schnorrsig: add algolen argument to nonce_function_hardened

This avoids having to remove trailing NUL bytes in the nonce function

Author: jonasnick

include/secp256k1_schnorrsig.h

@@ -23,14 +23,15 @@ extern "C" {
  *
  *  Returns: 1 if a nonce was successfully generated. 0 will cause signing to
  *           return an error.
- *  Out:     nonce32:   pointer to a 32-byte array to be filled by the function.
- *  In:      msg32:     the 32-byte message hash being verified (will not be NULL)
- *           key32:     pointer to a 32-byte secret key (will not be NULL)
- *      xonly_pk32:     the 32-byte serialized xonly pubkey corresponding to key32
- *                      (will not be NULL)
- *           algo16:    pointer to a 16-byte array describing the signature
- *                      algorithm (will not be NULL).
- *           data:      Arbitrary data pointer that is passed through.
+ *  Out:  nonce32: pointer to a 32-byte array to be filled by the function
+ *  In:     msg32: the 32-byte message hash being verified (will not be NULL)
+ *          key32: pointer to a 32-byte secret key (will not be NULL)
+ *     xonly_pk32: the 32-byte serialized xonly pubkey corresponding to key32
+ *                 (will not be NULL)
+ *           algo: pointer to an array describing the signature
+ *                 algorithm (will not be NULL)
+ *        algolen: the length of the algo array
+ *           data: arbitrary data pointer that is passed through
  *
  *  Except for test cases, this function should compute some cryptographic hash of
  *  the message, the key, the pubkey, the algorithm description, and data.
@@ -40,7 +41,8 @@ typedef int (*secp256k1_nonce_function_hardened)(
     const unsigned char *msg32,
     const unsigned char *key32,
     const unsigned char *xonly_pk32,
-    const unsigned char *algo16,
+    const unsigned char *algo,
+    size_t algolen,
     void *data
 );
 
@@ -51,10 +53,10 @@ typedef int (*secp256k1_nonce_function_hardened)(
  *  If a data pointer is passed, it is assumed to be a pointer to 32 bytes of
  *  auxiliary random data as defined in BIP-340. If the data pointer is NULL,
  *  the nonce derivation procedure follows BIP-340 by setting the auxiliary
- *  random data to zero. The algo16 argument must be non-NULL, otherwise the
- *  function will fail and return 0. The hash will be tagged with algo16 after
- *  removing all terminating null bytes. Therefore, to create BIP-340 compliant
- *  signatures, algo16 must be set to "BIP0340/nonce\0\0\0"
+ *  random data to zero. The algo argument must be non-NULL, otherwise the
+ *  function will fail and return 0. The hash will be tagged with algo.
+ *  Therefore, to create BIP-340 compliant signatures, algo must be set to
+ *  "BIP0340/nonce" and algolen to 13.
  */
 SECP256K1_API extern const secp256k1_nonce_function_hardened secp256k1_nonce_function_bip340;
 

src/modules/schnorrsig/main_impl.h

@@ -43,16 +43,16 @@ static void secp256k1_nonce_function_bip340_sha256_tagged_aux(secp256k1_sha256 *
     sha->bytes = 64;
 }
 
-/* algo16 argument for nonce_function_bip340 to derive the nonce exactly as stated in BIP-340
+/* algo argument for nonce_function_bip340 to derive the nonce exactly as stated in BIP-340
  * by using the correct tagged hash function. */
-static const unsigned char bip340_algo16[16] = "BIP0340/nonce\0\0\0";
+static const unsigned char bip340_algo[13] = "BIP0340/nonce";
 
-static int nonce_function_bip340(unsigned char *nonce32, const unsigned char *msg32, const unsigned char *key32, const unsigned char *xonly_pk32, const unsigned char *algo16, void *data) {
+static int nonce_function_bip340(unsigned char *nonce32, const unsigned char *msg32, const unsigned char *key32, const unsigned char *xonly_pk32, const unsigned char *algo, size_t algolen, void *data) {
     secp256k1_sha256 sha;
     unsigned char masked_key[32];
     int i;
 
-    if (algo16 == NULL) {
+    if (algo == NULL) {
         return 0;
     }
 
@@ -65,18 +65,14 @@ static int nonce_function_bip340(unsigned char *nonce32, const unsigned char *ms
         }
     }
 
