allow creating and verifying Schnorr sign-to-contract commitments

Adapted from bitcoin-core#589.

The data is hashed using a tagged hash with the "s2c/schnorr/data"
tag, which is consistent with the data hashing done in the ECDSA
version of sign-to-contract (in ElementsProject/secp256k1-zkp), where
the "s2c/ecdsa/data" tag is used.

Co-authored-by: Jonas Nick <[email protected]>

Author: benma

include/secp256k1_schnorrsig.h

@@ -125,26 +125,36 @@ SECP256K1_API extern const secp256k1_nonce_function_hardened secp256k1_nonce_fun
  *  setting it to SECP256K1_SCHNORRSIG_EXTRAPARAMS_INIT.
  *
  *  Members:
- *      magic: set to SECP256K1_SCHNORRSIG_EXTRAPARAMS_MAGIC at initialization
- *             and has no other function than making sure the object is
- *             initialized.
- *    noncefp: pointer to a nonce generation function. If NULL,
- *             secp256k1_nonce_function_bip340 is used
- *      ndata: pointer to arbitrary data used by the nonce generation function
- *             (can be NULL). If it is non-NULL and
- *             secp256k1_nonce_function_bip340 is used, then ndata must be a
- *             pointer to 32-byte auxiliary randomness as per BIP-340.
+ *         magic: set to SECP256K1_SCHNORRSIG_EXTRAPARAMS_MAGIC at initialization
+ *                and has no other function than making sure the object is
+ *                initialized.
+ *       noncefp: pointer to a nonce generation function. If NULL,
+ *                secp256k1_nonce_function_bip340 is used
+ *         ndata: pointer to arbitrary data used by the nonce generation function
+ *                (can be NULL). If it is non-NULL and
+ *                secp256k1_nonce_function_bip340 is used, then ndata must be a
+ *                pointer to 32-byte auxiliary randomness as per BIP-340.
+ *   s2c_opening: pointer to an secp256k1_schnorr_s2c_opening structure which can be
+ *                NULL but is required to be not NULL if this signature creates
+ *                a sign-to-contract commitment (i.e. the `s2c_data32` argument
+ *                is not NULL).
+ *    s2c_data32: pointer to a 32-byte data to create an optional
+ *                sign-to-contract commitment to if not NULL (can be NULL).
  */
 typedef struct {
     unsigned char magic[4];
     secp256k1_nonce_function_hardened noncefp;
     void* ndata;
+    secp256k1_schnorr_s2c_opening* s2c_opening;
+    const unsigned char* s2c_data32;
 } secp256k1_schnorrsig_extraparams;
 
 #define SECP256K1_SCHNORRSIG_EXTRAPARAMS_MAGIC { 0xda, 0x6f, 0xb3, 0x8c }
 #define SECP256K1_SCHNORRSIG_EXTRAPARAMS_INIT {\
     SECP256K1_SCHNORRSIG_EXTRAPARAMS_MAGIC,\
     NULL,\
+    NULL,\
+    NULL,\
     NULL\
 }
 
@@ -231,6 +241,22 @@ SECP256K1_API SECP256K1_WARN_UNUSED_RESULT int secp256k1_schnorrsig_verify(
     const secp256k1_xonly_pubkey *pubkey
 ) SECP256K1_ARG_NONNULL(1) SECP256K1_ARG_NONNULL(2) SECP256K1_ARG_NONNULL(5);
 
+/** Verify a sign-to-contract commitment.
+ *
+ *  Returns: 1: the signature contains a commitment to data32
+ *           0: incorrect opening
+ *  Args:    ctx: a secp256k1 context object, initialized for verification.
+ *  In:    sig64: the signature containing the sign-to-contract commitment (cannot be NULL)
+ *        data32: the 32-byte data that was committed to (cannot be NULL)
+ *       opening: pointer to the opening created during signing (cannot be NULL)
+ */
+SECP256K1_API int secp256k1_schnorrsig_verify_s2c_commit(
+    const secp256k1_context* ctx,
+    const unsigned char *sig64,
+    const unsigned char *data32,
+    const secp256k1_schnorr_s2c_opening *opening
+) SECP256K1_ARG_NONNULL(1) SECP256K1_ARG_NONNULL(2) SECP256K1_ARG_NONNULL(3) SECP256K1_ARG_NONNULL(4);
+
 #ifdef __cplusplus
 }
 #endif

src/modules/schnorrsig/main_impl.h

@@ -13,6 +13,8 @@
 
 static uint64_t s2c_opening_magic = 0x5d0520b8b7f2b168ULL;
 
