text: /*
   *
   * Copyright 2015 gRPC authors.
   *
   * 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.
   *
   */
  
  #include <grpc/support/port_platform.h>
  
  #include "src/core/tsi/ssl_transport_security.h"
  
  #include <limits.h>
  #include <string.h>
  
  /* TODO(jboeuf): refactor inet_ntop into a portability header. */
  /* Note: for whomever reads this and tries to refactor this, this
     can't be in grpc, it has to be in gpr. */
  #ifdef GPR_WINDOWS
  #include <ws2tcpip.h>
  #else
  #include <arpa/inet.h>
  #include <sys/socket.h>
  #endif
  
  #include <string>
  
  #include <grpc/grpc_security.h>
  #include <grpc/support/alloc.h>
  #include <grpc/support/log.h>
  #include <grpc/support/string_util.h>
  #include <grpc/support/sync.h>
  #include <grpc/support/thd_id.h>
  
  #include "absl/strings/match.h"
  #include "absl/strings/string_view.h"
  
  #pragma clang diagnostic push
  #pragma clang diagnostic ignored "-Wmodule-import-in-extern-c"
  extern "C" {
  #if COCOAPODS==1
    #include <openssl_grpc/bio.h>
  #else
    #include <openssl/bio.h>
  #endif
  #if COCOAPODS==1
    #include <openssl_grpc/crypto.h>
  #else
    #include <openssl/crypto.h>
  #endif /* For OPENSSL_free */
  #if COCOAPODS==1
    #include <openssl_grpc/engine.h>
  #else
    #include <openssl/engine.h>
  #endif
  #if COCOAPODS==1
    #include <openssl_grpc/err.h>
  #else
    #include <openssl/err.h>
  #endif
  #if COCOAPODS==1
    #include <openssl_grpc/ssl.h>
  #else
    #include <openssl/ssl.h>
  #endif
  #if COCOAPODS==1
    #include <openssl_grpc/tls1.h>
  #else
    #include <openssl/tls1.h>
  #endif
  #if COCOAPODS==1
    #include <openssl_grpc/x509.h>
  #else
    #include <openssl/x509.h>
  #endif
  #if COCOAPODS==1
    #include <openssl_grpc/x509v3.h>
  #else
    #include <openssl/x509v3.h>
  #endif
  }
  #pragma clang diagnostic pop
  
  #include "src/core/lib/gpr/useful.h"
  #include "src/core/tsi/ssl/session_cache/ssl_session_cache.h"
  #include "src/core/tsi/ssl_types.h"
  #include "src/core/tsi/transport_security.h"
  
  /* --- Constants. ---*/
  
  #define TSI_SSL_MAX_PROTECTED_FRAME_SIZE_UPPER_BOUND 16384
  #define TSI_SSL_MAX_PROTECTED_FRAME_SIZE_LOWER_BOUND 1024
  #define TSI_SSL_HANDSHAKER_OUTGOING_BUFFER_INITIAL_SIZE 1024
  
  /* Putting a macro like this and littering the source file with #if is really
     bad practice.
     TODO(jboeuf): refactor all the #if / #endif in a separate module. */
  #ifndef TSI_OPENSSL_ALPN_SUPPORT
  #define TSI_OPENSSL_ALPN_SUPPORT 1
  #endif
  
  /* TODO(jboeuf): I have not found a way to get this number dynamically from the
     SSL structure. This is what we would ultimately want though... */
  #define TSI_SSL_MAX_PROTECTION_OVERHEAD 100
  
  /* --- Structure definitions. ---*/
  
  struct tsi_ssl_root_certs_store {
    X509_STORE* store;
  };
  
  struct tsi_ssl_handshaker_factory {
    const tsi_ssl_handshaker_factory_vtable* vtable;
    gpr_refcount refcount;
  };
  
  struct tsi_ssl_client_handshaker_factory {
    tsi_ssl_handshaker_factory base;
    SSL_CTX* ssl_context;
    unsigned char* alpn_protocol_list;
    size_t alpn_protocol_list_length;
    grpc_core::RefCountedPtr<tsi::SslSessionLRUCache> session_cache;
  };
  
  struct tsi_ssl_server_handshaker_factory {
    /* Several contexts to support SNI.
       The tsi_peer array contains the subject names of the server certificates
       associated with the contexts at the same index.  */
    tsi_ssl_handshaker_factory base;
    SSL_CTX** ssl_contexts;
    tsi_peer* ssl_context_x509_subject_names;
    size_t ssl_context_count;
    unsigned char* alpn_protocol_list;
    size_t alpn_protocol_list_length;
  };
  
  struct tsi_ssl_handshaker {
    tsi_handshaker base;
    SSL* ssl;
    BIO* network_io;
    tsi_result result;
    unsigned char* outgoing_bytes_buffer;
    size_t outgoing_bytes_buffer_size;
    tsi_ssl_handshaker_factory* factory_ref;
  };
  struct tsi_ssl_handshaker_result {
    tsi_handshaker_result base;
    SSL* ssl;
    BIO* network_io;
    unsigned char* unused_bytes;
    size_t unused_bytes_size;
  };
  struct tsi_ssl_frame_protector {
    tsi_frame_protector base;
    SSL* ssl;
    BIO* network_io;
    unsigned char* buffer;
    size_t buffer_size;
    size_t buffer_offset;
  };
  /* --- Library Initialization. ---*/
  
  static gpr_once g_init_openssl_once = GPR_ONCE_INIT;
  static int g_ssl_ctx_ex_factory_index = -1;
  static const unsigned char kSslSessionIdContext[] = {'g', 'r', 'p', 'c'};
  #if !defined(OPENSSL_IS_BORINGSSL) && !defined(OPENSSL_NO_ENGINE)
  static const char kSslEnginePrefix[] = "engine:";
  #endif
  
  #if OPENSSL_VERSION_NUMBER < 0x10100000
  static gpr_mu* g_openssl_mutexes = nullptr;
  static void openssl_locking_cb(int mode, int type, const char* file,
                                 int line) GRPC_UNUSED;
  static unsigned long openssl_thread_id_cb(void) GRPC_UNUSED;
  
  static void openssl_locking_cb(int mode, int type, const char* file, int line) {
    if (mode & CRYPTO_LOCK) {
      gpr_mu_lock(&g_openssl_mutexes[type]);
    } else {
      gpr_mu_unlock(&g_openssl_mutexes[type]);
    }
  }
  
  static unsigned long openssl_thread_id_cb(void) {
    return static_cast<unsigned long>(gpr_thd_currentid());
  }
  #endif
  
  static void init_openssl(void) {
  #if OPENSSL_VERSION_NUMBER >= 0x10100000
    OPENSSL_init_ssl(0, nullptr);
  #else
    SSL_library_init();
    SSL_load_error_strings();
    OpenSSL_add_all_algorithms();
  #endif
  #if OPENSSL_VERSION_NUMBER < 0x10100000
    if (!CRYPTO_get_locking_callback()) {
      int num_locks = CRYPTO_num_locks();
      GPR_ASSERT(num_locks > 0);
      g_openssl_mutexes = static_cast<gpr_mu*>(
          gpr_malloc(static_cast<size_t>(num_locks) * sizeof(gpr_mu)));
      for (int i = 0; i < num_locks; i++) {
        gpr_mu_init(&g_openssl_mutexes[i]);
      }
      CRYPTO_set_locking_callback(openssl_locking_cb);
      CRYPTO_set_id_callback(openssl_thread_id_cb);
    } else {
      gpr_log(GPR_INFO, "OpenSSL callback has already been set.");
    }
  #endif
    g_ssl_ctx_ex_factory_index =
        SSL_CTX_get_ex_new_index(0, nullptr, nullptr, nullptr, nullptr);
    GPR_ASSERT(g_ssl_ctx_ex_factory_index != -1);
  }
  
  /* --- Ssl utils. ---*/
  
  static const char* ssl_error_string(int error) {
    switch (error) {
      case SSL_ERROR_NONE:
        return "SSL_ERROR_NONE";
      case SSL_ERROR_ZERO_RETURN:
        return "SSL_ERROR_ZERO_RETURN";
      case SSL_ERROR_WANT_READ:
        return "SSL_ERROR_WANT_READ";
      case SSL_ERROR_WANT_WRITE:
        return "SSL_ERROR_WANT_WRITE";
      case SSL_ERROR_WANT_CONNECT:
        return "SSL_ERROR_WANT_CONNECT";
      case SSL_ERROR_WANT_ACCEPT:
        return "SSL_ERROR_WANT_ACCEPT";
      case SSL_ERROR_WANT_X509_LOOKUP:
        return "SSL_ERROR_WANT_X509_LOOKUP";
      case SSL_ERROR_SYSCALL:
        return "SSL_ERROR_SYSCALL";
      case SSL_ERROR_SSL:
        return "SSL_ERROR_SSL";
      default:
        return "Unknown error";
    }
  }
  
  /* TODO(jboeuf): Remove when we are past the debugging phase with this code. */
  static void ssl_log_where_info(const SSL* ssl, int where, int flag,
                                 const char* msg) {
    if ((where & flag) && GRPC_TRACE_FLAG_ENABLED(tsi_tracing_enabled)) {
      gpr_log(GPR_INFO, "%20.20s - %30.30s  - %5.10s", msg,
              SSL_state_string_long(ssl), SSL_state_string(ssl));
    }
  }
  
  /* Used for debugging. TODO(jboeuf): Remove when code is mature enough. */
  static void ssl_info_callback(const SSL* ssl, int where, int ret) {
    if (ret == 0) {
      gpr_log(GPR_ERROR, "ssl_info_callback: error occurred.\n");
      return;
    }
  
    ssl_log_where_info(ssl, where, SSL_CB_LOOP, "LOOP");
    ssl_log_where_info(ssl, where, SSL_CB_HANDSHAKE_START, "HANDSHAKE START");
    ssl_log_where_info(ssl, where, SSL_CB_HANDSHAKE_DONE, "HANDSHAKE DONE");
  }
  
  /* Returns 1 if name looks like an IP address, 0 otherwise.
     This is a very rough heuristic, and only handles IPv6 in hexadecimal form. */
  static int looks_like_ip_address(absl::string_view name) {
    size_t dot_count = 0;
    size_t num_size = 0;
    for (size_t i = 0; i < name.size(); ++i) {
      if (name[i] == ':') {
        /* IPv6 Address in hexadecimal form, : is not allowed in DNS names. */
        return 1;
      }
      if (name[i] >= '0' && name[i] <= '9') {
        if (num_size > 3) return 0;
        num_size++;
      } else if (name[i] == '.') {
        if (dot_count > 3 || num_size == 0) return 0;
        dot_count++;
        num_size = 0;
      } else {
        return 0;
      }
    }
    if (dot_count < 3 || num_size == 0) return 0;
    return 1;
  }
  
  /* Gets the subject CN from an X509 cert. */
  static tsi_result ssl_get_x509_common_name(X509* cert, unsigned char** utf8,
                                             size_t* utf8_size) {
    int common_name_index = -1;
    X509_NAME_ENTRY* common_name_entry = nullptr;
    ASN1_STRING* common_name_asn1 = nullptr;
    X509_NAME* subject_name = X509_get_subject_name(cert);
    int utf8_returned_size = 0;
    if (subject_name == nullptr) {
      gpr_log(GPR_INFO, "Could not get subject name from certificate.");
      return TSI_NOT_FOUND;
    }
    common_name_index =
        X509_NAME_get_index_by_NID(subject_name, NID_commonName, -1);
    if (common_name_index == -1) {
      gpr_log(GPR_INFO, "Could not get common name of subject from certificate.");
      return TSI_NOT_FOUND;
    }
    common_name_entry = X509_NAME_get_entry(subject_name, common_name_index);
    if (common_name_entry == nullptr) {
      gpr_log(GPR_ERROR, "Could not get common name entry from certificate.");
      return TSI_INTERNAL_ERROR;
    }
    common_name_asn1 = X509_NAME_ENTRY_get_data(common_name_entry);
    if (common_name_asn1 == nullptr) {
      gpr_log(GPR_ERROR,
              "Could not get common name entry asn1 from certificate.");
      return TSI_INTERNAL_ERROR;
    }
    utf8_returned_size = ASN1_STRING_to_UTF8(utf8, common_name_asn1);
    if (utf8_returned_size < 0) {
      gpr_log(GPR_ERROR, "Could not extract utf8 from asn1 string.");
      return TSI_OUT_OF_RESOURCES;
    }
    *utf8_size = static_cast<size_t>(utf8_returned_size);
    return TSI_OK;
  }
  
