secp256k1_musig.h

#ifndef SECP256K1_MUSIG_H
#define SECP256K1_MUSIG_H

#include "secp256k1_extrakeys.h"

#ifdef __cplusplus
extern "C" {
#endif

#include <stddef.h>

/** This module implements a Schnorr-based multi-signature scheme called MuSig2
 * (https://eprint.iacr.org/2020/1261, see Appendix B for the exact variant).
 * Signatures are compatible with BIP-340 ("Schnorr").
 * There's an example C source file in the module's directory
 * (examples/musig.c) that demonstrates how it can be used.
 *
 * The module also supports BIP-341 ("Taproot") public key tweaking and adaptor
 * signatures as described in
 * https://github.com/ElementsProject/scriptless-scripts/pull/24.
 *
 * It is recommended to read the documentation in this include file carefully.
 * Further notes on API usage can be found in src/modules/musig/musig.md
 *
 * You may know that the MuSig2 scheme uses two "nonces" instead of one. This
 * is not wrong, but only a technical detail we don't want to bother the user
 * with. Therefore, the API only uses the singular term "nonce".
 *
 * Since the first version of MuSig is essentially replaced by MuSig2, when
 * writing MuSig or musig here we mean MuSig2.
 */

/** Opaque data structures
 *
 *  The exact representation of data inside is implementation defined and not
 *  guaranteed to be portable between different platforms or versions. If you
 *  need to convert to a format suitable for storage, transmission, or
 *  comparison, use the corresponding serialization and parsing functions.
 */

/** Opaque data structure that caches information about public key aggregation.
 *
 *  Guaranteed to be 165 bytes in size. It can be safely copied/moved. No
 *  serialization and parsing functions (yet).
 */
typedef struct {
    unsigned char data[165];
} secp256k1_musig_keyagg_cache;

/** Opaque data structure that holds a signer's _secret_ nonce.
 *
 *  Guaranteed to be 68 bytes in size.
 *
 *  WARNING: This structure MUST NOT be copied or read or written to directly. A
 *  signer who is online throughout the whole process and can keep this
 *  structure in memory can use the provided API functions for a safe standard
 *  workflow. See
 *  https://blockstream.com/2019/02/18/musig-a-new-multisignature-standard/ for
 *  more details about the risks associated with serializing or deserializing
 *  this structure.
 *
 *  We repeat, copying this data structure can result in nonce reuse which will
 *  leak the secret signing key.
 */
typedef struct {
    unsigned char data[68];
} secp256k1_musig_secnonce;

/** Opaque data structure that holds a signer's public nonce.
*
*  Guaranteed to be 132 bytes in size. It can be safely copied/moved. Serialized
*  and parsed with `musig_pubnonce_serialize` and `musig_pubnonce_parse`.
*/
typedef struct {
    unsigned char data[132];
} secp256k1_musig_pubnonce;

/** Opaque data structure that holds an aggregate public nonce.
 *
 *  Guaranteed to be 132 bytes in size. It can be safely copied/moved.
 *  Serialized and parsed with `musig_aggnonce_serialize` and
 *  `musig_aggnonce_parse`.
 */
typedef struct {
    unsigned char data[132];
} secp256k1_musig_aggnonce;

/** Opaque data structure that holds a MuSig session.
 *
 *  This structure is not required to be kept secret for the signing protocol to
 *  be secure. Guaranteed to be 133 bytes in size. It can be safely
 *  copied/moved. No serialization and parsing functions (yet).
 */
typedef struct {
    unsigned char data[133];
} secp256k1_musig_session;

/** Opaque data structure that holds a partial MuSig signature.
 *
 *  Guaranteed to be 36 bytes in size. Serialized and parsed with
 *  `musig_partial_sig_serialize` and `musig_partial_sig_parse`.
 */
typedef struct {
    unsigned char data[36];
} secp256k1_musig_partial_sig;

/** Parse a signer's public nonce.
 *
 *  Returns: 1 when the nonce could be parsed, 0 otherwise.
 *  Args:    ctx: a secp256k1 context object
 *  Out:   nonce: pointer to a nonce object
 *  In:     in66: pointer to the 66-byte nonce to be parsed
 */
SECP256K1_API int secp256k1_musig_pubnonce_parse(
    const secp256k1_context* ctx,
    secp256k1_musig_pubnonce* nonce,
    const unsigned char *in66
) SECP256K1_ARG_NONNULL(1) SECP256K1_ARG_NONNULL(2) SECP256K1_ARG_NONNULL(3);

