text: #ifndef SECP256K1_H
  #define SECP256K1_H
  
  #ifdef __cplusplus
  extern "C" {
  #endif
  
  #include <stddef.h>
  
  /* These rules specify the order of arguments in API calls:
   *
   * 1. Context pointers go first, followed by output arguments, combined
   *    output/input arguments, and finally input-only arguments.
   * 2. Array lengths always immediately the follow the argument whose length
   *    they describe, even if this violates rule 1.
   * 3. Within the OUT/OUTIN/IN groups, pointers to data that is typically generated
   *    later go first. This means: signatures, public nonces, private nonces,
   *    messages, public keys, secret keys, tweaks.
   * 4. Arguments that are not data pointers go last, from more complex to less
   *    complex: function pointers, algorithm names, messages, void pointers,
   *    counts, flags, booleans.
   * 5. Opaque data pointers follow the function pointer they are to be passed to.
   */
  
  /** Opaque data structure that holds context information (precomputed tables etc.).
   *
   *  The purpose of context structures is to cache large precomputed data tables
   *  that are expensive to construct, and also to maintain the randomization data
   *  for blinding.
   *
   *  Do not create a new context object for each operation, as construction is
   *  far slower than all other API calls (~100 times slower than an ECDSA
   *  verification).
   *
   *  A constructed context can safely be used from multiple threads
   *  simultaneously, but API calls that take a non-const pointer to a context
   *  need exclusive access to it. In particular this is the case for
   *  secp256k1_context_destroy, secp256k1_context_preallocated_destroy,
   *  and secp256k1_context_randomize.
   *
   *  Regarding randomization, either do it once at creation time (in which case
   *  you do not need any locking for the other calls), or use a read-write lock.
   */
  typedef struct secp256k1_context_struct secp256k1_context;
  
  /** Opaque data structure that holds rewriteable "scratch space"
   *
   *  The purpose of this structure is to replace dynamic memory allocations,
   *  because we target architectures where this may not be available. It is
   *  essentially a resizable (within specified parameters) block of bytes,
   *  which is initially created either by memory allocation or TODO as a pointer
   *  into some fixed rewritable space.
   *
   *  Unlike the context object, this cannot safely be shared between threads
   *  without additional synchronization logic.
   */
  typedef struct secp256k1_scratch_space_struct secp256k1_scratch_space;
  
  /** Opaque data structure that holds a parsed and valid public key.
   *
   *  The exact representation of data inside is implementation defined and not
   *  guaranteed to be portable between different platforms or versions. It is
   *  however guaranteed to be 64 bytes in size, and can be safely copied/moved.
   *  If you need to convert to a format suitable for storage, transmission, or
   *  comparison, use secp256k1_ec_pubkey_serialize and secp256k1_ec_pubkey_parse.
   */
  typedef struct {
      unsigned char data[64];
  } secp256k1_pubkey;
  
  /** Opaque data structured that holds a parsed ECDSA signature.
   *
   *  The exact representation of data inside is implementation defined and not
   *  guaranteed to be portable between different platforms or versions. It is
   *  however guaranteed to be 64 bytes in size, and can be safely copied/moved.
   *  If you need to convert to a format suitable for storage, transmission, or
   *  comparison, use the secp256k1_ecdsa_signature_serialize_* and
   *  secp256k1_ecdsa_signature_parse_* functions.
   */
  typedef struct {
      unsigned char data[64];
  } secp256k1_ecdsa_signature;
  
  /** A pointer to a function to deterministically generate a nonce.
   *
   * Returns: 1 if a nonce was successfully generated. 0 will cause signing to fail.
   * Out:     nonce32:   pointer to a 32-byte array to be filled by the function.
   * In:      msg32:     the 32-byte message hash being verified (will not be NULL)
   *          key32:     pointer to a 32-byte secret key (will not be NULL)
   *          algo16:    pointer to a 16-byte array describing the signature
   *                     algorithm (will be NULL for ECDSA for compatibility).
   *          data:      Arbitrary data pointer that is passed through.
   *          attempt:   how many iterations we have tried to find a nonce.
   *                     This will almost always be 0, but different attempt values
   *                     are required to result in a different nonce.
   *
   * Except for test cases, this function should compute some cryptographic hash of
   * the message, the algorithm, the key and the attempt.
   */
  typedef int (*secp256k1_nonce_function)(
      unsigned char *nonce32,
      const unsigned char *msg32,
      const unsigned char *key32,
      const unsigned char *algo16,
      void *data,
      unsigned int attempt
  );
  
  # if !defined(SECP256K1_GNUC_PREREQ)
  #  if defined(__GNUC__)&&defined(__GNUC_MINOR__)
  #   define SECP256K1_GNUC_PREREQ(_maj,_min) \
   ((__GNUC__<<16)+__GNUC_MINOR__>=((_maj)<<16)+(_min))
  #  else
  #   define SECP256K1_GNUC_PREREQ(_maj,_min) 0
  #  endif
  # endif
  
  # if (!defined(__STDC_VERSION__) || (__STDC_VERSION__ < 199901L) )
  #  if SECP256K1_GNUC_PREREQ(2,7)
  #   define SECP256K1_INLINE __inline__
  #  elif (defined(_MSC_VER))
  #   define SECP256K1_INLINE __inline
  #  else
  #   define SECP256K1_INLINE
  #  endif
  # else
  #  define SECP256K1_INLINE inline
  # endif
  
