Add MuSig module which allows creating n-of-n multisignatures and adaptor signatures.

Makefile.am

@@ -182,6 +182,10 @@ if ENABLE_MODULE_SCHNORRSIG
 include src/modules/schnorrsig/Makefile.am.include
 endif
 
+if ENABLE_MODULE_MUSIG
+include src/modules/musig/Makefile.am.include
+endif
+
 if ENABLE_MODULE_RECOVERY
 include src/modules/recovery/Makefile.am.include
 endif

configure.ac

@@ -134,6 +134,11 @@ AC_ARG_ENABLE(module_schnorrsig,
     [enable_module_schnorrsig=$enableval],
     [enable_module_schnorrsig=no])
 
+AC_ARG_ENABLE(module_musig,
+    AS_HELP_STRING([--enable-module-musig],[enable MuSig module (experimental)]),
+    [enable_module_musig=$enableval],
+    [enable_module_musig=no])
+
 AC_ARG_ENABLE(module_recovery,
     AS_HELP_STRING([--enable-module-recovery],[enable ECDSA pubkey recovery module [default=no]]),
     [enable_module_recovery=$enableval],
@@ -529,6 +534,10 @@ if test x"$enable_module_schnorrsig" = x"yes"; then
   AC_DEFINE(ENABLE_MODULE_SCHNORRSIG, 1, [Define this symbol to enable the schnorrsig module])
 fi
 
+if test x"$enable_module_musig" = x"yes"; then
+  AC_DEFINE(ENABLE_MODULE_MUSIG, 1, [Define this symbol to enable the MuSig module])
+fi
+
 if test x"$enable_module_recovery" = x"yes"; then
   AC_DEFINE(ENABLE_MODULE_RECOVERY, 1, [Define this symbol to enable the ECDSA pubkey recovery module])
 fi
@@ -569,8 +578,16 @@ if test x"$enable_experimental" = x"yes"; then
   AC_MSG_NOTICE([Building key whitelisting module: $enable_module_whitelist])
   AC_MSG_NOTICE([Building surjection proof module: $enable_module_surjectionproof])
   AC_MSG_NOTICE([Building schnorrsig module: $enable_module_schnorrsig])
+  AC_MSG_NOTICE([Building MuSig module: $enable_module_musig])
   AC_MSG_NOTICE([******])
 
+
+  if test x"$enable_module_schnorrsig" != x"yes"; then
+    if test x"$enable_module_musig" = x"yes"; then
+      AC_MSG_ERROR([MuSig module requires the schnorrsig module. Use --enable-module-schnorrsig to allow.])
+    fi
+  fi
+
   if test x"$enable_module_generator" != x"yes"; then
     if test x"$enable_module_rangeproof" = x"yes"; then
       AC_MSG_ERROR([Rangeproof module requires the generator module. Use --enable-module-generator to allow.])
@@ -592,6 +609,9 @@ else
   if test x"$enable_module_schnorrsig" = x"yes"; then
     AC_MSG_ERROR([schnorrsig module is experimental. Use --enable-experimental to allow.])
   fi
+  if test x"$enable_module_musig" = x"yes"; then
+    AC_MSG_ERROR([MuSig module is experimental. Use --enable-experimental to allow.])
+  fi
   if test x"$set_asm" = x"arm"; then
     AC_MSG_ERROR([ARM assembly optimization is experimental. Use --enable-experimental to allow.])
   fi
@@ -623,6 +643,7 @@ AM_CONDITIONAL([USE_BENCHMARK], [test x"$use_benchmark" = x"yes"])
 AM_CONDITIONAL([USE_ECMULT_STATIC_PRECOMPUTATION], [test x"$set_precomp" = x"yes"])
 AM_CONDITIONAL([ENABLE_MODULE_ECDH], [test x"$enable_module_ecdh" = x"yes"])
 AM_CONDITIONAL([ENABLE_MODULE_SCHNORRSIG], [test x"$enable_module_schnorrsig" = x"yes"])
+AM_CONDITIONAL([ENABLE_MODULE_MUSIG], [test x"$enable_module_musig" = x"yes"])
 AM_CONDITIONAL([ENABLE_MODULE_RECOVERY], [test x"$enable_module_recovery" = x"yes"])
 AM_CONDITIONAL([ENABLE_MODULE_GENERATOR], [test x"$enable_module_generator" = x"yes"])
 AM_CONDITIONAL([ENABLE_MODULE_RANGEPROOF], [test x"$enable_module_rangeproof" = x"yes"])