  /* Gets the subject CN of an X509 cert as a tsi_peer_property. */
  static tsi_result peer_property_from_x509_common_name(
      X509* cert, tsi_peer_property* property) {
    unsigned char* common_name;
    size_t common_name_size;
    tsi_result result =
        ssl_get_x509_common_name(cert, &common_name, &common_name_size);
    if (result != TSI_OK) {
      if (result == TSI_NOT_FOUND) {
        common_name = nullptr;
        common_name_size = 0;
      } else {
        return result;
      }
    }
    result = tsi_construct_string_peer_property(
        TSI_X509_SUBJECT_COMMON_NAME_PEER_PROPERTY,
        common_name == nullptr ? "" : reinterpret_cast<const char*>(common_name),
        common_name_size, property);
    OPENSSL_free(common_name);
    return result;
  }
  
  /* Gets the X509 cert in PEM format as a tsi_peer_property. */
  static tsi_result add_pem_certificate(X509* cert, tsi_peer_property* property) {
    BIO* bio = BIO_new(BIO_s_mem());
    if (!PEM_write_bio_X509(bio, cert)) {
      BIO_free(bio);
      return TSI_INTERNAL_ERROR;
    }
    char* contents;
    long len = BIO_get_mem_data(bio, &contents);
    if (len <= 0) {
      BIO_free(bio);
      return TSI_INTERNAL_ERROR;
    }
    tsi_result result = tsi_construct_string_peer_property(
        TSI_X509_PEM_CERT_PROPERTY, contents, static_cast<size_t>(len), property);
    BIO_free(bio);
    return result;
  }
  
  /* Gets the subject SANs from an X509 cert as a tsi_peer_property. */
  static tsi_result add_subject_alt_names_properties_to_peer(
      tsi_peer* peer, GENERAL_NAMES* subject_alt_names,
      size_t subject_alt_name_count, int* current_insert_index) {
    size_t i;
    tsi_result result = TSI_OK;
  
    for (i = 0; i < subject_alt_name_count; i++) {
      GENERAL_NAME* subject_alt_name =
          sk_GENERAL_NAME_value(subject_alt_names, TSI_SIZE_AS_SIZE(i));
      if (subject_alt_name->type == GEN_DNS ||
          subject_alt_name->type == GEN_EMAIL ||
          subject_alt_name->type == GEN_URI) {
        unsigned char* name = nullptr;
        int name_size;
        if (subject_alt_name->type == GEN_DNS) {
          name_size = ASN1_STRING_to_UTF8(&name, subject_alt_name->d.dNSName);
        } else if (subject_alt_name->type == GEN_EMAIL) {
          name_size = ASN1_STRING_to_UTF8(&name, subject_alt_name->d.rfc822Name);
        } else {
          name_size = ASN1_STRING_to_UTF8(
              &name, subject_alt_name->d.uniformResourceIdentifier);
        }
        if (name_size < 0) {
          gpr_log(GPR_ERROR, "Could not get utf8 from asn1 string.");
          result = TSI_INTERNAL_ERROR;
          break;
        }
        result = tsi_construct_string_peer_property(
            TSI_X509_SUBJECT_ALTERNATIVE_NAME_PEER_PROPERTY,
            reinterpret_cast<const char*>(name), static_cast<size_t>(name_size),
            &peer->properties[(*current_insert_index)++]);
        if (result != TSI_OK) {
          OPENSSL_free(name);
          break;
        }
        if (subject_alt_name->type == GEN_URI) {
          result = tsi_construct_string_peer_property(
              TSI_X509_URI_PEER_PROPERTY, reinterpret_cast<const char*>(name),
              static_cast<size_t>(name_size),
              &peer->properties[(*current_insert_index)++]);
        }
        OPENSSL_free(name);
      } else if (subject_alt_name->type == GEN_IPADD) {
        char ntop_buf[INET6_ADDRSTRLEN];
        int af;
  
        if (subject_alt_name->d.iPAddress->length == 4) {
          af = AF_INET;
        } else if (subject_alt_name->d.iPAddress->length == 16) {
          af = AF_INET6;
        } else {
          gpr_log(GPR_ERROR, "SAN IP Address contained invalid IP");
          result = TSI_INTERNAL_ERROR;
          break;
        }
        const char* name = inet_ntop(af, subject_alt_name->d.iPAddress->data,
                                     ntop_buf, INET6_ADDRSTRLEN);
        if (name == nullptr) {
          gpr_log(GPR_ERROR, "Could not get IP string from asn1 octet.");
          result = TSI_INTERNAL_ERROR;
          break;
        }
  
        result = tsi_construct_string_peer_property_from_cstring(
            TSI_X509_SUBJECT_ALTERNATIVE_NAME_PEER_PROPERTY, name,
            &peer->properties[(*current_insert_index)++]);
      } else {
        result = tsi_construct_string_peer_property_from_cstring(
            TSI_X509_SUBJECT_ALTERNATIVE_NAME_PEER_PROPERTY, "other types of SAN",
            &peer->properties[(*current_insert_index)++]);
      }
      if (result != TSI_OK) break;
    }
    return result;
  }
  
  /* Gets information about the peer's X509 cert as a tsi_peer object. */
  static tsi_result peer_from_x509(X509* cert, int include_certificate_type,
                                   tsi_peer* peer) {
    /* TODO(jboeuf): Maybe add more properties. */
    GENERAL_NAMES* subject_alt_names = static_cast<GENERAL_NAMES*>(
        X509_get_ext_d2i(cert, NID_subject_alt_name, nullptr, nullptr));
    int subject_alt_name_count =
        (subject_alt_names != nullptr)
            ? static_cast<int>(sk_GENERAL_NAME_num(subject_alt_names))
            : 0;
    size_t property_count;
    tsi_result result;
    GPR_ASSERT(subject_alt_name_count >= 0);
    property_count = (include_certificate_type ? static_cast<size_t>(1) : 0) +
                     2 /* common name, certificate */ +
                     static_cast<size_t>(subject_alt_name_count);
    for (int i = 0; i < subject_alt_name_count; i++) {
      GENERAL_NAME* subject_alt_name =
          sk_GENERAL_NAME_value(subject_alt_names, TSI_SIZE_AS_SIZE(i));
      if (subject_alt_name->type == GEN_URI) {
        property_count += 1;
      }
    }
    result = tsi_construct_peer(property_count, peer);
    if (result != TSI_OK) return result;
    int current_insert_index = 0;
    do {
      if (include_certificate_type) {
        result = tsi_construct_string_peer_property_from_cstring(
            TSI_CERTIFICATE_TYPE_PEER_PROPERTY, TSI_X509_CERTIFICATE_TYPE,
            &peer->properties[current_insert_index++]);
        if (result != TSI_OK) break;
      }
      result = peer_property_from_x509_common_name(
          cert, &peer->properties[current_insert_index++]);
      if (result != TSI_OK) break;
  
      result =
          add_pem_certificate(cert, &peer->properties[current_insert_index++]);
      if (result != TSI_OK) break;
  
      if (subject_alt_name_count != 0) {
        result = add_subject_alt_names_properties_to_peer(
            peer, subject_alt_names, static_cast<size_t>(subject_alt_name_count),
            &current_insert_index);
        if (result != TSI_OK) break;
      }
    } while (false);
  
    if (subject_alt_names != nullptr) {
      sk_GENERAL_NAME_pop_free(subject_alt_names, GENERAL_NAME_free);
    }
    if (result != TSI_OK) tsi_peer_destruct(peer);
  
    GPR_ASSERT((int)peer->property_count == current_insert_index);
    return result;
  }
  
  /* Logs the SSL error stack. */
  static void log_ssl_error_stack(void) {
    unsigned long err;
    while ((err = ERR_get_error()) != 0) {
      char details[256];
      ERR_error_string_n(static_cast<uint32_t>(err), details, sizeof(details));
      gpr_log(GPR_ERROR, "%s", details);
    }
  }
  
  /* Performs an SSL_read and handle errors. */
  static tsi_result do_ssl_read(SSL* ssl, unsigned char* unprotected_bytes,
                                size_t* unprotected_bytes_size) {
    int read_from_ssl;
    GPR_ASSERT(*unprotected_bytes_size <= INT_MAX);
    read_from_ssl = SSL_read(ssl, unprotected_bytes,
                             static_cast<int>(*unprotected_bytes_size));
    if (read_from_ssl <= 0) {
      read_from_ssl = SSL_get_error(ssl, read_from_ssl);
      switch (read_from_ssl) {
        case SSL_ERROR_ZERO_RETURN: /* Received a close_notify alert. */
        case SSL_ERROR_WANT_READ:   /* We need more data to finish the frame. */
          *unprotected_bytes_size = 0;
          return TSI_OK;
        case SSL_ERROR_WANT_WRITE:
          gpr_log(
              GPR_ERROR,
              "Peer tried to renegotiate SSL connection. This is unsupported.");
          return TSI_UNIMPLEMENTED;
        case SSL_ERROR_SSL:
          gpr_log(GPR_ERROR, "Corruption detected.");
          log_ssl_error_stack();
          return TSI_DATA_CORRUPTED;
        default:
          gpr_log(GPR_ERROR, "SSL_read failed with error %s.",
                  ssl_error_string(read_from_ssl));
          return TSI_PROTOCOL_FAILURE;
      }
    }
    *unprotected_bytes_size = static_cast<size_t>(read_from_ssl);
    return TSI_OK;
  }
  
  /* Performs an SSL_write and handle errors. */
  static tsi_result do_ssl_write(SSL* ssl, unsigned char* unprotected_bytes,
                                 size_t unprotected_bytes_size) {
    int ssl_write_result;
    GPR_ASSERT(unprotected_bytes_size <= INT_MAX);
    ssl_write_result = SSL_write(ssl, unprotected_bytes,
                                 static_cast<int>(unprotected_bytes_size));
    if (ssl_write_result < 0) {
      ssl_write_result = SSL_get_error(ssl, ssl_write_result);
      if (ssl_write_result == SSL_ERROR_WANT_READ) {
        gpr_log(GPR_ERROR,
                "Peer tried to renegotiate SSL connection. This is unsupported.");
        return TSI_UNIMPLEMENTED;
      } else {
        gpr_log(GPR_ERROR, "SSL_write failed with error %s.",
                ssl_error_string(ssl_write_result));
        return TSI_INTERNAL_ERROR;
      }
    }
    return TSI_OK;
  }
  
  /* Loads an in-memory PEM certificate chain into the SSL context. */
  static tsi_result ssl_ctx_use_certificate_chain(SSL_CTX* context,
                                                  const char* pem_cert_chain,
                                                  size_t pem_cert_chain_size) {
    tsi_result result = TSI_OK;
    X509* certificate = nullptr;
    BIO* pem;
    GPR_ASSERT(pem_cert_chain_size <= INT_MAX);
    pem = BIO_new_mem_buf(pem_cert_chain, static_cast<int>(pem_cert_chain_size));
    if (pem == nullptr) return TSI_OUT_OF_RESOURCES;
  
    do {
      certificate =
          PEM_read_bio_X509_AUX(pem, nullptr, nullptr, const_cast<char*>(""));
      if (certificate == nullptr) {
        result = TSI_INVALID_ARGUMENT;
        break;
      }
      if (!SSL_CTX_use_certificate(context, certificate)) {
        result = TSI_INVALID_ARGUMENT;
        break;
      }
      while (true) {
        X509* certificate_authority =
            PEM_read_bio_X509(pem, nullptr, nullptr, const_cast<char*>(""));
        if (certificate_authority == nullptr) {
          ERR_clear_error();
          break; /* Done reading. */
        }
        if (!SSL_CTX_add_extra_chain_cert(context, certificate_authority)) {
          X509_free(certificate_authority);
          result = TSI_INVALID_ARGUMENT;
          break;
        }
        /* We don't need to free certificate_authority as its ownership has been
           transferred to the context. That is not the case for certificate
           though.
         */
      }
    } while (false);
  
    if (certificate != nullptr) X509_free(certificate);
    BIO_free(pem);
    return result;
  }
  
