or call \c mbedtls_ssl_session_reset()
   *                 on it before re-using it for a new connection; the current
   *                 connection must be closed.
   *
   * \note           If DTLS is in use, then you may choose to handle
   *                 #MBEDTLS_ERR_SSL_HELLO_VERIFY_REQUIRED specially for logging
   *                 purposes, as it is an expected return value rather than an
   *                 actual error, but you still need to reset/free the context.
   *
   * \note           Remarks regarding event-driven DTLS:
   *                 If the function returns #MBEDTLS_ERR_SSL_WANT_READ, no datagram
   *                 from the underlying transport layer is currently being processed,
   *                 and it is safe to idle until the timer or the underlying transport
   *                 signal a new event. This is not true for a successful handshake,
   *                 in which case the datagram of the underlying transport that is
   *                 currently being processed might or might not contain further
   *                 DTLS records.
   */
  int mbedtls_ssl_handshake( mbedtls_ssl_context *ssl );
  
  /**
   * \brief          Perform a single step of the SSL handshake
   *
   * \note           The state of the context (ssl->state) will be at
   *                 the next state after this function returns \c 0. Do not
   *                 call this function if state is MBEDTLS_SSL_HANDSHAKE_OVER.
   *
   * \param ssl      SSL context
   *
   * \return         See mbedtls_ssl_handshake().
   *
   * \warning        If this function returns something other than \c 0,
   *                 #MBEDTLS_ERR_SSL_WANT_READ, #MBEDTLS_ERR_SSL_WANT_WRITE,
   *                 #MBEDTLS_ERR_SSL_ASYNC_IN_PROGRESS or
   *                 #MBEDTLS_ERR_SSL_CRYPTO_IN_PROGRESS, you must stop using
   *                 the SSL context for reading or writing, and either free it
   *                 or call \c mbedtls_ssl_session_reset() on it before
   *                 re-using it for a new connection; the current connection
   *                 must be closed.
   */
  int mbedtls_ssl_handshake_step( mbedtls_ssl_context *ssl );
  
  #if defined(MBEDTLS_SSL_RENEGOTIATION)
  /**
   * \brief          Initiate an SSL renegotiation on the running connection.
   *                 Client: perform the renegotiation right now.
   *                 Server: request renegotiation, which will be performed
   *                 during the next call to mbedtls_ssl_read() if honored by
   *                 client.
   *
   * \param ssl      SSL context
   *
   * \return         0 if successful, or any mbedtls_ssl_handshake() return
   *                 value except #MBEDTLS_ERR_SSL_CLIENT_RECONNECT that can't
   *                 happen during a renegotiation.
   *
   * \warning        If this function returns something other than \c 0,
   *                 #MBEDTLS_ERR_SSL_WANT_READ, #MBEDTLS_ERR_SSL_WANT_WRITE,
   *                 #MBEDTLS_ERR_SSL_ASYNC_IN_PROGRESS or
   *                 #MBEDTLS_ERR_SSL_CRYPTO_IN_PROGRESS, you must stop using
   *                 the SSL context for reading or writing, and either free it
   *                 or call \c mbedtls_ssl_session_reset() on it before
   *                 re-using it for a new connection; the current connection
   *                 must be closed.
   *
   */
  int mbedtls_ssl_renegotiate( mbedtls_ssl_context *ssl );
  #endif /* MBEDTLS_SSL_RENEGOTIATION */
  