include/secp256k1_musig.h

@@ -0,0 +1,429 @@
+#ifndef SECP256K1_MUSIG_H
+#define SECP256K1_MUSIG_H
+
+#include <stdint.h>
+
+/** This module implements a Schnorr-based multi-signature scheme called MuSig
+ * (https://eprint.iacr.org/2018/068.pdf).
+ */
+
+/** Data structure containing data related to a signing session resulting in a single
+ * signature.
+ *
+ * This structure is not opaque, but it MUST NOT be copied or read or written to it
+ * 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.
+ *
+ * A signer who goes offline and needs to import/export or save/load this structure
+ * **must** take measures prevent replay attacks wherein an old state is loaded and
+ * the signing protocol forked from that point. One straightforward way to accomplish
+ * this is to attach the output of a monotonic non-resettable counter (hardware
+ * support is needed for this). Increment the counter before each output and
+ * encrypt+sign the entire package. If a package is deserialized with an old counter
+ * state or bad signature it should be rejected.
+ *
+ * Observe that an independent counter is needed for each concurrent signing session
+ * such a device is involved in. To avoid fragility, it is therefore recommended that
+ * any offline signer be usable for only a single session at once.
+ *
+ * Given access to such a counter, its output should be used as (or mixed into) the
+ * session ID to ensure uniqueness.
+ *
+ * Fields:
+ *      combined_pk: MuSig-computed combined public key
+ *        n_signers: Number of signers
+ *          pk_hash: The 32-byte hash of the original public keys
+ *   combined_nonce: Summed combined public nonce (undefined if `nonce_is_set` is false)
+ *     nonce_is_set: Whether the above nonce has been set
+ * nonce_is_negated: If `nonce_is_set`, whether the above nonce was negated after
+ *                   summing the participants' nonces. Needed to ensure the nonce's y
+ *                   coordinate has a quadratic-residue y coordinate
+ *              msg: The 32-byte message (hash) to be signed
+ *       msg_is_set: Whether the above message has been set
+ *  has_secret_data: Whether this session object has a signers' secret data; if this
+ *                   is `false`, it may still be used for verification purposes.
+ *           seckey: If `has_secret_data`, the signer's secret key
+ *         secnonce: If `has_secret_data`, the signer's secret nonce
+ *            nonce: If `has_secret_data`, the signer's public nonce
+ * nonce_commitments_hash: If `has_secret_data` and `nonce_commitments_hash_is_set`,
+ *                   the hash of all signers' commitments
+ * nonce_commitments_hash_is_set: If `has_secret_data`, whether the
+ *                   nonce_commitments_hash has been set
+ */
+typedef struct {
+    secp256k1_pubkey combined_pk;
+    uint32_t n_signers;
+    unsigned char pk_hash[32];
+    secp256k1_pubkey combined_nonce;
+    int nonce_is_set;
+    int nonce_is_negated;
+    unsigned char msg[32];
+    int msg_is_set;
+    int has_secret_data;
+    unsigned char seckey[32];
+    unsigned char secnonce[32];
+    secp256k1_pubkey nonce;
+    unsigned char nonce_commitments_hash[32];
+    int nonce_commitments_hash_is_set;
+} secp256k1_musig_session;
+
+/** Data structure containing data on all signers in a single session.
+ *
+ * The workflow for this structure is as follows:
+ *
+ * 1. This structure is initialized with `musig_session_initialize` or
+ *    `musig_session_initialize_verifier`, which set the `index` field, and zero out
+ *    all other fields. The public session is initialized with the signers'
+ *    nonce_commitments.
+ *
+ * 2. In a non-public session the nonce_commitments are set with the function
+ *    `musig_get_public_nonce`, which also returns the signer's public nonce. This
+ *    ensures that the public nonce is not exposed until all commitments have been
+ *    received.
+ *
+ * 3. Each individual data struct should be updated with `musig_set_nonce` once a
+ *    nonce is available. This function takes a single signer data struct rather than
+ *    an array because it may fail in the case that the provided nonce does not match
+ *    the commitment. In this case, it is desirable to identify the exact party whose
+ *    nonce was inconsistent.
+ *
+ * Fields:
+ *   present: indicates whether the signer's nonce is set
+ *     index: index of the signer in the MuSig key aggregation
+ *     nonce: public nonce, must be a valid curvepoint if the signer is `present`
+ * nonce_commitment: commitment to the nonce, or all-bits zero if a commitment
+ *                   has not yet been set
+ */
+typedef struct {
+    int present;
+    uint32_t index;
+    secp256k1_pubkey nonce;
+    unsigned char nonce_commitment[32];
+} secp256k1_musig_session_signer_data;
+
+/** Opaque data structure that holds a MuSig partial 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 32 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
+ *  `musig_partial_signature_serialize` and `musig_partial_signature_parse`
+ *  functions.
+ */
+typedef struct {
+    unsigned char data[32];
+} secp256k1_musig_partial_signature;
+
+/** Computes a combined public key and the hash of the given public keys
+ *
+ *  Returns: 1 if the public keys were successfully combined, 0 otherwise
+ *  Args:        ctx: pointer to a context object initialized for verification
+ *                    (cannot be NULL)
+ *           scratch: scratch space used to compute the combined pubkey by
+ *                    multiexponentiation. If NULL, an inefficient algorithm is used.
+ *  Out: combined_pk: the MuSig-combined public key (cannot be NULL)
+ *         pk_hash32: if non-NULL, filled with the 32-byte hash of all input public
+ *                    keys in order to be used in `musig_session_initialize`.
+ *   In:     pubkeys: input array of public keys to combine. The order is important;
+ *                    a different order will result in a different combined public
+ *                    key (cannot be NULL)
+ *         n_pubkeys: length of pubkeys array
+ */
+SECP256K1_API int secp256k1_musig_pubkey_combine(
+    const secp256k1_context* ctx,
+    secp256k1_scratch_space *scratch,
+    secp256k1_pubkey *combined_pk,
+    unsigned char *pk_hash32,
+    const secp256k1_pubkey *pubkeys,
+    size_t n_pubkeys
+) SECP256K1_ARG_NONNULL(1) SECP256K1_ARG_NONNULL(3) SECP256K1_ARG_NONNULL(5);
+
+/** Initializes a signing session for a signer
+ *
+ *  Returns: 1: session is successfully initialized
+ *           0: session could not be initialized: secret key or secret nonce overflow
+ *  Args:         ctx: pointer to a context object, initialized for signing (cannot
+ *                     be NULL)
+ *  Out:      session: the session structure to initialize (cannot be NULL)
+ *            signers: an array of signers' data to be initialized. Array length must
+ *                     equal to `n_signers` (cannot be NULL)
+ * nonce_commitment32: filled with a 32-byte commitment to the generated nonce
+ *                     (cannot be NULL)
+ *  In:  session_id32: a *unique* 32-byte ID to assign to this session (cannot be
+ *                     NULL). If a non-unique session_id32 was given then a partial
+ *                     signature will LEAK THE SECRET KEY.
+ *              msg32: the 32-byte message to be signed. Shouldn't be NULL unless you
+ *                     require sharing public nonces before the message is known
+ *                     because it reduces nonce misuse resistance. If NULL, must be
+ *                     set with `musig_session_set_msg` before signing and verifying.
+ *        combined_pk: the combined public key of all signers (cannot be NULL)
+ *          pk_hash32: the 32-byte hash of the signers' individual keys (cannot be
+ *                     NULL)
+ *          n_signers: length of signers array. Number of signers participating in
+ *                     the MuSig. Must be greater than 0 and at most 2^32 - 1.
+ *           my_index: index of this signer in the signers array
+ *             seckey: the signer's 32-byte secret key (cannot be NULL)
+ */
+SECP256K1_API int secp256k1_musig_session_initialize(
+    const secp256k1_context* ctx,
+    secp256k1_musig_session *session,
+    secp256k1_musig_session_signer_data *signers,
+    unsigned char *nonce_commitment32,
+    const unsigned char *session_id32,
+    const unsigned char *msg32,
+    const secp256k1_pubkey *combined_pk,
+    const unsigned char *pk_hash32,
+    size_t n_signers,
+    size_t my_index,
+    const unsigned char *seckey
+) SECP256K1_ARG_NONNULL(1) SECP256K1_ARG_NONNULL(2) SECP256K1_ARG_NONNULL(3) SECP256K1_ARG_NONNULL(4) SECP256K1_ARG_NONNULL(5) SECP256K1_ARG_NONNULL(7) SECP256K1_ARG_NONNULL(8) SECP256K1_ARG_NONNULL(11);
+
+/** Gets the signer's public nonce given a list of all signers' data with commitments
+ *
+ *  Returns: 1: public nonce is written in nonce
+ *           0: signer data is missing commitments or session isn't initialized
+ *              for signing
+ *  Args:        ctx: pointer to a context object (cannot be NULL)
+ *           session: the signing session to get the nonce from (cannot be NULL)
+ *           signers: an array of signers' data initialized with
+ *                    `musig_session_initialize`. Array length must equal to
+ *                    `n_commitments` (cannot be NULL)
+ *  Out:       nonce: the nonce (cannot be NULL)
+ *  In:  commitments: array of 32-byte nonce commitments (cannot be NULL)
+ *     n_commitments: the length of commitments and signers array. Must be the total
+ *                    number of signers participating in the MuSig.
+ */
+SECP256K1_API SECP256K1_WARN_UNUSED_RESULT int secp256k1_musig_session_get_public_nonce(
+    const secp256k1_context* ctx,
+    secp256k1_musig_session *session,
+    secp256k1_musig_session_signer_data *signers,
+    secp256k1_pubkey *nonce,
+    const unsigned char *const *commitments,
+    size_t n_commitments
+) SECP256K1_ARG_NONNULL(1) SECP256K1_ARG_NONNULL(2) SECP256K1_ARG_NONNULL(3) SECP256K1_ARG_NONNULL(4) SECP256K1_ARG_NONNULL(5);
+
+/** Initializes a verifier session that can be used for verifying nonce commitments
+ *  and partial signatures. It does not have secret key material and therefore can not
+ *  be used to create signatures.
+ *
+ *  Returns: 1 when session is successfully initialized, 0 otherwise
+ *  Args:        ctx: pointer to a context object (cannot be NULL)
+ *  Out:     session: the session structure to initialize (cannot be NULL)
+ *           signers: an array of signers' data to be initialized. Array length must
+ *                    equal to `n_signers`(cannot be NULL)
+ *  In:        msg32: the 32-byte message to be signed If NULL, must be set with
+ *                    `musig_session_set_msg` before using the session for verifying
+ *                    partial signatures.
+ *       combined_pk: the combined public key of all signers (cannot be NULL)
+ *         pk_hash32: the 32-byte hash of the signers' individual keys (cannot be NULL)
+ *       commitments: array of 32-byte nonce commitments. Array length must equal to
+ *                    `n_signers` (cannot be NULL)
+ *         n_signers: length of signers and commitments array. Number of signers
+ *                    participating in the MuSig. Must be greater than 0 and at most
+ *                    2^32 - 1.
+ */
+SECP256K1_API int secp256k1_musig_session_initialize_verifier(
+    const secp256k1_context* ctx,
+    secp256k1_musig_session *session,
+    secp256k1_musig_session_signer_data *signers,
+    const unsigned char *msg32,
+    const secp256k1_pubkey *combined_pk,
+    const unsigned char *pk_hash32,
+    const unsigned char *const *commitments,
+    size_t n_signers
+) SECP256K1_ARG_NONNULL(1) SECP256K1_ARG_NONNULL(2) SECP256K1_ARG_NONNULL(3) SECP256K1_ARG_NONNULL(5) SECP256K1_ARG_NONNULL(6) SECP256K1_ARG_NONNULL(7);
+
+/** Checks a signer's public nonce against a commitment to said nonce, and update
+ *  data structure if they match
+ *
+ *  Returns: 1: commitment was valid, data structure updated
+ *           0: commitment was invalid, nothing happened
+ *  Args:      ctx: pointer to a context object (cannot be NULL)
+ *          signer: pointer to the signer data to update (cannot be NULL). Must have
+ *                  been used with `musig_session_get_public_nonce` or initialized
+ *                  with `musig_session_initialize_verifier`.
+ *  In:     nonce: signer's alleged public nonce (cannot be NULL)
+ */
+SECP256K1_API SECP256K1_WARN_UNUSED_RESULT int secp256k1_musig_set_nonce(
+    const secp256k1_context* ctx,
+    secp256k1_musig_session_signer_data *signer,
+    const secp256k1_pubkey *nonce
+) SECP256K1_ARG_NONNULL(1) SECP256K1_ARG_NONNULL(2) SECP256K1_ARG_NONNULL(3);
+
+/** Updates a session with the combined public nonce of all signers. The combined
+ * public nonce is the sum of every signer's public nonce.
+ *
+ *  Returns: 1: nonces are successfully combined
+ *           0: a signer's nonce is missing
+ *  Args:        ctx: pointer to a context object (cannot be NULL)
+ *           session: session to update with the combined public nonce (cannot be
+ *                    NULL)
+ *           signers: an array of signers' data, which must have had public nonces
+ *                    set with `musig_set_nonce`. Array length must equal to `n_signers`
+ *                    (cannot be NULL)
+ *         n_signers: the length of the signers array. Must be the total number of
+ *                    signers participating in the MuSig.
+ *  Out: nonce_is_negated: a pointer to an integer that indicates if the combined
+ *                    public nonce had to be negated.
+ *           adaptor: point to add to the combined public nonce. If NULL, nothing is
+ *                    added to the combined nonce.
+ */
+SECP256K1_API int secp256k1_musig_session_combine_nonces(
+    const secp256k1_context* ctx,
+    secp256k1_musig_session *session,
+    const secp256k1_musig_session_signer_data *signers,
+    size_t n_signers,
+    int *nonce_is_negated,
+    const secp256k1_pubkey *adaptor
+) SECP256K1_ARG_NONNULL(1) SECP256K1_ARG_NONNULL(2) SECP256K1_ARG_NONNULL(4);
+
+/** Sets the message of a session if previously unset
+ *
+ *  Returns 1 if the message was not set yet and is now successfully set
+ *          0 otherwise
+ *  Args:    ctx: pointer to a context object (cannot be NULL)
+ *       session: the session structure to update with the message (cannot be NULL)
+ *  In:    msg32: the 32-byte message to be signed (cannot be NULL)
+ */
+SECP256K1_API SECP256K1_WARN_UNUSED_RESULT int secp256k1_musig_session_set_msg(
+    const secp256k1_context* ctx,
+    secp256k1_musig_session *session,
+    const unsigned char *msg32
+) SECP256K1_ARG_NONNULL(1) SECP256K1_ARG_NONNULL(2) SECP256K1_ARG_NONNULL(3);
+
+/** Serialize a MuSig partial signature or adaptor 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_signature_serialize(
+    const secp256k1_context* ctx,
+    unsigned char *out32,
+    const secp256k1_musig_partial_signature* sig
+) SECP256K1_ARG_NONNULL(1) SECP256K1_ARG_NONNULL(2) SECP256K1_ARG_NONNULL(3);
+
+/** Parse and verify 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_signature_parse(
+    const secp256k1_context* ctx,
+    secp256k1_musig_partial_signature* sig,
+    const unsigned char *in32
+) SECP256K1_ARG_NONNULL(1) SECP256K1_ARG_NONNULL(2) SECP256K1_ARG_NONNULL(3);
+
+/** Produces a partial signature
+ *
+ *  Returns: 1: partial signature constructed
+ *           0: session in incorrect or inconsistent state
+ *  Args:         ctx: pointer to a context object (cannot be NULL)
+ *            session: active signing session for which the combined nonce has been
+ *                     computed (cannot be NULL)
+ *  Out:  partial_sig: partial signature (cannot be NULL)
+ */
+SECP256K1_API int secp256k1_musig_partial_sign(
+    const secp256k1_context* ctx,
+    const secp256k1_musig_session *session,
+    secp256k1_musig_partial_signature *partial_sig
+) SECP256K1_ARG_NONNULL(1) SECP256K1_ARG_NONNULL(2) SECP256K1_ARG_NONNULL(3);
+
+/** Checks that an individual partial signature verifies
+ *
+ *  This function is essential when using protocols with adaptor signatures.
+ *  However, it is not essential for regular MuSig's, in the sense that if any
+ *  partial signatures does not verify, the full signature will also not verify, so the
+ *  problem will be caught. But this function allows determining the specific party
+ *  who produced an invalid signature, so that signing can be restarted without them.
+ *
+ *  Returns: 1: partial signature verifies
+ *           0: invalid signature or bad data
+ *  Args:         ctx: pointer to a context object (cannot be NULL)
+ *            session: active session for which the combined nonce has been computed
+ *                     (cannot be NULL)
+ *             signer: data for the signer who produced this signature (cannot be NULL)
+ *  In:   partial_sig: signature to verify (cannot be NULL)
+ *             pubkey: public key of the signer who produced the signature (cannot be NULL)
+ */
+SECP256K1_API SECP256K1_WARN_UNUSED_RESULT int secp256k1_musig_partial_sig_verify(
+    const secp256k1_context* ctx,
+    const secp256k1_musig_session *session,
+    const secp256k1_musig_session_signer_data *signer,
+    const secp256k1_musig_partial_signature *partial_sig,
+    const secp256k1_pubkey *pubkey
+) SECP256K1_ARG_NONNULL(1) SECP256K1_ARG_NONNULL(2) SECP256K1_ARG_NONNULL(3) SECP256K1_ARG_NONNULL(4) SECP256K1_ARG_NONNULL(5);
+
+/** Combines partial signatures
+ *
+ *  Returns: 1: all partial signatures have values in range. Does NOT mean the
+ *              resulting signature verifies.
+ *           0: some partial signature had s/r out of range
+ *  Args:         ctx: pointer to a context object (cannot be NULL)
+ *            session: initialized session for which the combined nonce has been
+ *                     computed (cannot be NULL)
+ *  Out:          sig: complete signature (cannot be NULL)
+ *  In:  partial_sigs: array of partial signatures to combine (cannot be NULL)
+ *             n_sigs: number of signatures in the partial_sigs array
+ */
+SECP256K1_API SECP256K1_WARN_UNUSED_RESULT int secp256k1_musig_partial_sig_combine(
+    const secp256k1_context* ctx,
+    const secp256k1_musig_session *session,
+    secp256k1_schnorrsig *sig,
+    const secp256k1_musig_partial_signature *partial_sigs,
+    size_t n_sigs
+) SECP256K1_ARG_NONNULL(1) SECP256K1_ARG_NONNULL(2) SECP256K1_ARG_NONNULL(3) SECP256K1_ARG_NONNULL(4);
+
+/** Converts a partial signature to an adaptor signature by adding a given secret
+ *  adaptor.
+ *
+ *  Returns: 1: signature and secret adaptor contained valid values
+ *           0: otherwise
+ *  Args:         ctx: pointer to a context object (cannot be NULL)
+ *  Out:  adaptor_sig: adaptor signature to produce (cannot be NULL)
+ *  In:   partial_sig: partial signature to tweak with secret adaptor (cannot be NULL)
+ *      sec_adaptor32: 32-byte secret adaptor to add to the partial signature (cannot
+ *                     be NULL)
+ *   nonce_is_negated: the `nonce_is_negated` output of `musig_session_combine_nonces`
+ */
+SECP256K1_API int secp256k1_musig_partial_sig_adapt(
+    const secp256k1_context* ctx,
+    secp256k1_musig_partial_signature *adaptor_sig,
+    const secp256k1_musig_partial_signature *partial_sig,
+    const unsigned char *sec_adaptor32,
+    int nonce_is_negated
+) SECP256K1_ARG_NONNULL(1) SECP256K1_ARG_NONNULL(2) SECP256K1_ARG_NONNULL(3) SECP256K1_ARG_NONNULL(4);
+
+/** Extracts a secret adaptor from a MuSig, given all parties' partial
+ *  signatures. 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.
+ *
+ *  Returns: 1: signatures contained valid data such that an adaptor could be extracted
+ *           0: otherwise
+ *  Args:         ctx: pointer to a context object (cannot be NULL)
+ *  Out:sec_adaptor32: 32-byte secret adaptor (cannot be NULL)
+ *  In:           sig: complete 2-of-2 signature (cannot be NULL)
+ *       partial_sigs: array of partial signatures (cannot be NULL)
+ *     n_partial_sigs: number of elements in partial_sigs array
+ *   nonce_is_negated: the `nonce_is_negated` output of `musig_session_combine_nonces`
+ */
+SECP256K1_API SECP256K1_WARN_UNUSED_RESULT int secp256k1_musig_extract_secret_adaptor(
+    const secp256k1_context* ctx,
+    unsigned char *sec_adaptor32,
+    const secp256k1_schnorrsig *sig,
+    const secp256k1_musig_partial_signature *partial_sigs,
+    size_t n_partial_sigs,
+    int nonce_is_negated
+) SECP256K1_ARG_NONNULL(1) SECP256K1_ARG_NONNULL(2) SECP256K1_ARG_NONNULL(3) SECP256K1_ARG_NONNULL(4);
+
+#endif

src/modules/musig/Makefile.am.include

@@ -0,0 +1,3 @@
+include_HEADERS += include/secp256k1_musig.h
+noinst_HEADERS += src/modules/musig/main_impl.h
+noinst_HEADERS += src/modules/musig/tests_impl.h