  #if !defined(OPENSSL_IS_BORINGSSL) && !defined(OPENSSL_NO_ENGINE)
  static tsi_result ssl_ctx_use_engine_private_key(SSL_CTX* context,
                                                   const char* pem_key,
                                                   size_t pem_key_size) {
    tsi_result result = TSI_OK;
    EVP_PKEY* private_key = nullptr;
    ENGINE* engine = nullptr;
    char* engine_name = nullptr;
    // Parse key which is in following format engine:<engine_id>:<key_id>
    do {
      char* engine_start = (char*)pem_key + strlen(kSslEnginePrefix);
      char* engine_end = (char*)strchr(engine_start, ':');
      if (engine_end == nullptr) {
        result = TSI_INVALID_ARGUMENT;
        break;
      }
      char* key_id = engine_end + 1;
      int engine_name_length = engine_end - engine_start;
      if (engine_name_length == 0) {
        result = TSI_INVALID_ARGUMENT;
        break;
      }
      engine_name = static_cast<char*>(gpr_zalloc(engine_name_length + 1));
      memcpy(engine_name, engine_start, engine_name_length);
      gpr_log(GPR_DEBUG, "ENGINE key: %s", engine_name);
      ENGINE_load_dynamic();
      engine = ENGINE_by_id(engine_name);
      if (engine == nullptr) {
        // If not available at ENGINE_DIR, use dynamic to load from
        // current working directory.
        engine = ENGINE_by_id("dynamic");
        if (engine == nullptr) {
          gpr_log(GPR_ERROR, "Cannot load dynamic engine");
          result = TSI_INVALID_ARGUMENT;
          break;
        }
        if (!ENGINE_ctrl_cmd_string(engine, "ID", engine_name, 0) ||
            !ENGINE_ctrl_cmd_string(engine, "DIR_LOAD", "2", 0) ||
            !ENGINE_ctrl_cmd_string(engine, "DIR_ADD", ".", 0) ||
            !ENGINE_ctrl_cmd_string(engine, "LIST_ADD", "1", 0) ||
            !ENGINE_ctrl_cmd_string(engine, "LOAD", NULL, 0)) {
          gpr_log(GPR_ERROR, "Cannot find engine");
          result = TSI_INVALID_ARGUMENT;
          break;
        }
      }
      if (!ENGINE_set_default(engine, ENGINE_METHOD_ALL)) {
        gpr_log(GPR_ERROR, "ENGINE_set_default with ENGINE_METHOD_ALL failed");
        result = TSI_INVALID_ARGUMENT;
        break;
      }
      if (!ENGINE_init(engine)) {
        gpr_log(GPR_ERROR, "ENGINE_init failed");
        result = TSI_INVALID_ARGUMENT;
        break;
      }
      private_key = ENGINE_load_private_key(engine, key_id, 0, 0);
      if (private_key == nullptr) {
        gpr_log(GPR_ERROR, "ENGINE_load_private_key failed");
        result = TSI_INVALID_ARGUMENT;
        break;
      }
      if (!SSL_CTX_use_PrivateKey(context, private_key)) {
        gpr_log(GPR_ERROR, "SSL_CTX_use_PrivateKey failed");
        result = TSI_INVALID_ARGUMENT;
        break;
      }
    } while (0);
    if (engine != nullptr) ENGINE_free(engine);
    if (private_key != nullptr) EVP_PKEY_free(private_key);
    if (engine_name != nullptr) gpr_free(engine_name);
    return result;
  }
  #endif /* !defined(OPENSSL_IS_BORINGSSL) && !defined(OPENSSL_NO_ENGINE) */
  
  static tsi_result ssl_ctx_use_pem_private_key(SSL_CTX* context,
                                                const char* pem_key,
                                                size_t pem_key_size) {
    tsi_result result = TSI_OK;
    EVP_PKEY* private_key = nullptr;
    BIO* pem;
    GPR_ASSERT(pem_key_size <= INT_MAX);
    pem = BIO_new_mem_buf(pem_key, static_cast<int>(pem_key_size));
    if (pem == nullptr) return TSI_OUT_OF_RESOURCES;
    do {
      private_key =
          PEM_read_bio_PrivateKey(pem, nullptr, nullptr, const_cast<char*>(""));
      if (private_key == nullptr) {
        result = TSI_INVALID_ARGUMENT;
        break;
      }
      if (!SSL_CTX_use_PrivateKey(context, private_key)) {
        result = TSI_INVALID_ARGUMENT;
        break;
      }
    } while (false);
    if (private_key != nullptr) EVP_PKEY_free(private_key);
    BIO_free(pem);
    return result;
  }
  
  /* Loads an in-memory PEM private key into the SSL context. */
  static tsi_result ssl_ctx_use_private_key(SSL_CTX* context, const char* pem_key,
                                            size_t pem_key_size) {
  // BoringSSL does not have ENGINE support
  #if !defined(OPENSSL_IS_BORINGSSL) && !defined(OPENSSL_NO_ENGINE)
    if (strncmp(pem_key, kSslEnginePrefix, strlen(kSslEnginePrefix)) == 0) {
      return ssl_ctx_use_engine_private_key(context, pem_key, pem_key_size);
    } else
  #endif /* !defined(OPENSSL_IS_BORINGSSL) && !defined(OPENSSL_NO_ENGINE) */
    {
      return ssl_ctx_use_pem_private_key(context, pem_key, pem_key_size);
    }
  }
  
  /* Loads in-memory PEM verification certs into the SSL context and optionally
     returns the verification cert names (root_names can be NULL). */
  static tsi_result x509_store_load_certs(X509_STORE* cert_store,
                                          const char* pem_roots,
                                          size_t pem_roots_size,
                                          STACK_OF(X509_NAME) * *root_names) {
    tsi_result result = TSI_OK;
    size_t num_roots = 0;
    X509* root = nullptr;
    X509_NAME* root_name = nullptr;
    BIO* pem;
    GPR_ASSERT(pem_roots_size <= INT_MAX);
    pem = BIO_new_mem_buf(pem_roots, static_cast<int>(pem_roots_size));
    if (cert_store == nullptr) return TSI_INVALID_ARGUMENT;
    if (pem == nullptr) return TSI_OUT_OF_RESOURCES;
    if (root_names != nullptr) {
      *root_names = sk_X509_NAME_new_null();
      if (*root_names == nullptr) return TSI_OUT_OF_RESOURCES;
    }
  
    while (true) {
      root = PEM_read_bio_X509_AUX(pem, nullptr, nullptr, const_cast<char*>(""));
      if (root == nullptr) {
        ERR_clear_error();
        break; /* We're at the end of stream. */
      }
      if (root_names != nullptr) {
        root_name = X509_get_subject_name(root);
        if (root_name == nullptr) {
          gpr_log(GPR_ERROR, "Could not get name from root certificate.");
          result = TSI_INVALID_ARGUMENT;
          break;
        }
        root_name = X509_NAME_dup(root_name);
        if (root_name == nullptr) {
          result = TSI_OUT_OF_RESOURCES;
          break;
        }
        sk_X509_NAME_push(*root_names, root_name);
        root_name = nullptr;
      }
      ERR_clear_error();
      if (!X509_STORE_add_cert(cert_store, root)) {
        unsigned long error = ERR_get_error();
        if (ERR_GET_LIB(error) != ERR_LIB_X509 ||
            ERR_GET_REASON(error) != X509_R_CERT_ALREADY_IN_HASH_TABLE) {
          gpr_log(GPR_ERROR, "Could not add root certificate to ssl context.");
          result = TSI_INTERNAL_ERROR;
          break;
        }
      }
      X509_free(root);
      num_roots++;
    }
    if (num_roots == 0) {
      gpr_log(GPR_ERROR, "Could not load any root certificate.");
      result = TSI_INVALID_ARGUMENT;
    }
  
    if (result != TSI_OK) {
      if (root != nullptr) X509_free(root);
      if (root_names != nullptr) {
        sk_X509_NAME_pop_free(*root_names, X509_NAME_free);
        *root_names = nullptr;
        if (root_name != nullptr) X509_NAME_free(root_name);
      }
    }
    BIO_free(pem);
    return result;
  }
  
  static tsi_result ssl_ctx_load_verification_certs(SSL_CTX* context,
                                                    const char* pem_roots,
                                                    size_t pem_roots_size,
                                                    STACK_OF(X509_NAME) *
                                                        *root_name) {
    X509_STORE* cert_store = SSL_CTX_get_cert_store(context);
    X509_STORE_set_flags(cert_store,
                         X509_V_FLAG_PARTIAL_CHAIN | X509_V_FLAG_TRUSTED_FIRST);
    return x509_store_load_certs(cert_store, pem_roots, pem_roots_size,
                                 root_name);
  }
  
  /* Populates the SSL context with a private key and a cert chain, and sets the
     cipher list and the ephemeral ECDH key. */
  static tsi_result populate_ssl_context(
      SSL_CTX* context, const tsi_ssl_pem_key_cert_pair* key_cert_pair,
      const char* cipher_list) {
    tsi_result result = TSI_OK;
    if (key_cert_pair != nullptr) {
      if (key_cert_pair->cert_chain != nullptr) {
        result = ssl_ctx_use_certificate_chain(context, key_cert_pair->cert_chain,
                                               strlen(key_cert_pair->cert_chain));
        if (result != TSI_OK) {
          gpr_log(GPR_ERROR, "Invalid cert chain file.");
          return result;
        }
      }
      if (key_cert_pair->private_key != nullptr) {
        result = ssl_ctx_use_private_key(context, key_cert_pair->private_key,
                                         strlen(key_cert_pair->private_key));
        if (result != TSI_OK || !SSL_CTX_check_private_key(context)) {
          gpr_log(GPR_ERROR, "Invalid private key.");
          return result != TSI_OK ? result : TSI_INVALID_ARGUMENT;
        }
      }
    }
    if ((cipher_list != nullptr) &&
        !SSL_CTX_set_cipher_list(context, cipher_list)) {
      gpr_log(GPR_ERROR, "Invalid cipher list: %s.", cipher_list);
      return TSI_INVALID_ARGUMENT;
    }
    {
      EC_KEY* ecdh = EC_KEY_new_by_curve_name(NID_X9_62_prime256v1);
      if (!SSL_CTX_set_tmp_ecdh(context, ecdh)) {
        gpr_log(GPR_ERROR, "Could not set ephemeral ECDH key.");
        EC_KEY_free(ecdh);
        return TSI_INTERNAL_ERROR;
      }
      SSL_CTX_set_options(context, SSL_OP_SINGLE_ECDH_USE);
      EC_KEY_free(ecdh);
    }
    return TSI_OK;
  }
  
  /* Extracts the CN and the SANs from an X509 cert as a peer object. */
  tsi_result tsi_ssl_extract_x509_subject_names_from_pem_cert(
      const char* pem_cert, tsi_peer* peer) {
    tsi_result result = TSI_OK;
    X509* cert = nullptr;
    BIO* pem;
    pem = BIO_new_mem_buf(pem_cert, static_cast<int>(strlen(pem_cert)));
    if (pem == nullptr) return TSI_OUT_OF_RESOURCES;
  
    cert = PEM_read_bio_X509(pem, nullptr, nullptr, const_cast<char*>(""));
    if (cert == nullptr) {
      gpr_log(GPR_ERROR, "Invalid certificate");
      result = TSI_INVALID_ARGUMENT;
    } else {
      result = peer_from_x509(cert, 0, peer);
    }
    if (cert != nullptr) X509_free(cert);
    BIO_free(pem);
    return result;
  }
  