  #ifndef SECP256K1_API
  # if defined(_WIN32)
  #  ifdef SECP256K1_BUILD
  #   define SECP256K1_API __declspec(dllexport)
  #  else
  #   define SECP256K1_API
  #  endif
  # elif defined(__GNUC__) && defined(SECP256K1_BUILD)
  #  define SECP256K1_API __attribute__ ((visibility ("default")))
  # else
  #  define SECP256K1_API
  # endif
  #endif
  
  /**Warning attributes
    * NONNULL is not used if SECP256K1_BUILD is set to avoid the compiler optimizing out
    * some paranoid null checks. */
  # if defined(__GNUC__) && SECP256K1_GNUC_PREREQ(3, 4)
  #  define SECP256K1_WARN_UNUSED_RESULT __attribute__ ((__warn_unused_result__))
  # else
  #  define SECP256K1_WARN_UNUSED_RESULT
  # endif
  # if !defined(SECP256K1_BUILD) && defined(__GNUC__) && SECP256K1_GNUC_PREREQ(3, 4)
  #  define SECP256K1_ARG_NONNULL(_x)  __attribute__ ((__nonnull__(_x)))
  # else
  #  define SECP256K1_ARG_NONNULL(_x)
  # endif
  
  /** All flags' lower 8 bits indicate what they're for. Do not use directly. */
  #define SECP256K1_FLAGS_TYPE_MASK ((1 << 8) - 1)
  #define SECP256K1_FLAGS_TYPE_CONTEXT (1 << 0)
  #define SECP256K1_FLAGS_TYPE_COMPRESSION (1 << 1)
  /** The higher bits contain the actual data. Do not use directly. */
  #define SECP256K1_FLAGS_BIT_CONTEXT_VERIFY (1 << 8)
  #define SECP256K1_FLAGS_BIT_CONTEXT_SIGN (1 << 9)
  #define SECP256K1_FLAGS_BIT_COMPRESSION (1 << 8)
  
  /** Flags to pass to secp256k1_context_create, secp256k1_context_preallocated_size, and
   *  secp256k1_context_preallocated_create. */
  #define SECP256K1_CONTEXT_VERIFY (SECP256K1_FLAGS_TYPE_CONTEXT | SECP256K1_FLAGS_BIT_CONTEXT_VERIFY)
  #define SECP256K1_CONTEXT_SIGN (SECP256K1_FLAGS_TYPE_CONTEXT | SECP256K1_FLAGS_BIT_CONTEXT_SIGN)
  #define SECP256K1_CONTEXT_NONE (SECP256K1_FLAGS_TYPE_CONTEXT)
  
  /** Flag to pass to secp256k1_ec_pubkey_serialize and secp256k1_ec_privkey_export. */
  #define SECP256K1_EC_COMPRESSED (SECP256K1_FLAGS_TYPE_COMPRESSION | SECP256K1_FLAGS_BIT_COMPRESSION)
  #define SECP256K1_EC_UNCOMPRESSED (SECP256K1_FLAGS_TYPE_COMPRESSION)
  
  /** Prefix byte used to tag various encoded curvepoints for specific purposes */
  #define SECP256K1_TAG_PUBKEY_EVEN 0x02
  #define SECP256K1_TAG_PUBKEY_ODD 0x03
  #define SECP256K1_TAG_PUBKEY_UNCOMPRESSED 0x04
  #define SECP256K1_TAG_PUBKEY_HYBRID_EVEN 0x06
  #define SECP256K1_TAG_PUBKEY_HYBRID_ODD 0x07
  
  /** Length in bytes of a secp256k1_xonly_pubkey as serialized by
   * secp256k1_xonly_pubkey_serialize. */
  #define SECP256K1_LEN_XONLY_PUBKEY 32
  
  /** A simple secp256k1 context object with no precomputed tables. These are useful for
   *  type serialization/parsing functions which require a context object to maintain
   *  API consistency, but currently do not require expensive precomputations or dynamic
   *  allocations.
   */
  SECP256K1_API extern const secp256k1_context *secp256k1_context_no_precomp;
  
  /** Create a secp256k1 context object (in dynamically allocated memory).
   *
   *  This function uses malloc to allocate memory. It is guaranteed that malloc is
   *  called at most once for every call of this function. If you need to avoid dynamic
   *  memory allocation entirely, see the functions in secp256k1_preallocated.h.
   *
   *  Returns: a newly created context object.
   *  In:      flags: which parts of the context to initialize.
   *
   *  See also secp256k1_context_randomize.
   */
  SECP256K1_API secp256k1_context* secp256k1_context_create(
      unsigned int flags
  ) SECP256K1_WARN_UNUSED_RESULT;
  
  /** Copy a secp256k1 context object (into dynamically allocated memory).
   *
   *  This function uses malloc to allocate memory. It is guaranteed that malloc is
   *  called at most once for every call of this function. If you need to avoid dynamic
   *  memory allocation entirely, see the functions in secp256k1_preallocated.h.
   *
   *  Returns: a newly created context object.
   *  Args:    ctx: an existing context to copy (cannot be NULL)
   */
  SECP256K1_API secp256k1_context* secp256k1_context_clone(
      const secp256k1_context* ctx
  ) SECP256K1_ARG_NONNULL(1) SECP256K1_WARN_UNUSED_RESULT;
  
  /** Destroy a secp256k1 context object (created in dynamically allocated memory).
   *
   *  The context pointer may not be used afterwards.
   *
   *  The context to destroy must have been created using secp256k1_context_create
   *  or secp256k1_context_clone. If the context has instead been created using
   *  secp256k1_context_preallocated_create or secp256k1_context_preallocated_clone, the
   *  behaviour is undefined. In that case, secp256k1_context_preallocated_destroy must
   *  be used instead.
   *
   *  Args:   ctx: an existing context to destroy, constructed using
   *               secp256k1_context_create or secp256k1_context_clone
   */
  SECP256K1_API void secp256k1_context_destroy(
      secp256k1_context* ctx
  );
  