src/modules/musig/main_impl.h

@@ -0,0 +1,629 @@
+/**********************************************************************
+ * Copyright (c) 2018 Andrew Poelstra, Jonas Nick                     *
+ * Distributed under the MIT software license, see the accompanying   *
+ * file COPYING or http://www.opensource.org/licenses/mit-license.php.*
+ **********************************************************************/
+
+#ifndef _SECP256K1_MODULE_MUSIG_MAIN_
+#define _SECP256K1_MODULE_MUSIG_MAIN_
+
+#include "include/secp256k1.h"
+#include "include/secp256k1_musig.h"
+#include "hash.h"
+
+/* Computes ell = SHA256(pk[0], ..., pk[np-1]) */
+static int secp256k1_musig_compute_ell(const secp256k1_context *ctx, unsigned char *ell, const secp256k1_pubkey *pk, size_t np) {
+    secp256k1_sha256 sha;
+    size_t i;
+
+    secp256k1_sha256_initialize(&sha);
+    for (i = 0; i < np; i++) {
+        unsigned char ser[33];
+        size_t serlen = sizeof(ser);
+        if (!secp256k1_ec_pubkey_serialize(ctx, ser, &serlen, &pk[i], SECP256K1_EC_COMPRESSED)) {
+            return 0;
+        }
+        secp256k1_sha256_write(&sha, ser, serlen);
+    }
+    secp256k1_sha256_finalize(&sha, ell);
+    return 1;
+}
+
+/* Initializes SHA256 with fixed midstate. This midstate was computed by applying
+ * SHA256 to SHA256("MuSig coefficient")||SHA256("MuSig coefficient"). */
+static void secp256k1_musig_sha256_init_tagged(secp256k1_sha256 *sha) {
+    secp256k1_sha256_initialize(sha);
+
+    sha->s[0] = 0x0fd0690cul;
+    sha->s[1] = 0xfefeae97ul;
+    sha->s[2] = 0x996eac7ful;
+    sha->s[3] = 0x5c30d864ul;
+    sha->s[4] = 0x8c4a0573ul;
+    sha->s[5] = 0xaca1a22ful;
+    sha->s[6] = 0x6f43b801ul;
+    sha->s[7] = 0x85ce27cdul;
+    sha->bytes = 64;
+}
+
+/* Compute r = SHA256(ell, idx). The four bytes of idx are serialized least significant byte first. */
+static void secp256k1_musig_coefficient(secp256k1_scalar *r, const unsigned char *ell, uint32_t idx) {
+    secp256k1_sha256 sha;
+    unsigned char buf[32];
+    size_t i;
+
+    secp256k1_musig_sha256_init_tagged(&sha);
+    secp256k1_sha256_write(&sha, ell, 32);
+    /* We're hashing the index of the signer instead of its public key as specified
+     * in the MuSig paper. This reduces the total amount of data that needs to be
+     * hashed.
+     * Additionally, it prevents creating identical musig_coefficients for identical
+     * public keys. A participant Bob could choose his public key to be the same as
+     * Alice's, then replay Alice's messages (nonce and partial signature) to create
+     * a valid partial signature. This is not a problem for MuSig per se, but could
+     * result in subtle issues with protocols building on threshold signatures.
+     * With the assumption that public keys are unique, hashing the index is
+     * equivalent to hashing the public key. Because the public key can be
+     * identified by the index given the ordered list of public keys (included in
+     * ell), the index is just a different encoding of the public key.*/
+    for (i = 0; i < sizeof(uint32_t); i++) {
+        unsigned char c = idx;
+        secp256k1_sha256_write(&sha, &c, 1);
+        idx >>= 8;
+    }
+    secp256k1_sha256_finalize(&sha, buf);
+    secp256k1_scalar_set_b32(r, buf, NULL);
+}
+
+typedef struct {
+    const secp256k1_context *ctx;
+    unsigned char ell[32];
+    const secp256k1_pubkey *pks;
+} secp256k1_musig_pubkey_combine_ecmult_data;
+
+/* Callback for batch EC multiplication to compute ell_0*P0 + ell_1*P1 + ...  */
+static int secp256k1_musig_pubkey_combine_callback(secp256k1_scalar *sc, secp256k1_ge *pt, size_t idx, void *data) {
+    secp256k1_musig_pubkey_combine_ecmult_data *ctx = (secp256k1_musig_pubkey_combine_ecmult_data *) data;
+    secp256k1_musig_coefficient(sc, ctx->ell, idx);
+    return secp256k1_pubkey_load(ctx->ctx, pt, &ctx->pks[idx]);
+}
+
+
+static void secp256k1_musig_signers_init(secp256k1_musig_session_signer_data *signers, uint32_t n_signers) {
+    uint32_t i;
+    for (i = 0; i < n_signers; i++) {
+        memset(&signers[i], 0, sizeof(signers[i]));
+        signers[i].index = i;
+        signers[i].present = 0;
+    }
+}
+
+int secp256k1_musig_pubkey_combine(const secp256k1_context* ctx, secp256k1_scratch_space *scratch, secp256k1_pubkey *combined_pk, unsigned char *pk_hash32, const secp256k1_pubkey *pubkeys, size_t n_pubkeys) {
+    secp256k1_musig_pubkey_combine_ecmult_data ecmult_data;
+    secp256k1_gej pkj;
+    secp256k1_ge pkp;
+
+    VERIFY_CHECK(ctx != NULL);
+    ARG_CHECK(combined_pk != NULL);
+    ARG_CHECK(secp256k1_ecmult_context_is_built(&ctx->ecmult_ctx));
+    ARG_CHECK(pubkeys != NULL);
+    ARG_CHECK(n_pubkeys > 0);
+
+    ecmult_data.ctx = ctx;
+    ecmult_data.pks = pubkeys;
+    if (!secp256k1_musig_compute_ell(ctx, ecmult_data.ell, pubkeys, n_pubkeys)) {
+        return 0;
+    }
+    if (!secp256k1_ecmult_multi_var(&ctx->ecmult_ctx, scratch, &pkj, NULL, secp256k1_musig_pubkey_combine_callback, (void *) &ecmult_data, n_pubkeys)) {
+        return 0;
+    }
+    secp256k1_ge_set_gej(&pkp, &pkj);
+    secp256k1_pubkey_save(combined_pk, &pkp);
+
+    if (pk_hash32 != NULL) {
+        memcpy(pk_hash32, ecmult_data.ell, 32);
+    }
+    return 1;
+}
+
+int secp256k1_musig_session_initialize(const secp256k1_context* ctx, secp256k1_musig_session *session, secp256k1_musig_session_signer_data *signers, unsigned char *nonce_commitment32, const unsigned char *session_id32, const unsigned char *msg32, const secp256k1_pubkey *combined_pk, const unsigned char *pk_hash32, size_t n_signers, size_t my_index, const unsigned char *seckey) {
+    unsigned char combined_ser[33];
+    size_t combined_ser_size = sizeof(combined_ser);
+    int overflow;
+    secp256k1_scalar secret;
+    secp256k1_scalar mu;
+    secp256k1_sha256 sha;
+    secp256k1_gej rj;
+    secp256k1_ge rp;
+
+    VERIFY_CHECK(ctx != NULL);
+    ARG_CHECK(secp256k1_ecmult_gen_context_is_built(&ctx->ecmult_gen_ctx));
+    ARG_CHECK(session != NULL);
+    ARG_CHECK(signers != NULL);
+    ARG_CHECK(nonce_commitment32 != NULL);
+    ARG_CHECK(session_id32 != NULL);
+    ARG_CHECK(combined_pk != NULL);
+    ARG_CHECK(pk_hash32 != NULL);
+    ARG_CHECK(seckey != NULL);
+
+    memset(session, 0, sizeof(*session));
+
+    if (msg32 != NULL) {
+        memcpy(session->msg, msg32, 32);
+        session->msg_is_set = 1;
+    } else {
+        session->msg_is_set = 0;
+    }
+    memcpy(&session->combined_pk, combined_pk, sizeof(*combined_pk));
+    memcpy(session->pk_hash, pk_hash32, 32);
+    session->nonce_is_set = 0;
+    session->has_secret_data = 1;
+    if (n_signers == 0 || my_index >= n_signers) {
+        return 0;
+    }
+    if (n_signers > UINT32_MAX) {
+        return 0;
+    }
+    session->n_signers = (uint32_t) n_signers;
+    secp256k1_musig_signers_init(signers, session->n_signers);
+    session->nonce_commitments_hash_is_set = 0;
+
+    /* Compute secret key */
+    secp256k1_scalar_set_b32(&secret, seckey, &overflow);
+    if (overflow) {
+        secp256k1_scalar_clear(&secret);
+        return 0;
+    }
+    secp256k1_musig_coefficient(&mu, pk_hash32, (uint32_t) my_index);
+    secp256k1_scalar_mul(&secret, &secret, &mu);
+    secp256k1_scalar_get_b32(session->seckey, &secret);
+
+    /* Compute secret nonce */
+    secp256k1_sha256_initialize(&sha);
+    secp256k1_sha256_write(&sha, session_id32, 32);
+    if (session->msg_is_set) {
+        secp256k1_sha256_write(&sha, msg32, 32);
+    }
+    secp256k1_ec_pubkey_serialize(ctx, combined_ser, &combined_ser_size, combined_pk, SECP256K1_EC_COMPRESSED);
+    secp256k1_sha256_write(&sha, combined_ser, combined_ser_size);
+    secp256k1_sha256_write(&sha, seckey, 32);
+    secp256k1_sha256_finalize(&sha, session->secnonce);
+    secp256k1_scalar_set_b32(&secret, session->secnonce, &overflow);
+    if (overflow) {
+        secp256k1_scalar_clear(&secret);
+        return 0;
+    }
+
+    /* Compute public nonce and commitment */
+    secp256k1_ecmult_gen(&ctx->ecmult_gen_ctx, &rj, &secret);
+    secp256k1_ge_set_gej(&rp, &rj);
+    secp256k1_pubkey_save(&session->nonce, &rp);
+
+    if (nonce_commitment32 != NULL) {
+        unsigned char commit[33];
+        size_t commit_size = sizeof(commit);
+        secp256k1_sha256_initialize(&sha);
+        secp256k1_ec_pubkey_serialize(ctx, commit, &commit_size, &session->nonce, SECP256K1_EC_COMPRESSED);
+        secp256k1_sha256_write(&sha, commit, commit_size);
+        secp256k1_sha256_finalize(&sha, nonce_commitment32);
+    }
+
+    secp256k1_scalar_clear(&secret);
+    return 1;
+}
+
+int secp256k1_musig_session_get_public_nonce(const secp256k1_context* ctx, secp256k1_musig_session *session, secp256k1_musig_session_signer_data *signers, secp256k1_pubkey *nonce, const unsigned char *const *commitments, size_t n_commitments) {
+    secp256k1_sha256 sha;
+    unsigned char nonce_commitments_hash[32];
+    size_t i;
+    (void) ctx;
+
+    VERIFY_CHECK(ctx != NULL);
+    ARG_CHECK(session != NULL);
+    ARG_CHECK(signers != NULL);
+    ARG_CHECK(nonce != NULL);
+    ARG_CHECK(commitments != NULL);
+
+    if (!session->has_secret_data || n_commitments != session->n_signers) {
+        return 0;
+    }
+    for (i = 0; i < n_commitments; i++) {
+        ARG_CHECK(commitments[i] != NULL);
+    }
+
+    secp256k1_sha256_initialize(&sha);
+    for (i = 0; i < n_commitments; i++) {
+        memcpy(signers[i].nonce_commitment, commitments[i], 32);
+        secp256k1_sha256_write(&sha, commitments[i], 32);
+    }
+    secp256k1_sha256_finalize(&sha, nonce_commitments_hash);
+    if (session->nonce_commitments_hash_is_set
+            && memcmp(session->nonce_commitments_hash, nonce_commitments_hash, 32) != 0) {
+        /* Abort if get_public_nonce has been called before with a different array of
+         * commitments. */
+        return 0;
+    }
+    memcpy(session->nonce_commitments_hash, nonce_commitments_hash, 32);
+    session->nonce_commitments_hash_is_set = 1;
+    memcpy(nonce, &session->nonce, sizeof(*nonce));
+    return 1;
+}
+
+int secp256k1_musig_session_initialize_verifier(const secp256k1_context* ctx, secp256k1_musig_session *session, secp256k1_musig_session_signer_data *signers, const unsigned char *msg32, const secp256k1_pubkey *combined_pk, const unsigned char *pk_hash32, const unsigned char *const *commitments, size_t n_signers) {
+    size_t i;
+
+    VERIFY_CHECK(ctx != NULL);
+    ARG_CHECK(session != NULL);
+    ARG_CHECK(signers != NULL);
+    ARG_CHECK(combined_pk != NULL);
+    ARG_CHECK(pk_hash32 != NULL);
+    ARG_CHECK(commitments != NULL);
+    /* Check n_signers before checking commitments to allow testing the case where
+     * n_signers is big without allocating the space. */
+    if (n_signers > UINT32_MAX) {
+        return 0;
+    }
+    for (i = 0; i < n_signers; i++) {
+        ARG_CHECK(commitments[i] != NULL);
+    }
+    (void) ctx;
+
+    memset(session, 0, sizeof(*session));
+
+    memcpy(&session->combined_pk, combined_pk, sizeof(*combined_pk));
+    if (n_signers == 0) {
+        return 0;
+    }
+    session->n_signers = (uint32_t) n_signers;
+    secp256k1_musig_signers_init(signers, session->n_signers);
+
+    memcpy(session->pk_hash, pk_hash32, 32);
+    session->nonce_is_set = 0;
+    session->msg_is_set = 0;
+    if (msg32 != NULL) {
+        memcpy(session->msg, msg32, 32);
+        session->msg_is_set = 1;
+    }
+    session->has_secret_data = 0;
+    session->nonce_commitments_hash_is_set = 0;
+
