Merge bitcoin-core/secp256k1#966: Make aux_rand32 arg to secp256k1_sc…

…hnorrsig_sign const

db4667d Make aux_rand32 arg to secp256k1_schnorrsig_sign const (Pieter Wuille)

Pull request description:

ACKs for top commit:
  real-or-random:
    ACK db4667d diff looks good
  jonasnick:
    ACK db4667d

Tree-SHA512: 6f003c856b0e11f3f41f6d8007470129f02f9075416c6a5d3904f8efb5fa461f38e600a6b31d326314b2961946c8c6b3bca1a8e9b333b52e099a6f023a04c698

include/secp256k1_schnorrsig.h

@@ -120,7 +120,7 @@ SECP256K1_API int secp256k1_schnorrsig_sign(
     unsigned char *sig64,
     const unsigned char *msg32,
     const secp256k1_keypair *keypair,
-    unsigned char *aux_rand32
+    const unsigned char *aux_rand32
 ) SECP256K1_ARG_NONNULL(1) SECP256K1_ARG_NONNULL(2) SECP256K1_ARG_NONNULL(3) SECP256K1_ARG_NONNULL(4);
 
 /** Create a Schnorr signature with a more flexible API.

src/modules/schnorrsig/main_impl.h

@@ -185,8 +185,9 @@ static int secp256k1_schnorrsig_sign_internal(const secp256k1_context* ctx, unsi
     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(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);
 }
 
 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) {

src/modules/schnorrsig/tests_impl.h

@@ -230,7 +230,7 @@ void test_schnorrsig_sha256_tagged(void) {
 
 /* Helper function for schnorrsig_bip_vectors
  * Signs the message and checks that it's the same as expected_sig. */
-void test_schnorrsig_bip_vectors_check_signing(const unsigned char *sk, const unsigned char *pk_serialized, unsigned char *aux_rand, const unsigned char *msg32, const unsigned char *expected_sig) {
+void test_schnorrsig_bip_vectors_check_signing(const unsigned char *sk, const unsigned char *pk_serialized, const unsigned char *aux_rand, const unsigned char *msg32, const unsigned char *expected_sig) {
     unsigned char sig[64];
     secp256k1_keypair keypair;
     secp256k1_xonly_pubkey pk, pk_expected;