  /** Set a callback function to be called when an illegal argument is passed to
   *  an API call. It will only trigger for violations that are mentioned
   *  explicitly in the header.
   *
   *  The philosophy is that these shouldn't be dealt with through a
   *  specific return value, as calling code should not have branches to deal with
   *  the case that this code itself is broken.
   *
   *  On the other hand, during debug stage, one would want to be informed about
   *  such mistakes, and the default (crashing) may be inadvisable.
   *  When this callback is triggered, the API function called is guaranteed not
   *  to cause a crash, though its return value and output arguments are
   *  undefined.
   *
   *  When this function has not been called (or called with fn==NULL), then the
   *  default handler will be used. The library provides a default handler which
   *  writes the message to stderr and calls abort. This default handler can be
   *  replaced at link time if the preprocessor macro
   *  USE_EXTERNAL_DEFAULT_CALLBACKS is defined, which is the case if the build
   *  has been configured with --enable-external-default-callbacks. Then the
   *  following two symbols must be provided to link against:
   *   - void secp256k1_default_illegal_callback_fn(const char* message, void* data);
   *   - void secp256k1_default_error_callback_fn(const char* message, void* data);
   *  The library can call these default handlers even before a proper callback data
   *  pointer could have been set using secp256k1_context_set_illegal_callback or
   *  secp256k1_context_set_illegal_callback, e.g., when the creation of a context
   *  fails. In this case, the corresponding default handler will be called with
   *  the data pointer argument set to NULL.
   *
   *  Args: ctx:  an existing context object (cannot be NULL)
   *  In:   fun:  a pointer to a function to call when an illegal argument is
   *              passed to the API, taking a message and an opaque pointer.
   *              (NULL restores the default handler.)
   *        data: the opaque pointer to pass to fun above.
   *
   *  See also secp256k1_context_set_error_callback.
   */
  SECP256K1_API void secp256k1_context_set_illegal_callback(
      secp256k1_context* ctx,
      void (*fun)(const char* message, void* data),
      const void* data
  ) SECP256K1_ARG_NONNULL(1);
  
  /** Set a callback function to be called when an internal consistency check
   *  fails. The default is crashing.
   *
   *  This can only trigger in case of a hardware failure, miscompilation,
   *  memory corruption, serious bug in the library, or other error would can
   *  otherwise result in undefined behaviour. It will not trigger due to mere
   *  incorrect usage of the API (see secp256k1_context_set_illegal_callback
   *  for that). After this callback returns, anything may happen, including
   *  crashing.
   *
   *  Args: ctx:  an existing context object (cannot be NULL)
   *  In:   fun:  a pointer to a function to call when an internal error occurs,
   *              taking a message and an opaque pointer (NULL restores the
   *              default handler, see secp256k1_context_set_illegal_callback
   *              for details).
   *        data: the opaque pointer to pass to fun above.
   *
   *  See also secp256k1_context_set_illegal_callback.
   */
  SECP256K1_API void secp256k1_context_set_error_callback(
      secp256k1_context* ctx,
      void (*fun)(const char* message, void* data),
      const void* data
  ) SECP256K1_ARG_NONNULL(1);
  
  /** Create a secp256k1 scratch space object.
   *
   *  Returns: a newly created scratch space.
   *  Args: ctx:  an existing context object (cannot be NULL)
   *  In:   size: amount of memory to be available as scratch space. Some extra
   *              (<100 bytes) will be allocated for extra accounting.
   */
  SECP256K1_API SECP256K1_WARN_UNUSED_RESULT secp256k1_scratch_space* secp256k1_scratch_space_create(
      const secp256k1_context* ctx,
      size_t size
  ) SECP256K1_ARG_NONNULL(1);
  
  /** Destroy a secp256k1 scratch space.
   *
   *  The pointer may not be used afterwards.
   *  Args:       ctx: a secp256k1 context object.
   *          scratch: space to destroy
   */
  SECP256K1_API void secp256k1_scratch_space_destroy(
      const secp256k1_context* ctx,
      secp256k1_scratch_space* scratch
  ) SECP256K1_ARG_NONNULL(1);
  
  /** Parse a variable-length public key into the pubkey object.
   *
   *  Returns: 1 if the public key was fully valid.
   *           0 if the public key could not be parsed or is invalid.
   *  Args: ctx:      a secp256k1 context object.
   *  Out:  pubkey:   pointer to a pubkey object. If 1 is returned, it is set to a
   *                  parsed version of input. If not, its value is undefined.
   *  In:   input:    pointer to a serialized public key
   *        inputlen: length of the array pointed to by input
   *
   *  This function supports parsing compressed (33 bytes, header byte 0x02 or
   *  0x03), uncompressed (65 bytes, header byte 0x04), or hybrid (65 bytes, header
   *  byte 0x06 or 0x07) format public keys.
   */
  SECP256K1_API SECP256K1_WARN_UNUSED_RESULT int secp256k1_ec_pubkey_parse(
      const secp256k1_context* ctx,
      secp256k1_pubkey* pubkey,
      const unsigned char *input,
      size_t inputlen
  ) SECP256K1_ARG_NONNULL(1) SECP256K1_ARG_NONNULL(2) SECP256K1_ARG_NONNULL(3);
  