/** Serialize a signer's public nonce
 *
 *  Returns: 1 when the nonce could be serialized, 0 otherwise
 *  Args:    ctx: a secp256k1 context object
 *  Out:   out66: pointer to a 66-byte array to store the serialized nonce
 *  In:    nonce: pointer to the nonce
 */
SECP256K1_API int secp256k1_musig_pubnonce_serialize(
    const secp256k1_context* ctx,
    unsigned char *out66,
    const secp256k1_musig_pubnonce* nonce
) SECP256K1_ARG_NONNULL(1) SECP256K1_ARG_NONNULL(2) SECP256K1_ARG_NONNULL(3);

/** Parse an aggregate public nonce.
 *
 *  Returns: 1 when the nonce could be parsed, 0 otherwise.
 *  Args:    ctx: a secp256k1 context object
 *  Out:   nonce: pointer to a nonce object
 *  In:     in66: pointer to the 66-byte nonce to be parsed
 */
SECP256K1_API int secp256k1_musig_aggnonce_parse(
    const secp256k1_context* ctx,
    secp256k1_musig_aggnonce* nonce,
    const unsigned char *in66
) SECP256K1_ARG_NONNULL(1) SECP256K1_ARG_NONNULL(2) SECP256K1_ARG_NONNULL(3);

/** Serialize an aggregate public nonce
 *
 *  Returns: 1 when the nonce could be serialized, 0 otherwise
 *  Args:    ctx: a secp256k1 context object
 *  Out:   out66: pointer to a 66-byte array to store the serialized nonce
 *  In:    nonce: pointer to the nonce
 */
SECP256K1_API int secp256k1_musig_aggnonce_serialize(
    const secp256k1_context* ctx,
    unsigned char *out66,
    const secp256k1_musig_aggnonce* nonce
) SECP256K1_ARG_NONNULL(1) SECP256K1_ARG_NONNULL(2) SECP256K1_ARG_NONNULL(3);

/** Serialize a MuSig partial signature
 *
 *  Returns: 1 when the signature could be serialized, 0 otherwise
 *  Args:    ctx: a secp256k1 context object
 *  Out:   out32: pointer to a 32-byte array to store the serialized signature
 *  In:      sig: pointer to the signature
 */
SECP256K1_API int secp256k1_musig_partial_sig_serialize(
    const secp256k1_context* ctx,
    unsigned char *out32,
    const secp256k1_musig_partial_sig* sig
) SECP256K1_ARG_NONNULL(1) SECP256K1_ARG_NONNULL(2) SECP256K1_ARG_NONNULL(3);

/** Parse a MuSig partial signature.
 *
 *  Returns: 1 when the signature could be parsed, 0 otherwise.
 *  Args:    ctx: a secp256k1 context object
 *  Out:     sig: pointer to a signature object
 *  In:     in32: pointer to the 32-byte signature to be parsed
 *
 *  After the call, sig will always be initialized. If parsing failed or the
 *  encoded numbers are out of range, signature verification with it is
 *  guaranteed to fail for every message and public key.
 */
SECP256K1_API int secp256k1_musig_partial_sig_parse(
    const secp256k1_context* ctx,
    secp256k1_musig_partial_sig* sig,
    const unsigned char *in32
) SECP256K1_ARG_NONNULL(1) SECP256K1_ARG_NONNULL(2) SECP256K1_ARG_NONNULL(3);

