tests: impl bitflip test for schnorrsig randomizer gen

- refactor `secp256k1_batch_add_schnorrsig_randomizer` to support testing
    - `randomizer_gen` -> generates a 32-byte array randomizer
    - `randomizer_set` -> converts the 32-byte array to secp256k1_scalar
- impl bitflip test for `secp256k1_batch_add_schnorrsig_randomizer_gen`

src/modules/batch/tests_impl.h

@@ -5,11 +5,16 @@
 
 #define MAX_TERMS 10
 
+/*
+ todo: separate tests of batch_add_* from (create, destroy, verify)
+ since, compiling libsecp with batch module and without
+ schnorrsig, won't compile add functions. So, can't use them in tests
+*/
 
 /* Tests for the equality of two sha256 structs. This function only produces a
  * correct result if an integer multiple of 64 many bytes have been written
  * into the hash functions. */
-void batch_test_sha256_eq(const secp256k1_sha256 *sha1, const secp256k1_sha256 *sha2) {
+void test_batch_sha256_eq(const secp256k1_sha256 *sha1, const secp256k1_sha256 *sha2) {
     /* Is buffer fully consumed? */
     CHECK((sha1->bytes & 0x3F) == 0);
 
@@ -26,7 +31,67 @@ void test_batch_sha256_tagged(void) {
 
     secp256k1_sha256_initialize_tagged(&sha, (unsigned char *) tag, sizeof(tag));
     secp256k1_batch_sha256_tagged(&sha_optimized);
-    batch_test_sha256_eq(&sha, &sha_optimized);
+    test_batch_sha256_eq(&sha, &sha_optimized);
+}
+
+/* Checks that a bit flip in the n_flip-th argument (that has n_bytes many
+ * bytes) changes the hash function
+ */
+void batch_schnorrsig_randomizer_gen_bitflip(secp256k1_sha256 *sha, unsigned char **args, size_t n_flip, size_t n_bytes, size_t msglen) {
+    unsigned char randomizers[2][32];
+    secp256k1_sha256 sha_cpy;
+    sha_cpy = *sha;
+    CHECK(secp256k1_batch_schnorrsig_randomizer_gen(randomizers[0], &sha_cpy, args[0], args[1], msglen, args[2]) == 1);
+    secp256k1_testrand_flip(args[n_flip], n_bytes);
+    sha_cpy = *sha;
+    CHECK(secp256k1_batch_schnorrsig_randomizer_gen(randomizers[1], &sha_cpy, args[0], args[1], msglen, args[2]) == 1);
+    CHECK(secp256k1_memcmp_var(randomizers[0], randomizers[1], 32) != 0);
+}
+
+/*todo: make n_sigs var global macro? then, wouldn't it affects n_sigs api tests?*/
+void run_batch_schnorrsig_randomizer_gen_tests(void) {
+    secp256k1_sha256 sha;
+    size_t n_sigs = 20;
+    unsigned char msg[32];
+    size_t msglen = sizeof(msg[0]);
+    unsigned char sig[64];
+    unsigned char compressed_pk[33];
+    unsigned char *args[3];
+    uint8_t rand;
+    size_t i; /* loops through n_sigs */
+    int j; /* loops through count */
+
+    secp256k1_batch_sha256_tagged(&sha);
+
+    for (i = 0; i < n_sigs; i++) {
+        /* generate i-th schnorrsig verify data */
+        secp256k1_testrand256(msg);
+        secp256k1_testrand256(&sig[0]);
+        secp256k1_testrand256(&sig[32]);
+        secp256k1_testrand256(&compressed_pk[1]);
+        rand = secp256k1_testrand_int(2) + 2; /* randomly choose 2 or 3 */
+        compressed_pk[0] = (unsigned char)rand;
+
+        /* check that bitflip in an argument results in different nonces */
+        args[0] = sig;
+        args[1] = msg;
+        args[2] = compressed_pk;
+
+        for (j = 0; j < count; j++) {
+            batch_schnorrsig_randomizer_gen_bitflip(&sha, args, 0, 64, msglen);
+            batch_schnorrsig_randomizer_gen_bitflip(&sha, args, 1, 32, msglen);
+            batch_schnorrsig_randomizer_gen_bitflip(&sha, args, 2, 33, msglen);
+        }
+
+        /* write i-th schnorrsig verify data to the sha object
+         * this is required for generating the next randomizer */
+        secp256k1_sha256_write(&sha, sig, 64);
+        secp256k1_sha256_write(&sha, msg, msglen);
+        secp256k1_sha256_write(&sha, compressed_pk, 33);
+
+    }
+
+    /* todo: msglen difference test?? */
 }
 