  /** Serialize a pubkey object into a serialized byte sequence.
   *
   *  Returns: 1 always.
   *  Args:   ctx:        a secp256k1 context object.
   *  Out:    output:     a pointer to a 65-byte (if compressed==0) or 33-byte (if
   *                      compressed==1) byte array to place the serialized key
   *                      in.
   *  In/Out: outputlen:  a pointer to an integer which is initially set to the
   *                      size of output, and is overwritten with the written
   *                      size.
   *  In:     pubkey:     a pointer to a secp256k1_pubkey containing an
   *                      initialized public key.
   *          flags:      SECP256K1_EC_COMPRESSED if serialization should be in
   *                      compressed format, otherwise SECP256K1_EC_UNCOMPRESSED.
   */
  SECP256K1_API int secp256k1_ec_pubkey_serialize(
      const secp256k1_context* ctx,
      unsigned char *output,
      size_t *outputlen,
      const secp256k1_pubkey* pubkey,
      unsigned int flags
  ) SECP256K1_ARG_NONNULL(1) SECP256K1_ARG_NONNULL(2) SECP256K1_ARG_NONNULL(3) SECP256K1_ARG_NONNULL(4);
  
  /** Parse an ECDSA signature in compact (64 bytes) format.
   *
   *  Returns: 1 when the signature could be parsed, 0 otherwise.
   *  Args: ctx:      a secp256k1 context object
   *  Out:  sig:      a pointer to a signature object
   *  In:   input64:  a pointer to the 64-byte array to parse
   *
   *  The signature must consist of a 32-byte big endian R value, followed by a
   *  32-byte big endian S value. If R or S fall outside of [0..order-1], the
   *  encoding is invalid. R and S with value 0 are allowed in the encoding.
   *
   *  After the call, sig will always be initialized. If parsing failed or R or
   *  S are zero, the resulting sig value is guaranteed to fail validation for any
   *  message and public key.
   */
  SECP256K1_API int secp256k1_ecdsa_signature_parse_compact(
      const secp256k1_context* ctx,
      secp256k1_ecdsa_signature* sig,
      const unsigned char *input64
  ) SECP256K1_ARG_NONNULL(1) SECP256K1_ARG_NONNULL(2) SECP256K1_ARG_NONNULL(3);
  
  /** Parse a DER ECDSA signature.
   *
   *  Returns: 1 when the signature could be parsed, 0 otherwise.
   *  Args: ctx:      a secp256k1 context object
   *  Out:  sig:      a pointer to a signature object
   *  In:   input:    a pointer to the signature to be parsed
   *        inputlen: the length of the array pointed to be input
   *
   *  This function will accept any valid DER encoded signature, even if the
   *  encoded numbers are out of range.
   *
   *  After the call, sig will always be initialized. If parsing failed or the
   *  encoded numbers are out of range, signature validation with it is
   *  guaranteed to fail for every message and public key.
   */
  SECP256K1_API int secp256k1_ecdsa_signature_parse_der(
      const secp256k1_context* ctx,
      secp256k1_ecdsa_signature* sig,
      const unsigned char *input,
      size_t inputlen
  ) SECP256K1_ARG_NONNULL(1) SECP256K1_ARG_NONNULL(2) SECP256K1_ARG_NONNULL(3);
  
  /** Serialize an ECDSA signature in DER format.
   *
   *  Returns: 1 if enough space was available to serialize, 0 otherwise
   *  Args:   ctx:       a secp256k1 context object
   *  Out:    output:    a pointer to an array to store the DER serialization
   *  In/Out: outputlen: a pointer to a length integer. Initially, this integer
   *                     should be set to the length of output. After the call
   *                     it will be set to the length of the serialization (even
   *                     if 0 was returned).
   *  In:     sig:       a pointer to an initialized signature object
   */
  SECP256K1_API int secp256k1_ecdsa_signature_serialize_der(
      const secp256k1_context* ctx,
      unsigned char *output,
      size_t *outputlen,
      const secp256k1_ecdsa_signature* sig
  ) SECP256K1_ARG_NONNULL(1) SECP256K1_ARG_NONNULL(2) SECP256K1_ARG_NONNULL(3) SECP256K1_ARG_NONNULL(4);
  
  /** Serialize an ECDSA signature in compact (64 byte) format.
   *
   *  Returns: 1
   *  Args:   ctx:       a secp256k1 context object
   *  Out:    output64:  a pointer to a 64-byte array to store the compact serialization
   *  In:     sig:       a pointer to an initialized signature object
   *
   *  See secp256k1_ecdsa_signature_parse_compact for details about the encoding.
   */
  SECP256K1_API int secp256k1_ecdsa_signature_serialize_compact(
      const secp256k1_context* ctx,
      unsigned char *output64,
      const secp256k1_ecdsa_signature* sig
  ) SECP256K1_ARG_NONNULL(1) SECP256K1_ARG_NONNULL(2) SECP256K1_ARG_NONNULL(3);
  
  /** Verify an ECDSA signature.
   *
   *  Returns: 1: correct signature
   *           0: incorrect or unparseable signature
   *  Args:    ctx:       a secp256k1 context object, initialized for verification.
   *  In:      sig:       the signature being verified (cannot be NULL)
   *           msg32:     the 32-byte message hash being verified (cannot be NULL)
   *           pubkey:    pointer to an initialized public key to verify with (cannot be NULL)
   *
   * To avoid accepting malleable signatures, only ECDSA signatures in lower-S
   * form are accepted.
   *
   * If you need to accept ECDSA signatures from sources that do not obey this
   * rule, apply secp256k1_ecdsa_signature_normalize to the signature prior to
   * validation, but be aware that doing so results in malleable signatures.
   *
   * For details, see the comments for that function.
   */
  SECP256K1_API SECP256K1_WARN_UNUSED_RESULT int secp256k1_ecdsa_verify(
      const secp256k1_context* ctx,
      const secp256k1_ecdsa_signature *sig,
      const unsigned char *msg32,
      const secp256k1_pubkey *pubkey
  ) SECP256K1_ARG_NONNULL(1) SECP256K1_ARG_NONNULL(2) SECP256K1_ARG_NONNULL(3) SECP256K1_ARG_NONNULL(4);
  
