schnorrsig: add extra parameter struct for sign_custom

This simplifies the interface of sign_custom and allows adding more parameters
later in a backward compatible way.

include/secp256k1_schnorrsig.h

@@ -63,6 +63,35 @@ typedef int (*secp256k1_nonce_function_hardened)(
  */
 SECP256K1_API extern const secp256k1_nonce_function_hardened secp256k1_nonce_function_bip340;
 
+/** Data structure that contains additional arguments for schnorrsig_sign_custom.
+ *
+ *  A schnorrsig_extraparams structure object can be initialized correctly by
+ *  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.
+ */
+typedef struct {
+    unsigned char magic[4];
+    secp256k1_nonce_function_hardened noncefp;
+    void* ndata;
+} secp256k1_schnorrsig_extraparams;
+
+#define SECP256K1_SCHNORRSIG_EXTRAPARAMS_MAGIC "\xda\x6f\xb3\x8c"
+#define SECP256K1_SCHNORRSIG_EXTRAPARAMS_INIT {\
+    SECP256K1_SCHNORRSIG_EXTRAPARAMS_MAGIC,\
+    NULL,\
+    NULL\
+}
+
 /** Create a Schnorr signature.
  *
  *  Does _not_ strictly follow BIP-340 because it does not verify the resulting
@@ -97,26 +126,23 @@ SECP256K1_API int secp256k1_schnorrsig_sign(
 /** Create a Schnorr signature with a more flexible API.
  *
  *  Same arguments as secp256k1_schnorrsig_sign except that it allows signing
- *  variable length messages and allows providing a different nonce derivation
- *  function with its own data argument.
+ *  variable length messages and accepts a pointer to an extraparams object that
+ *  allows customizing signing by passing additional arguments.
+ *
+ *  Creates the same signatures as schnorrsig_sign if msglen is 32 and the
+ *  extraparams.ndata is the same as aux_rand32.
  *
  *  In:     msg: the message being signed. Can only be NULL if msglen is 0.
  *       msglen: length of the message
- *      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.
+ *  extraparams: pointer to a extraparams object (can be NULL)
  */
 SECP256K1_API int secp256k1_schnorrsig_sign_custom(
     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_schnorrsig_extraparams *extraparams
 ) SECP256K1_ARG_NONNULL(1) SECP256K1_ARG_NONNULL(2) SECP256K1_ARG_NONNULL(5);
 
 /** Verify a Schnorr signature.

src/modules/schnorrsig/main_impl.h

@@ -120,11 +120,7 @@ static void secp256k1_schnorrsig_challenge(secp256k1_scalar* e, const unsigned c
     secp256k1_scalar_set_b32(e, buf, NULL);
 }
 
-int secp256k1_schnorrsig_sign(const secp256k1_context* ctx, unsigned char *sig64, const unsigned char *msg32, const secp256k1_keypair *keypair, unsigned char *aux_rand32) {
-    return secp256k1_schnorrsig_sign_custom(ctx, sig64, msg32, 32, keypair, NULL, aux_rand32);
-}
-
-int secp256k1_schnorrsig_sign_custom(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) {
+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_scalar sk;
     secp256k1_scalar e;
     secp256k1_scalar k;
@@ -187,6 +183,25 @@ int secp256k1_schnorrsig_sign_custom(const secp256k1_context* ctx, unsigned char
     return ret;
 }
 
+int secp256k1_schnorrsig_sign(const secp256k1_context* ctx, unsigned char *sig64, const unsigned char *msg32, const secp256k1_keypair *keypair, unsigned char *aux_rand32) {
+    return secp256k1_schnorrsig_sign_internal(ctx, sig64, msg32, 32, keypair, secp256k1_nonce_function_bip340, aux_rand32);
+}
+
+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;
+    VERIFY_CHECK(ctx != NULL);
+
+    if (extraparams != NULL) {
+        ARG_CHECK(secp256k1_memcmp_var(extraparams->magic,
+                                       SECP256K1_SCHNORRSIG_EXTRAPARAMS_MAGIC,
+                                       sizeof(extraparams->magic)) == 0);
+        noncefp = extraparams->noncefp;
+        ndata = extraparams->ndata;
+    }
+    return secp256k1_schnorrsig_sign_internal(ctx, sig64, msg, msglen, keypair, noncefp, ndata);
+}
+
 int secp256k1_schnorrsig_verify(const secp256k1_context* ctx, const unsigned char *sig64, const unsigned char *msg, size_t msglen, const secp256k1_xonly_pubkey *pubkey) {
     secp256k1_scalar s;
     secp256k1_scalar e;