/** Computes an aggregate public key and uses it to initialize a keyagg_cache
 *
 *  Different orders of `pubkeys` result in different `agg_pk`s.
 *
 *  The pubkeys can be sorted before combining with `secp256k1_xonly_sort` which
 *  ensures the same `agg_pk` result for the same multiset of pubkeys.
 *  This is useful to do before `pubkey_agg`, such that the order of pubkeys
 *  does not affect the aggregate public key.
 *
 *  Returns: 0 if the arguments are invalid, 1 otherwise
 *  Args:        ctx: pointer to a context object initialized for verification
 *           scratch: should be NULL because it is not yet implemented. If it
 *                    was implemented then the scratch space would be used to
 *                    compute the aggregate pubkey by multiexponentiation.
 *                    Generally, the larger the scratch space, the faster this
 *                    function. However, the returns of providing a larger
 *                    scratch space are diminishing. If NULL, an inefficient
 *                    algorithm is used.
 *  Out:      agg_pk: the MuSig-aggregated x-only public key. If you do not need it,
 *                    this arg can be NULL.
 *      keyagg_cache: if non-NULL, pointer to a musig_keyagg_cache struct that
 *                    is required for signing (or observing the signing session
 *                    and verifying partial signatures).
 *   In:     pubkeys: input array of pointers to public keys to aggregate. The order
 *                    is important; a different order will result in a different
 *                    aggregate public key.
 *         n_pubkeys: length of pubkeys array. Must be greater than 0.
 */
SECP256K1_API int secp256k1_musig_pubkey_agg(
    const secp256k1_context* ctx,
    secp256k1_scratch_space *scratch,
    secp256k1_xonly_pubkey *agg_pk,
    secp256k1_musig_keyagg_cache *keyagg_cache,
    const secp256k1_xonly_pubkey * const* pubkeys,
    size_t n_pubkeys
) SECP256K1_ARG_NONNULL(1) SECP256K1_ARG_NONNULL(5);

/** Obtain the aggregate public key from a keyagg_cache.
 *
 *  This is only useful if you need the non-xonly public key, in particular for
 *  ordinary (non-xonly) tweaking or batch-verifying multiple key aggregations
 *  (not implemented).
 *
 *  Returns: 0 if the arguments are invalid, 1 otherwise
 *  Args:        ctx: pointer to a context object
 *  Out:      agg_pk: the MuSig-aggregated public key.
 *  In: keyagg_cache: pointer to a `musig_keyagg_cache` struct initialized by
 *                    `musig_pubkey_agg`
 */
SECP256K1_API SECP256K1_WARN_UNUSED_RESULT int secp256k1_musig_pubkey_get(
    const secp256k1_context* ctx,
    secp256k1_pubkey *agg_pk,
    secp256k1_musig_keyagg_cache *keyagg_cache
) SECP256K1_ARG_NONNULL(1) SECP256K1_ARG_NONNULL(2) SECP256K1_ARG_NONNULL(3);

/** Apply ordinary "EC" tweaking to a public key in a given keyagg_cache by
 *  adding the generator multiplied with `tweak32` to it. This is useful for
 *  deriving child keys from an aggregate public key via BIP32.
 *
 *  The tweaking method is the same as `secp256k1_ec_pubkey_tweak_add`. So after
 *  the following pseudocode buf and buf2 have identical contents (absent
 *  earlier failures).
 *
 *  secp256k1_musig_pubkey_agg(..., keyagg_cache, pubkeys, ...)
 *  secp256k1_musig_pubkey_get(..., agg_pk, keyagg_cache)
 *  secp256k1_musig_pubkey_ec_tweak_add(..., output_pk, tweak32, keyagg_cache)
 *  secp256k1_ec_pubkey_serialize(..., buf, output_pk)
 *  secp256k1_ec_pubkey_tweak_add(..., agg_pk, tweak32)
 *  secp256k1_ec_pubkey_serialize(..., buf2, agg_pk)
 *
 *  This function is required if you want to _sign_ for a tweaked aggregate key.
 *  On the other hand, if you are only computing a public key, but not intending
 *  to create a signature for it, you can just use
 *  `secp256k1_ec_pubkey_tweak_add`.
 *
 *  Returns: 0 if the arguments are invalid or the resulting public key would be
 *           invalid (only when the tweak is the negation of the corresponding
 *           secret key). 1 otherwise.
 *  Args:            ctx: pointer to a context object initialized for verification
 *  Out:   output_pubkey: pointer to a public key to store the result. Will be set
 *                        to an invalid value if this function returns 0. If you
 *                        do not need it, this arg can be NULL.
 *  In/Out: keyagg_cache: pointer to a `musig_keyagg_cache` struct initialized by
 *                       `musig_pubkey_agg`
 *  In:          tweak32: pointer to a 32-byte tweak. If the tweak is invalid
 *                        according to `secp256k1_ec_seckey_verify`, this function
 *                        returns 0. For uniformly random 32-byte arrays the
 *                        chance of being invalid is negligible (around 1 in
 *                        2^128).
 */
SECP256K1_API SECP256K1_WARN_UNUSED_RESULT int secp256k1_musig_pubkey_ec_tweak_add(
    const secp256k1_context* ctx,
    secp256k1_pubkey *output_pubkey,
    secp256k1_musig_keyagg_cache *keyagg_cache,
    const unsigned char *tweak32
) SECP256K1_ARG_NONNULL(1) SECP256K1_ARG_NONNULL(3) SECP256K1_ARG_NONNULL(4);