  /**
   * \brief          Read at most 'len' application data bytes
   *
   * \param ssl      SSL context
   * \param buf      buffer that will hold the data
   * \param len      maximum number of bytes to read
   *
   * \return         The (positive) number of bytes read if successful.
   * \return         \c 0 if the read end of the underlying transport was closed
   *                 without sending a CloseNotify beforehand, which might happen
   *                 because of various reasons (internal error of an underlying
   *                 stack, non-conformant peer not sending a CloseNotify and
   *                 such) - in this case you must stop using the context
   *                 (see below).
   * \return         #MBEDTLS_ERR_SSL_PEER_CLOSE_NOTIFY if the underlying
   *                 transport is still functional, but the peer has
   *                 acknowledged to not send anything anymore.
   * \return         #MBEDTLS_ERR_SSL_WANT_READ or #MBEDTLS_ERR_SSL_WANT_WRITE
   *                 if the handshake is incomplete and waiting for data to
   *                 be available for reading from or writing to the underlying
   *                 transport - in this case you must call this function again
   *                 when the underlying transport is ready for the operation.
   * \return         #MBEDTLS_ERR_SSL_ASYNC_IN_PROGRESS if an asynchronous
   *                 operation is in progress (see
   *                 mbedtls_ssl_conf_async_private_cb()) - in this case you
   *                 must call this function again when the operation is ready.
   * \return         #MBEDTLS_ERR_SSL_CRYPTO_IN_PROGRESS if a cryptographic
   *                 operation is in progress (see mbedtls_ecp_set_max_ops()) -
   *                 in this case you must call this function again to complete
   *                 the handshake when you're done attending other tasks.
   * \return         #MBEDTLS_ERR_SSL_CLIENT_RECONNECT if we're at the server
   *                 side of a DTLS connection and the client is initiating a
   *                 new connection using the same source port. See below.
   * \return         Another SSL error code - in this case you must stop using
   *                 the context (see below).
   *
   * \warning        If this function returns something other than
   *                 a positive value,
   *                 #MBEDTLS_ERR_SSL_WANT_READ,
   *                 #MBEDTLS_ERR_SSL_WANT_WRITE,
   *                 #MBEDTLS_ERR_SSL_ASYNC_IN_PROGRESS,
   *                 #MBEDTLS_ERR_SSL_CRYPTO_IN_PROGRESS or
   *                 #MBEDTLS_ERR_SSL_CLIENT_RECONNECT,
   *                 you must stop using the SSL context for reading or writing,
   *                 and either free it or call \c mbedtls_ssl_session_reset()
   *                 on it before re-using it for a new connection; the current
   *                 connection must be closed.
   *
   * \note           When this function returns #MBEDTLS_ERR_SSL_CLIENT_RECONNECT
   *                 (which can only happen server-side), it means that a client
   *                 is initiating a new connection using the same source port.
   *                 You can either treat that as a connection close and wait
   *                 for the client to resend a ClientHello, or directly
   *                 continue with \c mbedtls_ssl_handshake() with the same
   *                 context (as it has been reset internally). Either way, you
   *                 must make sure this is seen by the application as a new
   *                 connection: application state, if any, should be reset, and
   *                 most importantly the identity of the client must be checked
   *                 again. WARNING: not validating the identity of the client
   *                 again, or not transmitting the new identity to the
   *                 application layer, would allow authentication bypass!
   *
   * \note           Remarks regarding event-driven DTLS:
   *                 - If the function returns #MBEDTLS_ERR_SSL_WANT_READ, no datagram
   *                   from the underlying transport layer is currently being processed,
   *                   and it is safe to idle until the timer or the underlying transport
   *                   signal a new event.
   *                 - This function may return MBEDTLS_ERR_SSL_WANT_READ even if data was
   *                   initially available on the underlying transport, as this data may have
   *                   been only e.g. duplicated messages or a renegotiation request.
   *                   Therefore, you must be prepared to receive MBEDTLS_ERR_SSL_WANT_READ even
   *                   when reacting to an incoming-data event from the underlying transport.
   *                 - On success, the datagram of the underlying transport that is currently
   *                   being processed may contain further DTLS records. You should call
   *                   \c mbedtls_ssl_check_pending to check for remaining records.
   *
   */
  int mbedtls_ssl_read( mbedtls_ssl_context *ssl, unsigned char *buf, size_t len );
  