  /** Convert a signature to a normalized lower-S form.
   *
   *  Returns: 1 if sigin was not normalized, 0 if it already was.
   *  Args: ctx:    a secp256k1 context object
   *  Out:  sigout: a pointer to a signature to fill with the normalized form,
   *                or copy if the input was already normalized. (can be NULL if
   *                you're only interested in whether the input was already
   *                normalized).
   *  In:   sigin:  a pointer to a signature to check/normalize (cannot be NULL,
   *                can be identical to sigout)
   *
   *  With ECDSA a third-party can forge a second distinct signature of the same
   *  message, given a single initial signature, but without knowing the key. This
   *  is done by negating the S value modulo the order of the curve, 'flipping'
   *  the sign of the random point R which is not included in the signature.
   *
   *  Forgery of the same message isn't universally problematic, but in systems
   *  where message malleability or uniqueness of signatures is important this can
   *  cause issues. This forgery can be blocked by all verifiers forcing signers
   *  to use a normalized form.
   *
   *  The lower-S form reduces the size of signatures slightly on average when
   *  variable length encodings (such as DER) are used and is cheap to verify,
   *  making it a good choice. Security of always using lower-S is assured because
   *  anyone can trivially modify a signature after the fact to enforce this
   *  property anyway.
   *
   *  The lower S value is always between 0x1 and
   *  0x7FFFFFFFFFFFFFFFFFFFFFFFFFFFFFFF5D576E7357A4501DDFE92F46681B20A0,
   *  inclusive.
   *
   *  No other forms of ECDSA malleability are known and none seem likely, but
   *  there is no formal proof that ECDSA, even with this additional restriction,
   *  is free of other malleability. Commonly used serialization schemes will also
   *  accept various non-unique encodings, so care should be taken when this
   *  property is required for an application.
   *
   *  The secp256k1_ecdsa_sign function will by default create signatures in the
   *  lower-S form, and secp256k1_ecdsa_verify will not accept others. In case
   *  signatures come from a system that cannot enforce this property,
   *  secp256k1_ecdsa_signature_normalize must be called before verification.
   */
  SECP256K1_API int secp256k1_ecdsa_signature_normalize(
      const secp256k1_context* ctx,
      secp256k1_ecdsa_signature *sigout,
      const secp256k1_ecdsa_signature *sigin
  ) SECP256K1_ARG_NONNULL(1) SECP256K1_ARG_NONNULL(3);
  
  /** An implementation of RFC6979 (using HMAC-SHA256) as nonce generation function.
   * If a data pointer is passed, it is assumed to be a pointer to 32 bytes of
   * extra entropy.
   */
  SECP256K1_API extern const secp256k1_nonce_function secp256k1_nonce_function_rfc6979;
  
  /** An implementation of the nonce generation function as defined in BIP-schnorr.
   *
   * If a data pointer is passed, it is assumed to be a pointer to 32 bytes of
   * extra entropy. If the data pointer is NULL and this function is used in
   * schnorrsig_sign, it produces BIP-schnorr compliant signatures.
   * When this function is used in ecdsa_sign, it generates a nonce using an
   * analogue of the bip-schnorr nonce generation algorithm, but with tag
   * "BIPSchnorrNULL" instead of "BIPSchnorrDerive".
   * The attempt argument must be 0 or the function will fail and return 0.
   */
  SECP256K1_API extern const secp256k1_nonce_function secp256k1_nonce_function_bipschnorr;
  
  /** A default safe nonce generation function (currently equal to secp256k1_nonce_function_rfc6979). */
  SECP256K1_API extern const secp256k1_nonce_function secp256k1_nonce_function_default;
  
  /** Create an ECDSA signature.
   *
   *  Returns: 1: signature created
   *           0: the nonce generation function failed, or the private key was invalid.
   *  Args:    ctx:    pointer to a context object, initialized for signing (cannot be NULL)
   *  Out:     sig:    pointer to an array where the signature will be placed (cannot be NULL)
   *  In:      msg32:  the 32-byte message hash being signed (cannot be NULL)
   *           seckey: pointer to a 32-byte secret key (cannot be NULL)
   *           noncefp:pointer to a nonce generation function. If NULL, secp256k1_nonce_function_default is used
   *           ndata:  pointer to arbitrary data used by the nonce generation function (can be NULL)
   *
   * The created signature is always in lower-S form. See
   * secp256k1_ecdsa_signature_normalize for more details.
   */
  SECP256K1_API int secp256k1_ecdsa_sign(
      const secp256k1_context* ctx,
      secp256k1_ecdsa_signature *sig,
      const unsigned char *msg32,
      const unsigned char *seckey,
      secp256k1_nonce_function noncefp,
      const void *ndata
  ) SECP256K1_ARG_NONNULL(1) SECP256K1_ARG_NONNULL(2) SECP256K1_ARG_NONNULL(3) SECP256K1_ARG_NONNULL(4);
  
  /** Verify an ECDSA secret key.
   *
   *  Returns: 1: secret key is valid
   *           0: secret key is invalid
   *  Args:    ctx: pointer to a context object (cannot be NULL)
   *  In:      seckey: pointer to a 32-byte secret key (cannot be NULL)
   */
  SECP256K1_API SECP256K1_WARN_UNUSED_RESULT int secp256k1_ec_seckey_verify(
      const secp256k1_context* ctx,
      const unsigned char *seckey
  ) SECP256K1_ARG_NONNULL(1) SECP256K1_ARG_NONNULL(2);
  