+static const unsigned char s2c_data_tag[16] = "s2c/schnorr/data";
+
 static void secp256k1_schnorr_s2c_opening_init(secp256k1_schnorr_s2c_opening *opening) {
     opening->magic = s2c_opening_magic;
     opening->nonce_is_negated = 0;
@@ -183,23 +185,31 @@ static void secp256k1_schnorrsig_challenge(secp256k1_scalar* e, const unsigned c
     secp256k1_scalar_set_b32(e, buf, NULL);
 }
 
-static int secp256k1_schnorrsig_sign_internal(const secp256k1_context* ctx, unsigned char *sig64, const unsigned char *msg, size_t msglen, const secp256k1_keypair *keypair, secp256k1_nonce_function_hardened noncefp, void *ndata) {
+static int secp256k1_schnorrsig_sign_internal(const secp256k1_context* ctx, unsigned char *sig64, const unsigned char *msg, size_t msglen, const secp256k1_keypair *keypair, secp256k1_nonce_function_hardened noncefp, void *ndata, secp256k1_schnorr_s2c_opening *s2c_opening, const unsigned char *s2c_data32) {
     secp256k1_scalar sk;
     secp256k1_scalar e;
     secp256k1_scalar k;
     secp256k1_gej rj;
     secp256k1_ge pk;
     secp256k1_ge r;
+    secp256k1_sha256 sha;
     unsigned char buf[32] = { 0 };
     unsigned char pk_buf[32];
     unsigned char seckey[32];
+    unsigned char noncedata[32];
     int ret = 1;
 
     VERIFY_CHECK(ctx != NULL);
     ARG_CHECK(secp256k1_ecmult_gen_context_is_built(&ctx->ecmult_gen_ctx));
     ARG_CHECK(sig64 != NULL);
     ARG_CHECK(msg != NULL || msglen == 0);
     ARG_CHECK(keypair != NULL);
+    /* sign-to-contract commitments only work with the default nonce function,
+     * because we need to ensure that s2c_data is actually hashed into the nonce and
+     * not just ignored because otherwise this could result in nonce reuse. */
+    ARG_CHECK(s2c_data32 == NULL || (noncefp == NULL || noncefp == secp256k1_nonce_function_bip340));
+    /* s2c_opening and s2c_data32 should be either both non-NULL or both NULL. */
+    ARG_CHECK((s2c_opening != NULL) == (s2c_data32 != NULL));
 
     if (noncefp == NULL) {
         noncefp = secp256k1_nonce_function_bip340;
@@ -215,6 +225,25 @@ static int secp256k1_schnorrsig_sign_internal(const secp256k1_context* ctx, unsi
 
     secp256k1_scalar_get_b32(seckey, &sk);
     secp256k1_fe_get_b32(pk_buf, &pk.x);
+
+    if (s2c_data32 != NULL) {
+        /* Provide s2c_data32 and ndata (if not NULL) to the the nonce function
+         * as additional data to derive the nonce from. If both pointers are
+         * not NULL, they need to be hashed to get the nonce data 32 bytes.
+         * Even if only s2c_data32 is not NULL, it's hashed because it should
+         * be possible to derive nonces even if only a SHA256 commitment to the
+         * data is known. This is for example important in the anti nonce
+         * sidechannel protocol.
+         */
+        secp256k1_sha256_initialize(&sha);
+        secp256k1_sha256_write(&sha, s2c_data32, 32);
+        if (ndata != NULL) {
+            secp256k1_sha256_write(&sha, ndata, 32);
+        }
+        secp256k1_sha256_finalize(&sha, noncedata);
+        ndata = &noncedata;
+    }
+
     ret &= !!noncefp(buf, msg, msglen, seckey, pk_buf, bip340_algo, sizeof(bip340_algo), ndata);
     secp256k1_scalar_set_b32(&k, buf, NULL);
     ret &= !secp256k1_scalar_is_zero(&k);
@@ -223,12 +252,27 @@ static int secp256k1_schnorrsig_sign_internal(const secp256k1_context* ctx, unsi
     secp256k1_ecmult_gen(&ctx->ecmult_gen_ctx, &rj, &k);
     secp256k1_ge_set_gej(&r, &rj);
 
+    if (s2c_opening != NULL) {
+        secp256k1_schnorr_s2c_opening_init(s2c_opening);
+        if (s2c_data32 != NULL) {
+            secp256k1_sha256_initialize_tagged(&sha, s2c_data_tag, sizeof(s2c_data_tag));
+            /* Create sign-to-contract commitment */
+            secp256k1_pubkey_save(&s2c_opening->original_pubnonce, &r);
+            secp256k1_ec_commit_seckey(&k, &r, &sha, s2c_data32, 32);
+            secp256k1_ecmult_gen(&ctx->ecmult_gen_ctx, &rj, &k);
+            secp256k1_ge_set_gej(&r, &rj);
+        }
+    }
+
     /* We declassify r to allow using it as a branch point. This is fine
      * because r is not a secret. */
     secp256k1_declassify(ctx, &r, sizeof(r));
     secp256k1_fe_normalize_var(&r.y);
     if (secp256k1_fe_is_odd(&r.y)) {
         secp256k1_scalar_negate(&k, &k);
+        if (s2c_opening != NULL) {
+            s2c_opening->nonce_is_negated = 1;
+        }
     }
     secp256k1_fe_normalize_var(&r.x);
     secp256k1_fe_get_b32(&sig64[0], &r.x);
@@ -248,7 +292,7 @@ static int secp256k1_schnorrsig_sign_internal(const secp256k1_context* ctx, unsi
 