  /* Builds the alpn protocol name list according to rfc 7301. */
  static tsi_result build_alpn_protocol_name_list(
      const char** alpn_protocols, uint16_t num_alpn_protocols,
      unsigned char** protocol_name_list, size_t* protocol_name_list_length) {
    uint16_t i;
    unsigned char* current;
    *protocol_name_list = nullptr;
    *protocol_name_list_length = 0;
    if (num_alpn_protocols == 0) return TSI_INVALID_ARGUMENT;
    for (i = 0; i < num_alpn_protocols; i++) {
      size_t length =
          alpn_protocols[i] == nullptr ? 0 : strlen(alpn_protocols[i]);
      if (length == 0 || length > 255) {
        gpr_log(GPR_ERROR, "Invalid protocol name length: %d.",
                static_cast<int>(length));
        return TSI_INVALID_ARGUMENT;
      }
      *protocol_name_list_length += length + 1;
    }
    *protocol_name_list =
        static_cast<unsigned char*>(gpr_malloc(*protocol_name_list_length));
    if (*protocol_name_list == nullptr) return TSI_OUT_OF_RESOURCES;
    current = *protocol_name_list;
    for (i = 0; i < num_alpn_protocols; i++) {
      size_t length = strlen(alpn_protocols[i]);
      *(current++) = static_cast<uint8_t>(length); /* max checked above. */
      memcpy(current, alpn_protocols[i], length);
      current += length;
    }
    /* Safety check. */
    if ((current < *protocol_name_list) ||
        (static_cast<uintptr_t>(current - *protocol_name_list) !=
         *protocol_name_list_length)) {
      return TSI_INTERNAL_ERROR;
    }
    return TSI_OK;
  }
  
  // The verification callback is used for clients that don't really care about
  // the server's certificate, but we need to pull it anyway, in case a higher
  // layer wants to look at it. In this case the verification may fail, but
  // we don't really care.
  static int NullVerifyCallback(int /*preverify_ok*/, X509_STORE_CTX* /*ctx*/) {
    return 1;
  }
  
  // Sets the min and max TLS version of |ssl_context| to |min_tls_version| and
  // |max_tls_version|, respectively. Calling this method is a no-op when using
  // OpenSSL versions < 1.1.
  static tsi_result tsi_set_min_and_max_tls_versions(
      SSL_CTX* ssl_context, tsi_tls_version min_tls_version,
      tsi_tls_version max_tls_version) {
    if (ssl_context == nullptr) {
      gpr_log(GPR_INFO,
              "Invalid nullptr argument to |tsi_set_min_and_max_tls_versions|.");
      return TSI_INVALID_ARGUMENT;
    }
  #if OPENSSL_VERSION_NUMBER >= 0x10100000
    // Set the min TLS version of the SSL context if using OpenSSL version
    // >= 1.1.0. This OpenSSL version is required because the
    // |SSL_CTX_set_min_proto_version| and |SSL_CTX_set_max_proto_version| APIs
    // only exist in this version range.
    switch (min_tls_version) {
      case tsi_tls_version::TSI_TLS1_2:
        SSL_CTX_set_min_proto_version(ssl_context, TLS1_2_VERSION);
        break;
  #if defined(TLS1_3_VERSION)
      // If the library does not support TLS 1.3 and the caller requests a minimum
      // of TLS 1.3, then return an error because the caller's request cannot be
      // satisfied.
      case tsi_tls_version::TSI_TLS1_3:
        SSL_CTX_set_min_proto_version(ssl_context, TLS1_3_VERSION);
        break;
  #endif
      default:
        gpr_log(GPR_INFO, "TLS version is not supported.");
        return TSI_FAILED_PRECONDITION;
    }
  
    // Set the max TLS version of the SSL context.
    switch (max_tls_version) {
      case tsi_tls_version::TSI_TLS1_2:
        SSL_CTX_set_max_proto_version(ssl_context, TLS1_2_VERSION);
        break;
      case tsi_tls_version::TSI_TLS1_3:
  #if defined(TLS1_3_VERSION)
        SSL_CTX_set_max_proto_version(ssl_context, TLS1_3_VERSION);
  #else
        // If the library does not support TLS 1.3, then set the max TLS version
        // to TLS 1.2 instead.
        SSL_CTX_set_max_proto_version(ssl_context, TLS1_2_VERSION);
  #endif
        break;
      default:
        gpr_log(GPR_INFO, "TLS version is not supported.");
        return TSI_FAILED_PRECONDITION;
    }
  #endif
    return TSI_OK;
  }
  
  /* --- tsi_ssl_root_certs_store methods implementation. ---*/
  
  tsi_ssl_root_certs_store* tsi_ssl_root_certs_store_create(
      const char* pem_roots) {
    if (pem_roots == nullptr) {
      gpr_log(GPR_ERROR, "The root certificates are empty.");
      return nullptr;
    }
    tsi_ssl_root_certs_store* root_store = static_cast<tsi_ssl_root_certs_store*>(
        gpr_zalloc(sizeof(tsi_ssl_root_certs_store)));
    if (root_store == nullptr) {
      gpr_log(GPR_ERROR, "Could not allocate buffer for ssl_root_certs_store.");
      return nullptr;
    }
    root_store->store = X509_STORE_new();
    if (root_store->store == nullptr) {
      gpr_log(GPR_ERROR, "Could not allocate buffer for X509_STORE.");
      gpr_free(root_store);
      return nullptr;
    }
    tsi_result result = x509_store_load_certs(root_store->store, pem_roots,
                                              strlen(pem_roots), nullptr);
    if (result != TSI_OK) {
      gpr_log(GPR_ERROR, "Could not load root certificates.");
      X509_STORE_free(root_store->store);
      gpr_free(root_store);
      return nullptr;
    }
    return root_store;
  }
  
  void tsi_ssl_root_certs_store_destroy(tsi_ssl_root_certs_store* self) {
    if (self == nullptr) return;
    X509_STORE_free(self->store);
    gpr_free(self);
  }
  
  /* --- tsi_ssl_session_cache methods implementation. ---*/
  
  tsi_ssl_session_cache* tsi_ssl_session_cache_create_lru(size_t capacity) {
    /* Pointer will be dereferenced by unref call. */
    return reinterpret_cast<tsi_ssl_session_cache*>(
        tsi::SslSessionLRUCache::Create(capacity).release());
  }
  
  void tsi_ssl_session_cache_ref(tsi_ssl_session_cache* cache) {
    /* Pointer will be dereferenced by unref call. */
    reinterpret_cast<tsi::SslSessionLRUCache*>(cache)->Ref().release();
  }
  
  void tsi_ssl_session_cache_unref(tsi_ssl_session_cache* cache) {
    reinterpret_cast<tsi::SslSessionLRUCache*>(cache)->Unref();
  }
  
  /* --- tsi_frame_protector methods implementation. ---*/
  
  static tsi_result ssl_protector_protect(tsi_frame_protector* self,
                                          const unsigned char* unprotected_bytes,
                                          size_t* unprotected_bytes_size,
                                          unsigned char* protected_output_frames,
                                          size_t* protected_output_frames_size) {
    tsi_ssl_frame_protector* impl =
        reinterpret_cast<tsi_ssl_frame_protector*>(self);
    int read_from_ssl;
    size_t available;
    tsi_result result = TSI_OK;
  
    /* First see if we have some pending data in the SSL BIO. */
    int pending_in_ssl = static_cast<int>(BIO_pending(impl->network_io));
    if (pending_in_ssl > 0) {
      *unprotected_bytes_size = 0;
      GPR_ASSERT(*protected_output_frames_size <= INT_MAX);
      read_from_ssl = BIO_read(impl->network_io, protected_output_frames,
                               static_cast<int>(*protected_output_frames_size));
      if (read_from_ssl < 0) {
        gpr_log(GPR_ERROR,
                "Could not read from BIO even though some data is pending");
        return TSI_INTERNAL_ERROR;
      }
      *protected_output_frames_size = static_cast<size_t>(read_from_ssl);
      return TSI_OK;
    }
  
    /* Now see if we can send a complete frame. */
    available = impl->buffer_size - impl->buffer_offset;
    if (available > *unprotected_bytes_size) {
      /* If we cannot, just copy the data in our internal buffer. */
      memcpy(impl->buffer + impl->buffer_offset, unprotected_bytes,
             *unprotected_bytes_size);
      impl->buffer_offset += *unprotected_bytes_size;
      *protected_output_frames_size = 0;
      return TSI_OK;
    }
  
    /* If we can, prepare the buffer, send it to SSL_write and read. */
    memcpy(impl->buffer + impl->buffer_offset, unprotected_bytes, available);
    result = do_ssl_write(impl->ssl, impl->buffer, impl->buffer_size);
    if (result != TSI_OK) return result;
  
    GPR_ASSERT(*protected_output_frames_size <= INT_MAX);
    read_from_ssl = BIO_read(impl->network_io, protected_output_frames,
                             static_cast<int>(*protected_output_frames_size));
    if (read_from_ssl < 0) {
      gpr_log(GPR_ERROR, "Could not read from BIO after SSL_write.");
      return TSI_INTERNAL_ERROR;
    }
    *protected_output_frames_size = static_cast<size_t>(read_from_ssl);
    *unprotected_bytes_size = available;
    impl->buffer_offset = 0;
    return TSI_OK;
  }
  
  static tsi_result ssl_protector_protect_flush(
      tsi_frame_protector* self, unsigned char* protected_output_frames,
      size_t* protected_output_frames_size, size_t* still_pending_size) {
    tsi_result result = TSI_OK;
    tsi_ssl_frame_protector* impl =
        reinterpret_cast<tsi_ssl_frame_protector*>(self);
    int read_from_ssl = 0;
    int pending;
  
    if (impl->buffer_offset != 0) {
      result = do_ssl_write(impl->ssl, impl->buffer, impl->buffer_offset);
      if (result != TSI_OK) return result;
      impl->buffer_offset = 0;
    }
  
    pending = static_cast<int>(BIO_pending(impl->network_io));
    GPR_ASSERT(pending >= 0);
    *still_pending_size = static_cast<size_t>(pending);
    if (*still_pending_size == 0) return TSI_OK;
  
    GPR_ASSERT(*protected_output_frames_size <= INT_MAX);
    read_from_ssl = BIO_read(impl->network_io, protected_output_frames,
                             static_cast<int>(*protected_output_frames_size));
    if (read_from_ssl <= 0) {
      gpr_log(GPR_ERROR, "Could not read from BIO after SSL_write.");
      return TSI_INTERNAL_ERROR;
    }
    *protected_output_frames_size = static_cast<size_t>(read_from_ssl);
    pending = static_cast<int>(BIO_pending(impl->network_io));
    GPR_ASSERT(pending >= 0);
    *still_pending_size = static_cast<size_t>(pending);
    return TSI_OK;
  }
  
  static tsi_result ssl_protector_unprotect(
      tsi_frame_protector* self, const unsigned char* protected_frames_bytes,
      size_t* protected_frames_bytes_size, unsigned char* unprotected_bytes,
      size_t* unprotected_bytes_size) {
    tsi_result result = TSI_OK;
    int written_into_ssl = 0;
    size_t output_bytes_size = *unprotected_bytes_size;
    size_t output_bytes_offset = 0;
    tsi_ssl_frame_protector* impl =
        reinterpret_cast<tsi_ssl_frame_protector*>(self);
  
    /* First, try to read remaining data from ssl. */
    result = do_ssl_read(impl->ssl, unprotected_bytes, unprotected_bytes_size);
    if (result != TSI_OK) return result;
    if (*unprotected_bytes_size == output_bytes_size) {
      /* We have read everything we could and cannot process any more input. */
      *protected_frames_bytes_size = 0;
      return TSI_OK;
    }
    output_bytes_offset = *unprotected_bytes_size;
    unprotected_bytes += output_bytes_offset;
    *unprotected_bytes_size = output_bytes_size - output_bytes_offset;
  
    /* Then, try to write some data to ssl. */
    GPR_ASSERT(*protected_frames_bytes_size <= INT_MAX);
    written_into_ssl = BIO_write(impl->network_io, protected_frames_bytes,
                                 static_cast<int>(*protected_frames_bytes_size));
    if (written_into_ssl < 0) {
      gpr_log(GPR_ERROR, "Sending protected frame to ssl failed with %d",
              written_into_ssl);
      return TSI_INTERNAL_ERROR;
    }
    *protected_frames_bytes_size = static_cast<size_t>(written_into_ssl);
  
    /* Now try to read some data again. */
    result = do_ssl_read(impl->ssl, unprotected_bytes, unprotected_bytes_size);
    if (result == TSI_OK) {
      /* Don't forget to output the total number of bytes read. */
      *unprotected_bytes_size += output_bytes_offset;
    }
    return result;
  }
  
  static void ssl_protector_destroy(tsi_frame_protector* self) {
    tsi_ssl_frame_protector* impl =
        reinterpret_cast<tsi_ssl_frame_protector*>(self);
    if (impl->buffer != nullptr) gpr_free(impl->buffer);
    if (impl->ssl != nullptr) SSL_free(impl->ssl);
    if (impl->network_io != nullptr) BIO_free(impl->network_io);
    gpr_free(self);
  }
  
  static const tsi_frame_protector_vtable frame_protector_vtable = {
      ssl_protector_protect,
      ssl_protector_protect_flush,
      ssl_protector_unprotect,
      ssl_protector_destroy,
  };
  