  /** Compute the public key for a secret key.
   *
   *  Returns: 1: secret was valid, public key stores
   *           0: secret was invalid, try again
   *  Args:   ctx:        pointer to a context object, initialized for signing (cannot be NULL)
   *  Out:    pubkey:     pointer to the created public key (cannot be NULL)
   *  In:     seckey:     pointer to a 32-byte private key (cannot be NULL)
   */
  SECP256K1_API SECP256K1_WARN_UNUSED_RESULT int secp256k1_ec_pubkey_create(
      const secp256k1_context* ctx,
      secp256k1_pubkey *pubkey,
      const unsigned char *seckey
  ) SECP256K1_ARG_NONNULL(1) SECP256K1_ARG_NONNULL(2) SECP256K1_ARG_NONNULL(3);
  
  /** Negates a private key in place.
   *
   *  Returns: 1 always
   *  Args:   ctx:        pointer to a context object
   *  In/Out: seckey:     pointer to the 32-byte private key to be negated (cannot be NULL)
   */
  SECP256K1_API SECP256K1_WARN_UNUSED_RESULT int secp256k1_ec_privkey_negate(
      const secp256k1_context* ctx,
      unsigned char *seckey
  ) SECP256K1_ARG_NONNULL(1) SECP256K1_ARG_NONNULL(2);
  
  /** Negates a public key in place.
   *
   *  Returns: 1 always
   *  Args:   ctx:        pointer to a context object
   *  In/Out: pubkey:     pointer to the public key to be negated (cannot be NULL)
   */
  SECP256K1_API SECP256K1_WARN_UNUSED_RESULT int secp256k1_ec_pubkey_negate(
      const secp256k1_context* ctx,
      secp256k1_pubkey *pubkey
  ) SECP256K1_ARG_NONNULL(1) SECP256K1_ARG_NONNULL(2);
  
  /** Tweak a private key by adding tweak to it.
   * Returns: 0 if the tweak was out of range (chance of around 1 in 2^128 for
   *          uniformly random 32-byte arrays, or if the resulting private key
   *          would be invalid (only when the tweak is the complement of the
   *          private key). 1 otherwise.
   * Args:    ctx:    pointer to a context object (cannot be NULL).
   * In/Out:  seckey: pointer to a 32-byte private key.
   * In:      tweak:  pointer to a 32-byte tweak.
   */
  SECP256K1_API SECP256K1_WARN_UNUSED_RESULT int secp256k1_ec_privkey_tweak_add(
      const secp256k1_context* ctx,
      unsigned char *seckey,
      const unsigned char *tweak
  ) SECP256K1_ARG_NONNULL(1) SECP256K1_ARG_NONNULL(2) SECP256K1_ARG_NONNULL(3);
  
  /** Tweak a public key by adding tweak times the generator to it.
   * Returns: 0 if the tweak was out of range (chance of around 1 in 2^128 for
   *          uniformly random 32-byte arrays, or if the resulting public key
   *          would be invalid (only when the tweak is the complement of the
   *          corresponding private key). 1 otherwise.
   * Args:    ctx:    pointer to a context object initialized for validation
   *                  (cannot be NULL).
   * In/Out:  pubkey: pointer to a public key object.
   * In:      tweak:  pointer to a 32-byte tweak.
   */
  SECP256K1_API SECP256K1_WARN_UNUSED_RESULT int secp256k1_ec_pubkey_tweak_add(
      const secp256k1_context* ctx,
      secp256k1_pubkey *pubkey,
      const unsigned char *tweak
  ) SECP256K1_ARG_NONNULL(1) SECP256K1_ARG_NONNULL(2) SECP256K1_ARG_NONNULL(3);
  
  /** Tweak a private key by multiplying it by a tweak.
   * Returns: 0 if the tweak was out of range (chance of around 1 in 2^128 for
   *          uniformly random 32-byte arrays, or equal to zero. 1 otherwise.
   * Args:   ctx:    pointer to a context object (cannot be NULL).
   * In/Out: seckey: pointer to a 32-byte private key.
   * In:     tweak:  pointer to a 32-byte tweak.
   */
  SECP256K1_API SECP256K1_WARN_UNUSED_RESULT int secp256k1_ec_privkey_tweak_mul(
      const secp256k1_context* ctx,
      unsigned char *seckey,
      const unsigned char *tweak
  ) SECP256K1_ARG_NONNULL(1) SECP256K1_ARG_NONNULL(2) SECP256K1_ARG_NONNULL(3);
  
  /** Tweak a public key by multiplying it by a tweak value.
   * Returns: 0 if the tweak was out of range (chance of around 1 in 2^128 for
   *          uniformly random 32-byte arrays, or equal to zero. 1 otherwise.
   * Args:    ctx:    pointer to a context object initialized for validation
   *                 (cannot be NULL).
   * In/Out:  pubkey: pointer to a public key obkect.
   * In:      tweak:  pointer to a 32-byte tweak.
   */
  SECP256K1_API SECP256K1_WARN_UNUSED_RESULT int secp256k1_ec_pubkey_tweak_mul(
      const secp256k1_context* ctx,
      secp256k1_pubkey *pubkey,
      const unsigned char *tweak
  ) SECP256K1_ARG_NONNULL(1) SECP256K1_ARG_NONNULL(2) SECP256K1_ARG_NONNULL(3);
  
