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schnorrsig: allow signing and verification of variable length msgs

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Varlen message support for the default sign function comes from recommending
tagged_sha256. sign_custom on the other hand gets the ability to directly sign
message of any length. This also implies signing and verification support for
the empty message (NULL) with msglen 0.

Tests for variable lengths follow in a later commit.
This commit is contained in:
Jonas Nick
2021-01-15 21:19:34 +00:00
parent 5a8e4991ad
commit a0c3fc177f
5 changed files with 111 additions and 81 deletions
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27
src/modules/schnorrsig/main_impl.h
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@@ -47,7 +47,7 @@ static void secp256k1_nonce_function_bip340_sha256_tagged_aux(secp256k1_sha256 *
* by using the correct tagged hash function. */
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 *algo, size_t algolen, void *data) {
static int nonce_function_bip340(unsigned char *nonce32, const unsigned char *msg, size_t msglen, 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;
@@ -82,7 +82,7 @@ static int nonce_function_bip340(unsigned char *nonce32, const unsigned char *ms
secp256k1_sha256_write(&sha, key32, 32);
}
secp256k1_sha256_write(&sha, xonly_pk32, 32);
secp256k1_sha256_write(&sha, msg32, 32);
secp256k1_sha256_write(&sha, msg, msglen);
secp256k1_sha256_finalize(&sha, nonce32);
return 1;
}
@@ -104,28 +104,27 @@ static void secp256k1_schnorrsig_sha256_tagged(secp256k1_sha256 *sha) {
sha->bytes = 64;
}
static void secp256k1_schnorrsig_challenge(secp256k1_scalar* e, const unsigned char *r32, const unsigned char *msg32, const unsigned char *pubkey32)
static void secp256k1_schnorrsig_challenge(secp256k1_scalar* e, const unsigned char *r32, const unsigned char *msg, size_t msglen, const unsigned char *pubkey32)
{
unsigned char buf[32];
secp256k1_sha256 sha;
/* tagged hash(r.x, pk.x, msg32) */
/* tagged hash(r.x, pk.x, msg) */
secp256k1_schnorrsig_sha256_tagged(&sha);
secp256k1_sha256_write(&sha, r32, 32);
secp256k1_sha256_write(&sha, pubkey32, 32);
secp256k1_sha256_write(&sha, msg32, 32);
secp256k1_sha256_write(&sha, msg, msglen);
secp256k1_sha256_finalize(&sha, buf);
/* Set scalar e to the challenge hash modulo the curve order as per
* BIP340. */
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, keypair, NULL, 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 *msg32, const secp256k1_keypair *keypair, secp256k1_nonce_function_hardened noncefp, void *ndata) {
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_scalar sk;
secp256k1_scalar e;
secp256k1_scalar k;
@@ -140,7 +139,7 @@ int secp256k1_schnorrsig_sign_custom(const secp256k1_context* ctx, unsigned char
VERIFY_CHECK(ctx != NULL);
ARG_CHECK(secp256k1_ecmult_gen_context_is_built(&ctx->ecmult_gen_ctx));
ARG_CHECK(sig64 != NULL);
ARG_CHECK(msg32 != NULL);
ARG_CHECK(msg != NULL || msglen == 0);
ARG_CHECK(keypair != NULL);
if (noncefp == NULL) {
@@ -157,7 +156,7 @@ int secp256k1_schnorrsig_sign_custom(const secp256k1_context* ctx, unsigned char
secp256k1_scalar_get_b32(seckey, &sk);
secp256k1_fe_get_b32(pk_buf, &pk.x);
ret &= !!noncefp(buf, msg32, seckey, pk_buf, bip340_algo, sizeof(bip340_algo), ndata);
ret &= !!noncefp(buf, msg, msglen, 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);
@@ -175,7 +174,7 @@ int secp256k1_schnorrsig_sign_custom(const secp256k1_context* ctx, unsigned char
secp256k1_fe_normalize_var(&r.x);
secp256k1_fe_get_b32(&sig64[0], &r.x);
secp256k1_schnorrsig_challenge(&e, &sig64[0], msg32, pk_buf);
secp256k1_schnorrsig_challenge(&e, &sig64[0], msg, msglen, pk_buf);
secp256k1_scalar_mul(&e, &e, &sk);
secp256k1_scalar_add(&e, &e, &k);
secp256k1_scalar_get_b32(&sig64[32], &e);
@@ -188,7 +187,7 @@ int secp256k1_schnorrsig_sign_custom(const secp256k1_context* ctx, unsigned char
return ret;
}
int secp256k1_schnorrsig_verify(const secp256k1_context* ctx, const unsigned char *sig64, const unsigned char *msg32, const secp256k1_xonly_pubkey *pubkey) {
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;
secp256k1_gej rj;
@@ -202,7 +201,7 @@ int secp256k1_schnorrsig_verify(const secp256k1_context* ctx, const unsigned cha
VERIFY_CHECK(ctx != NULL);
ARG_CHECK(secp256k1_ecmult_context_is_built(&ctx->ecmult_ctx));
ARG_CHECK(sig64 != NULL);
ARG_CHECK(msg32 != NULL);
ARG_CHECK(msg != NULL || msglen == 0);
ARG_CHECK(pubkey != NULL);
if (!secp256k1_fe_set_b32(&rx, &sig64[0])) {
@@ -220,7 +219,7 @@ int secp256k1_schnorrsig_verify(const secp256k1_context* ctx, const unsigned cha
/* Compute e. */
secp256k1_fe_get_b32(buf, &pk.x);
secp256k1_schnorrsig_challenge(&e, &sig64[0], msg32, buf);
secp256k1_schnorrsig_challenge(&e, &sig64[0], msg, msglen, buf);
/* Compute rj = s*G + (-e)*pkj */
secp256k1_scalar_negate(&e, &e);