  /* --- tsi_server_handshaker_factory methods implementation. --- */
  
  static void tsi_ssl_handshaker_factory_destroy(
      tsi_ssl_handshaker_factory* factory) {
    if (factory == nullptr) return;
  
    if (factory->vtable != nullptr && factory->vtable->destroy != nullptr) {
      factory->vtable->destroy(factory);
    }
    /* Note, we don't free(self) here because this object is always directly
     * embedded in another object. If tsi_ssl_handshaker_factory_init allocates
     * any memory, it should be free'd here. */
  }
  
  static tsi_ssl_handshaker_factory* tsi_ssl_handshaker_factory_ref(
      tsi_ssl_handshaker_factory* factory) {
    if (factory == nullptr) return nullptr;
    gpr_refn(&factory->refcount, 1);
    return factory;
  }
  
  static void tsi_ssl_handshaker_factory_unref(
      tsi_ssl_handshaker_factory* factory) {
    if (factory == nullptr) return;
  
    if (gpr_unref(&factory->refcount)) {
      tsi_ssl_handshaker_factory_destroy(factory);
    }
  }
  
  static tsi_ssl_handshaker_factory_vtable handshaker_factory_vtable = {nullptr};
  
  /* Initializes a tsi_ssl_handshaker_factory object. Caller is responsible for
   * allocating memory for the factory. */
  static void tsi_ssl_handshaker_factory_init(
      tsi_ssl_handshaker_factory* factory) {
    GPR_ASSERT(factory != nullptr);
  
    factory->vtable = &handshaker_factory_vtable;
    gpr_ref_init(&factory->refcount, 1);
  }
  
  /* Gets the X509 cert chain in PEM format as a tsi_peer_property. */
  tsi_result tsi_ssl_get_cert_chain_contents(STACK_OF(X509) * peer_chain,
                                             tsi_peer_property* property) {
    BIO* bio = BIO_new(BIO_s_mem());
    const auto peer_chain_len = sk_X509_num(peer_chain);
    for (auto i = decltype(peer_chain_len){0}; i < peer_chain_len; i++) {
      if (!PEM_write_bio_X509(bio, sk_X509_value(peer_chain, i))) {
        BIO_free(bio);
        return TSI_INTERNAL_ERROR;
      }
    }
    char* contents;
    long len = BIO_get_mem_data(bio, &contents);
    if (len <= 0) {
      BIO_free(bio);
      return TSI_INTERNAL_ERROR;
    }
    tsi_result result = tsi_construct_string_peer_property(
        TSI_X509_PEM_CERT_CHAIN_PROPERTY, contents, static_cast<size_t>(len),
        property);
    BIO_free(bio);
    return result;
  }
  
  /* --- tsi_handshaker_result methods implementation. ---*/
  static tsi_result ssl_handshaker_result_extract_peer(
      const tsi_handshaker_result* self, tsi_peer* peer) {
    tsi_result result = TSI_OK;
    const unsigned char* alpn_selected = nullptr;
    unsigned int alpn_selected_len;
    const tsi_ssl_handshaker_result* impl =
        reinterpret_cast<const tsi_ssl_handshaker_result*>(self);
    X509* peer_cert = SSL_get_peer_certificate(impl->ssl);
    if (peer_cert != nullptr) {
      result = peer_from_x509(peer_cert, 1, peer);
      X509_free(peer_cert);
      if (result != TSI_OK) return result;
    }
  #if TSI_OPENSSL_ALPN_SUPPORT
    SSL_get0_alpn_selected(impl->ssl, &alpn_selected, &alpn_selected_len);
  #endif /* TSI_OPENSSL_ALPN_SUPPORT */
    if (alpn_selected == nullptr) {
      /* Try npn. */
      SSL_get0_next_proto_negotiated(impl->ssl, &alpn_selected,
                                     &alpn_selected_len);
    }
    // When called on the client side, the stack also contains the
    // peer's certificate; When called on the server side,
    // the peer's certificate is not present in the stack
    STACK_OF(X509)* peer_chain = SSL_get_peer_cert_chain(impl->ssl);
    // 1 is for session reused property.
    size_t new_property_count = peer->property_count + 3;
    if (alpn_selected != nullptr) new_property_count++;
    if (peer_chain != nullptr) new_property_count++;
    tsi_peer_property* new_properties = static_cast<tsi_peer_property*>(
        gpr_zalloc(sizeof(*new_properties) * new_property_count));
    for (size_t i = 0; i < peer->property_count; i++) {
      new_properties[i] = peer->properties[i];
    }
    if (peer->properties != nullptr) gpr_free(peer->properties);
    peer->properties = new_properties;
    // Add peer chain if available
    if (peer_chain != nullptr) {
      result = tsi_ssl_get_cert_chain_contents(
          peer_chain, &peer->properties[peer->property_count]);
      if (result == TSI_OK) peer->property_count++;
    }
    if (alpn_selected != nullptr) {
      result = tsi_construct_string_peer_property(
          TSI_SSL_ALPN_SELECTED_PROTOCOL,
          reinterpret_cast<const char*>(alpn_selected), alpn_selected_len,
          &peer->properties[peer->property_count]);
      if (result != TSI_OK) return result;
      peer->property_count++;
    }
    // Add security_level peer property.
    result = tsi_construct_string_peer_property_from_cstring(
        TSI_SECURITY_LEVEL_PEER_PROPERTY,
        tsi_security_level_to_string(TSI_PRIVACY_AND_INTEGRITY),
        &peer->properties[peer->property_count]);
    if (result != TSI_OK) return result;
    peer->property_count++;
  
    const char* session_reused = SSL_session_reused(impl->ssl) ? "true" : "false";
    result = tsi_construct_string_peer_property_from_cstring(
        TSI_SSL_SESSION_REUSED_PEER_PROPERTY, session_reused,
        &peer->properties[peer->property_count]);
    if (result != TSI_OK) return result;
    peer->property_count++;
    return result;
  }
  
  static tsi_result ssl_handshaker_result_create_frame_protector(
      const tsi_handshaker_result* self, size_t* max_output_protected_frame_size,
      tsi_frame_protector** protector) {
    size_t actual_max_output_protected_frame_size =
        TSI_SSL_MAX_PROTECTED_FRAME_SIZE_UPPER_BOUND;
    tsi_ssl_handshaker_result* impl =
        reinterpret_cast<tsi_ssl_handshaker_result*>(
            const_cast<tsi_handshaker_result*>(self));
    tsi_ssl_frame_protector* protector_impl =
        static_cast<tsi_ssl_frame_protector*>(
            gpr_zalloc(sizeof(*protector_impl)));
  
    if (max_output_protected_frame_size != nullptr) {
      if (*max_output_protected_frame_size >
          TSI_SSL_MAX_PROTECTED_FRAME_SIZE_UPPER_BOUND) {
        *max_output_protected_frame_size =
            TSI_SSL_MAX_PROTECTED_FRAME_SIZE_UPPER_BOUND;
      } else if (*max_output_protected_frame_size <
                 TSI_SSL_MAX_PROTECTED_FRAME_SIZE_LOWER_BOUND) {
        *max_output_protected_frame_size =
            TSI_SSL_MAX_PROTECTED_FRAME_SIZE_LOWER_BOUND;
      }
      actual_max_output_protected_frame_size = *max_output_protected_frame_size;
    }
    protector_impl->buffer_size =
        actual_max_output_protected_frame_size - TSI_SSL_MAX_PROTECTION_OVERHEAD;
    protector_impl->buffer =
        static_cast<unsigned char*>(gpr_malloc(protector_impl->buffer_size));
    if (protector_impl->buffer == nullptr) {
      gpr_log(GPR_ERROR,
              "Could not allocated buffer for tsi_ssl_frame_protector.");
      gpr_free(protector_impl);
      return TSI_INTERNAL_ERROR;
    }
  
    /* Transfer ownership of ssl and network_io to the frame protector. */
    protector_impl->ssl = impl->ssl;
    impl->ssl = nullptr;
    protector_impl->network_io = impl->network_io;
    impl->network_io = nullptr;
    protector_impl->base.vtable = &frame_protector_vtable;
    *protector = &protector_impl->base;
    return TSI_OK;
  }
  
  static tsi_result ssl_handshaker_result_get_unused_bytes(
      const tsi_handshaker_result* self, const unsigned char** bytes,
      size_t* bytes_size) {
    const tsi_ssl_handshaker_result* impl =
        reinterpret_cast<const tsi_ssl_handshaker_result*>(self);
    *bytes_size = impl->unused_bytes_size;
    *bytes = impl->unused_bytes;
    return TSI_OK;
  }
  
  static void ssl_handshaker_result_destroy(tsi_handshaker_result* self) {
    tsi_ssl_handshaker_result* impl =
        reinterpret_cast<tsi_ssl_handshaker_result*>(self);
    SSL_free(impl->ssl);
    BIO_free(impl->network_io);
    gpr_free(impl->unused_bytes);
    gpr_free(impl);
  }
  
  static const tsi_handshaker_result_vtable handshaker_result_vtable = {
      ssl_handshaker_result_extract_peer,
      nullptr, /* create_zero_copy_grpc_protector */
      ssl_handshaker_result_create_frame_protector,
      ssl_handshaker_result_get_unused_bytes,
      ssl_handshaker_result_destroy,
  };
  
  static tsi_result ssl_handshaker_result_create(
      tsi_ssl_handshaker* handshaker, unsigned char* unused_bytes,
      size_t unused_bytes_size, tsi_handshaker_result** handshaker_result) {
    if (handshaker == nullptr || handshaker_result == nullptr ||
        (unused_bytes_size > 0 && unused_bytes == nullptr)) {
      return TSI_INVALID_ARGUMENT;
    }
    tsi_ssl_handshaker_result* result =
        static_cast<tsi_ssl_handshaker_result*>(gpr_zalloc(sizeof(*result)));
    result->base.vtable = &handshaker_result_vtable;
    /* Transfer ownership of ssl and network_io to the handshaker result. */
    result->ssl = handshaker->ssl;
    handshaker->ssl = nullptr;
    result->network_io = handshaker->network_io;
    handshaker->network_io = nullptr;
    /* Transfer ownership of |unused_bytes| to the handshaker result. */
    result->unused_bytes = unused_bytes;
    result->unused_bytes_size = unused_bytes_size;
    *handshaker_result = &result->base;
    return TSI_OK;
  }
  