-    /* Tag the hash with algo16 which is important to avoid nonce reuse across
+    /* Tag the hash with algo which is important to avoid nonce reuse across
      * algorithms. If this nonce function is used in BIP-340 signing as defined
      * in the spec, an optimized tagging implementation is used. */
-    if (secp256k1_memcmp_var(algo16, bip340_algo16, 16) == 0) {
+    if (algolen == sizeof(bip340_algo)
+            && secp256k1_memcmp_var(algo, bip340_algo, algolen) == 0) {
         secp256k1_nonce_function_bip340_sha256_tagged(&sha);
     } else {
-        int algo16_len = 16;
-        /* Remove terminating null bytes */
-        while (algo16_len > 0 && !algo16[algo16_len - 1]) {
-            algo16_len--;
-        }
-        secp256k1_sha256_initialize_tagged(&sha, algo16, algo16_len);
+        secp256k1_sha256_initialize_tagged(&sha, algo, algolen);
     }
 
     /* Hash (masked-)key||pk||msg using the tagged hash as per the spec */
@@ -156,7 +152,7 @@ int secp256k1_schnorrsig_sign(const secp256k1_context* ctx, unsigned char *sig64
 
     secp256k1_scalar_get_b32(seckey, &sk);
     secp256k1_fe_get_b32(pk_buf, &pk.x);
-    ret &= !!noncefp(buf, msg32, seckey, pk_buf, bip340_algo16, ndata);
+    ret &= !!noncefp(buf, msg32, seckey, pk_buf, bip340_algo, sizeof(bip340_algo), ndata);
     secp256k1_scalar_set_b32(&k, buf, NULL);
     ret &= !secp256k1_scalar_is_zero(&k);
     secp256k1_scalar_cmov(&k, &secp256k1_scalar_one, !ret);

src/modules/schnorrsig/tests_exhaustive_impl.h

@@ -60,13 +60,15 @@ static const unsigned char invalid_pubkey_bytes[][32] = {
 
 static int secp256k1_hardened_nonce_function_smallint(unsigned char *nonce32, const unsigned char *msg32,
                                                       const unsigned char *key32, const unsigned char *xonly_pk32,
-                                                      const unsigned char *algo16, void* data) {
+                                                      const unsigned char *algo, size_t algolen,
+                                                      void* data) {
     secp256k1_scalar s;
     int *idata = data;
     (void)msg32;
     (void)key32;
     (void)xonly_pk32;
-    (void)algo16;
+    (void)algo;
+    (void)algolen;
     secp256k1_scalar_set_int(&s, *idata);
     secp256k1_scalar_get_b32(nonce32, &s);
     return 1;

src/modules/schnorrsig/tests_impl.h

@@ -12,11 +12,11 @@
 /* Checks that a bit flip in the n_flip-th argument (that has n_bytes many
  * bytes) changes the hash function
  */
-void nonce_function_bip340_bitflip(unsigned char **args, size_t n_flip, size_t n_bytes) {
+void nonce_function_bip340_bitflip(unsigned char **args, size_t n_flip, size_t n_bytes, size_t algolen) {
     unsigned char nonces[2][32];
-    CHECK(nonce_function_bip340(nonces[0], args[0], args[1], args[2], args[3], args[4]) == 1);
+    CHECK(nonce_function_bip340(nonces[0], args[0], args[1], args[2], args[3], algolen, args[4]) == 1);
     secp256k1_testrand_flip(args[n_flip], n_bytes);
-    CHECK(nonce_function_bip340(nonces[1], args[0], args[1], args[2], args[3], args[4]) == 1);
+    CHECK(nonce_function_bip340(nonces[1], args[0], args[1], args[2], args[3], algolen, args[4]) == 1);
     CHECK(secp256k1_memcmp_var(nonces[0], nonces[1], 32) != 0);
 }
 
@@ -34,7 +34,8 @@ void test_sha256_eq(const secp256k1_sha256 *sha1, const secp256k1_sha256 *sha2)
 void run_nonce_function_bip340_tests(void) {
     unsigned char tag[13] = "BIP0340/nonce";
     unsigned char aux_tag[11] = "BIP0340/aux";
-    unsigned char algo16[16] = "BIP0340/nonce\0\0\0";
+    unsigned char algo[13] = "BIP0340/nonce";
+    size_t algolen = sizeof(algo);
     secp256k1_sha256 sha;
     secp256k1_sha256 sha_optimized;
     unsigned char nonce[32];
@@ -68,33 +69,37 @@ void run_nonce_function_bip340_tests(void) {
     args[0] = msg;
     args[1] = key;
     args[2] = pk;
-    args[3] = algo16;
+    args[3] = algo;
     args[4] = aux_rand;
     for (i = 0; i < count; i++) {
-        nonce_function_bip340_bitflip(args, 0, 32);
-        nonce_function_bip340_bitflip(args, 1, 32);
-        nonce_function_bip340_bitflip(args, 2, 32);
-        /* Flip algo16 special case "BIP0340/nonce" */
-        nonce_function_bip340_bitflip(args, 3, 16);
-        /* Flip algo16 again */
-        nonce_function_bip340_bitflip(args, 3, 16);
-        nonce_function_bip340_bitflip(args, 4, 32);
+        nonce_function_bip340_bitflip(args, 0, 32, algolen);
+        nonce_function_bip340_bitflip(args, 1, 32, algolen);
+        nonce_function_bip340_bitflip(args, 2, 32, algolen);
+        /* Flip algo special case "BIP0340/nonce" */
+        nonce_function_bip340_bitflip(args, 3, algolen, algolen);
+        /* Flip algo again */
+        nonce_function_bip340_bitflip(args, 3, algolen, algolen);
+        nonce_function_bip340_bitflip(args, 4, 32, algolen);
     }
 