+    for (i = 0; i < n_signers; i++) {
+        memcpy(signers[i].nonce_commitment, commitments[i], 32);
+    }
+    return 1;
+}
+
+int secp256k1_musig_set_nonce(const secp256k1_context* ctx, secp256k1_musig_session_signer_data *signer, const secp256k1_pubkey *nonce) {
+    unsigned char commit[33];
+    size_t commit_size = sizeof(commit);
+    secp256k1_sha256 sha;
+
+    VERIFY_CHECK(ctx != NULL);
+    ARG_CHECK(signer != NULL);
+    ARG_CHECK(nonce != NULL);
+
+    secp256k1_sha256_initialize(&sha);
+    secp256k1_ec_pubkey_serialize(ctx, commit, &commit_size, nonce, SECP256K1_EC_COMPRESSED);
+    secp256k1_sha256_write(&sha, commit, commit_size);
+    secp256k1_sha256_finalize(&sha, commit);
+
+    if (memcmp(commit, signer->nonce_commitment, 32) != 0) {
+        return 0;
+    }
+    memcpy(&signer->nonce, nonce, sizeof(*nonce));
+    signer->present = 1;
+    return 1;
+}
+
+int secp256k1_musig_session_combine_nonces(const secp256k1_context* ctx, secp256k1_musig_session *session, const secp256k1_musig_session_signer_data *signers, size_t n_signers, int *nonce_is_negated, const secp256k1_pubkey *adaptor) {
+    secp256k1_gej combined_noncej;
+    secp256k1_ge combined_noncep;
+    secp256k1_ge noncep;
+    secp256k1_sha256 sha;
+    unsigned char nonce_commitments_hash[32];
+    size_t i;
+
+    VERIFY_CHECK(ctx != NULL);
+    ARG_CHECK(session != NULL);
+    ARG_CHECK(signers != NULL);
+
+    if (n_signers != session->n_signers) {
+        return 0;
+    }
+    secp256k1_sha256_initialize(&sha);
+    secp256k1_gej_set_infinity(&combined_noncej);
+    for (i = 0; i < n_signers; i++) {
+        if (!signers[i].present) {
+            return 0;
+        }
+        secp256k1_sha256_write(&sha, signers[i].nonce_commitment, 32);
+        secp256k1_pubkey_load(ctx, &noncep, &signers[i].nonce);
+        secp256k1_gej_add_ge_var(&combined_noncej, &combined_noncej, &noncep, NULL);
+    }
+    secp256k1_sha256_finalize(&sha, nonce_commitments_hash);
+    /* Either the session is a verifier session or or the nonce_commitments_hash has
+     * been set in `musig_session_get_public_nonce`. */
+    VERIFY_CHECK(!session->has_secret_data || session->nonce_commitments_hash_is_set);
+    if (session->has_secret_data
+            && memcmp(session->nonce_commitments_hash, nonce_commitments_hash, 32) != 0) {
+        /* If the signers' commitments changed between get_public_nonce and now we
+         * have to abort because in that case they may have seen our nonce before
+         * creating their commitment. That can happen if the signer_data given to
+         * this function is different to the signer_data given to get_public_nonce.
+         * */
+        return 0;
+    }
+
+    /* Add public adaptor to nonce */
+    if (adaptor != NULL) {
+        secp256k1_pubkey_load(ctx, &noncep, adaptor);
+        secp256k1_gej_add_ge_var(&combined_noncej, &combined_noncej, &noncep, NULL);
+    }
+    secp256k1_ge_set_gej(&combined_noncep, &combined_noncej);
+    if (secp256k1_fe_is_quad_var(&combined_noncep.y)) {
+        session->nonce_is_negated = 0;
+    } else {
+        session->nonce_is_negated = 1;
+        secp256k1_ge_neg(&combined_noncep, &combined_noncep);
+    }
+    if (nonce_is_negated != NULL) {
+        *nonce_is_negated = session->nonce_is_negated;
+    }
+    secp256k1_pubkey_save(&session->combined_nonce, &combined_noncep);
+    session->nonce_is_set = 1;
+    return 1;
+}
+
+int secp256k1_musig_session_set_msg(const secp256k1_context* ctx, secp256k1_musig_session *session, const unsigned char *msg32) {
+    VERIFY_CHECK(ctx != NULL);
+    ARG_CHECK(session != NULL);
+    ARG_CHECK(msg32 != NULL);
+
+    if (session->msg_is_set) {
+        return 0;
+    }
+    memcpy(session->msg, msg32, 32);
+    session->msg_is_set = 1;
+    return 1;
+}
+
+int secp256k1_musig_partial_signature_serialize(const secp256k1_context* ctx, unsigned char *out32, const secp256k1_musig_partial_signature* sig) {
+    VERIFY_CHECK(ctx != NULL);
+    ARG_CHECK(out32 != NULL);
+    ARG_CHECK(sig != NULL);
+    memcpy(out32, sig->data, 32);
+    return 1;
+}
+
+int secp256k1_musig_partial_signature_parse(const secp256k1_context* ctx, secp256k1_musig_partial_signature* sig, const unsigned char *in32) {
+    VERIFY_CHECK(ctx != NULL);
+    ARG_CHECK(sig != NULL);
+    ARG_CHECK(in32 != NULL);
+    memcpy(sig->data, in32, 32);
+    return 1;
+}
+
+/* Compute msghash = SHA256(combined_nonce, combined_pk, msg) */
+static int secp256k1_musig_compute_messagehash(const secp256k1_context *ctx, unsigned char *msghash, const secp256k1_musig_session *session) {
+    unsigned char buf[33];
+    size_t bufsize = 33;
+    secp256k1_ge rp;
+    secp256k1_sha256 sha;
+
+    secp256k1_sha256_initialize(&sha);
+    if (!session->nonce_is_set) {
+        return 0;
+    }
+    secp256k1_pubkey_load(ctx, &rp, &session->combined_nonce);
+    secp256k1_fe_get_b32(buf, &rp.x);
+    secp256k1_sha256_write(&sha, buf, 32);
+    secp256k1_ec_pubkey_serialize(ctx, buf, &bufsize, &session->combined_pk, SECP256K1_EC_COMPRESSED);
+    VERIFY_CHECK(bufsize == 33);
+    secp256k1_sha256_write(&sha, buf, bufsize);
+    if (!session->msg_is_set) {
+        return 0;
+    }
+    secp256k1_sha256_write(&sha, session->msg, 32);
+    secp256k1_sha256_finalize(&sha, msghash);
+    return 1;
+}
+
+int secp256k1_musig_partial_sign(const secp256k1_context* ctx, const secp256k1_musig_session *session, secp256k1_musig_partial_signature *partial_sig) {
+    unsigned char msghash[32];
+    int overflow;
+    secp256k1_scalar sk;
+    secp256k1_scalar e, k;
+
+    VERIFY_CHECK(ctx != NULL);
+    ARG_CHECK(partial_sig != NULL);
+    ARG_CHECK(session != NULL);
+
+    if (!session->nonce_is_set || !session->has_secret_data) {
+        return 0;
+    }
+
+    /* build message hash */
+    if (!secp256k1_musig_compute_messagehash(ctx, msghash, session)) {
+        return 0;
+    }
+    secp256k1_scalar_set_b32(&e, msghash, NULL);
+
+    secp256k1_scalar_set_b32(&sk, session->seckey, &overflow);
+    if (overflow) {
+        secp256k1_scalar_clear(&sk);
+        return 0;
+    }
+
+    secp256k1_scalar_set_b32(&k, session->secnonce, &overflow);
+    if (overflow || secp256k1_scalar_is_zero(&k)) {
+        secp256k1_scalar_clear(&sk);
+        secp256k1_scalar_clear(&k);
+        return 0;
+    }
+    if (session->nonce_is_negated) {
+        secp256k1_scalar_negate(&k, &k);
+    }
+
+    /* Sign */
+    secp256k1_scalar_mul(&e, &e, &sk);
+    secp256k1_scalar_add(&e, &e, &k);
+    secp256k1_scalar_get_b32(&partial_sig->data[0], &e);
+    secp256k1_scalar_clear(&sk);
+    secp256k1_scalar_clear(&k);
+
+    return 1;
+}
+
+int secp256k1_musig_partial_sig_combine(const secp256k1_context* ctx, const secp256k1_musig_session *session, secp256k1_schnorrsig *sig, const secp256k1_musig_partial_signature *partial_sigs, size_t n_sigs) {
+    size_t i;
+    secp256k1_scalar s;
+    secp256k1_ge noncep;
+    (void) ctx;
+
+    VERIFY_CHECK(ctx != NULL);
+    ARG_CHECK(sig != NULL);
+    ARG_CHECK(partial_sigs != NULL);
+    ARG_CHECK(session != NULL);
+
+    if (!session->nonce_is_set) {
+        return 0;
+    }
+    if (n_sigs != session->n_signers) {
+        return 0;
+    }
+    secp256k1_scalar_clear(&s);
+    for (i = 0; i < n_sigs; i++) {
+        int overflow;
+        secp256k1_scalar term;
+
+        secp256k1_scalar_set_b32(&term, partial_sigs[i].data, &overflow);
+        if (overflow) {
+            return 0;
+        }
+        secp256k1_scalar_add(&s, &s, &term);
+    }
+
+    secp256k1_pubkey_load(ctx, &noncep, &session->combined_nonce);
+    VERIFY_CHECK(secp256k1_fe_is_quad_var(&noncep.y));
+    secp256k1_fe_normalize(&noncep.x);
+    secp256k1_fe_get_b32(&sig->data[0], &noncep.x);
+    secp256k1_scalar_get_b32(&sig->data[32], &s);
+
+    return 1;
+}
+
+int secp256k1_musig_partial_sig_verify(const secp256k1_context* ctx, const secp256k1_musig_session *session, const secp256k1_musig_session_signer_data *signer, const secp256k1_musig_partial_signature *partial_sig, const secp256k1_pubkey *pubkey) {
+    unsigned char msghash[32];
+    secp256k1_scalar s;
+    secp256k1_scalar e;
+    secp256k1_scalar mu;
+    secp256k1_gej rj;
+    secp256k1_ge rp;
+    int overflow;
+
+    VERIFY_CHECK(ctx != NULL);
+    ARG_CHECK(secp256k1_ecmult_context_is_built(&ctx->ecmult_ctx));
+    ARG_CHECK(session != NULL);
+    ARG_CHECK(signer != NULL);
+    ARG_CHECK(partial_sig != NULL);
+    ARG_CHECK(pubkey != NULL);
+
+    if (!session->nonce_is_set || !signer->present) {
+        return 0;
+    }
+    secp256k1_scalar_set_b32(&s, partial_sig->data, &overflow);
+    if (overflow) {
+        return 0;
+    }
+    if (!secp256k1_musig_compute_messagehash(ctx, msghash, session)) {
+        return 0;
+    }
+    secp256k1_scalar_set_b32(&e, msghash, NULL);
+
+    /* Multiplying the messagehash by the musig coefficient is equivalent
+     * to multiplying the signer's public key by the coefficient, except
+     * much easier to do. */
+    secp256k1_musig_coefficient(&mu, session->pk_hash, signer->index);
+    secp256k1_scalar_mul(&e, &e, &mu);
+
+    if (!secp256k1_pubkey_load(ctx, &rp, &signer->nonce)) {
+        return 0;
+    }
+
+    if (!secp256k1_schnorrsig_real_verify(ctx, &rj, &s, &e, pubkey)) {
+        return 0;
+    }
+    if (!session->nonce_is_negated) {
+        secp256k1_ge_neg(&rp, &rp);
+    }
+    secp256k1_gej_add_ge_var(&rj, &rj, &rp, NULL);
+
+    return secp256k1_gej_is_infinity(&rj);
+}
+
+int secp256k1_musig_partial_sig_adapt(const secp256k1_context* ctx, secp256k1_musig_partial_signature *adaptor_sig, const secp256k1_musig_partial_signature *partial_sig, const unsigned char *sec_adaptor32, int nonce_is_negated) {
+    secp256k1_scalar s;
+    secp256k1_scalar t;
+    int overflow;
+
+    (void) ctx;
+    VERIFY_CHECK(ctx != NULL);
+    ARG_CHECK(adaptor_sig != NULL);
+    ARG_CHECK(partial_sig != NULL);
+    ARG_CHECK(sec_adaptor32 != NULL);
+
+    secp256k1_scalar_set_b32(&s, partial_sig->data, &overflow);
+    if (overflow) {
+        return 0;
+    }
+    secp256k1_scalar_set_b32(&t, sec_adaptor32, &overflow);
+    if (overflow) {
+        secp256k1_scalar_clear(&t);
+        return 0;
+    }
+
+    if (nonce_is_negated) {
+        secp256k1_scalar_negate(&t, &t);
+    }
+
+    secp256k1_scalar_add(&s, &s, &t);
+    secp256k1_scalar_get_b32(adaptor_sig->data, &s);
+    secp256k1_scalar_clear(&t);
+    return 1;
+}
+
+int secp256k1_musig_extract_secret_adaptor(const secp256k1_context* ctx, unsigned char *sec_adaptor32, const secp256k1_schnorrsig *sig, const secp256k1_musig_partial_signature *partial_sigs, size_t n_partial_sigs, int nonce_is_negated) {
+    secp256k1_scalar t;
+    secp256k1_scalar s;
+    int overflow;
+    size_t i;
+
+    (void) ctx;
+    VERIFY_CHECK(ctx != NULL);
+    ARG_CHECK(sec_adaptor32 != NULL);
+    ARG_CHECK(sig != NULL);
+    ARG_CHECK(partial_sigs != NULL);
+
+    secp256k1_scalar_set_b32(&t, &sig->data[32], &overflow);
+    if (overflow) {
+        return 0;
+    }
+    secp256k1_scalar_negate(&t, &t);
+
+    for (i = 0; i < n_partial_sigs; i++) {
+        secp256k1_scalar_set_b32(&s, partial_sigs[i].data, &overflow);
+        if (overflow) {
+            secp256k1_scalar_clear(&t);
+            return 0;
+        }
+        secp256k1_scalar_add(&t, &t, &s);
+    }
+
+    if (!nonce_is_negated) {
+        secp256k1_scalar_negate(&t, &t);
+    }
+    secp256k1_scalar_get_b32(sec_adaptor32, &t);
+    secp256k1_scalar_clear(&t);
+    return 1;
+}
+
+#endif