  /* --- tsi_handshaker methods implementation. ---*/
  
  static tsi_result ssl_handshaker_get_bytes_to_send_to_peer(
      tsi_ssl_handshaker* impl, unsigned char* bytes, size_t* bytes_size) {
    int bytes_read_from_ssl = 0;
    if (bytes == nullptr || bytes_size == nullptr || *bytes_size == 0 ||
        *bytes_size > INT_MAX) {
      return TSI_INVALID_ARGUMENT;
    }
    GPR_ASSERT(*bytes_size <= INT_MAX);
    bytes_read_from_ssl =
        BIO_read(impl->network_io, bytes, static_cast<int>(*bytes_size));
    if (bytes_read_from_ssl < 0) {
      *bytes_size = 0;
      if (!BIO_should_retry(impl->network_io)) {
        impl->result = TSI_INTERNAL_ERROR;
        return impl->result;
      } else {
        return TSI_OK;
      }
    }
    *bytes_size = static_cast<size_t>(bytes_read_from_ssl);
    return BIO_pending(impl->network_io) == 0 ? TSI_OK : TSI_INCOMPLETE_DATA;
  }
  
  static tsi_result ssl_handshaker_get_result(tsi_ssl_handshaker* impl) {
    if ((impl->result == TSI_HANDSHAKE_IN_PROGRESS) &&
        SSL_is_init_finished(impl->ssl)) {
      impl->result = TSI_OK;
    }
    return impl->result;
  }
  
  static tsi_result ssl_handshaker_process_bytes_from_peer(
      tsi_ssl_handshaker* impl, const unsigned char* bytes, size_t* bytes_size) {
    int bytes_written_into_ssl_size = 0;
    if (bytes == nullptr || bytes_size == nullptr || *bytes_size > INT_MAX) {
      return TSI_INVALID_ARGUMENT;
    }
    GPR_ASSERT(*bytes_size <= INT_MAX);
    bytes_written_into_ssl_size =
        BIO_write(impl->network_io, bytes, static_cast<int>(*bytes_size));
    if (bytes_written_into_ssl_size < 0) {
      gpr_log(GPR_ERROR, "Could not write to memory BIO.");
      impl->result = TSI_INTERNAL_ERROR;
      return impl->result;
    }
    *bytes_size = static_cast<size_t>(bytes_written_into_ssl_size);
  
    if (ssl_handshaker_get_result(impl) != TSI_HANDSHAKE_IN_PROGRESS) {
      impl->result = TSI_OK;
      return impl->result;
    } else {
      /* Get ready to get some bytes from SSL. */
      int ssl_result = SSL_do_handshake(impl->ssl);
      ssl_result = SSL_get_error(impl->ssl, ssl_result);
      switch (ssl_result) {
        case SSL_ERROR_WANT_READ:
          if (BIO_pending(impl->network_io) == 0) {
            /* We need more data. */
            return TSI_INCOMPLETE_DATA;
          } else {
            return TSI_OK;
          }
        case SSL_ERROR_NONE:
          return TSI_OK;
        default: {
          char err_str[256];
          ERR_error_string_n(ERR_get_error(), err_str, sizeof(err_str));
          gpr_log(GPR_ERROR, "Handshake failed with fatal error %s: %s.",
                  ssl_error_string(ssl_result), err_str);
          impl->result = TSI_PROTOCOL_FAILURE;
          return impl->result;
        }
      }
    }
  }
  
  static void ssl_handshaker_destroy(tsi_handshaker* self) {
    tsi_ssl_handshaker* impl = reinterpret_cast<tsi_ssl_handshaker*>(self);
    SSL_free(impl->ssl);
    BIO_free(impl->network_io);
    gpr_free(impl->outgoing_bytes_buffer);
    tsi_ssl_handshaker_factory_unref(impl->factory_ref);
    gpr_free(impl);
  }
  
  // Removes the bytes remaining in |impl->SSL|'s read BIO and writes them to
  // |bytes_remaining|.
  static tsi_result ssl_bytes_remaining(tsi_ssl_handshaker* impl,
                                        unsigned char** bytes_remaining,
                                        size_t* bytes_remaining_size) {
    if (impl == nullptr || bytes_remaining == nullptr ||
        bytes_remaining_size == nullptr) {
      return TSI_INVALID_ARGUMENT;
    }
    // Atempt to read all of the bytes in SSL's read BIO. These bytes should
    // contain application data records that were appended to a handshake record
    // containing the ClientFinished or ServerFinished message.
    size_t bytes_in_ssl = BIO_pending(SSL_get_rbio(impl->ssl));
    if (bytes_in_ssl == 0) return TSI_OK;
    *bytes_remaining = static_cast<uint8_t*>(gpr_malloc(bytes_in_ssl));
    int bytes_read = BIO_read(SSL_get_rbio(impl->ssl), *bytes_remaining,
                              static_cast<int>(bytes_in_ssl));
    // If an unexpected number of bytes were read, return an error status and free
    // all of the bytes that were read.
    if (bytes_read < 0 || static_cast<size_t>(bytes_read) != bytes_in_ssl) {
      gpr_log(GPR_ERROR,
              "Failed to read the expected number of bytes from SSL object.");
      gpr_free(*bytes_remaining);
      *bytes_remaining = nullptr;
      return TSI_INTERNAL_ERROR;
    }
    *bytes_remaining_size = static_cast<size_t>(bytes_read);
    return TSI_OK;
  }
  
  static tsi_result ssl_handshaker_next(
      tsi_handshaker* self, const unsigned char* received_bytes,
      size_t received_bytes_size, const unsigned char** bytes_to_send,
      size_t* bytes_to_send_size, tsi_handshaker_result** handshaker_result,
      tsi_handshaker_on_next_done_cb /*cb*/, void* /*user_data*/) {
    /* Input sanity check.  */
    if ((received_bytes_size > 0 && received_bytes == nullptr) ||
        bytes_to_send == nullptr || bytes_to_send_size == nullptr ||
        handshaker_result == nullptr) {
      return TSI_INVALID_ARGUMENT;
    }
    /* If there are received bytes, process them first.  */
    tsi_ssl_handshaker* impl = reinterpret_cast<tsi_ssl_handshaker*>(self);
    tsi_result status = TSI_OK;
    size_t bytes_consumed = received_bytes_size;
    if (received_bytes_size > 0) {
      status = ssl_handshaker_process_bytes_from_peer(impl, received_bytes,
                                                      &bytes_consumed);
      if (status != TSI_OK) return status;
    }
    /* Get bytes to send to the peer, if available.  */
    size_t offset = 0;
    do {
      size_t to_send_size = impl->outgoing_bytes_buffer_size - offset;
      status = ssl_handshaker_get_bytes_to_send_to_peer(
          impl, impl->outgoing_bytes_buffer + offset, &to_send_size);
      offset += to_send_size;
      if (status == TSI_INCOMPLETE_DATA) {
        impl->outgoing_bytes_buffer_size *= 2;
        impl->outgoing_bytes_buffer = static_cast<unsigned char*>(gpr_realloc(
            impl->outgoing_bytes_buffer, impl->outgoing_bytes_buffer_size));
      }
    } while (status == TSI_INCOMPLETE_DATA);
    if (status != TSI_OK) return status;
    *bytes_to_send = impl->outgoing_bytes_buffer;
    *bytes_to_send_size = offset;
    /* If handshake completes, create tsi_handshaker_result.  */
    if (ssl_handshaker_get_result(impl) == TSI_HANDSHAKE_IN_PROGRESS) {
      *handshaker_result = nullptr;
    } else {
      // Any bytes that remain in |impl->ssl|'s read BIO after the handshake is
      // complete must be extracted and set to the unused bytes of the handshaker
      // result. This indicates to the gRPC stack that there are bytes from the
      // peer that must be processed.
      unsigned char* unused_bytes = nullptr;
      size_t unused_bytes_size = 0;
      status = ssl_bytes_remaining(impl, &unused_bytes, &unused_bytes_size);
      if (status != TSI_OK) return status;
      if (unused_bytes_size > received_bytes_size) {
        gpr_log(GPR_ERROR, "More unused bytes than received bytes.");
        gpr_free(unused_bytes);
        return TSI_INTERNAL_ERROR;
      }
      status = ssl_handshaker_result_create(impl, unused_bytes, unused_bytes_size,
                                            handshaker_result);
      if (status == TSI_OK) {
        /* Indicates that the handshake has completed and that a handshaker_result
         * has been created. */
        self->handshaker_result_created = true;
      }
    }
    return status;
  }
  
  static const tsi_handshaker_vtable handshaker_vtable = {
      nullptr, /* get_bytes_to_send_to_peer -- deprecated */
      nullptr, /* process_bytes_from_peer   -- deprecated */
      nullptr, /* get_result                -- deprecated */
      nullptr, /* extract_peer              -- deprecated */
      nullptr, /* create_frame_protector    -- deprecated */
      ssl_handshaker_destroy,
      ssl_handshaker_next,
      nullptr, /* shutdown */
  };
  
  /* --- tsi_ssl_handshaker_factory common methods. --- */
  
  static void tsi_ssl_handshaker_resume_session(
      SSL* ssl, tsi::SslSessionLRUCache* session_cache) {
    const char* server_name = SSL_get_servername(ssl, TLSEXT_NAMETYPE_host_name);
    if (server_name == nullptr) {
      return;
    }
    tsi::SslSessionPtr session = session_cache->Get(server_name);
    if (session != nullptr) {
      // SSL_set_session internally increments reference counter.
      SSL_set_session(ssl, session.get());
    }
  }
  
  static tsi_result create_tsi_ssl_handshaker(SSL_CTX* ctx, int is_client,
                                              const char* server_name_indication,
                                              tsi_ssl_handshaker_factory* factory,
                                              tsi_handshaker** handshaker) {
    SSL* ssl = SSL_new(ctx);
    BIO* network_io = nullptr;
    BIO* ssl_io = nullptr;
    tsi_ssl_handshaker* impl = nullptr;
    *handshaker = nullptr;
    if (ctx == nullptr) {
      gpr_log(GPR_ERROR, "SSL Context is null. Should never happen.");
      return TSI_INTERNAL_ERROR;
    }
    if (ssl == nullptr) {
      return TSI_OUT_OF_RESOURCES;
    }
    SSL_set_info_callback(ssl, ssl_info_callback);
  
    if (!BIO_new_bio_pair(&network_io, 0, &ssl_io, 0)) {
      gpr_log(GPR_ERROR, "BIO_new_bio_pair failed.");
      SSL_free(ssl);
      return TSI_OUT_OF_RESOURCES;
    }
    SSL_set_bio(ssl, ssl_io, ssl_io);
    if (is_client) {
      int ssl_result;
      SSL_set_connect_state(ssl);
      if (server_name_indication != nullptr) {
        if (!SSL_set_tlsext_host_name(ssl, server_name_indication)) {
          gpr_log(GPR_ERROR, "Invalid server name indication %s.",
                  server_name_indication);
          SSL_free(ssl);
          BIO_free(network_io);
          return TSI_INTERNAL_ERROR;
        }
      }
      tsi_ssl_client_handshaker_factory* client_factory =
          reinterpret_cast<tsi_ssl_client_handshaker_factory*>(factory);
      if (client_factory->session_cache != nullptr) {
        tsi_ssl_handshaker_resume_session(ssl,
                                          client_factory->session_cache.get());
      }
      ssl_result = SSL_do_handshake(ssl);
      ssl_result = SSL_get_error(ssl, ssl_result);
      if (ssl_result != SSL_ERROR_WANT_READ) {
        gpr_log(GPR_ERROR,
                "Unexpected error received from first SSL_do_handshake call: %s",
                ssl_error_string(ssl_result));
        SSL_free(ssl);
        BIO_free(network_io);
        return TSI_INTERNAL_ERROR;
      }
    } else {
      SSL_set_accept_state(ssl);
    }
  
    impl = static_cast<tsi_ssl_handshaker*>(gpr_zalloc(sizeof(*impl)));
    impl->ssl = ssl;
    impl->network_io = network_io;
    impl->result = TSI_HANDSHAKE_IN_PROGRESS;
    impl->outgoing_bytes_buffer_size =
        TSI_SSL_HANDSHAKER_OUTGOING_BUFFER_INITIAL_SIZE;
    impl->outgoing_bytes_buffer =
        static_cast<unsigned char*>(gpr_zalloc(impl->outgoing_bytes_buffer_size));
    impl->base.vtable = &handshaker_vtable;
    impl->factory_ref = tsi_ssl_handshaker_factory_ref(factory);
    *handshaker = &impl->base;
    return TSI_OK;
  }
  
  static int select_protocol_list(const unsigned char** out,
                                  unsigned char* outlen,
                                  const unsigned char* client_list,
                                  size_t client_list_len,
                                  const unsigned char* server_list,
                                  size_t server_list_len) {
    const unsigned char* client_current = client_list;
    while (static_cast<unsigned int>(client_current - client_list) <
           client_list_len) {
      unsigned char client_current_len = *(client_current++);
      const unsigned char* server_current = server_list;
      while ((server_current >= server_list) &&
             static_cast<uintptr_t>(server_current - server_list) <
                 server_list_len) {
        unsigned char server_current_len = *(server_current++);
        if ((client_current_len == server_current_len) &&
            !memcmp(client_current, server_current, server_current_len)) {
          *out = server_current;
          *outlen = server_current_len;
          return SSL_TLSEXT_ERR_OK;
        }
        server_current += server_current_len;
      }
      client_current += client_current_len;
    }
    return SSL_TLSEXT_ERR_NOACK;
  }
  