 int secp256k1_schnorrsig_sign32(const secp256k1_context* ctx, unsigned char *sig64, const unsigned char *msg32, const secp256k1_keypair *keypair, const unsigned char *aux_rand32) {
     /* We cast away const from the passed aux_rand32 argument since we know the default nonce function does not modify it. */
-    return secp256k1_schnorrsig_sign_internal(ctx, sig64, msg32, 32, keypair, secp256k1_nonce_function_bip340, (unsigned char*)aux_rand32);
+    return secp256k1_schnorrsig_sign_internal(ctx, sig64, msg32, 32, keypair, secp256k1_nonce_function_bip340, (unsigned char*)aux_rand32, NULL, NULL);
 }
 
 int secp256k1_schnorrsig_sign(const secp256k1_context* ctx, unsigned char *sig64, const unsigned char *msg32, const secp256k1_keypair *keypair, const unsigned char *aux_rand32) {
@@ -258,6 +302,8 @@ int secp256k1_schnorrsig_sign(const secp256k1_context* ctx, unsigned char *sig64
 int secp256k1_schnorrsig_sign_custom(const secp256k1_context* ctx, unsigned char *sig64, const unsigned char *msg, size_t msglen, const secp256k1_keypair *keypair, secp256k1_schnorrsig_extraparams *extraparams) {
     secp256k1_nonce_function_hardened noncefp = NULL;
     void *ndata = NULL;
+    secp256k1_schnorr_s2c_opening *s2c_opening = NULL;
+    const unsigned char *s2c_data32 = NULL;
     VERIFY_CHECK(ctx != NULL);
 
     if (extraparams != NULL) {
@@ -266,8 +312,10 @@ int secp256k1_schnorrsig_sign_custom(const secp256k1_context* ctx, unsigned char
                                        sizeof(extraparams->magic)) == 0);
         noncefp = extraparams->noncefp;
         ndata = extraparams->ndata;
+        s2c_opening = extraparams->s2c_opening;
+        s2c_data32 = extraparams->s2c_data32;
     }
-    return secp256k1_schnorrsig_sign_internal(ctx, sig64, msg, msglen, keypair, noncefp, ndata);
+    return secp256k1_schnorrsig_sign_internal(ctx, sig64, msg, msglen, keypair, noncefp, ndata, s2c_opening, s2c_data32);
 }
 
 int secp256k1_schnorrsig_verify(const secp256k1_context* ctx, const unsigned char *sig64, const unsigned char *msg, size_t msglen, const secp256k1_xonly_pubkey *pubkey) {
@@ -318,4 +366,34 @@ int secp256k1_schnorrsig_verify(const secp256k1_context* ctx, const unsigned cha
            secp256k1_fe_equal_var(&rx, &r.x);
 }
 
+int secp256k1_schnorrsig_verify_s2c_commit(const secp256k1_context* ctx, const unsigned char *sig64, const unsigned char *data32, const secp256k1_schnorr_s2c_opening *opening) {
+    secp256k1_fe rx;
+    secp256k1_ge original_r;
+    secp256k1_ge r;
+    secp256k1_sha256 sha;
+
+    VERIFY_CHECK(ctx != NULL);
+    ARG_CHECK(sig64 != NULL);
+    ARG_CHECK(data32 != NULL);
+    ARG_CHECK(opening != NULL);
+    ARG_CHECK(secp256k1_s2c_commit_is_init(opening));
+
+    if (!secp256k1_fe_set_b32(&rx, &sig64[0])) {
+        return 0;
+    }
+    if (!secp256k1_ge_set_xo_var(&r, &rx, 0)) {
+        return 0;
+    }
+    if (opening->nonce_is_negated) {
+        secp256k1_ge_neg(&r, &r);
+    }
+
+    if (!secp256k1_pubkey_load(ctx, &original_r, &opening->original_pubnonce)) {
+        return 0;
+    }
+
+    secp256k1_sha256_initialize_tagged(&sha, s2c_data_tag, sizeof(s2c_data_tag));
+    return secp256k1_ec_commit_verify(&r, &original_r, &sha, data32, 32);
+}
+
 #endif