src/modules/musig/tests_impl.h

@@ -0,0 +1,757 @@
+/**********************************************************************
+ * Copyright (c) 2018 Andrew Poelstra                                 *
+ * Distributed under the MIT software license, see the accompanying   *
+ * file COPYING or http://www.opensource.org/licenses/mit-license.php.*
+ **********************************************************************/
+
+#ifndef _SECP256K1_MODULE_MUSIG_TESTS_
+#define _SECP256K1_MODULE_MUSIG_TESTS_
+
+#include "secp256k1_musig.h"
+
+void musig_api_tests(secp256k1_scratch_space *scratch) {
+    secp256k1_scratch_space *scratch_small;
+    secp256k1_musig_session session[2];
+    secp256k1_musig_session verifier_session;
+    secp256k1_musig_session_signer_data signer0[2];
+    secp256k1_musig_session_signer_data signer1[2];
+    secp256k1_musig_session_signer_data verifier_signer_data[2];
+    secp256k1_musig_partial_signature partial_sig[2];
+    secp256k1_musig_partial_signature partial_sig_adapted[2];
+    secp256k1_musig_partial_signature partial_sig_overflow;
+    secp256k1_schnorrsig final_sig;
+    secp256k1_schnorrsig final_sig_cmp;
+
+    unsigned char buf[32];
+    unsigned char sk[2][32];
+    unsigned char ones[32];
+    unsigned char session_id[2][32];
+    unsigned char nonce_commitment[2][32];
+    int nonce_is_negated;
+    const unsigned char *ncs[2];
+    unsigned char msg[32];
+    unsigned char msghash[32];
+    secp256k1_pubkey combined_pk;
+    unsigned char pk_hash[32];
+    secp256k1_pubkey pk[2];
+
+    unsigned char sec_adaptor[32];
+    unsigned char sec_adaptor1[32];
+    secp256k1_pubkey adaptor;
+
+    /** setup **/
+    secp256k1_context *none = secp256k1_context_create(SECP256K1_CONTEXT_NONE);
+    secp256k1_context *sign = secp256k1_context_create(SECP256K1_CONTEXT_SIGN);
+    secp256k1_context *vrfy = secp256k1_context_create(SECP256K1_CONTEXT_VERIFY);
+    int ecount;
+
+    secp256k1_context_set_error_callback(none, counting_illegal_callback_fn, &ecount);
+    secp256k1_context_set_error_callback(sign, counting_illegal_callback_fn, &ecount);
+    secp256k1_context_set_error_callback(vrfy, counting_illegal_callback_fn, &ecount);
+    secp256k1_context_set_illegal_callback(none, counting_illegal_callback_fn, &ecount);
+    secp256k1_context_set_illegal_callback(sign, counting_illegal_callback_fn, &ecount);
+    secp256k1_context_set_illegal_callback(vrfy, counting_illegal_callback_fn, &ecount);
+
+    memset(ones, 0xff, 32);
+
+    secp256k1_rand256(session_id[0]);
+    secp256k1_rand256(session_id[1]);
+    secp256k1_rand256(sk[0]);
+    secp256k1_rand256(sk[1]);
+    secp256k1_rand256(msg);
+    secp256k1_rand256(sec_adaptor);
+
+    CHECK(secp256k1_ec_pubkey_create(ctx, &pk[0], sk[0]) == 1);
+    CHECK(secp256k1_ec_pubkey_create(ctx, &pk[1], sk[1]) == 1);
+    CHECK(secp256k1_ec_pubkey_create(ctx, &adaptor, sec_adaptor) == 1);
+
+    /** main test body **/
+
+    /* Key combination */
+    ecount = 0;
+    CHECK(secp256k1_musig_pubkey_combine(none, scratch, &combined_pk, pk_hash, pk, 2) == 0);
+    CHECK(ecount == 1);
+    CHECK(secp256k1_musig_pubkey_combine(sign, scratch, &combined_pk, pk_hash, pk, 2) == 0);
+    CHECK(ecount == 2);
+    CHECK(secp256k1_musig_pubkey_combine(vrfy, scratch, &combined_pk, pk_hash, pk, 2) == 1);
+    CHECK(ecount == 2);
+    /* pubkey_combine does not require a scratch space */
+    CHECK(secp256k1_musig_pubkey_combine(vrfy, NULL, &combined_pk, pk_hash, pk, 2) == 1);
+    CHECK(ecount == 2);
+    /* If a scratch space is given it shouldn't be too small */
+    scratch_small = secp256k1_scratch_space_create(ctx, 1);
+    CHECK(secp256k1_musig_pubkey_combine(vrfy, scratch_small, &combined_pk, pk_hash, pk, 2) == 0);
+    secp256k1_scratch_space_destroy(scratch_small);
+    CHECK(ecount == 2);
+    CHECK(secp256k1_musig_pubkey_combine(vrfy, scratch, NULL, pk_hash, pk, 2) == 0);
+    CHECK(ecount == 3);
+    CHECK(secp256k1_musig_pubkey_combine(vrfy, scratch, &combined_pk, NULL, pk, 2) == 1);
+    CHECK(ecount == 3);
+    CHECK(secp256k1_musig_pubkey_combine(vrfy, scratch, &combined_pk, pk_hash, NULL, 2) == 0);
+    CHECK(ecount == 4);
+    CHECK(secp256k1_musig_pubkey_combine(vrfy, scratch, &combined_pk, pk_hash, pk, 0) == 0);
+    CHECK(ecount == 5);
+    CHECK(secp256k1_musig_pubkey_combine(vrfy, scratch, &combined_pk, pk_hash, NULL, 0) == 0);
+    CHECK(ecount == 6);
+
+    CHECK(secp256k1_musig_pubkey_combine(vrfy, scratch, &combined_pk, pk_hash, pk, 2) == 1);
+    CHECK(secp256k1_musig_pubkey_combine(vrfy, scratch, &combined_pk, pk_hash, pk, 2) == 1);
+    CHECK(secp256k1_musig_pubkey_combine(vrfy, scratch, &combined_pk, pk_hash, pk, 2) == 1);
+
+    /** Session creation **/
+    ecount = 0;
+    CHECK(secp256k1_musig_session_initialize(none, &session[0], signer0, nonce_commitment[0], session_id[0], msg, &combined_pk, pk_hash, 2, 0, sk[0]) == 0);
+    CHECK(ecount == 1);
+    CHECK(secp256k1_musig_session_initialize(vrfy, &session[0], signer0, nonce_commitment[0], session_id[0], msg, &combined_pk, pk_hash, 2, 0, sk[0]) == 0);
+    CHECK(ecount == 2);
+    CHECK(secp256k1_musig_session_initialize(sign, &session[0], signer0, nonce_commitment[0], session_id[0], msg, &combined_pk, pk_hash, 2, 0, sk[0]) == 1);
+    CHECK(ecount == 2);
+    CHECK(secp256k1_musig_session_initialize(sign, NULL, signer0, nonce_commitment[0], session_id[0], msg, &combined_pk, pk_hash, 2, 0, sk[0]) == 0);
+    CHECK(ecount == 3);
+    CHECK(secp256k1_musig_session_initialize(sign, &session[0], NULL, nonce_commitment[0], session_id[0], msg, &combined_pk, pk_hash, 2, 0, sk[0]) == 0);
+    CHECK(ecount == 4);
+    CHECK(secp256k1_musig_session_initialize(sign, &session[0], signer0, NULL, session_id[0], msg, &combined_pk, pk_hash, 2, 0, sk[0]) == 0);
+    CHECK(ecount == 5);
+    CHECK(secp256k1_musig_session_initialize(sign, &session[0], signer0, nonce_commitment[0], NULL, msg, &combined_pk, pk_hash, 2, 0, sk[0]) == 0);
+    CHECK(ecount == 6);
+    CHECK(secp256k1_musig_session_initialize(sign, &session[0], signer0, nonce_commitment[0], session_id[0], NULL, &combined_pk, pk_hash, 2, 0, sk[0]) == 1);
+    CHECK(ecount == 6);
+    CHECK(secp256k1_musig_session_initialize(sign, &session[0], signer0, nonce_commitment[0], session_id[0], msg, NULL, pk_hash, 2, 0, sk[0]) == 0);
+    CHECK(ecount == 7);
+    CHECK(secp256k1_musig_session_initialize(sign, &session[0], signer0, nonce_commitment[0], session_id[0], msg, &combined_pk, NULL, 2, 0, sk[0]) == 0);
+    CHECK(ecount == 8);
+    CHECK(secp256k1_musig_session_initialize(sign, &session[0], signer0, nonce_commitment[0], session_id[0], msg, &combined_pk, pk_hash, 0, 0, sk[0]) == 0);
+    CHECK(ecount == 8);
+    /* If more than UINT32_MAX fits in a size_t, test that session_initialize
+     * rejects n_signers that high. */
+    if (SIZE_MAX > UINT32_MAX) {
+        CHECK(secp256k1_musig_session_initialize(sign, &session[0], signer0, nonce_commitment[0], session_id[0], msg, &combined_pk, pk_hash, ((size_t) UINT32_MAX) + 2, 0, sk[0]) == 0);
+    }
+    CHECK(ecount == 8);
+    CHECK(secp256k1_musig_session_initialize(sign, &session[0], signer0, nonce_commitment[0], session_id[0], msg, &combined_pk, pk_hash, 2, 0, NULL) == 0);
+    CHECK(ecount == 9);
+    /* secret key overflows */
+    CHECK(secp256k1_musig_session_initialize(sign, &session[0], signer0, nonce_commitment[0], session_id[0], msg, &combined_pk, pk_hash, 2, 0, ones) == 0);
+    CHECK(ecount == 9);
+
+
+    {
+        secp256k1_musig_session session_without_msg;
+        CHECK(secp256k1_musig_session_initialize(sign, &session_without_msg, signer0, nonce_commitment[0], session_id[0], NULL, &combined_pk, pk_hash, 2, 0, sk[0]) == 1);
+        CHECK(secp256k1_musig_session_set_msg(none, &session_without_msg, msg) == 1);
+        CHECK(secp256k1_musig_session_set_msg(none, &session_without_msg, msg) == 0);
+    }
+    CHECK(secp256k1_musig_session_initialize(sign, &session[0], signer0, nonce_commitment[0], session_id[0], msg, &combined_pk, pk_hash, 2, 0, sk[0]) == 1);
+    CHECK(secp256k1_musig_session_initialize(sign, &session[1], signer1, nonce_commitment[1], session_id[1], msg, &combined_pk, pk_hash, 2, 1, sk[1]) == 1);
+    ncs[0] = nonce_commitment[0];
+    ncs[1] = nonce_commitment[1];
+
+    ecount = 0;
+    CHECK(secp256k1_musig_session_initialize_verifier(none, &verifier_session, verifier_signer_data, msg, &combined_pk, pk_hash, ncs, 2) == 1);
+    CHECK(ecount == 0);
+    CHECK(secp256k1_musig_session_initialize_verifier(none, NULL, verifier_signer_data, msg, &combined_pk, pk_hash, ncs, 2) == 0);
+    CHECK(ecount == 1);
+    CHECK(secp256k1_musig_session_initialize_verifier(none, &verifier_session, verifier_signer_data, NULL, &combined_pk, pk_hash, ncs, 2) == 1);
+    CHECK(ecount == 1);
+    CHECK(secp256k1_musig_session_initialize_verifier(none, &verifier_session, verifier_signer_data, msg, NULL, pk_hash, ncs, 2) == 0);
+    CHECK(ecount == 2);
+    CHECK(secp256k1_musig_session_initialize_verifier(none, &verifier_session, verifier_signer_data, msg, &combined_pk, NULL, ncs, 2) == 0);
+    CHECK(ecount == 3);
+    CHECK(secp256k1_musig_session_initialize_verifier(none, &verifier_session, verifier_signer_data, msg, &combined_pk, pk_hash, NULL, 2) == 0);
+    CHECK(ecount == 4);
+    CHECK(secp256k1_musig_session_initialize_verifier(none, &verifier_session, verifier_signer_data, msg, &combined_pk, pk_hash, ncs, 0) == 0);
+    CHECK(ecount == 4);
+    if (SIZE_MAX > UINT32_MAX) {
+        CHECK(secp256k1_musig_session_initialize_verifier(none, &verifier_session, verifier_signer_data, msg, &combined_pk, pk_hash, ncs, ((size_t) UINT32_MAX) + 2) == 0);
+    }
+    CHECK(ecount == 4);
+    CHECK(secp256k1_musig_session_initialize_verifier(none, &verifier_session, verifier_signer_data, msg, &combined_pk, pk_hash, ncs, 2) == 1);
+
+    CHECK(secp256k1_musig_compute_messagehash(none, msghash, &verifier_session) == 0);
+    CHECK(secp256k1_musig_compute_messagehash(none, msghash, &session[0]) == 0);
+
+    /** Signing step 0 -- exchange nonce commitments */
+    ecount = 0;
+    {
+        secp256k1_pubkey nonce;
+
+        /* Can obtain public nonce after commitments have been exchanged; still can't sign */
+        CHECK(secp256k1_musig_session_get_public_nonce(none, &session[0], signer0, &nonce, ncs, 2) == 1);
+        CHECK(secp256k1_musig_partial_sign(none, &session[0], &partial_sig[0]) == 0);
+        CHECK(ecount == 0);
+    }
+
+    /** Signing step 1 -- exchange nonces */
+    ecount = 0;
+    {
+        secp256k1_pubkey public_nonce[3];
+
+        CHECK(secp256k1_musig_session_get_public_nonce(none, &session[0], signer0, &public_nonce[0], ncs, 2) == 1);
+        CHECK(ecount == 0);
+        CHECK(secp256k1_musig_session_get_public_nonce(none, NULL, signer0, &public_nonce[0], ncs, 2) == 0);
+        CHECK(ecount == 1);
+        CHECK(secp256k1_musig_session_get_public_nonce(none, &session[0], NULL, &public_nonce[0], ncs, 2) == 0);
+        CHECK(ecount == 2);
+        CHECK(secp256k1_musig_session_get_public_nonce(none, &session[0], signer0, NULL, ncs, 2) == 0);
+        CHECK(ecount == 3);
+        CHECK(secp256k1_musig_session_get_public_nonce(none, &session[0], signer0, &public_nonce[0], NULL, 2) == 0);
