tests: refactor tagged hash tests

Move the sha256_tag_test_internal function out of the musig module
into tests.c. This makes it available to other modules wishing to verify tagged
hashes without needing to duplicate the function.

Change the function signature to expect a const unsigned char and update
the tagged hash tests to use static const unsigned char character
arrays (where necessary).

Add a comment for each tag. This is done as a convenience for checking
the strings against the protocol specifications, where the tags are
normally specified as strings.

Update tests in the ellswift and schnorrsig modules to use the
sha256_tag_test_internal helper function.

Author: josibake

src/modules/ellswift/tests_impl.h

@@ -405,31 +405,31 @@ void run_ellswift_tests(void) {
 
     /* Test hash initializers. */
     {
-        secp256k1_sha256 sha, sha_optimized;
+        secp256k1_sha256 sha_optimized;
+        /* "secp256k1_ellswift_encode" */
         static const unsigned char encode_tag[] = {'s', 'e', 'c', 'p', '2', '5', '6', 'k', '1', '_', 'e', 'l', 'l', 's', 'w', 'i', 'f', 't', '_', 'e', 'n', 'c', 'o', 'd', 'e'};
+        /* "secp256k1_ellswift_create" */
         static const unsigned char create_tag[] = {'s', 'e', 'c', 'p', '2', '5', '6', 'k', '1', '_', 'e', 'l', 'l', 's', 'w', 'i', 'f', 't', '_', 'c', 'r', 'e', 'a', 't', 'e'};
+        /* "bip324_ellswift_xonly_ecdh" */
         static const unsigned char bip324_tag[] = {'b', 'i', 'p', '3', '2', '4', '_', 'e', 'l', 'l', 's', 'w', 'i', 'f', 't', '_', 'x', 'o', 'n', 'l', 'y', '_', 'e', 'c', 'd', 'h'};
 
         /* Check that hash initialized by
          * secp256k1_ellswift_sha256_init_encode has the expected
          * state. */
-        secp256k1_sha256_initialize_tagged(&sha, encode_tag, sizeof(encode_tag));
         secp256k1_ellswift_sha256_init_encode(&sha_optimized);
-        test_sha256_eq(&sha, &sha_optimized);
+        test_sha256_tag_midstate(&sha_optimized, encode_tag, sizeof(encode_tag));
 
         /* Check that hash initialized by
          * secp256k1_ellswift_sha256_init_create has the expected
          * state. */
-        secp256k1_sha256_initialize_tagged(&sha, create_tag, sizeof(create_tag));
         secp256k1_ellswift_sha256_init_create(&sha_optimized);
-        test_sha256_eq(&sha, &sha_optimized);
+        test_sha256_tag_midstate(&sha_optimized, create_tag, sizeof(create_tag));
 
         /* Check that hash initialized by
          * secp256k1_ellswift_sha256_init_bip324 has the expected
          * state. */
-        secp256k1_sha256_initialize_tagged(&sha, bip324_tag, sizeof(bip324_tag));
         secp256k1_ellswift_sha256_init_bip324(&sha_optimized);
-        test_sha256_eq(&sha, &sha_optimized);
+        test_sha256_tag_midstate(&sha_optimized, bip324_tag, sizeof(bip324_tag));
     }
 }
 