  /** Updates the context randomization to protect against side-channel leakage.
   *  Returns: 1: randomization successfully updated or nothing to randomize
   *           0: error
   *  Args:    ctx:       pointer to a context object (cannot be NULL)
   *  In:      seed32:    pointer to a 32-byte random seed (NULL resets to initial state)
   *
   * While secp256k1 code is written to be constant-time no matter what secret
   * values are, it's possible that a future compiler may output code which isn't,
   * and also that the CPU may not emit the same radio frequencies or draw the same
   * amount power for all values.
   *
   * This function provides a seed which is combined into the blinding value: that
   * blinding value is added before each multiplication (and removed afterwards) so
   * that it does not affect function results, but shields against attacks which
   * rely on any input-dependent behaviour.
   *
   * This function has currently an effect only on contexts initialized for signing
   * because randomization is currently used only for signing. However, this is not
   * guaranteed and may change in the future. It is safe to call this function on
   * contexts not initialized for signing; then it will have no effect and return 1.
   *
   * You should call this after secp256k1_context_create or
   * secp256k1_context_clone (and secp256k1_context_preallocated_create or
   * secp256k1_context_clone, resp.), and you may call this repeatedly afterwards.
   */
  SECP256K1_API SECP256K1_WARN_UNUSED_RESULT int secp256k1_context_randomize(
      secp256k1_context* ctx,
      const unsigned char *seed32
  ) SECP256K1_ARG_NONNULL(1);
  
  /** Add a number of public keys together.
   *  Returns: 1: the sum of the public keys is valid.
   *           0: the sum of the public keys is not valid.
   *  Args:   ctx:        pointer to a context object
   *  Out:    out:        pointer to a public key object for placing the resulting public key
   *                      (cannot be NULL)
   *  In:     ins:        pointer to array of pointers to public keys (cannot be NULL)
   *          n:          the number of public keys to add together (must be at least 1)
   */
  SECP256K1_API SECP256K1_WARN_UNUSED_RESULT int secp256k1_ec_pubkey_combine(
      const secp256k1_context* ctx,
      secp256k1_pubkey *out,
      const secp256k1_pubkey * const * ins,
      size_t n
  ) SECP256K1_ARG_NONNULL(2) SECP256K1_ARG_NONNULL(3);
  
  /** Opaque data structure that holds a parsed and valid "x-only" public key.
   *  An x-only pubkey encodes a point whose Y coordinate is square. It is
   *  serialized using only its X coordinate (32 bytes). See bip-schnorr for more
   *  information about x-only pubkeys.
   *
   *  The exact representation of data inside is implementation defined and not
   *  guaranteed to be portable between different platforms or versions. It is
   *  however guaranteed to be 64 bytes in size, and can be safely copied/moved.
   *  If you need to convert to a format suitable for storage, transmission, or
   *  comparison, use secp256k1_xonly_pubkey_serialize and
   *  secp256k1_xonly_pubkey_parse.
   */
  typedef struct {
      unsigned char data[64];
  } secp256k1_xonly_pubkey;
  
  /** Parse a 32-byte public key into a xonly_pubkey object.
   *
   *  Returns: 1 if the public key was fully valid.
   *           0 if the public key could not be parsed or is invalid.
   *
   *  Args:   ctx: a secp256k1 context object.
   *  Out: pubkey: pointer to a pubkey object. If 1 is returned, it is set to a
   *               parsed version of input. If not, its value is undefined.
   *  In: input32: pointer to a serialized xonly public key
   */
  SECP256K1_API SECP256K1_WARN_UNUSED_RESULT int secp256k1_xonly_pubkey_parse(
      const secp256k1_context* ctx,
      secp256k1_xonly_pubkey* pubkey,
      const unsigned char *input32
  ) SECP256K1_ARG_NONNULL(1) SECP256K1_ARG_NONNULL(2) SECP256K1_ARG_NONNULL(3);
  
  /** Serialize an xonly_pubkey object into a 32-byte sequence. Use the
   *  SECP256K1_LEN_XONLY_PUBKEY macro if you want to avoid the magic number 32.
   *
   *  Returns: 1 always.
   *
   *  Args:     ctx: a secp256k1 context object.
   *  Out: output32: a pointer to a 32-byte array to place the
   *                 serialized key in.
   *  In:    pubkey: a pointer to a secp256k1_xonly_pubkey containing an
   *                 initialized public key.
   */
  SECP256K1_API int secp256k1_xonly_pubkey_serialize(
      const secp256k1_context* ctx,
      unsigned char *output32,
      const secp256k1_xonly_pubkey* pubkey
  ) SECP256K1_ARG_NONNULL(1) SECP256K1_ARG_NONNULL(2) SECP256K1_ARG_NONNULL(3);
  
  /** Compute the xonly public key for a secret key. Same as ec_pubkey_create, but
   *  for xonly public keys.
   *
   *  Returns: 1 if secret was valid, public key stores
   *           0 if secret was invalid, try again
   *
   *  Args:   ctx: pointer to a context object, initialized for signing (cannot be NULL)
   *  Out: pubkey: pointer to the created xonly public key (cannot be NULL)
   *  In:  seckey: pointer to a 32-byte private key (cannot be NULL)
   */
  SECP256K1_API SECP256K1_WARN_UNUSED_RESULT int secp256k1_xonly_pubkey_create(
      const secp256k1_context* ctx,
      secp256k1_xonly_pubkey *pubkey,
      const unsigned char *seckey
  ) SECP256K1_ARG_NONNULL(1) SECP256K1_ARG_NONNULL(2) SECP256K1_ARG_NONNULL(3);
  