  /**
   * \brief          Try to write exactly 'len' application data bytes
   *
   * \warning        This function will do partial writes in some cases. If the
   *                 return value is non-negative but less than length, the
   *                 function must be called again with updated arguments:
   *                 buf + ret, len - ret (if ret is the return value) until
   *                 it returns a value equal to the last 'len' argument.
   *
   * \param ssl      SSL context
   * \param buf      buffer holding the data
   * \param len      how many bytes must be written
   *
   * \return         The (non-negative) number of bytes actually written if
   *                 successful (may be less than \p len).
   * \return         #MBEDTLS_ERR_SSL_WANT_READ or #MBEDTLS_ERR_SSL_WANT_WRITE
   *                 if the handshake is incomplete and waiting for data to
   *                 be available for reading from or writing to the underlying
   *                 transport - in this case you must call this function again
   *                 when the underlying transport is ready for the operation.
   * \return         #MBEDTLS_ERR_SSL_ASYNC_IN_PROGRESS if an asynchronous
   *                 operation is in progress (see
   *                 mbedtls_ssl_conf_async_private_cb()) - in this case you
   *                 must call this function again when the operation is ready.
   * \return         #MBEDTLS_ERR_SSL_CRYPTO_IN_PROGRESS if a cryptographic
   *                 operation is in progress (see mbedtls_ecp_set_max_ops()) -
   *                 in this case you must call this function again to complete
   *                 the handshake when you're done attending other tasks.
   * \return         Another SSL error code - in this case you must stop using
   *                 the context (see below).
   *
   * \warning        If this function returns something other than
   *                 a non-negative value,
   *                 #MBEDTLS_ERR_SSL_WANT_READ,
   *                 #MBEDTLS_ERR_SSL_WANT_WRITE,
   *                 #MBEDTLS_ERR_SSL_ASYNC_IN_PROGRESS or
   *                 #MBEDTLS_ERR_SSL_CRYPTO_IN_PROGRESS,
   *                 you must stop using the SSL context for reading or writing,
   *                 and either free it or call \c mbedtls_ssl_session_reset()
   *                 on it before re-using it for a new connection; the current
   *                 connection must be closed.
   *
   * \note           When this function returns #MBEDTLS_ERR_SSL_WANT_WRITE/READ,
   *                 it must be called later with the *same* arguments,
   *                 until it returns a value greater that or equal to 0. When
   *                 the function returns #MBEDTLS_ERR_SSL_WANT_WRITE there may be
   *                 some partial data in the output buffer, however this is not
   *                 yet sent.
   *
   * \note           If the requested length is greater than the maximum
   *                 fragment length (either the built-in limit or the one set
   *                 or negotiated with the peer), then:
   *                 - with TLS, less bytes than requested are written.
   *                 - with DTLS, MBEDTLS_ERR_SSL_BAD_INPUT_DATA is returned.
   *                 \c mbedtls_ssl_get_output_max_frag_len() may be used to
   *                 query the active maximum fragment length.
   *
   * \note           Attempting to write 0 bytes will result in an empty TLS
   *                 application record being sent.
   */
  int mbedtls_ssl_write( mbedtls_ssl_context *ssl, const unsigned char *buf, size_t len );
  
  /**
   * \brief           Send an alert message
   *
   * \param ssl       SSL context
   * \param level     The alert level of the message
   *                  (MBEDTLS_SSL_ALERT_LEVEL_WARNING or MBEDTLS_SSL_ALERT_LEVEL_FATAL)
   * \param message   The alert message (SSL_ALERT_MSG_*)
   *
   * \return          0 if successful, or a specific SSL error code.
   *
   * \note           If this function returns something other than 0 or
   *                 MBEDTLS_ERR_SSL_WANT_READ/WRITE, you must stop using
   *                 the SSL context for reading or writing, and either free it or
   *                 call \c mbedtls_ssl_session_reset() on it before re-using it
   *                 for a new connection; the current connection must be closed.
   */
  int mbedtls_ssl_send_alert_message( mbedtls_ssl_context *ssl,
                              unsigned char level,
                              unsigned char message );
  /**
   * \brief          Notify the peer that the connection is being closed
   *
   * \param ssl      SSL context
   *
   * \return          0 if successful, or a specific SSL error code.
   *
   * \note           If this function returns something other than 0 or
   *                 MBEDTLS_ERR_SSL_WANT_READ/WRITE, you must stop using
   *                 the SSL context for reading or writing, and either free it or
   *                 call \c mbedtls_ssl_session_reset() on it before re-using it
   *                 for a new connection; the current connection must be closed.
   */
  int mbedtls_ssl_close_notify( mbedtls_ssl_context *ssl );
  
  /**
   * \brief          Free referenced items in an SSL context and clear memory
   *
   * \param ssl      SSL context
   */
  void mbedtls_ssl_free( mbedtls_ssl_context *ssl );
  
