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schnorrsig: add algolen argument to nonce_function_hardened
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This avoids having to remove trailing NUL bytes in the nonce function
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@jonasnick
jonasnick committed May 28, 2021
1 parent df3bfa1 commit 442cee5
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Showing 4 changed files with 67 additions and 59 deletions.
28 changes: 15 additions & 13 deletions include/secp256k1_schnorrsig.h
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Original file line number Diff line number Diff line change
Expand Up @@ -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.
Expand All @@ -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
);

Expand All @@ -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;

Expand Down
22 changes: 9 additions & 13 deletions src/modules/schnorrsig/main_impl.h
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Original file line number Diff line number Diff line change
Expand Up @@ -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;
}

Expand All @@ -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 */
Expand Down Expand Up @@ -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);
Expand Down
6 changes: 4 additions & 2 deletions src/modules/schnorrsig/tests_exhaustive_impl.h
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Original file line number Diff line number Diff line change
Expand Up @@ -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;
Expand Down
70 changes: 39 additions & 31 deletions src/modules/schnorrsig/tests_impl.h
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Original file line number Diff line number Diff line change
Expand Up @@ -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);
}

Expand All @@ -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];
Expand Down Expand Up @@ -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) {
Expand Down Expand Up @@ -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);
Expand Down

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