  /* --- tsi_ssl_client_handshaker_factory methods implementation. --- */
  
  tsi_result tsi_ssl_client_handshaker_factory_create_handshaker(
      tsi_ssl_client_handshaker_factory* factory,
      const char* server_name_indication, tsi_handshaker** handshaker) {
    return create_tsi_ssl_handshaker(factory->ssl_context, 1,
                                     server_name_indication, &factory->base,
                                     handshaker);
  }
  
  void tsi_ssl_client_handshaker_factory_unref(
      tsi_ssl_client_handshaker_factory* factory) {
    if (factory == nullptr) return;
    tsi_ssl_handshaker_factory_unref(&factory->base);
  }
  
  static void tsi_ssl_client_handshaker_factory_destroy(
      tsi_ssl_handshaker_factory* factory) {
    if (factory == nullptr) return;
    tsi_ssl_client_handshaker_factory* self =
        reinterpret_cast<tsi_ssl_client_handshaker_factory*>(factory);
    if (self->ssl_context != nullptr) SSL_CTX_free(self->ssl_context);
    if (self->alpn_protocol_list != nullptr) gpr_free(self->alpn_protocol_list);
    self->session_cache.reset();
    gpr_free(self);
  }
  
  static int client_handshaker_factory_npn_callback(
      SSL* /*ssl*/, unsigned char** out, unsigned char* outlen,
      const unsigned char* in, unsigned int inlen, void* arg) {
    tsi_ssl_client_handshaker_factory* factory =
        static_cast<tsi_ssl_client_handshaker_factory*>(arg);
    return select_protocol_list(const_cast<const unsigned char**>(out), outlen,
                                factory->alpn_protocol_list,
                                factory->alpn_protocol_list_length, in, inlen);
  }
  
  /* --- tsi_ssl_server_handshaker_factory methods implementation. --- */
  
  tsi_result tsi_ssl_server_handshaker_factory_create_handshaker(
      tsi_ssl_server_handshaker_factory* factory, tsi_handshaker** handshaker) {
    if (factory->ssl_context_count == 0) return TSI_INVALID_ARGUMENT;
    /* Create the handshaker with the first context. We will switch if needed
       because of SNI in ssl_server_handshaker_factory_servername_callback.  */
    return create_tsi_ssl_handshaker(factory->ssl_contexts[0], 0, nullptr,
                                     &factory->base, handshaker);
  }
  
  void tsi_ssl_server_handshaker_factory_unref(
      tsi_ssl_server_handshaker_factory* factory) {
    if (factory == nullptr) return;
    tsi_ssl_handshaker_factory_unref(&factory->base);
  }
  
  static void tsi_ssl_server_handshaker_factory_destroy(
      tsi_ssl_handshaker_factory* factory) {
    if (factory == nullptr) return;
    tsi_ssl_server_handshaker_factory* self =
        reinterpret_cast<tsi_ssl_server_handshaker_factory*>(factory);
    size_t i;
    for (i = 0; i < self->ssl_context_count; i++) {
      if (self->ssl_contexts[i] != nullptr) {
        SSL_CTX_free(self->ssl_contexts[i]);
        tsi_peer_destruct(&self->ssl_context_x509_subject_names[i]);
      }
    }
    if (self->ssl_contexts != nullptr) gpr_free(self->ssl_contexts);
    if (self->ssl_context_x509_subject_names != nullptr) {
      gpr_free(self->ssl_context_x509_subject_names);
    }
    if (self->alpn_protocol_list != nullptr) gpr_free(self->alpn_protocol_list);
    gpr_free(self);
  }
  
  static int does_entry_match_name(absl::string_view entry,
                                   absl::string_view name) {
    if (entry.empty()) return 0;
  
    /* Take care of '.' terminations. */
    if (name.back() == '.') {
      name.remove_suffix(1);
    }
    if (entry.back() == '.') {
      entry.remove_suffix(1);
      if (entry.empty()) return 0;
    }
  
    if (absl::EqualsIgnoreCase(name, entry)) {
      return 1; /* Perfect match. */
    }
    if (entry.front() != '*') return 0;
  
    /* Wildchar subdomain matching. */
    if (entry.size() < 3 || entry[1] != '.') { /* At least *.x */
      gpr_log(GPR_ERROR, "Invalid wildchar entry.");
      return 0;
    }
    size_t name_subdomain_pos = name.find('.');
    if (name_subdomain_pos == absl::string_view::npos) return 0;
    if (name_subdomain_pos >= name.size() - 2) return 0;
    absl::string_view name_subdomain =
        name.substr(name_subdomain_pos + 1); /* Starts after the dot. */
    entry.remove_prefix(2);                  /* Remove *. */
    size_t dot = name_subdomain.find('.');
    if (dot == absl::string_view::npos || dot == name_subdomain.size() - 1) {
      gpr_log(GPR_ERROR, "Invalid toplevel subdomain: %s",
              std::string(name_subdomain).c_str());
      return 0;
    }
    if (name_subdomain.back() == '.') {
      name_subdomain.remove_suffix(1);
    }
    return !entry.empty() && absl::EqualsIgnoreCase(name_subdomain, entry);
  }
  
  static int ssl_server_handshaker_factory_servername_callback(SSL* ssl,
                                                               int* /*ap*/,
                                                               void* arg) {
    tsi_ssl_server_handshaker_factory* impl =
        static_cast<tsi_ssl_server_handshaker_factory*>(arg);
    size_t i = 0;
    const char* servername = SSL_get_servername(ssl, TLSEXT_NAMETYPE_host_name);
    if (servername == nullptr || strlen(servername) == 0) {
      return SSL_TLSEXT_ERR_NOACK;
    }
  
    for (i = 0; i < impl->ssl_context_count; i++) {
      if (tsi_ssl_peer_matches_name(&impl->ssl_context_x509_subject_names[i],
                                    servername)) {
        SSL_set_SSL_CTX(ssl, impl->ssl_contexts[i]);
        return SSL_TLSEXT_ERR_OK;
      }
    }
    gpr_log(GPR_ERROR, "No match found for server name: %s.", servername);
    return SSL_TLSEXT_ERR_NOACK;
  }
  
  #if TSI_OPENSSL_ALPN_SUPPORT
  static int server_handshaker_factory_alpn_callback(
      SSL* /*ssl*/, const unsigned char** out, unsigned char* outlen,
      const unsigned char* in, unsigned int inlen, void* arg) {
    tsi_ssl_server_handshaker_factory* factory =
        static_cast<tsi_ssl_server_handshaker_factory*>(arg);
    return select_protocol_list(out, outlen, in, inlen,
                                factory->alpn_protocol_list,
                                factory->alpn_protocol_list_length);
  }
  #endif /* TSI_OPENSSL_ALPN_SUPPORT */
  
  static int server_handshaker_factory_npn_advertised_callback(
      SSL* /*ssl*/, const unsigned char** out, unsigned int* outlen, void* arg) {
    tsi_ssl_server_handshaker_factory* factory =
        static_cast<tsi_ssl_server_handshaker_factory*>(arg);
    *out = factory->alpn_protocol_list;
    GPR_ASSERT(factory->alpn_protocol_list_length <= UINT_MAX);
    *outlen = static_cast<unsigned int>(factory->alpn_protocol_list_length);
    return SSL_TLSEXT_ERR_OK;
  }
  
  /// This callback is called when new \a session is established and ready to
  /// be cached. This session can be reused for new connections to similar
  /// servers at later point of time.
  /// It's intended to be used with SSL_CTX_sess_set_new_cb function.
  ///
  /// It returns 1 if callback takes ownership over \a session and 0 otherwise.
  static int server_handshaker_factory_new_session_callback(
      SSL* ssl, SSL_SESSION* session) {
    SSL_CTX* ssl_context = SSL_get_SSL_CTX(ssl);
    if (ssl_context == nullptr) {
      return 0;
    }
    void* arg = SSL_CTX_get_ex_data(ssl_context, g_ssl_ctx_ex_factory_index);
    tsi_ssl_client_handshaker_factory* factory =
        static_cast<tsi_ssl_client_handshaker_factory*>(arg);
    const char* server_name = SSL_get_servername(ssl, TLSEXT_NAMETYPE_host_name);
    if (server_name == nullptr) {
      return 0;
    }
    factory->session_cache->Put(server_name, tsi::SslSessionPtr(session));
    // Return 1 to indicate transferred ownership over the given session.
    return 1;
  }
  
  /* --- tsi_ssl_handshaker_factory constructors. --- */
  
  static tsi_ssl_handshaker_factory_vtable client_handshaker_factory_vtable = {
      tsi_ssl_client_handshaker_factory_destroy};
  
  tsi_result tsi_create_ssl_client_handshaker_factory(
      const tsi_ssl_pem_key_cert_pair* pem_key_cert_pair,
      const char* pem_root_certs, const char* cipher_suites,
      const char** alpn_protocols, uint16_t num_alpn_protocols,
      tsi_ssl_client_handshaker_factory** factory) {
    tsi_ssl_client_handshaker_options options;
    options.pem_key_cert_pair = pem_key_cert_pair;
    options.pem_root_certs = pem_root_certs;
    options.cipher_suites = cipher_suites;
    options.alpn_protocols = alpn_protocols;
    options.num_alpn_protocols = num_alpn_protocols;
    return tsi_create_ssl_client_handshaker_factory_with_options(&options,
                                                                 factory);
  }
  
  tsi_result tsi_create_ssl_client_handshaker_factory_with_options(
      const tsi_ssl_client_handshaker_options* options,
      tsi_ssl_client_handshaker_factory** factory) {
    SSL_CTX* ssl_context = nullptr;
    tsi_ssl_client_handshaker_factory* impl = nullptr;
    tsi_result result = TSI_OK;
  
    gpr_once_init(&g_init_openssl_once, init_openssl);
  
    if (factory == nullptr) return TSI_INVALID_ARGUMENT;
    *factory = nullptr;
    if (options->pem_root_certs == nullptr && options->root_store == nullptr) {
      return TSI_INVALID_ARGUMENT;
    }
  
  #if OPENSSL_VERSION_NUMBER >= 0x10100000
    ssl_context = SSL_CTX_new(TLS_method());
  #else
    ssl_context = SSL_CTX_new(TLSv1_2_method());
  #endif
    result = tsi_set_min_and_max_tls_versions(
        ssl_context, options->min_tls_version, options->max_tls_version);
    if (result != TSI_OK) return result;
    if (ssl_context == nullptr) {
      gpr_log(GPR_ERROR, "Could not create ssl context.");
      return TSI_INVALID_ARGUMENT;
    }
  
    impl = static_cast<tsi_ssl_client_handshaker_factory*>(
        gpr_zalloc(sizeof(*impl)));
    tsi_ssl_handshaker_factory_init(&impl->base);
    impl->base.vtable = &client_handshaker_factory_vtable;
    impl->ssl_context = ssl_context;
    if (options->session_cache != nullptr) {
      // Unref is called manually on factory destruction.
      impl->session_cache =
          reinterpret_cast<tsi::SslSessionLRUCache*>(options->session_cache)
              ->Ref();
      SSL_CTX_set_ex_data(ssl_context, g_ssl_ctx_ex_factory_index, impl);
      SSL_CTX_sess_set_new_cb(ssl_context,
                              server_handshaker_factory_new_session_callback);
      SSL_CTX_set_session_cache_mode(ssl_context, SSL_SESS_CACHE_CLIENT);
    }
  
    do {
      result = populate_ssl_context(ssl_context, options->pem_key_cert_pair,
                                    options->cipher_suites);
      if (result != TSI_OK) break;
  