  /** Converts a secp256k1_pubkey into a secp256k1_xonly_pubkey.
   *
   *  Returns: 1 if the public key was successfully converted
   *           0 otherwise
   *
   *  Args:         ctx: pointer to a context object
   *  Out: xonly_pubkey: pointer to an x-only public key object for placing the
   *                     converted public key (cannot be NULL)
   *         is_negated: pointer to an integer that will be set to 1 if the point
   *                     encoded by `xonly_pubkey` is the negation of `pubkey`
   *                     and set to 0 otherwise. (can be NULL)
   *  In:       pubkey: pointer to a public key that is converted (cannot be
   *                    NULL)
   */
  SECP256K1_API SECP256K1_WARN_UNUSED_RESULT int secp256k1_xonly_pubkey_from_pubkey(
      const secp256k1_context* ctx,
      secp256k1_xonly_pubkey *xonly_pubkey,
      int *is_negated,
      const secp256k1_pubkey *pubkey
  ) SECP256K1_ARG_NONNULL(1) SECP256K1_ARG_NONNULL(2) SECP256K1_ARG_NONNULL(4);
  
  /** Tweak the private key of an x-only pubkey by adding a tweak to it. The public
   *  key of the resulting private key will be the same as the output of
   *  secp256k1_xonly_pubkey_tweak_add called with the same tweak and corresponding
   *  input public key.
   *
   *  If the public key corresponds to a point with square Y, tweak32 is added to
   *  the seckey (modulo the group order). Otherwise, tweak32 is added to the
   *  negation of the seckey (modulo the group order).
   *
   *  Returns: 1 if the tweak was successfully added to seckey
   *           0 if the tweak was out of range or the resulting private key would be
   *             invalid (only when the tweak is the complement of the private key) or
   *             seckey is 0.
   *
   *  Args:      ctx: pointer to a context object, initialized for signing (cannot be NULL)
   *  In/Out: seckey: pointer to a 32-byte private key
   *  In:    tweak32: pointer to a 32-byte tweak
   */
  SECP256K1_API SECP256K1_WARN_UNUSED_RESULT int secp256k1_xonly_privkey_tweak_add(
      const secp256k1_context* ctx,
      unsigned char *seckey,
      const unsigned char *tweak32
  ) SECP256K1_ARG_NONNULL(1) SECP256K1_ARG_NONNULL(2) SECP256K1_ARG_NONNULL(3);
  
  /** Tweak an x-only public key by adding tweak times the generator to it.
   *
   *  Because the resulting point may have a non-square Y coordinate, it may not
   *  be representable by an x-only pubkey. Instead, `output_pubkey` will be set
   *  to the negation of that point. Therefore this function outputs `is_negated`
   *  which is required for `xonly_pubkey_tweak_test`.
   *
   *  Returns: 1 if tweak times the generator was successfully added to pubkey
   *           0 if the tweak was out of range or the resulting public key would be
   *             invalid (only when the tweak is the complement of the corresponding
   *             private key).
   *
   *  Args:           ctx: pointer to a context object initialized for validation
   *                       (cannot be NULL)
   *  Out:  output_pubkey: pointer to a public key object (cannot be NULL)
   *           is_negated: pointer to an integer that will be set to 1 if
   *                       `output_pubkey` is the negation of the point that
   *                       resulted from adding the tweak. (cannot be NULL)
   *  In: internal_pubkey: pointer to an x-only public key object to apply the
   *                       tweak to (cannot be NULL)
   *              tweak32: pointer to a 32-byte tweak (cannot be NULL)
   */
  SECP256K1_API SECP256K1_WARN_UNUSED_RESULT int secp256k1_xonly_pubkey_tweak_add(
      const secp256k1_context* ctx,
      secp256k1_xonly_pubkey *output_pubkey,
      int *is_negated,
      const secp256k1_xonly_pubkey *internal_pubkey,
      const unsigned char *tweak32
  ) SECP256K1_ARG_NONNULL(1) SECP256K1_ARG_NONNULL(2) SECP256K1_ARG_NONNULL(3) SECP256K1_ARG_NONNULL(4) SECP256K1_ARG_NONNULL(5);
  
  /** Tests that output_pubkey and is_negated are the result of calling
   *  secp256k1_xonly_pubkey_tweak_add with internal_pubkey and tweak32. Note
   *  that this alone does _not_ verify that output_pubkey is a commitment. If the
   *  tweak is not chosen in a specific way, the output_pubkey can easily be the
   *  result of a different internal_pubkey and tweak.
   *
   *  Returns: 1 if output_pubkey is the result of tweaking the internal_pubkey with
   *             tweak32
   *           0 otherwise
   *
   *  Args:           ctx: pointer to a context object initialized for validation
   *                       (cannot be NULL)
   *  In:   output_pubkey: pointer to a public key object (cannot be NULL)
   *           is_negated: 1 if `output_pubkey is the negation of the point that
   *                       resulted from adding the tweak and 0 otherwise.
   *      internal_pubkey: pointer to an x-only public key object to apply the
   *                       tweak to (cannot be NULL)
   *              tweak32: pointer to a 32-byte tweak (cannot be NULL)
   */
  SECP256K1_API SECP256K1_WARN_UNUSED_RESULT int secp256k1_xonly_pubkey_tweak_test(
      const secp256k1_context* ctx,
      const secp256k1_xonly_pubkey *output_pubkey,
      int is_negated,
      const secp256k1_xonly_pubkey *internal_pubkey,
      const unsigned char *tweak32
  ) SECP256K1_ARG_NONNULL(1) SECP256K1_ARG_NONNULL(2) SECP256K1_ARG_NONNULL(4) SECP256K1_ARG_NONNULL(5);
  
  #ifdef __cplusplus
  }
  #endif
  
  #endif /* SECP256K1_H */
  