src/modules/musig/tests_impl.h

@@ -548,40 +548,39 @@ static void musig_nonce_test(void) {
     }
 }
 
-static void sha256_tag_test_internal(secp256k1_sha256 *sha_tagged, unsigned char *tag, size_t taglen) {
-    secp256k1_sha256 sha;
-    secp256k1_sha256_initialize_tagged(&sha, tag, taglen);
-    test_sha256_eq(&sha, sha_tagged);
-}
-
 /* Checks that the initialized tagged hashes have the expected
  * state. */
 static void sha256_tag_test(void) {
     secp256k1_sha256 sha;
     {
-        char tag[] = "KeyAgg list";
+        /* "KeyAgg list" */
+        static const unsigned char tag[] = {'K', 'e', 'y', 'A', 'g', 'g', ' ', 'l', 'i', 's', 't'};
         secp256k1_musig_keyagglist_sha256(&sha);
-        sha256_tag_test_internal(&sha, (unsigned char*)tag, sizeof(tag) - 1);
+        test_sha256_tag_midstate(&sha, tag, sizeof(tag));
     }
     {
-        char tag[] = "KeyAgg coefficient";
+        /* "KeyAgg coefficient" */
+        static const unsigned char tag[] = {'K', 'e', 'y', 'A', 'g', 'g', ' ', 'c', 'o', 'e', 'f', 'f', 'i', 'c', 'i', 'e', 'n', 't'};
         secp256k1_musig_keyaggcoef_sha256(&sha);
-        sha256_tag_test_internal(&sha, (unsigned char*)tag, sizeof(tag) - 1);
+        test_sha256_tag_midstate(&sha, tag, sizeof(tag));
     }
     {
-        unsigned char tag[] = "MuSig/aux";
+        /* "MuSig/aux" */
+        static const unsigned char tag[] = { 'M', 'u', 'S', 'i', 'g', '/', 'a', 'u', 'x' };
         secp256k1_nonce_function_musig_sha256_tagged_aux(&sha);
-        sha256_tag_test_internal(&sha, (unsigned char*)tag, sizeof(tag) - 1);
+        test_sha256_tag_midstate(&sha, tag, sizeof(tag));
     }
     {
-        unsigned char tag[] = "MuSig/nonce";
+        /* "MuSig/nonce" */
+        static const unsigned char tag[] = { 'M', 'u', 'S', 'i', 'g', '/', 'n', 'o', 'n', 'c', 'e' };
         secp256k1_nonce_function_musig_sha256_tagged(&sha);
-        sha256_tag_test_internal(&sha, (unsigned char*)tag, sizeof(tag) - 1);
+        test_sha256_tag_midstate(&sha, tag, sizeof(tag));
     }
     {
-        unsigned char tag[] = "MuSig/noncecoef";
+        /* "MuSig/noncecoef" */
+        static const unsigned char tag[] = { 'M', 'u', 'S', 'i', 'g', '/', 'n', 'o', 'n', 'c', 'e', 'c', 'o', 'e', 'f' };
         secp256k1_musig_compute_noncehash_sha256_tagged(&sha);
-        sha256_tag_test_internal(&sha, (unsigned char*)tag, sizeof(tag) - 1);
+        test_sha256_tag_midstate(&sha, tag, sizeof(tag));
     }
 }
 

src/modules/schnorrsig/tests_impl.h

@@ -21,11 +21,12 @@ static void nonce_function_bip340_bitflip(unsigned char **args, size_t n_flip, s
 }
 
 static void run_nonce_function_bip340_tests(void) {
-    unsigned char tag[] = {'B', 'I', 'P', '0', '3', '4', '0', '/', 'n', 'o', 'n', 'c', 'e'};
-    unsigned char aux_tag[] = {'B', 'I', 'P', '0', '3', '4', '0', '/', 'a', 'u', 'x'};
+    /* "BIP0340/nonce" */
+    static const unsigned char tag[] = {'B', 'I', 'P', '0', '3', '4', '0', '/', 'n', 'o', 'n', 'c', 'e'};
+    /* "BIP0340/aux" */
+    static const unsigned char aux_tag[] = {'B', 'I', 'P', '0', '3', '4', '0', '/', 'a', 'u', 'x'};
     unsigned char algo[] = {'B', 'I', 'P', '0', '3', '4', '0', '/', 'n', 'o', 'n', 'c', 'e'};
     size_t algolen = sizeof(algo);
-    secp256k1_sha256 sha;
     secp256k1_sha256 sha_optimized;
     unsigned char nonce[32], nonce_z[32];
     unsigned char msg[32];
@@ -39,16 +40,15 @@ static void run_nonce_function_bip340_tests(void) {
     /* Check that hash initialized by
      * secp256k1_nonce_function_bip340_sha256_tagged has the expected
      * state. */
-    secp256k1_sha256_initialize_tagged(&sha, tag, sizeof(tag));
     secp256k1_nonce_function_bip340_sha256_tagged(&sha_optimized);
-    test_sha256_eq(&sha, &sha_optimized);
+    test_sha256_tag_midstate(&sha_optimized, tag, sizeof(tag));
+
 
    /* Check that hash initialized by
     * secp256k1_nonce_function_bip340_sha256_tagged_aux has the expected
     * state. */
-    secp256k1_sha256_initialize_tagged(&sha, aux_tag, sizeof(aux_tag));
     secp256k1_nonce_function_bip340_sha256_tagged_aux(&sha_optimized);
-    test_sha256_eq(&sha, &sha_optimized);
+    test_sha256_tag_midstate(&sha_optimized, aux_tag, sizeof(aux_tag));
 
     testrand256(msg);
     testrand256(key);

src/tests.c

@@ -609,6 +609,13 @@ static void test_sha256_eq(const secp256k1_sha256 *sha1, const secp256k1_sha256
     CHECK(sha1->bytes == sha2->bytes);
     CHECK(secp256k1_memcmp_var(sha1->s, sha2->s, sizeof(sha1->s)) == 0);
 }
+/* Convenience function for using test_sha256_eq to verify the correctness of a
+ * tagged hash midstate. This function is used by some module tests. */
+static void test_sha256_tag_midstate(secp256k1_sha256 *sha_tagged, const unsigned char *tag, size_t taglen) {
+    secp256k1_sha256 sha;
+    secp256k1_sha256_initialize_tagged(&sha, tag, taglen);
+    test_sha256_eq(&sha, sha_tagged);
+}
 
 static void run_hmac_sha256_tests(void) {
     static const char *keys[6] = {