/** Apply x-only tweaking to a public key in a given keyagg_cache by adding the
 *  generator multiplied with `tweak32` to it. This is useful for creating
 *  Taproot outputs.
 *
 *  The tweaking method is the same as `secp256k1_xonly_pubkey_tweak_add`. So in
 *  the following pseudocode xonly_pubkey_tweak_add_check (absent earlier
 *  failures) returns 1.
 *
 *  secp256k1_musig_pubkey_agg(..., agg_pk, keyagg_cache, pubkeys, ...)
 *  secp256k1_musig_pubkey_xonly_tweak_add(..., output_pk, tweak32, keyagg_cache)
 *  secp256k1_xonly_pubkey_serialize(..., buf, output_pk)
 *  secp256k1_xonly_pubkey_tweak_add_check(..., buf, ..., agg_pk, tweak32)
 *
 *  This function is required if you want to _sign_ for a tweaked aggregate key.
 *  On the other hand, if you are only computing a public key, but not intending
 *  to create a signature for it, you can just use
 *  `secp256k1_xonly_pubkey_tweak_add`.
 *
 *  Returns: 0 if the arguments are invalid or the resulting public key would be
 *           invalid (only when the tweak is the negation of the corresponding
 *           secret key). 1 otherwise.
 *  Args:            ctx: pointer to a context object initialized for verification
 *  Out:   output_pubkey: pointer to a public key to store the result. Will be set
 *                        to an invalid value if this function returns 0. If you
 *                        do not need it, this arg can be NULL.
 *  In/Out: keyagg_cache: pointer to a `musig_keyagg_cache` struct initialized by
 *                       `musig_pubkey_agg`
 *  In:          tweak32: pointer to a 32-byte tweak. If the tweak is invalid
 *                        according to secp256k1_ec_seckey_verify, this function
 *                        returns 0. For uniformly random 32-byte arrays the
 *                        chance of being invalid is negligible (around 1 in
 *                        2^128).
 */
SECP256K1_API SECP256K1_WARN_UNUSED_RESULT int secp256k1_musig_pubkey_xonly_tweak_add(
    const secp256k1_context* ctx,
    secp256k1_pubkey *output_pubkey,
    secp256k1_musig_keyagg_cache *keyagg_cache,
    const unsigned char *tweak32
) SECP256K1_ARG_NONNULL(1) SECP256K1_ARG_NONNULL(3) SECP256K1_ARG_NONNULL(4);