src/modules/schnorrsig/tests_exhaustive_impl.h

@@ -141,6 +141,8 @@ static void test_exhaustive_schnorrsig_verify(const secp256k1_context *ctx, cons
 static void test_exhaustive_schnorrsig_sign(const secp256k1_context *ctx, unsigned char (*xonly_pubkey_bytes)[32], const secp256k1_keypair* keypairs, const int* parities) {
     int d, k;
     uint64_t iter = 0;
+    secp256k1_schnorrsig_extraparams extraparams = SECP256K1_SCHNORRSIG_EXTRAPARAMS_INIT;
+
     /* Loop over keys. */
     for (d = 1; d < EXHAUSTIVE_TEST_ORDER; ++d) {
         int actual_d = d;
@@ -153,6 +155,8 @@ static void test_exhaustive_schnorrsig_sign(const secp256k1_context *ctx, unsign
             unsigned char sig64[64];
             int actual_k = k;
             if (skip_section(&iter)) continue;
+            extraparams.noncefp = secp256k1_hardened_nonce_function_smallint;
+            extraparams.ndata = &k;
             if (parities[k - 1]) actual_k = EXHAUSTIVE_TEST_ORDER - k;
             /* Generate random messages until all challenges have been tried. */
             while (e_count_done < EXHAUSTIVE_TEST_ORDER) {
@@ -165,7 +169,7 @@ static void test_exhaustive_schnorrsig_sign(const secp256k1_context *ctx, unsign
                     unsigned char expected_s_bytes[32];
                     secp256k1_scalar_get_b32(expected_s_bytes, &expected_s);
                     /* Invoke the real function to construct a signature. */
-                    CHECK(secp256k1_schnorrsig_sign_custom(ctx, sig64, msg32, sizeof(msg32), &keypairs[d - 1], secp256k1_hardened_nonce_function_smallint, &k));
+                    CHECK(secp256k1_schnorrsig_sign_custom(ctx, sig64, msg32, sizeof(msg32), &keypairs[d - 1], &extraparams));
                     /* The first 32 bytes must match the xonly pubkey for the specified k. */
                     CHECK(secp256k1_memcmp_var(sig64, xonly_pubkey_bytes[k - 1], 32) == 0);
                     /* The last 32 bytes must match the expected s value. */

src/modules/schnorrsig/tests_impl.h

@@ -690,24 +690,34 @@ static int nonce_function_overflowing(unsigned char *nonce32, const unsigned cha
 
 void test_schnorrsig_sign(void) {
     unsigned char sk[32];
+    secp256k1_xonly_pubkey pk;
     secp256k1_keypair keypair;
     const unsigned char msg[32] = "this is a msg for a schnorrsig..";
     unsigned char sig[64];
     unsigned char zeros64[64] = { 0 };
+    secp256k1_schnorrsig_extraparams extraparams = SECP256K1_SCHNORRSIG_EXTRAPARAMS_INIT;
 
     secp256k1_testrand256(sk);
     CHECK(secp256k1_keypair_create(ctx, &keypair, sk));
+    CHECK(secp256k1_keypair_xonly_pub(ctx, &pk, NULL, &keypair));
     CHECK(secp256k1_schnorrsig_sign(ctx, sig, msg, &keypair, NULL) == 1);
+    CHECK(secp256k1_schnorrsig_verify(ctx, sig, msg, sizeof(msg), &pk));
 
     /* Test different nonce functions */
+    CHECK(secp256k1_schnorrsig_sign_custom(ctx, sig, msg, sizeof(msg), &keypair, &extraparams) == 1);
+    CHECK(secp256k1_schnorrsig_verify(ctx, sig, msg, sizeof(msg), &pk));
     memset(sig, 1, sizeof(sig));
-    CHECK(secp256k1_schnorrsig_sign_custom(ctx, sig, msg, sizeof(msg), &keypair, nonce_function_failing, NULL) == 0);
+    extraparams.noncefp = nonce_function_failing;
+    CHECK(secp256k1_schnorrsig_sign_custom(ctx, sig, msg, sizeof(msg), &keypair, &extraparams) == 0);
     CHECK(secp256k1_memcmp_var(sig, zeros64, sizeof(sig)) == 0);
     memset(&sig, 1, sizeof(sig));
-    CHECK(secp256k1_schnorrsig_sign_custom(ctx, sig, msg, sizeof(msg), &keypair, nonce_function_0, NULL) == 0);
+    extraparams.noncefp = nonce_function_0;
+    CHECK(secp256k1_schnorrsig_sign_custom(ctx, sig, msg, sizeof(msg), &keypair, &extraparams) == 0);
     CHECK(secp256k1_memcmp_var(sig, zeros64, sizeof(sig)) == 0);
-    CHECK(secp256k1_schnorrsig_sign_custom(ctx, sig, msg, sizeof(msg), &keypair, nonce_function_overflowing, NULL) == 1);
-    CHECK(secp256k1_memcmp_var(sig, zeros64, sizeof(sig)) != 0);
+    memset(&sig, 1, sizeof(sig));
+    extraparams.noncefp = nonce_function_overflowing;
+    CHECK(secp256k1_schnorrsig_sign_custom(ctx, sig, msg, sizeof(msg), &keypair, &extraparams) == 1);
+    CHECK(secp256k1_schnorrsig_verify(ctx, sig, msg, sizeof(msg), &pk));
 }
 
 #define N_SIGS 3