  #if defined(MBEDTLS_SSL_CONTEXT_SERIALIZATION)
  /**
   * \brief          Save an active connection as serialized data in a buffer.
   *                 This allows the freeing or re-using of the SSL context
   *                 while still picking up the connection later in a way that
   *                 it entirely transparent to the peer.
   *
   * \see            mbedtls_ssl_context_load()
   *
   * \note           This feature is currently only available under certain
   *                 conditions, see the documentation of the return value
   *                 #MBEDTLS_ERR_SSL_BAD_INPUT_DATA for details.
   *
   * \note           When this function succeeds, it calls
   *                 mbedtls_ssl_session_reset() on \p ssl which as a result is
   *                 no longer associated with the connection that has been
   *                 serialized. This avoids creating copies of the connection
   *                 state. You're then free to either re-use the context
   *                 structure for a different connection, or call
   *                 mbedtls_ssl_free() on it. See the documentation of
   *                 mbedtls_ssl_session_reset() for more details.
   *
   * \param ssl      The SSL context to save. On success, it is no longer
   *                 associated with the connection that has been serialized.
   * \param buf      The buffer to write the serialized data to. It must be a
   *                 writeable buffer of at least \p buf_len bytes, or may be \c
   *                 NULL if \p buf_len is \c 0.
   * \param buf_len  The number of bytes available for writing in \p buf.
   * \param olen     The size in bytes of the data that has been or would have
   *                 been written. It must point to a valid \c size_t.
   *
   * \note           \p olen is updated to the correct value regardless of
   *                 whether \p buf_len was large enough. This makes it possible
   *                 to determine the necessary size by calling this function
   *                 with \p buf set to \c NULL and \p buf_len to \c 0. However,
   *                 the value of \p olen is only guaranteed to be correct when
   *                 the function returns #MBEDTLS_ERR_SSL_BUFFER_TOO_SMALL or
   *                 \c 0. If the return value is different, then the value of
   *                 \p olen is undefined.
   *
   * \return         \c 0 if successful.
   * \return         #MBEDTLS_ERR_SSL_BUFFER_TOO_SMALL if \p buf is too small.
   * \return         #MBEDTLS_ERR_SSL_ALLOC_FAILED if memory allocation failed
   *                 while reseting the context.
   * \return         #MBEDTLS_ERR_SSL_BAD_INPUT_DATA if a handshake is in
   *                 progress, or there is pending data for reading or sending,
   *                 or the connection does not use DTLS 1.2 with an AEAD
   *                 ciphersuite, or renegotiation is enabled.
   */
  int mbedtls_ssl_context_save( mbedtls_ssl_context *ssl,
                                unsigned char *buf,
                                size_t buf_len,
                                size_t *olen );
  
  /**
   * \brief          Load serialized connection data to an SSL context.
   *
   * \see            mbedtls_ssl_context_save()
   *
   * \warning        The same serialized data must never be loaded into more
   *                 that one context. In order to ensure that, after
   *                 successfully loading serialized data to an SSL context, you
   *                 should immediately destroy or invalidate all copies of the
   *                 serialized data that was loaded. Loading the same data in
   *                 more than one context would cause severe security failures
   *                 including but not limited to loss of confidentiality.
   *
   * \note           Before calling this function, the SSL context must be
   *                 prepared in one of the two following ways. The first way is
   *                 to take a context freshly initialised with
   *                 mbedtls_ssl_init() and call mbedtls_ssl_setup() on it with
   *                 the same ::mbedtls_ssl_config structure that was used in
   *                 the original connection. The second way is to
   *                 call mbedtls_ssl_session_reset() on a context that was
   *                 previously prepared as above but used in the meantime.
   *                 Either way, you must not use the context to perform a
   *                 handshake between calling mbedtls_ssl_setup() or
   *                 mbedtls_ssl_session_reset() and calling this function. You
   *                 may however call other setter functions in that time frame
   *                 as indicated in the note below.
   *
   * \note           Before or after calling this function successfully, you
   *                 also need to configure some connection-specific callbacks
   *                 and settings before you can use the connection again
   *                 (unless they were already set before calling
   *                 mbedtls_ssl_session_reset() and the values are suitable for
   *                 the present connection). Specifically, you want to call
   *                 at least mbedtls_ssl_set_bio() and
   *                 mbedtls_ssl_set_timer_cb(). All other SSL setter functions
   *                 are not necessary to call, either because they're only used
   *                 in handshakes, or because the setting is already saved. You
   *                 might choose to call them anyway, for example in order to
   *                 share code between the cases of establishing a new
   *                 connection and the case of loading an already-established
   *                 connection.
   *
   * \note           If you have new information about the path MTU, you want to
   *                 call mbedtls_ssl_set_mtu() after calling this function, as
   *                 otherwise this function would overwrite your
   *                 newly-configured value with the value that was active when
   *                 the context was saved.
   *
   * \note           When this function returns an error code, it calls
   *                 mbedtls_ssl_free() on \p ssl. In this case, you need to
   *                 prepare the context with the usual sequence starting with a
   *                 call to mbedtls_ssl_init() if you want to use it again.
   *
   * \param ssl      The SSL context structure to be populated. It must have
   *                 been prepared as described in the note above.
   * \param buf      The buffer holding the serialized connection data. It must
   *                 be a readable buffer of at least \p len bytes.
   * \param len      The size of the serialized data in bytes.
   *
   * \return         \c 0 if successful.
   * \return         #MBEDTLS_ERR_SSL_ALLOC_FAILED if memory allocation failed.
   * \return         #MBEDTLS_ERR_SSL_VERSION_MISMATCH if the serialized data
   *                 comes from a different Mbed TLS version or build.
   * \return         #MBEDTLS_ERR_SSL_BAD_INPUT_DATA if input data is invalid.
   */
  int mbedtls_ssl_context_load( mbedtls_ssl_context *ssl,
                                const unsigned char *buf,
                                size_t len );
  #endif /* MBEDTLS_SSL_CONTEXT_SERIALIZATION */
  