 void test_batch_api(void) {
@@ -113,6 +178,7 @@ void test_batch_api(void) {
 void run_batch_tests(void) {
     test_batch_api();
     test_batch_sha256_tagged();
+    run_batch_schnorrsig_randomizer_gen_tests();
 }
 
 #endif /* SECP256K1_MODULE_BATCH_TESTS_H */

src/modules/schnorrsig/batch_add_impl.h

@@ -5,12 +5,26 @@
 #include "src/hash.h"
 #include "src/modules/batch/main_impl.h"
 
-static int secp256k1_batch_schnorrsig_randomizer(const secp256k1_context *ctx, secp256k1_batch *batch, secp256k1_scalar *r, const unsigned char *sig64, const unsigned char *msg, size_t msglen, const secp256k1_xonly_pubkey *pubkey) {
+static int secp256k1_batch_schnorrsig_randomizer_gen(unsigned char *randomizer32, secp256k1_sha256 *sha256, const unsigned char *sig64, const unsigned char *msg, size_t msglen, const unsigned char *compressed_pk33) {
     secp256k1_sha256 sha256_cpy;
+
+    /* add schnorrsig data to sha256 object */
+    secp256k1_sha256_write(sha256, sig64, 64);
+    secp256k1_sha256_write(sha256, msg, msglen);
+    secp256k1_sha256_write(sha256, compressed_pk33, 33);
+
+    /* generate randomizer */
+    sha256_cpy = *sha256;
+    secp256k1_sha256_finalize(&sha256_cpy, randomizer32);
+
+    return 1;
+}
+
+static int secp256k1_batch_schnorrsig_randomizer_set(const secp256k1_context *ctx, secp256k1_batch *batch, secp256k1_scalar *r, const unsigned char *sig64, const unsigned char *msg, size_t msglen, const secp256k1_xonly_pubkey *pubkey) {
+    int overflow;
     unsigned char randomizer[32];
     unsigned char buf[33];
     size_t buflen = sizeof(buf);
-    int overflow;
 
     /* We use compressed serialization here. If we would use
     * xonly_pubkey serialization and a user would wrongly memcpy
@@ -20,14 +34,9 @@ static int secp256k1_batch_schnorrsig_randomizer(const secp256k1_context *ctx, s
         return 0;
     }
 
-    /* add schnorrsig data to sha256 object */
-    secp256k1_sha256_write(&batch->sha256, sig64, 64);
-    secp256k1_sha256_write(&batch->sha256, msg, msglen);
-    secp256k1_sha256_write(&batch->sha256, buf, buflen);
-
-    /* generate randomizer */
-    sha256_cpy = batch->sha256;
-    secp256k1_sha256_finalize(&sha256_cpy, randomizer);
+    if(!secp256k1_batch_schnorrsig_randomizer_gen(randomizer, &batch->sha256, sig64, msg, msglen, buf)) {
+        return 0;
+    }
     secp256k1_scalar_set_b32(r, randomizer, &overflow);
     VERIFY_CHECK(overflow == 0);
 
@@ -112,7 +121,7 @@ int secp256k1_batch_add_schnorrsig(const secp256k1_context* ctx, secp256k1_batch
     secp256k1_schnorrsig_challenge(&e, &sig64[0], msg, msglen, buf);
 
     /* Compute ai */
-    if (!secp256k1_batch_schnorrsig_randomizer(ctx, batch, &ai, sig64, msg, msglen, pubkey)) {
+    if (!secp256k1_batch_schnorrsig_randomizer_set(ctx, batch, &ai, sig64, msg, msglen, pubkey)) {
         return 0;
     }