/** Starts a signing session by generating a nonce
 *
 *  This function outputs a secret nonce that will be required for signing and a
 *  corresponding public nonce that is intended to be sent to other signers.
 *
 *  MuSig differs from regular Schnorr signing in that implementers _must_ take
 *  special care to not reuse a nonce. This can be ensured by following these rules:
 *
 *  1. Each call to this function must have a UNIQUE session_id32 that must NOT BE
 *     REUSED in subsequent calls to this function.
 *     If you do not provide a seckey, session_id32 _must_ be UNIFORMLY RANDOM
 *     AND KEPT SECRET (even from other signers). If you do provide a seckey,
 *     session_id32 can instead be a counter (that must never repeat!). However,
 *     it is recommended to always choose session_id32 uniformly at random.
 *  2. If you already know the seckey, message or aggregate public key
 *     cache, they can be optionally provided to derive the nonce and increase
 *     misuse-resistance. The extra_input32 argument can be used to provide
 *     additional data that does not repeat in normal scenarios, such as the
 *     current time.
 *  3. Avoid copying (or serializing) the secnonce. This reduces the possibility
 *     that it is used more than once for signing.
 *
 *  Remember that nonce reuse will leak the secret key!
 *  Note that using the same seckey for multiple MuSig sessions is fine.
 *
 *  Returns: 0 if the arguments are invalid and 1 otherwise
 *  Args:         ctx: pointer to a context object, initialized for signing
 *  Out:     secnonce: pointer to a structure to store the secret nonce
 *           pubnonce: pointer to a structure to store the public nonce
 *  In:  session_id32: a 32-byte session_id32 as explained above. Must be unique to this
 *                     call to secp256k1_musig_nonce_gen and must be uniformly random
 *                     unless you really know what you are doing.
 *             seckey: the 32-byte secret key that will later be used for signing, if
 *                     already known (can be NULL)
 *              msg32: the 32-byte message that will later be signed, if already known
 *                     (can be NULL)
 *       keyagg_cache: pointer to the keyagg_cache that was used to create the aggregate
 *                     (and potentially tweaked) public key if already known
 *                     (can be NULL)
 *      extra_input32: an optional 32-byte array that is input to the nonce
 *                     derivation function (can be NULL)
 */
SECP256K1_API int secp256k1_musig_nonce_gen(
    const secp256k1_context* ctx,
    secp256k1_musig_secnonce *secnonce,
    secp256k1_musig_pubnonce *pubnonce,
    const unsigned char *session_id32,
    const unsigned char *seckey,
    const unsigned char *msg32,
    const secp256k1_musig_keyagg_cache *keyagg_cache,
    const unsigned char *extra_input32
) SECP256K1_ARG_NONNULL(1) SECP256K1_ARG_NONNULL(2) SECP256K1_ARG_NONNULL(3) SECP256K1_ARG_NONNULL(4);

/** Aggregates the nonces of all signers into a single nonce
 *
 *  This can be done by an untrusted party to reduce the communication
 *  between signers. Instead of everyone sending nonces to everyone else, there
 *  can be one party receiving all nonces, aggregating the nonces with this
 *  function and then sending only the aggregate nonce back to the signers.
 *
 *  Returns: 0 if the arguments are invalid, 1 otherwise
 *  Args:           ctx: pointer to a context object
 *  Out:       aggnonce: pointer to an aggregate public nonce object for
 *                       musig_nonce_process
 *  In:       pubnonces: array of pointers to public nonces sent by the
 *                       signers
 *          n_pubnonces: number of elements in the pubnonces array. Must be
 *                       greater than 0.
 */
SECP256K1_API int secp256k1_musig_nonce_agg(
    const secp256k1_context* ctx,
    secp256k1_musig_aggnonce  *aggnonce,
    const secp256k1_musig_pubnonce * const* pubnonces,
    size_t n_pubnonces
) SECP256K1_ARG_NONNULL(1) SECP256K1_ARG_NONNULL(2) SECP256K1_ARG_NONNULL(3);

/** Takes the public nonces of all signers and computes a session that is
 *  required for signing and verification of partial signatures.
 *
 *  If the adaptor argument is non-NULL, then the output of
 *  musig_partial_sig_agg will be a pre-signature which is not a valid Schnorr
 *  signature. In order to create a valid signature, the pre-signature and the
 *  secret adaptor must be provided to `musig_adapt`.
 *
 *  Returns: 0 if the arguments are invalid, 1 otherwise
 *  Args:          ctx: pointer to a context object, initialized for verification
 *  Out:       session: pointer to a struct to store the session
 *  In:       aggnonce: pointer to an aggregate public nonce object that is the
 *                      output of musig_nonce_agg
 *              msg32:  the 32-byte message to sign
 *       keyagg_cache:  pointer to the keyagg_cache that was used to create the
 *                      aggregate (and potentially tweaked) pubkey
 *            adaptor:  optional pointer to an adaptor point encoded as a public
 *                      key if this signing session is part of an adaptor
 *                      signature protocol (can be NULL)
 */
SECP256K1_API SECP256K1_WARN_UNUSED_RESULT int secp256k1_musig_nonce_process(
    const secp256k1_context* ctx,
    secp256k1_musig_session *session,
    const secp256k1_musig_aggnonce  *aggnonce,
    const unsigned char *msg32,
    const secp256k1_musig_keyagg_cache *keyagg_cache,
    const secp256k1_pubkey *adaptor
) SECP256K1_ARG_NONNULL(1) SECP256K1_ARG_NONNULL(2) SECP256K1_ARG_NONNULL(3) SECP256K1_ARG_NONNULL(4) SECP256K1_ARG_NONNULL(5);