-    /* NULL algo16 is disallowed */
-    CHECK(nonce_function_bip340(nonce, msg, key, pk, NULL, NULL) == 0);
-    /* Empty algo16 is fine */
-    memset(algo16, 0x00, 16);
-    CHECK(nonce_function_bip340(nonce, msg, key, pk, algo16, NULL) == 1);
-    /* algo16 with terminating null bytes is fine */
-    algo16[1] = 65;
-    CHECK(nonce_function_bip340(nonce, msg, key, pk, algo16, NULL) == 1);
-    /* Other algo16 is fine */
-    memset(algo16, 0xFF, 16);
-    CHECK(nonce_function_bip340(nonce, msg, key, pk, algo16, NULL) == 1);
+    /* NULL algo is disallowed */
+    CHECK(nonce_function_bip340(nonce, msg, key, pk, NULL, 0, NULL) == 0);
+    CHECK(nonce_function_bip340(nonce, msg, key, pk, algo, algolen, NULL) == 1);
+    /* Other algo is fine */
+    secp256k1_rfc6979_hmac_sha256_generate(&secp256k1_test_rng, algo, algolen);
+    CHECK(nonce_function_bip340(nonce, msg, key, pk, algo, algolen, NULL) == 1);
+
+    for (i = 0; i < count; i++) {
+        unsigned char nonce2[32];
+        /* Different algolen gives different nonce */
+        uint32_t offset = secp256k1_testrand_int(algolen - 1);
+        size_t algolen_tmp = (algolen + offset) % algolen;
+        CHECK(nonce_function_bip340(nonce2, msg, key, pk, algo, algolen_tmp, NULL) == 1);
+        CHECK(secp256k1_memcmp_var(nonce, nonce2, 32) != 0);
+    }
 
     /* NULL aux_rand argument is allowed. */
-    CHECK(nonce_function_bip340(nonce, msg, key, pk, algo16, NULL) == 1);
+    CHECK(nonce_function_bip340(nonce, msg, key, pk, algo, algolen, NULL) == 1);
 }
 
 void test_schnorrsig_api(void) {
@@ -634,34 +639,37 @@ void test_schnorrsig_bip_vectors(void) {
 }
 
 /* Nonce function that returns constant 0 */
-static int nonce_function_failing(unsigned char *nonce32, const unsigned char *msg32, const unsigned char *key32, const unsigned char *xonly_pk32, const unsigned char *algo16, void *data) {
+static int nonce_function_failing(unsigned char *nonce32, const unsigned char *msg32, const unsigned char *key32, const unsigned char *xonly_pk32, const unsigned char *algo, size_t algolen, void *data) {
     (void) msg32;
     (void) key32;
     (void) xonly_pk32;
-    (void) algo16;
+    (void) algo;
+    (void) algolen;
     (void) data;
     (void) nonce32;
     return 0;
 }
 
 /* Nonce function that sets nonce to 0 */
-static int nonce_function_0(unsigned char *nonce32, const unsigned char *msg32, const unsigned char *key32, const unsigned char *xonly_pk32, const unsigned char *algo16, void *data) {
+static int nonce_function_0(unsigned char *nonce32, const unsigned char *msg32, const unsigned char *key32, const unsigned char *xonly_pk32, const unsigned char *algo, size_t algolen, void *data) {
     (void) msg32;
     (void) key32;
     (void) xonly_pk32;
-    (void) algo16;
+    (void) algo;
+    (void) algolen;
     (void) data;
 
     memset(nonce32, 0, 32);
     return 1;
 }
 
 /* Nonce function that sets nonce to 0xFF...0xFF */
-static int nonce_function_overflowing(unsigned char *nonce32, const unsigned char *msg32, const unsigned char *key32, const unsigned char *xonly_pk32, const unsigned char *algo16, void *data) {
+static int nonce_function_overflowing(unsigned char *nonce32, const unsigned char *msg32, const unsigned char *key32, const unsigned char *xonly_pk32, const unsigned char *algo, size_t algolen, void *data) {
     (void) msg32;
     (void) key32;
     (void) xonly_pk32;
-    (void) algo16;
+    (void) algo;
+    (void) algolen;
     (void) data;
 
     memset(nonce32, 0xFF, 32);