  #if OPENSSL_VERSION_NUMBER >= 0x10100000
      // X509_STORE_up_ref is only available since OpenSSL 1.1.
      if (options->root_store != nullptr) {
        X509_STORE_up_ref(options->root_store->store);
        SSL_CTX_set_cert_store(ssl_context, options->root_store->store);
      }
  #endif
      if (OPENSSL_VERSION_NUMBER < 0x10100000 || options->root_store == nullptr) {
        result = ssl_ctx_load_verification_certs(
            ssl_context, options->pem_root_certs, strlen(options->pem_root_certs),
            nullptr);
        if (result != TSI_OK) {
          gpr_log(GPR_ERROR, "Cannot load server root certificates.");
          break;
        }
      }
  
      if (options->num_alpn_protocols != 0) {
        result = build_alpn_protocol_name_list(
            options->alpn_protocols, options->num_alpn_protocols,
            &impl->alpn_protocol_list, &impl->alpn_protocol_list_length);
        if (result != TSI_OK) {
          gpr_log(GPR_ERROR, "Building alpn list failed with error %s.",
                  tsi_result_to_string(result));
          break;
        }
  #if TSI_OPENSSL_ALPN_SUPPORT
        GPR_ASSERT(impl->alpn_protocol_list_length < UINT_MAX);
        if (SSL_CTX_set_alpn_protos(
                ssl_context, impl->alpn_protocol_list,
                static_cast<unsigned int>(impl->alpn_protocol_list_length))) {
          gpr_log(GPR_ERROR, "Could not set alpn protocol list to context.");
          result = TSI_INVALID_ARGUMENT;
          break;
        }
  #endif /* TSI_OPENSSL_ALPN_SUPPORT */
        SSL_CTX_set_next_proto_select_cb(
            ssl_context, client_handshaker_factory_npn_callback, impl);
      }
    } while (false);
    if (result != TSI_OK) {
      tsi_ssl_handshaker_factory_unref(&impl->base);
      return result;
    }
    if (options->skip_server_certificate_verification) {
      SSL_CTX_set_verify(ssl_context, SSL_VERIFY_PEER, NullVerifyCallback);
    } else {
      SSL_CTX_set_verify(ssl_context, SSL_VERIFY_PEER, nullptr);
    }
    /* TODO(jboeuf): Add revocation verification. */
  
    *factory = impl;
    return TSI_OK;
  }
  
  static tsi_ssl_handshaker_factory_vtable server_handshaker_factory_vtable = {
      tsi_ssl_server_handshaker_factory_destroy};
  
  tsi_result tsi_create_ssl_server_handshaker_factory(
      const tsi_ssl_pem_key_cert_pair* pem_key_cert_pairs,
      size_t num_key_cert_pairs, const char* pem_client_root_certs,
      int force_client_auth, const char* cipher_suites,
      const char** alpn_protocols, uint16_t num_alpn_protocols,
      tsi_ssl_server_handshaker_factory** factory) {
    return tsi_create_ssl_server_handshaker_factory_ex(
        pem_key_cert_pairs, num_key_cert_pairs, pem_client_root_certs,
        force_client_auth ? TSI_REQUEST_AND_REQUIRE_CLIENT_CERTIFICATE_AND_VERIFY
                          : TSI_DONT_REQUEST_CLIENT_CERTIFICATE,
        cipher_suites, alpn_protocols, num_alpn_protocols, factory);
  }
  
  tsi_result tsi_create_ssl_server_handshaker_factory_ex(
      const tsi_ssl_pem_key_cert_pair* pem_key_cert_pairs,
      size_t num_key_cert_pairs, const char* pem_client_root_certs,
      tsi_client_certificate_request_type client_certificate_request,
      const char* cipher_suites, const char** alpn_protocols,
      uint16_t num_alpn_protocols, tsi_ssl_server_handshaker_factory** factory) {
    tsi_ssl_server_handshaker_options options;
    options.pem_key_cert_pairs = pem_key_cert_pairs;
    options.num_key_cert_pairs = num_key_cert_pairs;
    options.pem_client_root_certs = pem_client_root_certs;
    options.client_certificate_request = client_certificate_request;
    options.cipher_suites = cipher_suites;
    options.alpn_protocols = alpn_protocols;
    options.num_alpn_protocols = num_alpn_protocols;
    return tsi_create_ssl_server_handshaker_factory_with_options(&options,
                                                                 factory);
  }
  
  tsi_result tsi_create_ssl_server_handshaker_factory_with_options(
      const tsi_ssl_server_handshaker_options* options,
      tsi_ssl_server_handshaker_factory** factory) {
    tsi_ssl_server_handshaker_factory* impl = nullptr;
    tsi_result result = TSI_OK;
    size_t i = 0;
  
    gpr_once_init(&g_init_openssl_once, init_openssl);
  
    if (factory == nullptr) return TSI_INVALID_ARGUMENT;
    *factory = nullptr;
    if (options->num_key_cert_pairs == 0 ||
        options->pem_key_cert_pairs == nullptr) {
      return TSI_INVALID_ARGUMENT;
    }
  
    impl = static_cast<tsi_ssl_server_handshaker_factory*>(
        gpr_zalloc(sizeof(*impl)));
    tsi_ssl_handshaker_factory_init(&impl->base);
    impl->base.vtable = &server_handshaker_factory_vtable;
  
    impl->ssl_contexts = static_cast<SSL_CTX**>(
        gpr_zalloc(options->num_key_cert_pairs * sizeof(SSL_CTX*)));
    impl->ssl_context_x509_subject_names = static_cast<tsi_peer*>(
        gpr_zalloc(options->num_key_cert_pairs * sizeof(tsi_peer)));
    if (impl->ssl_contexts == nullptr ||
        impl->ssl_context_x509_subject_names == nullptr) {
      tsi_ssl_handshaker_factory_unref(&impl->base);
      return TSI_OUT_OF_RESOURCES;
    }
    impl->ssl_context_count = options->num_key_cert_pairs;
  
    if (options->num_alpn_protocols > 0) {
      result = build_alpn_protocol_name_list(
          options->alpn_protocols, options->num_alpn_protocols,
          &impl->alpn_protocol_list, &impl->alpn_protocol_list_length);
      if (result != TSI_OK) {
        tsi_ssl_handshaker_factory_unref(&impl->base);
        return result;
      }
    }
  
    for (i = 0; i < options->num_key_cert_pairs; i++) {
      do {
  #if OPENSSL_VERSION_NUMBER >= 0x10100000
        impl->ssl_contexts[i] = SSL_CTX_new(TLS_method());
  #else
        impl->ssl_contexts[i] = SSL_CTX_new(TLSv1_2_method());
  #endif
        result = tsi_set_min_and_max_tls_versions(impl->ssl_contexts[i],
                                                  options->min_tls_version,
                                                  options->max_tls_version);
        if (result != TSI_OK) return result;
        if (impl->ssl_contexts[i] == nullptr) {
          gpr_log(GPR_ERROR, "Could not create ssl context.");
          result = TSI_OUT_OF_RESOURCES;
          break;
        }
        result = populate_ssl_context(impl->ssl_contexts[i],
                                      &options->pem_key_cert_pairs[i],
                                      options->cipher_suites);
        if (result != TSI_OK) break;
  
        // TODO(elessar): Provide ability to disable session ticket keys.
  
        // Allow client cache sessions (it's needed for OpenSSL only).
        int set_sid_ctx_result = SSL_CTX_set_session_id_context(
            impl->ssl_contexts[i], kSslSessionIdContext,
            GPR_ARRAY_SIZE(kSslSessionIdContext));
        if (set_sid_ctx_result == 0) {
          gpr_log(GPR_ERROR, "Failed to set session id context.");
          result = TSI_INTERNAL_ERROR;
          break;
        }
  
        if (options->session_ticket_key != nullptr) {
          if (SSL_CTX_set_tlsext_ticket_keys(
                  impl->ssl_contexts[i],
                  const_cast<char*>(options->session_ticket_key),
                  options->session_ticket_key_size) == 0) {
            gpr_log(GPR_ERROR, "Invalid STEK size.");
            result = TSI_INVALID_ARGUMENT;
            break;
          }
        }
  
        if (options->pem_client_root_certs != nullptr) {
          STACK_OF(X509_NAME)* root_names = nullptr;
          result = ssl_ctx_load_verification_certs(
              impl->ssl_contexts[i], options->pem_client_root_certs,
              strlen(options->pem_client_root_certs), &root_names);
          if (result != TSI_OK) {
            gpr_log(GPR_ERROR, "Invalid verification certs.");
            break;
          }
          SSL_CTX_set_client_CA_list(impl->ssl_contexts[i], root_names);
        }
        switch (options->client_certificate_request) {
          case TSI_DONT_REQUEST_CLIENT_CERTIFICATE:
            SSL_CTX_set_verify(impl->ssl_contexts[i], SSL_VERIFY_NONE, nullptr);
            break;
          case TSI_REQUEST_CLIENT_CERTIFICATE_BUT_DONT_VERIFY:
            SSL_CTX_set_verify(impl->ssl_contexts[i], SSL_VERIFY_PEER,
                               NullVerifyCallback);
            break;
          case TSI_REQUEST_CLIENT_CERTIFICATE_AND_VERIFY:
            SSL_CTX_set_verify(impl->ssl_contexts[i], SSL_VERIFY_PEER, nullptr);
            break;
          case TSI_REQUEST_AND_REQUIRE_CLIENT_CERTIFICATE_BUT_DONT_VERIFY:
            SSL_CTX_set_verify(impl->ssl_contexts[i],
                               SSL_VERIFY_PEER | SSL_VERIFY_FAIL_IF_NO_PEER_CERT,
                               NullVerifyCallback);
            break;
          case TSI_REQUEST_AND_REQUIRE_CLIENT_CERTIFICATE_AND_VERIFY:
            SSL_CTX_set_verify(impl->ssl_contexts[i],
                               SSL_VERIFY_PEER | SSL_VERIFY_FAIL_IF_NO_PEER_CERT,
                               nullptr);
            break;
        }
        /* TODO(jboeuf): Add revocation verification. */
  
        result = tsi_ssl_extract_x509_subject_names_from_pem_cert(
            options->pem_key_cert_pairs[i].cert_chain,
            &impl->ssl_context_x509_subject_names[i]);
        if (result != TSI_OK) break;
  
        SSL_CTX_set_tlsext_servername_callback(
            impl->ssl_contexts[i],
            ssl_server_handshaker_factory_servername_callback);
        SSL_CTX_set_tlsext_servername_arg(impl->ssl_contexts[i], impl);
  #if TSI_OPENSSL_ALPN_SUPPORT
        SSL_CTX_set_alpn_select_cb(impl->ssl_contexts[i],
                                   server_handshaker_factory_alpn_callback, impl);
  #endif /* TSI_OPENSSL_ALPN_SUPPORT */
        SSL_CTX_set_next_protos_advertised_cb(
            impl->ssl_contexts[i],
            server_handshaker_factory_npn_advertised_callback, impl);
      } while (false);
  
      if (result != TSI_OK) {
        tsi_ssl_handshaker_factory_unref(&impl->base);
        return result;
      }
    }
  
    *factory = impl;
    return TSI_OK;
  }
  
  /* --- tsi_ssl utils. --- */
  
  int tsi_ssl_peer_matches_name(const tsi_peer* peer, absl::string_view name) {
    size_t i = 0;
    size_t san_count = 0;
    const tsi_peer_property* cn_property = nullptr;
    int like_ip = looks_like_ip_address(name);
  
    /* Check the SAN first. */
    for (i = 0; i < peer->property_count; i++) {
      const tsi_peer_property* property = &peer->properties[i];
      if (property->name == nullptr) continue;
      if (strcmp(property->name,
                 TSI_X509_SUBJECT_ALTERNATIVE_NAME_PEER_PROPERTY) == 0) {
        san_count++;
  
        absl::string_view entry(property->value.data, property->value.length);
        if (!like_ip && does_entry_match_name(entry, name)) {
          return 1;
        } else if (like_ip && name == entry) {
          /* IP Addresses are exact matches only. */
          return 1;
        }
      } else if (strcmp(property->name,
                        TSI_X509_SUBJECT_COMMON_NAME_PEER_PROPERTY) == 0) {
        cn_property = property;
      }
    }
  
    /* If there's no SAN, try the CN, but only if its not like an IP Address */
    if (san_count == 0 && cn_property != nullptr && !like_ip) {
      if (does_entry_match_name(absl::string_view(cn_property->value.data,
                                                  cn_property->value.length),
                                name)) {
        return 1;
      }
    }
  
    return 0; /* Not found. */
  }
  
  /* --- Testing support. --- */
  const tsi_ssl_handshaker_factory_vtable* tsi_ssl_handshaker_factory_swap_vtable(
      tsi_ssl_handshaker_factory* factory,
      tsi_ssl_handshaker_factory_vtable* new_vtable) {
    GPR_ASSERT(factory != nullptr);
    GPR_ASSERT(factory->vtable != nullptr);
  
    const tsi_ssl_handshaker_factory_vtable* orig_vtable = factory->vtable;
    factory->vtable = new_vtable;
    return orig_vtable;
  }
  