/** Produces a partial signature
 *
 *  This function overwrites the given secnonce with zeros and will abort if given a
 *  secnonce that is all zeros. This is a best effort attempt to protect against nonce
 *  reuse. However, this is of course easily defeated if the secnonce has been
 *  copied (or serialized). Remember that nonce reuse will leak the secret key!
 *
 *  Returns: 0 if the arguments are invalid or the provided secnonce has already
 *           been used for signing, 1 otherwise
 *  Args:         ctx: pointer to a context object
 *  Out:  partial_sig: pointer to struct to store the partial signature
 *  In/Out:  secnonce: pointer to the secnonce struct created in
 *                     musig_nonce_gen that has been never used in a
 *                     partial_sign call before
 *  In:       keypair: pointer to keypair to sign the message with
 *       keyagg_cache: pointer to the keyagg_cache that was output when the
 *                     aggregate public key for this session
 *            session: pointer to the session that was created with
 *                     musig_nonce_process
 */
SECP256K1_API int secp256k1_musig_partial_sign(
    const secp256k1_context* ctx,
    secp256k1_musig_partial_sig *partial_sig,
    secp256k1_musig_secnonce *secnonce,
    const secp256k1_keypair *keypair,
    const secp256k1_musig_keyagg_cache *keyagg_cache,
    const secp256k1_musig_session *session
) SECP256K1_ARG_NONNULL(1) SECP256K1_ARG_NONNULL(2) SECP256K1_ARG_NONNULL(3) SECP256K1_ARG_NONNULL(4) SECP256K1_ARG_NONNULL(5) SECP256K1_ARG_NONNULL(6);

/** Verifies an individual signer's partial signature
 *
 *  The signature is verified for a specific signing session. In order to avoid
 *  accidentally verifying a signature from a different or non-existing signing
 *  session, you must ensure the following:
 *    1. The `keyagg_cache` argument is identical to the one used to create the
 *       `session` with `musig_nonce_process`.
 *    2. The `pubkey` argument must be identical to the one sent by the signer
 *       before aggregating it with `musig_pubkey_agg` to create the
 *       `keyagg_cache`.
 *    3. The `pubnonce` argument must be identical to the one sent by the signer
 *       before aggregating it with `musig_nonce_agg` and using the result to
 *       create the `session` with `musig_nonce_process`.
 *
 *  This function is essential when using protocols with adaptor signatures.
 *  However, it is not essential for regular MuSig sessions, in the sense that if any
 *  partial signature does not verify, the full signature will not verify either, so the
 *  problem will be caught. But this function allows determining the specific party
 *  who produced an invalid signature.
 *
 *  Returns: 0 if the arguments are invalid or the partial signature does not
 *           verify, 1 otherwise
 *  Args         ctx: pointer to a context object, initialized for verification
 *  In:  partial_sig: pointer to partial signature to verify, sent by
 *                    the signer associated with `pubnonce` and `pubkey`
 *          pubnonce: public nonce of the signer in the signing session
 *            pubkey: public key of the signer in the signing session
 *      keyagg_cache: pointer to the keyagg_cache that was output when the
 *                    aggregate public key for this signing session
 *           session: pointer to the session that was created with
 *                    `musig_nonce_process`
 */
SECP256K1_API SECP256K1_WARN_UNUSED_RESULT int secp256k1_musig_partial_sig_verify(
    const secp256k1_context* ctx,
    const secp256k1_musig_partial_sig *partial_sig,
    const secp256k1_musig_pubnonce *pubnonce,
    const secp256k1_xonly_pubkey *pubkey,
    const secp256k1_musig_keyagg_cache *keyagg_cache,
    const secp256k1_musig_session *session
) SECP256K1_ARG_NONNULL(1) SECP256K1_ARG_NONNULL(2) SECP256K1_ARG_NONNULL(3) SECP256K1_ARG_NONNULL(4) SECP256K1_ARG_NONNULL(5) SECP256K1_ARG_NONNULL(6);