+        CHECK(ecount == 4);
+        /* Number of commitments and number of signers are different */
+        CHECK(secp256k1_musig_session_get_public_nonce(none, &session[0], signer0, &public_nonce[0], ncs, 1) == 0);
+        CHECK(ecount == 4);
+
+        CHECK(secp256k1_musig_session_get_public_nonce(none, &session[0], signer0, &public_nonce[0], ncs, 2) == 1);
+        CHECK(secp256k1_musig_session_get_public_nonce(none, &session[1], signer1, &public_nonce[1], ncs, 2) == 1);
+
+        CHECK(secp256k1_musig_set_nonce(none, &signer0[0], &public_nonce[0]) == 1);
+        CHECK(secp256k1_musig_set_nonce(none, &signer0[1], &public_nonce[0]) == 0);
+        CHECK(secp256k1_musig_set_nonce(none, &signer0[1], &public_nonce[1]) == 1);
+        CHECK(secp256k1_musig_set_nonce(none, &signer0[1], &public_nonce[1]) == 1);
+        CHECK(ecount == 4);
+
+        CHECK(secp256k1_musig_set_nonce(none, NULL, &public_nonce[0]) == 0);
+        CHECK(ecount == 5);
+        CHECK(secp256k1_musig_set_nonce(none, &signer1[0], NULL) == 0);
+        CHECK(ecount == 6);
+
+        CHECK(secp256k1_musig_set_nonce(none, &signer1[0], &public_nonce[0]) == 1);
+        CHECK(secp256k1_musig_set_nonce(none, &signer1[1], &public_nonce[1]) == 1);
+        CHECK(secp256k1_musig_set_nonce(none, &verifier_signer_data[0], &public_nonce[0]) == 1);
+        CHECK(secp256k1_musig_set_nonce(none, &verifier_signer_data[1], &public_nonce[1]) == 1);
+
+        ecount = 0;
+        CHECK(secp256k1_musig_session_combine_nonces(none, &session[0], signer0, 2, &nonce_is_negated, &adaptor) == 1);
+        CHECK(secp256k1_musig_session_combine_nonces(none, NULL, signer0, 2, &nonce_is_negated, &adaptor) == 0);
+        CHECK(ecount == 1);
+        CHECK(secp256k1_musig_session_combine_nonces(none, &session[0], NULL, 2, &nonce_is_negated, &adaptor) == 0);
+        CHECK(ecount == 2);
+        /* Number of signers differs from number during intialization */
+        CHECK(secp256k1_musig_session_combine_nonces(none, &session[0], signer0, 1, &nonce_is_negated, &adaptor) == 0);
+        CHECK(ecount == 2);
+        CHECK(secp256k1_musig_session_combine_nonces(none, &session[0], signer0, 2, NULL, &adaptor) == 1);
+        CHECK(ecount == 2);
+        CHECK(secp256k1_musig_session_combine_nonces(none, &session[0], signer0, 2, &nonce_is_negated, NULL) == 1);
+
+        CHECK(secp256k1_musig_session_combine_nonces(none, &session[0], signer0, 2, &nonce_is_negated, &adaptor) == 1);
+        CHECK(secp256k1_musig_session_combine_nonces(none, &session[1], signer0, 2, &nonce_is_negated, &adaptor) == 1);
+        CHECK(secp256k1_musig_session_combine_nonces(none, &verifier_session, verifier_signer_data, 2, &nonce_is_negated, &adaptor) == 1);
+    }
+
+    /** Signing step 2 -- partial signatures */
+    ecount = 0;
+    CHECK(secp256k1_musig_partial_sign(none, &session[0], &partial_sig[0]) == 1);
+    CHECK(ecount == 0);
+    CHECK(secp256k1_musig_partial_sign(none, NULL, &partial_sig[0]) == 0);
+    CHECK(ecount == 1);
+    CHECK(secp256k1_musig_partial_sign(none, &session[0], NULL) == 0);
+    CHECK(ecount == 2);
+
+    CHECK(secp256k1_musig_partial_sign(none, &session[0], &partial_sig[0]) == 1);
+    CHECK(secp256k1_musig_partial_sign(none, &session[1], &partial_sig[1]) == 1);
+    /* observer can't sign */
+    CHECK(secp256k1_musig_partial_sign(none, &verifier_session, &partial_sig[2]) == 0);
+    CHECK(ecount == 2);
+
+    ecount = 0;
+    CHECK(secp256k1_musig_partial_signature_serialize(none, buf, &partial_sig[0]) == 1);
+    CHECK(secp256k1_musig_partial_signature_serialize(none, NULL, &partial_sig[0]) == 0);
+    CHECK(ecount == 1);
+    CHECK(secp256k1_musig_partial_signature_serialize(none, buf, NULL) == 0);
+    CHECK(ecount == 2);
+    CHECK(secp256k1_musig_partial_signature_parse(none, &partial_sig[0], buf) == 1);
+    CHECK(secp256k1_musig_partial_signature_parse(none, NULL, buf) == 0);
+    CHECK(ecount == 3);
+    CHECK(secp256k1_musig_partial_signature_parse(none, &partial_sig[0], NULL) == 0);
+    CHECK(ecount == 4);
+    CHECK(secp256k1_musig_partial_signature_parse(none, &partial_sig_overflow, ones) == 1);
+
+    /** Partial signature verification */
+    ecount = 0;
+    CHECK(secp256k1_musig_partial_sig_verify(none, &session[0], &signer0[0], &partial_sig[0], &pk[0]) == 0);
+    CHECK(ecount == 1);
+    CHECK(secp256k1_musig_partial_sig_verify(sign, &session[0], &signer0[0], &partial_sig[0], &pk[0]) == 0);
+    CHECK(ecount == 2);
+    CHECK(secp256k1_musig_partial_sig_verify(vrfy, &session[0], &signer0[0], &partial_sig[0], &pk[0]) == 1);
+    CHECK(ecount == 2);
+    CHECK(secp256k1_musig_partial_sig_verify(vrfy, &session[0], &signer0[0], &partial_sig[1], &pk[0]) == 0);
+    CHECK(ecount == 2);
+    CHECK(secp256k1_musig_partial_sig_verify(vrfy, NULL, &signer0[0], &partial_sig[0], &pk[0]) == 0);
+    CHECK(ecount == 3);
+    CHECK(secp256k1_musig_partial_sig_verify(vrfy, &session[0], NULL, &partial_sig[0], &pk[0]) == 0);
+    CHECK(ecount == 4);
+    CHECK(secp256k1_musig_partial_sig_verify(vrfy, &session[0], &signer0[0], NULL, &pk[0]) == 0);
+    CHECK(ecount == 5);
+    CHECK(secp256k1_musig_partial_sig_verify(vrfy, &session[0], &signer0[0], &partial_sig_overflow, &pk[0]) == 0);
+    CHECK(ecount == 5);
+    CHECK(secp256k1_musig_partial_sig_verify(vrfy, &session[0], &signer0[0], &partial_sig[0], NULL) == 0);
+    CHECK(ecount == 6);
+
+    CHECK(secp256k1_musig_partial_sig_verify(vrfy, &session[0], &signer0[0], &partial_sig[0], &pk[0]) == 1);
+    CHECK(secp256k1_musig_partial_sig_verify(vrfy, &session[1], &signer1[0], &partial_sig[0], &pk[0]) == 1);
+    CHECK(secp256k1_musig_partial_sig_verify(vrfy, &session[0], &signer0[1], &partial_sig[1], &pk[1]) == 1);
+    CHECK(secp256k1_musig_partial_sig_verify(vrfy, &session[1], &signer1[1], &partial_sig[1], &pk[1]) == 1);
+    CHECK(secp256k1_musig_partial_sig_verify(vrfy, &verifier_session, &verifier_signer_data[0], &partial_sig[0], &pk[0]) == 1);
+    CHECK(secp256k1_musig_partial_sig_verify(vrfy, &verifier_session, &verifier_signer_data[1], &partial_sig[1], &pk[1]) == 1);
+    CHECK(ecount == 6);
+
+    /** Adaptor signature verification */
+    memcpy(&partial_sig_adapted[1], &partial_sig[1], sizeof(partial_sig_adapted[1]));
+    ecount = 0;
+    CHECK(secp256k1_musig_partial_sig_adapt(none, &partial_sig_adapted[0], &partial_sig[0], sec_adaptor, nonce_is_negated) == 1);
+    CHECK(secp256k1_musig_partial_sig_adapt(none, NULL, &partial_sig[0], sec_adaptor, 0) == 0);
+    CHECK(ecount == 1);
+    CHECK(secp256k1_musig_partial_sig_adapt(none, &partial_sig_adapted[0], NULL, sec_adaptor, 0) == 0);
+    CHECK(ecount == 2);
+    CHECK(secp256k1_musig_partial_sig_adapt(none, &partial_sig_adapted[0], &partial_sig_overflow, sec_adaptor, nonce_is_negated) == 0);
+    CHECK(ecount == 2);
+    CHECK(secp256k1_musig_partial_sig_adapt(none, &partial_sig_adapted[0], &partial_sig[0], NULL, 0) == 0);
+    CHECK(ecount == 3);
+    CHECK(secp256k1_musig_partial_sig_adapt(none, &partial_sig_adapted[0], &partial_sig[0], ones, nonce_is_negated) == 0);
+    CHECK(ecount == 3);
+
+    /** Signing combining and verification */
+    ecount = 0;
+    CHECK(secp256k1_musig_partial_sig_combine(none, &session[0], &final_sig, partial_sig_adapted, 2) == 1);
+    CHECK(secp256k1_musig_partial_sig_combine(none, &session[0], &final_sig_cmp, partial_sig_adapted, 2) == 1);
+    CHECK(memcmp(&final_sig, &final_sig_cmp, sizeof(final_sig)) == 0);
+    CHECK(secp256k1_musig_partial_sig_combine(none, &session[0], &final_sig_cmp, partial_sig_adapted, 2) == 1);
+    CHECK(memcmp(&final_sig, &final_sig_cmp, sizeof(final_sig)) == 0);
+
+    CHECK(secp256k1_musig_partial_sig_combine(none, NULL, &final_sig, partial_sig_adapted, 2) == 0);
+    CHECK(ecount == 1);
+    CHECK(secp256k1_musig_partial_sig_combine(none, &session[0], NULL, partial_sig_adapted, 2) == 0);
+    CHECK(ecount == 2);
+    CHECK(secp256k1_musig_partial_sig_combine(none, &session[0], &final_sig, NULL, 2) == 0);
+    CHECK(ecount == 3);
+    {
+        secp256k1_musig_partial_signature partial_sig_tmp[2];
+        partial_sig_tmp[0] = partial_sig_adapted[0];
+        partial_sig_tmp[1] = partial_sig_overflow;
+        CHECK(secp256k1_musig_partial_sig_combine(none, &session[0], &final_sig, partial_sig_tmp, 2) == 0);
+    }
+    CHECK(ecount == 3);
+    /* Wrong number of partial sigs */
+    CHECK(secp256k1_musig_partial_sig_combine(none, &session[0], &final_sig, partial_sig_adapted, 1) == 0);
+    CHECK(ecount == 3);
+
+    CHECK(secp256k1_schnorrsig_verify(vrfy, &final_sig, msg, &combined_pk) == 1);
+
+    /** Secret adaptor can be extracted from signature */
+    ecount = 0;
+    CHECK(secp256k1_musig_extract_secret_adaptor(none, sec_adaptor1, &final_sig, partial_sig, 2, nonce_is_negated) == 1);
+    CHECK(memcmp(sec_adaptor, sec_adaptor1, 32) == 0);
+    CHECK(secp256k1_musig_extract_secret_adaptor(none, NULL, &final_sig, partial_sig, 2, 0) == 0);
+    CHECK(ecount == 1);
+    CHECK(secp256k1_musig_extract_secret_adaptor(none, sec_adaptor1, NULL, partial_sig, 2, 0) == 0);
+    CHECK(ecount == 2);
+    {
+        secp256k1_schnorrsig final_sig_tmp = final_sig;
+        memcpy(&final_sig_tmp.data[32], ones, 32);
+        CHECK(secp256k1_musig_extract_secret_adaptor(none, sec_adaptor1, &final_sig_tmp, partial_sig, 2, nonce_is_negated) == 0);
+    }
+    CHECK(ecount == 2);
+    CHECK(secp256k1_musig_extract_secret_adaptor(none, sec_adaptor1, &final_sig, NULL, 2, 0) == 0);
+    CHECK(ecount == 3);
+    {
+        secp256k1_musig_partial_signature partial_sig_tmp[2];
+        partial_sig_tmp[0] = partial_sig[0];
+        partial_sig_tmp[1] = partial_sig_overflow;
+        CHECK(secp256k1_musig_extract_secret_adaptor(none, sec_adaptor1, &final_sig, partial_sig_tmp, 2, nonce_is_negated) == 0);
+    }
+    CHECK(ecount == 3);
+    CHECK(secp256k1_musig_extract_secret_adaptor(none, sec_adaptor1, &final_sig, partial_sig, 0, 0) == 1);
+    CHECK(secp256k1_musig_extract_secret_adaptor(none, sec_adaptor1, &final_sig, partial_sig, 2, 1) == 1);
+
+    /** cleanup **/
+    memset(&session, 0, sizeof(session));
+    secp256k1_context_destroy(none);
+    secp256k1_context_destroy(sign);
+    secp256k1_context_destroy(vrfy);
+}
+
+/* Initializes two sessions, one use the given parameters (session_id,
+ * nonce_commitments, etc.) except that `session_tmp` uses new signers with different
+ * public keys. The point of this test is to call `musig_session_get_public_nonce`
+ * with signers from `session_tmp` who have different public keys than the correct
+ * ones and return the resulting messagehash. This should not result in a different
+ * messagehash because the public keys of the signers are only used during session
+ * initialization. */
+int musig_state_machine_diff_signer_msghash_test(unsigned char *msghash, secp256k1_pubkey *pks, secp256k1_pubkey *combined_pk, unsigned char *pk_hash, const unsigned char * const *nonce_commitments, unsigned char *msg, secp256k1_pubkey *nonce_other, unsigned char *sk, unsigned char *session_id) {
+    secp256k1_musig_session session;
+    secp256k1_musig_session session_tmp;
+    unsigned char nonce_commitment[32];
+    secp256k1_musig_session_signer_data signers[2];