  /**
   * \brief          Initialize an SSL configuration context
   *                 Just makes the context ready for
   *                 mbedtls_ssl_config_defaults() or mbedtls_ssl_config_free().
   *
   * \note           You need to call mbedtls_ssl_config_defaults() unless you
   *                 manually set all of the relevant fields yourself.
   *
   * \param conf     SSL configuration context
   */
  void mbedtls_ssl_config_init( mbedtls_ssl_config *conf );
  
  /**
   * \brief          Load reasonnable default SSL configuration values.
   *                 (You need to call mbedtls_ssl_config_init() first.)
   *
   * \param conf     SSL configuration context
   * \param endpoint MBEDTLS_SSL_IS_CLIENT or MBEDTLS_SSL_IS_SERVER
   * \param transport MBEDTLS_SSL_TRANSPORT_STREAM for TLS, or
   *                  MBEDTLS_SSL_TRANSPORT_DATAGRAM for DTLS
   * \param preset   a MBEDTLS_SSL_PRESET_XXX value
   *
   * \note           See \c mbedtls_ssl_conf_transport() for notes on DTLS.
   *
   * \return         0 if successful, or
   *                 MBEDTLS_ERR_XXX_ALLOC_FAILED on memory allocation error.
   */
  int mbedtls_ssl_config_defaults( mbedtls_ssl_config *conf,
                                   int endpoint, int transport, int preset );
  
  /**
   * \brief          Free an SSL configuration context
   *
   * \param conf     SSL configuration context
   */
  void mbedtls_ssl_config_free( mbedtls_ssl_config *conf );
  
  /**
   * \brief          Initialize SSL session structure
   *
   * \param session  SSL session
   */
  void mbedtls_ssl_session_init( mbedtls_ssl_session *session );
  
  /**
   * \brief          Free referenced items in an SSL session including the
   *                 peer certificate and clear memory
   *
   * \note           A session object can be freed even if the SSL context
   *                 that was used to retrieve the session is still in use.
   *
   * \param session  SSL session
   */
  void mbedtls_ssl_session_free( mbedtls_ssl_session *session );
  
  /**
   * \brief          TLS-PRF function for key derivation.
   *
   * \param prf      The tls_prf type funtion type to be used.
   * \param secret   Secret for the key derivation function.
   * \param slen     Length of the secret.
   * \param label    String label for the key derivation function,
   *                 terminated with null character.
   * \param random   Random bytes.
   * \param rlen     Length of the random bytes buffer.
   * \param dstbuf   The buffer holding the derived key.
   * \param dlen     Length of the output buffer.
   *
   * \return         0 on sucess. An SSL specific error on failure.
   */
  int  mbedtls_ssl_tls_prf( const mbedtls_tls_prf_types prf,
                            const unsigned char *secret, size_t slen,
                            const char *label,
                            const unsigned char *random, size_t rlen,
                            unsigned char *dstbuf, size_t dlen );
  
  #ifdef __cplusplus
  }
  #endif
  
  #endif /* ssl.h */
  