/** Aggregates partial signatures
 *
 *  Returns: 0 if the arguments are invalid, 1 otherwise (which does NOT mean
 *           the resulting signature verifies).
 *  Args:         ctx: pointer to a context object
 *  Out:        sig64: complete (but possibly invalid) Schnorr signature
 *  In:       session: pointer to the session that was created with
 *                     musig_nonce_process
 *       partial_sigs: array of pointers to partial signatures to aggregate
 *             n_sigs: number of elements in the partial_sigs array. Must be
 *                     greater than 0.
 */
SECP256K1_API int secp256k1_musig_partial_sig_agg(
    const secp256k1_context* ctx,
    unsigned char *sig64,
    const secp256k1_musig_session *session,
    const secp256k1_musig_partial_sig * const* partial_sigs,
    size_t n_sigs
) SECP256K1_ARG_NONNULL(1) SECP256K1_ARG_NONNULL(2) SECP256K1_ARG_NONNULL(3) SECP256K1_ARG_NONNULL(4);

/** Extracts the nonce_parity bit from a session
 *
 *  This is used for adaptor signatures.
 *
 *  Returns: 0 if the arguments are invalid, 1 otherwise
 *  Args:         ctx: pointer to a context object
 *  Out: nonce_parity: pointer to an integer that indicates the parity
 *                     of the aggregate public nonce. Used for adaptor
 *                     signatures.
 *  In:       session: pointer to the session that was created with
 *                     musig_nonce_process
 */
SECP256K1_API int secp256k1_musig_nonce_parity(
    const secp256k1_context* ctx,
    int *nonce_parity,
    const secp256k1_musig_session *session
) SECP256K1_ARG_NONNULL(1) SECP256K1_ARG_NONNULL(2) SECP256K1_ARG_NONNULL(3);

/** Creates a signature from a pre-signature and an adaptor.
 *
 *  If the sec_adaptor32 argument is incorrect, the output signature will be
 *  invalid. This function does not verify the signature.
 *
 *  Returns: 0 if the arguments are invalid, or pre_sig64 or sec_adaptor32 contain
 *           invalid (overflowing) values. 1 otherwise (which does NOT mean the
 *           signature or the adaptor are valid!)
 *  Args:         ctx: pointer to a context object
 *  Out:        sig64: 64-byte signature. This pointer may point to the same
 *                     memory area as `pre_sig`.
 *  In:     pre_sig64: 64-byte pre-signature
 *      sec_adaptor32: 32-byte secret adaptor to add to the pre-signature
 *       nonce_parity: the output of `musig_nonce_parity` called with the
 *                     session used for producing the pre-signature
 */
SECP256K1_API SECP256K1_WARN_UNUSED_RESULT int secp256k1_musig_adapt(
    const secp256k1_context* ctx,
    unsigned char *sig64,
    const unsigned char *pre_sig64,
    const unsigned char *sec_adaptor32,
    int nonce_parity
) SECP256K1_ARG_NONNULL(1) SECP256K1_ARG_NONNULL(2) SECP256K1_ARG_NONNULL(3) SECP256K1_ARG_NONNULL(4);

/** Extracts a secret adaptor from a MuSig pre-signature and corresponding
 *  signature
 *
 *  This function will not fail unless given grossly invalid data; if it is
 *  merely given signatures that do not verify, the returned value will be
 *  nonsense. It is therefore important that all data be verified at earlier
 *  steps of any protocol that uses this function. In particular, this includes
 *  verifying all partial signatures that were aggregated into pre_sig64.
 *
 *  Returns: 0 if the arguments are NULL, or sig64 or pre_sig64 contain
 *           grossly invalid (overflowing) values. 1 otherwise (which does NOT
 *           mean the signatures or the adaptor are valid!)
 *  Args:         ctx: pointer to a context object
 *  Out:sec_adaptor32: 32-byte secret adaptor
 *  In:         sig64: complete, valid 64-byte signature
 *          pre_sig64: the pre-signature corresponding to sig64, i.e., the
 *                     aggregate of partial signatures without the secret
 *                     adaptor
 *       nonce_parity: the output of `musig_nonce_parity` called with the
 *                     session used for producing sig64
 */
SECP256K1_API SECP256K1_WARN_UNUSED_RESULT int secp256k1_musig_extract_adaptor(
    const secp256k1_context* ctx,
    unsigned char *sec_adaptor32,
    const unsigned char *sig64,
    const unsigned char *pre_sig64,
    int nonce_parity
) SECP256K1_ARG_NONNULL(1) SECP256K1_ARG_NONNULL(2) SECP256K1_ARG_NONNULL(3) SECP256K1_ARG_NONNULL(4);

#ifdef __cplusplus
}
#endif

#endif