+    secp256k1_musig_session_signer_data signers_tmp[2];
+    unsigned char sk_dummy[32];
+    secp256k1_pubkey pks_tmp[2];
+    secp256k1_pubkey combined_pk_tmp;
+    unsigned char pk_hash_tmp[32];
+    secp256k1_pubkey nonce;
+
+    /* Set up signers with different public keys */
+    secp256k1_rand256(sk_dummy);
+    pks_tmp[0] = pks[0];
+    CHECK(secp256k1_ec_pubkey_create(ctx, &pks_tmp[1], sk_dummy) == 1);
+    CHECK(secp256k1_musig_pubkey_combine(ctx, NULL, &combined_pk_tmp, pk_hash_tmp, pks_tmp, 2) == 1);
+    CHECK(secp256k1_musig_session_initialize(ctx, &session_tmp, signers_tmp, nonce_commitment, session_id, msg, &combined_pk_tmp, pk_hash_tmp, 2, 0, sk_dummy) == 1);
+
+    CHECK(secp256k1_musig_session_initialize(ctx, &session, signers, nonce_commitment, session_id, msg, combined_pk, pk_hash, 2, 0, sk) == 1);
+    CHECK(memcmp(nonce_commitment, nonce_commitments[1], 32) == 0);
+    /* Call get_public_nonce with different signers than the signers the session was
+     * initialized with. */
+    CHECK(secp256k1_musig_session_get_public_nonce(ctx, &session_tmp, signers, &nonce, nonce_commitments, 2) == 1);
+    CHECK(secp256k1_musig_session_get_public_nonce(ctx, &session, signers_tmp, &nonce, nonce_commitments, 2) == 1);
+    CHECK(secp256k1_musig_set_nonce(ctx, &signers[0], nonce_other) == 1);
+    CHECK(secp256k1_musig_set_nonce(ctx, &signers[1], &nonce) == 1);
+    CHECK(secp256k1_musig_session_combine_nonces(ctx, &session, signers, 2, NULL, NULL) == 1);
+
+    return secp256k1_musig_compute_messagehash(ctx, msghash, &session);
+}
+
+/* Creates a new session (with a different session id) and tries to use that session
+ * to combine nonces with given signers_other. This should fail, because the nonce
+ * commitments of signers_other do not match the nonce commitments the new session
+ * was initialized with. If do_test is 0, the correct signers are being used and
+ * therefore the function should return 1. */
+int musig_state_machine_diff_signers_combine_nonce_test(secp256k1_pubkey *combined_pk, unsigned char *pk_hash, unsigned char *nonce_commitment_other, secp256k1_pubkey *nonce_other, unsigned char *msg, unsigned char *sk, secp256k1_musig_session_signer_data *signers_other, int do_test) {
+    secp256k1_musig_session session;
+    secp256k1_musig_session_signer_data signers[2];
+    secp256k1_musig_session_signer_data *signers_to_use;
+    unsigned char nonce_commitment[32];
+    unsigned char session_id[32];
+    secp256k1_pubkey nonce;
+    const unsigned char *ncs[2];
+
+    /* Initialize new signers */
+    secp256k1_rand256(session_id);
+    CHECK(secp256k1_musig_session_initialize(ctx, &session, signers, nonce_commitment, session_id, msg, combined_pk, pk_hash, 2, 1, sk) == 1);
+    ncs[0] = nonce_commitment_other;
+    ncs[1] = nonce_commitment;
+    CHECK(secp256k1_musig_session_get_public_nonce(ctx, &session, signers, &nonce, ncs, 2) == 1);
+    CHECK(secp256k1_musig_set_nonce(ctx, &signers[0], nonce_other) == 1);
+    CHECK(secp256k1_musig_set_nonce(ctx, &signers[1], &nonce) == 1);
+    CHECK(secp256k1_musig_set_nonce(ctx, &signers[1], &nonce) == 1);
+    secp256k1_musig_session_combine_nonces(ctx, &session, signers_other, 2, NULL, NULL);
+    if (do_test) {
+        signers_to_use = signers_other;
+    } else {
+        signers_to_use = signers;
+    }
+    return secp256k1_musig_session_combine_nonces(ctx, &session, signers_to_use, 2, NULL, NULL);
+}
+
+/* Recreates a session with the given session_id, signers, pk, msg etc. parameters
+ * and tries to sign and verify the other signers partial signature. Both should fail
+ * if msg is NULL. */
+int musig_state_machine_missing_msg_test(secp256k1_pubkey *pks, secp256k1_pubkey *combined_pk, unsigned char *pk_hash, unsigned char *nonce_commitment_other, secp256k1_pubkey *nonce_other, secp256k1_musig_partial_signature *partial_sig_other, unsigned char *sk, unsigned char *session_id, unsigned char *msg) {
+    secp256k1_musig_session session;
+    secp256k1_musig_session_signer_data signers[2];
+    unsigned char nonce_commitment[32];
+    const unsigned char *ncs[2];
+    secp256k1_pubkey nonce;
+    secp256k1_musig_partial_signature partial_sig;
+    int partial_sign, partial_verify;
+
+    CHECK(secp256k1_musig_session_initialize(ctx, &session, signers, nonce_commitment, session_id, msg, combined_pk, pk_hash, 2, 0, sk) == 1);
+    ncs[0] = nonce_commitment_other;
+    ncs[1] = nonce_commitment;
+    CHECK(secp256k1_musig_session_get_public_nonce(ctx, &session, signers, &nonce, ncs, 2) == 1);
+    CHECK(secp256k1_musig_set_nonce(ctx, &signers[0], nonce_other) == 1);
+    CHECK(secp256k1_musig_set_nonce(ctx, &signers[1], &nonce) == 1);
+
+    CHECK(secp256k1_musig_session_combine_nonces(ctx, &session, signers, 2, NULL, NULL) == 1);
+    partial_sign = secp256k1_musig_partial_sign(ctx, &session, &partial_sig);
+    partial_verify = secp256k1_musig_partial_sig_verify(ctx, &session, &signers[0], partial_sig_other, &pks[0]);
+    if (msg != NULL) {
+        /* Return 1 if both succeeded */
+        return partial_sign && partial_verify;
+    }
+    /* Return 0 if both failed */
+    return partial_sign || partial_verify;
+}
+
+/* Recreates a session with the given session_id, signers, pk, msg etc. parameters
+ * and tries to verify and combine partial sigs. If do_combine is 0, the
+ * combine_nonces step is left out. In that case verify and combine should fail and
+ * this function should return 0. */
+int musig_state_machine_missing_combine_test(secp256k1_pubkey *pks, secp256k1_pubkey *combined_pk, unsigned char *pk_hash, unsigned char *nonce_commitment_other, secp256k1_pubkey *nonce_other, secp256k1_musig_partial_signature *partial_sig_other, unsigned char *msg, unsigned char *sk, unsigned char *session_id, secp256k1_musig_partial_signature *partial_sig, int do_combine) {
+    secp256k1_musig_session session;
+    secp256k1_musig_session_signer_data signers[2];
+    unsigned char nonce_commitment[32];
+    const unsigned char *ncs[2];
+    secp256k1_pubkey nonce;
+    secp256k1_musig_partial_signature partial_sigs[2];
+    secp256k1_schnorrsig sig;
+    int partial_verify, sig_combine;
+
+    CHECK(secp256k1_musig_session_initialize(ctx, &session, signers, nonce_commitment, session_id, msg, combined_pk, pk_hash, 2, 0, sk) == 1);
+    ncs[0] = nonce_commitment_other;
+    ncs[1] = nonce_commitment;
+    CHECK(secp256k1_musig_session_get_public_nonce(ctx, &session, signers, &nonce, ncs, 2) == 1);
+    CHECK(secp256k1_musig_set_nonce(ctx, &signers[0], nonce_other) == 1);
+    CHECK(secp256k1_musig_set_nonce(ctx, &signers[1], &nonce) == 1);
+
+    partial_sigs[0] = *partial_sig_other;
+    partial_sigs[1] = *partial_sig;
+    if (do_combine != 0) {
+        CHECK(secp256k1_musig_session_combine_nonces(ctx, &session, signers, 2, NULL, NULL) == 1);
+    }
+    partial_verify = secp256k1_musig_partial_sig_verify(ctx, &session, signers, partial_sig_other, &pks[0]);
+    sig_combine = secp256k1_musig_partial_sig_combine(ctx, &session, &sig, partial_sigs, 2);
+    if (do_combine != 0) {
+        /* Return 1 if both succeeded */
+        return partial_verify && sig_combine;
+    }
+    /* Return 0 if both failed */
+    return partial_verify || sig_combine;
+}
+
+void musig_state_machine_tests(secp256k1_scratch_space *scratch) {
+    size_t i;
+    secp256k1_musig_session session[2];
+    secp256k1_musig_session_signer_data signers0[2];
+    secp256k1_musig_session_signer_data signers1[2];
+    unsigned char nonce_commitment[2][32];
+    unsigned char session_id[2][32];
+    unsigned char msg[32];
+    unsigned char sk[2][32];
+    secp256k1_pubkey pk[2];
+    secp256k1_pubkey combined_pk;
+    unsigned char pk_hash[32];
+    secp256k1_pubkey nonce[2];
+    const unsigned char *ncs[2];
+    secp256k1_musig_partial_signature partial_sig[2];
+    unsigned char msghash1[32];
+    unsigned char msghash2[32];
+
+    /* Run state machine with the same objects twice to test that it's allowed to
+     * reinitialize session and session_signer_data. */
+    for (i = 0; i < 2; i++) {
+        /* Setup */
+        secp256k1_rand256(session_id[0]);
+        secp256k1_rand256(session_id[1]);
+        secp256k1_rand256(sk[0]);
+        secp256k1_rand256(sk[1]);
+        secp256k1_rand256(msg);
+        CHECK(secp256k1_ec_pubkey_create(ctx, &pk[0], sk[0]) == 1);
+        CHECK(secp256k1_ec_pubkey_create(ctx, &pk[1], sk[1]) == 1);
+        CHECK(secp256k1_musig_pubkey_combine(ctx, scratch, &combined_pk, pk_hash, pk, 2) == 1);
+        CHECK(secp256k1_musig_session_initialize(ctx, &session[0], signers0, nonce_commitment[0], session_id[0], msg, &combined_pk, pk_hash, 2, 0, sk[0]) == 1);
+        CHECK(secp256k1_musig_session_initialize(ctx, &session[1], signers1, nonce_commitment[1], session_id[1], msg, &combined_pk, pk_hash, 2, 1, sk[1]) == 1);
+
+        /* Set nonce commitments */
+        ncs[0] = nonce_commitment[0];
+        ncs[1] = nonce_commitment[1];
+        CHECK(secp256k1_musig_session_get_public_nonce(ctx, &session[0], signers0, &nonce[0], ncs, 2) == 1);
+        /* Changing a nonce commitment is not okay */
+        ncs[1] = (unsigned char*) "this isn't a nonce commitment...";
+        CHECK(secp256k1_musig_session_get_public_nonce(ctx, &session[0], signers0, &nonce[0], ncs, 2) == 0);
+        /* Repeating with the same nonce commitments is okay */
+        ncs[1] = nonce_commitment[1];
+        CHECK(secp256k1_musig_session_get_public_nonce(ctx, &session[0], signers0, &nonce[0], ncs, 2) == 1);
+
+        /* Get nonce for signer 1 */
+        CHECK(secp256k1_musig_session_get_public_nonce(ctx, &session[1], signers1, &nonce[1], ncs, 2) == 1);
+
+        /* Set nonces */
+        CHECK(secp256k1_musig_set_nonce(ctx, &signers0[0], &nonce[0]) == 1);
+        /* Can't set nonce that doesn't match nonce commitment */
+        CHECK(secp256k1_musig_set_nonce(ctx, &signers0[1], &nonce[0]) == 0);
+        /* Set correct nonce */
+        CHECK(secp256k1_musig_set_nonce(ctx, &signers0[1], &nonce[1]) == 1);
+
+        /* Combine nonces */
+        CHECK(secp256k1_musig_session_combine_nonces(ctx, &session[0], signers0, 2, NULL, NULL) == 1);
+        /* Not everyone is present from signer 1's view */
+        CHECK(secp256k1_musig_session_combine_nonces(ctx, &session[1], signers1, 2, NULL, NULL) == 0);
+        /* Make everyone present */
+        CHECK(secp256k1_musig_set_nonce(ctx, &signers1[0], &nonce[0]) == 1);
+        CHECK(secp256k1_musig_set_nonce(ctx, &signers1[1], &nonce[1]) == 1);
+
+        /* Can't combine nonces from signers of a different session */
+        CHECK(musig_state_machine_diff_signers_combine_nonce_test(&combined_pk, pk_hash, nonce_commitment[0], &nonce[0], msg, sk[1], signers1, 1) == 0);
+        CHECK(musig_state_machine_diff_signers_combine_nonce_test(&combined_pk, pk_hash, nonce_commitment[0], &nonce[0], msg, sk[1], signers1, 0) == 1);
+
+        /* Partially sign */
+        CHECK(secp256k1_musig_partial_sign(ctx, &session[0], &partial_sig[0]) == 1);
+        /* Can't verify or sign until nonce is combined */
+        CHECK(secp256k1_musig_partial_sig_verify(ctx, &session[1], &signers1[0], &partial_sig[0], &pk[0]) == 0);
+        CHECK(secp256k1_musig_partial_sign(ctx, &session[1], &partial_sig[1]) == 0);
+        CHECK(secp256k1_musig_session_combine_nonces(ctx, &session[1], signers1, 2, NULL, NULL) == 1);
+        CHECK(secp256k1_musig_partial_sig_verify(ctx, &session[1], &signers1[0], &partial_sig[0], &pk[0]) == 1);
+        /* messagehash should be the same as a session whose get_public_nonce was called
+         * with different signers (i.e. they diff in public keys). This is because the
+         * public keys of the signers is set in stone when initializing the session. */
+        CHECK(secp256k1_musig_compute_messagehash(ctx, msghash1, &session[1]) == 1);
+        CHECK(musig_state_machine_diff_signer_msghash_test(msghash2, pk, &combined_pk, pk_hash, ncs, msg, &nonce[0], sk[1], session_id[1]) == 1);
+        CHECK(memcmp(msghash1, msghash2, 32) == 0);
+        CHECK(secp256k1_musig_partial_sign(ctx, &session[1], &partial_sig[1]) == 1);
+        CHECK(secp256k1_musig_partial_sig_verify(ctx, &session[1], &signers1[1], &partial_sig[1], &pk[1]) == 1);
+        /* Wrong signature */
+        CHECK(secp256k1_musig_partial_sig_verify(ctx, &session[1], &signers1[1], &partial_sig[0], &pk[1]) == 0);
+        /* Can't sign or verify until msg is set */
+        CHECK(musig_state_machine_missing_msg_test(pk, &combined_pk, pk_hash, nonce_commitment[0], &nonce[0], &partial_sig[0], sk[1], session_id[1], NULL) == 0);
+        CHECK(musig_state_machine_missing_msg_test(pk, &combined_pk, pk_hash, nonce_commitment[0], &nonce[0], &partial_sig[0], sk[1], session_id[1], msg) == 1);
+
+        /* Can't verify and combine partial sigs until nonces are combined */
+        CHECK(musig_state_machine_missing_combine_test(pk, &combined_pk, pk_hash, nonce_commitment[0], &nonce[0], &partial_sig[0], msg, sk[1], session_id[1], &partial_sig[1], 0) == 0);
+        CHECK(musig_state_machine_missing_combine_test(pk, &combined_pk, pk_hash, nonce_commitment[0], &nonce[0], &partial_sig[0], msg, sk[1], session_id[1], &partial_sig[1], 1) == 1);
+    }
+}
+
+void scriptless_atomic_swap(secp256k1_scratch_space *scratch) {
+    /* Throughout this test "a" and "b" refer to two hypothetical blockchains,
+     * while the indices 0 and 1 refer to the two signers. Here signer 0 is
+     * sending a-coins to signer 1, while signer 1 is sending b-coins to signer
+     * 0. Signer 0 produces the adaptor signatures. */
+    secp256k1_schnorrsig final_sig_a;
+    secp256k1_schnorrsig final_sig_b;
+    secp256k1_musig_partial_signature partial_sig_a[2];
+    secp256k1_musig_partial_signature partial_sig_b_adapted[2];
+    secp256k1_musig_partial_signature partial_sig_b[2];
+    unsigned char sec_adaptor[32];
+    unsigned char sec_adaptor_extracted[32];
+    secp256k1_pubkey pub_adaptor;
+
+    unsigned char seckey_a[2][32];
+    unsigned char seckey_b[2][32];
+    secp256k1_pubkey pk_a[2];
+    secp256k1_pubkey pk_b[2];
+    unsigned char pk_hash_a[32];
+    unsigned char pk_hash_b[32];
+    secp256k1_pubkey combined_pk_a;
+    secp256k1_pubkey combined_pk_b;
+    secp256k1_musig_session musig_session_a[2];
+    secp256k1_musig_session musig_session_b[2];
+    unsigned char noncommit_a[2][32];
+    unsigned char noncommit_b[2][32];
+    const unsigned char *noncommit_a_ptr[2];
+    const unsigned char *noncommit_b_ptr[2];
+    secp256k1_pubkey pubnon_a[2];
+    secp256k1_pubkey pubnon_b[2];
+    int nonce_is_negated_a;
+    int nonce_is_negated_b;
+    secp256k1_musig_session_signer_data data_a[2];
+    secp256k1_musig_session_signer_data data_b[2];
+
+    const unsigned char seed[32] = "still tired of choosing seeds...";
+    const unsigned char msg32_a[32] = "this is the message blockchain a";
+    const unsigned char msg32_b[32] = "this is the message blockchain b";
+
+    /* Step 1: key setup */
+    secp256k1_rand256(seckey_a[0]);
+    secp256k1_rand256(seckey_a[1]);
+    secp256k1_rand256(seckey_b[0]);
+    secp256k1_rand256(seckey_b[1]);
+    secp256k1_rand256(sec_adaptor);
+
+    CHECK(secp256k1_ec_pubkey_create(ctx, &pk_a[0], seckey_a[0]));
+    CHECK(secp256k1_ec_pubkey_create(ctx, &pk_a[1], seckey_a[1]));
+    CHECK(secp256k1_ec_pubkey_create(ctx, &pk_b[0], seckey_b[0]));
+    CHECK(secp256k1_ec_pubkey_create(ctx, &pk_b[1], seckey_b[1]));
+    CHECK(secp256k1_ec_pubkey_create(ctx, &pub_adaptor, sec_adaptor));
+
+    CHECK(secp256k1_musig_pubkey_combine(ctx, scratch, &combined_pk_a, pk_hash_a, pk_a, 2));
+    CHECK(secp256k1_musig_pubkey_combine(ctx, scratch, &combined_pk_b, pk_hash_b, pk_b, 2));
+
+    CHECK(secp256k1_musig_session_initialize(ctx, &musig_session_a[0], data_a, noncommit_a[0], seed, msg32_a, &combined_pk_a, pk_hash_a, 2, 0, seckey_a[0]));
+    CHECK(secp256k1_musig_session_initialize(ctx, &musig_session_a[1], data_a, noncommit_a[1], seed, msg32_a, &combined_pk_a, pk_hash_a, 2, 1, seckey_a[1]));
+    noncommit_a_ptr[0] = noncommit_a[0];
+    noncommit_a_ptr[1] = noncommit_a[1];
+
+    CHECK(secp256k1_musig_session_initialize(ctx, &musig_session_b[0], data_b, noncommit_b[0], seed, msg32_b, &combined_pk_b, pk_hash_b, 2, 0, seckey_b[0]));
+    CHECK(secp256k1_musig_session_initialize(ctx, &musig_session_b[1], data_b, noncommit_b[1], seed, msg32_b, &combined_pk_b, pk_hash_b, 2, 1, seckey_b[1]));
+    noncommit_b_ptr[0] = noncommit_b[0];
+    noncommit_b_ptr[1] = noncommit_b[1];
+
+    /* Step 2: Exchange nonces */
+    CHECK(secp256k1_musig_session_get_public_nonce(ctx, &musig_session_a[0], data_a, &pubnon_a[0], noncommit_a_ptr, 2));
+    CHECK(secp256k1_musig_session_get_public_nonce(ctx, &musig_session_a[1], data_a, &pubnon_a[1], noncommit_a_ptr, 2));
+    CHECK(secp256k1_musig_session_get_public_nonce(ctx, &musig_session_b[0], data_b, &pubnon_b[0], noncommit_b_ptr, 2));
+    CHECK(secp256k1_musig_session_get_public_nonce(ctx, &musig_session_b[1], data_b, &pubnon_b[1], noncommit_b_ptr, 2));
+    CHECK(secp256k1_musig_set_nonce(ctx, &data_a[0], &pubnon_a[0]));
+    CHECK(secp256k1_musig_set_nonce(ctx, &data_a[1], &pubnon_a[1]));
+    CHECK(secp256k1_musig_set_nonce(ctx, &data_b[0], &pubnon_b[0]));
+    CHECK(secp256k1_musig_set_nonce(ctx, &data_b[1], &pubnon_b[1]));
+    CHECK(secp256k1_musig_session_combine_nonces(ctx, &musig_session_a[0], data_a, 2, &nonce_is_negated_a, &pub_adaptor));
+    CHECK(secp256k1_musig_session_combine_nonces(ctx, &musig_session_a[1], data_a, 2, NULL, &pub_adaptor));
+    CHECK(secp256k1_musig_session_combine_nonces(ctx, &musig_session_b[0], data_b, 2, &nonce_is_negated_b, &pub_adaptor));
+    CHECK(secp256k1_musig_session_combine_nonces(ctx, &musig_session_b[1], data_b, 2, NULL, &pub_adaptor));
+
+    /* Step 3: Signer 0 produces partial signatures for both chains. */
+    CHECK(secp256k1_musig_partial_sign(ctx, &musig_session_a[0], &partial_sig_a[0]));
+    CHECK(secp256k1_musig_partial_sign(ctx, &musig_session_b[0], &partial_sig_b[0]));
+
+    /* Step 4: Signer 1 receives partial signatures, verifies them and creates a
+     * partial signature to send B-coins to signer 0. */
+    CHECK(secp256k1_musig_partial_sig_verify(ctx, &musig_session_a[1], data_a, &partial_sig_a[0], &pk_a[0]) == 1);
+    CHECK(secp256k1_musig_partial_sig_verify(ctx, &musig_session_b[1], data_b, &partial_sig_b[0], &pk_b[0]) == 1);
+    CHECK(secp256k1_musig_partial_sign(ctx, &musig_session_b[1], &partial_sig_b[1]));
+
+    /* Step 5: Signer 0 adapts its own partial signature and combines it with the
+     * partial signature from signer 1. This results in a complete signature which
+     * is broadcasted by signer 0 to take B-coins. */
+    CHECK(secp256k1_musig_partial_sig_adapt(ctx, &partial_sig_b_adapted[0], &partial_sig_b[0], sec_adaptor, nonce_is_negated_b));
+    memcpy(&partial_sig_b_adapted[1], &partial_sig_b[1], sizeof(partial_sig_b_adapted[1]));
+    CHECK(secp256k1_musig_partial_sig_combine(ctx, &musig_session_b[0], &final_sig_b, partial_sig_b_adapted, 2) == 1);
+    CHECK(secp256k1_schnorrsig_verify(ctx, &final_sig_b, msg32_b, &combined_pk_b) == 1);
+
+    /* Step 6: Signer 1 extracts adaptor from the published signature, applies it to
+     * other partial signature, and takes A-coins. */
+    CHECK(secp256k1_musig_extract_secret_adaptor(ctx, sec_adaptor_extracted, &final_sig_b, partial_sig_b, 2, nonce_is_negated_b) == 1);
+    CHECK(memcmp(sec_adaptor_extracted, sec_adaptor, sizeof(sec_adaptor)) == 0); /* in real life we couldn't check this, of course */
+    CHECK(secp256k1_musig_partial_sig_adapt(ctx, &partial_sig_a[0], &partial_sig_a[0], sec_adaptor_extracted, nonce_is_negated_a));
+    CHECK(secp256k1_musig_partial_sign(ctx, &musig_session_a[1], &partial_sig_a[1]));
+    CHECK(secp256k1_musig_partial_sig_combine(ctx, &musig_session_a[1], &final_sig_a, partial_sig_a, 2) == 1);
+    CHECK(secp256k1_schnorrsig_verify(ctx, &final_sig_a, msg32_a, &combined_pk_a) == 1);
+}
+
+/* Checks that hash initialized by secp256k1_musig_sha256_init_tagged has the
+ * expected state. */
+void sha256_tag_test(void) {
+    char tag[17] = "MuSig coefficient";
+    secp256k1_sha256 sha;
+    secp256k1_sha256 sha_tagged;
+    unsigned char buf[32];
+    unsigned char buf2[32];
+    size_t i;
+
+    secp256k1_sha256_initialize(&sha);
+    secp256k1_sha256_write(&sha, (unsigned char *) tag, 17);
+    secp256k1_sha256_finalize(&sha, buf);
+    /* buf = SHA256("MuSig coefficient") */
+
+    secp256k1_sha256_initialize(&sha);
+    secp256k1_sha256_write(&sha, buf, 32);
+    secp256k1_sha256_write(&sha, buf, 32);
+    /* Is buffer fully consumed? */
+    CHECK((sha.bytes & 0x3F) == 0);
+
+    /* Compare with tagged SHA */
+    secp256k1_musig_sha256_init_tagged(&sha_tagged);
+    for (i = 0; i < 8; i++) {
+        CHECK(sha_tagged.s[i] == sha.s[i]);
+    }
+    secp256k1_sha256_write(&sha, buf, 32);
+    secp256k1_sha256_write(&sha_tagged, buf, 32);
+    secp256k1_sha256_finalize(&sha, buf);
+    secp256k1_sha256_finalize(&sha_tagged, buf2);
+    CHECK(memcmp(buf, buf2, 32) == 0);
+}
+
+void run_musig_tests(void) {
+    int i;
+    secp256k1_scratch_space *scratch = secp256k1_scratch_space_create(ctx, 1024 * 1024);
+
+    musig_api_tests(scratch);
+    musig_state_machine_tests(scratch);
+    for (i = 0; i < count; i++) {
+        /* Run multiple times to ensure that the nonce is negated in some tests */
+        scriptless_atomic_swap(scratch);
+    }
+    sha256_tag_test();
+
+    secp256k1_scratch_space_destroy(scratch);
+}
+
+#endif

src/secp256k1.c

@@ -720,6 +720,10 @@ int secp256k1_ec_pubkey_combine(const secp256k1_context* ctx, secp256k1_pubkey *
 # include "modules/schnorrsig/main_impl.h"
 #endif
 
+#ifdef ENABLE_MODULE_MUSIG
+# include "modules/musig/main_impl.h"
+#endif
+
 #ifdef ENABLE_MODULE_RECOVERY
 # include "modules/recovery/main_impl.h"
 #endif

src/tests.c

@@ -5322,6 +5322,10 @@ void run_ecdsa_openssl(void) {
 # include "modules/schnorrsig/tests_impl.h"
 #endif
 
+#ifdef ENABLE_MODULE_MUSIG
+# include "modules/musig/tests_impl.h"
+#endif
+
 #ifdef ENABLE_MODULE_RECOVERY
 # include "modules/recovery/tests_impl.h"
 #endif
@@ -5457,6 +5461,10 @@ int main(int argc, char **argv) {
     run_schnorrsig_tests();
 #endif
 
+#ifdef ENABLE_MODULE_MUSIG
+    run_musig_tests();
+#endif
+
     /* ecdsa tests */
     run_random_pubkeys();
     run_ecdsa_der_parse();