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Code Issues Releases Wiki Activity

fix(crypto): Fix nonce bias in CoSi signing.

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This commit is contained in:
Andrew Kozlik 2022-06-24 22:55:25 +02:00 committed by matejcik
parent b7dde50d5f
commit fa5e7feda6
8 changed files with 82 additions and 25 deletions
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36
core/embed/extmod/modtrezorcrypto/modtrezorcrypto-ed25519.h
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@ -240,6 +240,25 @@ STATIC MP_DEFINE_CONST_FUN_OBJ_2(
mod_trezorcrypto_ed25519_cosi_combine_signatures_obj,
mod_trezorcrypto_ed25519_cosi_combine_signatures);
/// def cosi_commit() -> tuple[bytes, bytes]:
/// """
/// Generate a nonce and commitment for the CoSi cosigning scheme.
/// """
STATIC mp_obj_t mod_trezorcrypto_ed25519_cosi_commit() {
vstr_t nonce = {0};
vstr_t commitment = {0};
vstr_init_len(&nonce, 32);
vstr_init_len(&commitment, 32);
ed25519_cosi_commit(*(ed25519_secret_key *)nonce.buf,
*(ed25519_public_key *)commitment.buf);
mp_obj_tuple_t *tuple = MP_OBJ_TO_PTR(mp_obj_new_tuple(2, NULL));
tuple->items[0] = mp_obj_new_str_from_vstr(&mp_type_bytes, &nonce);
tuple->items[1] = mp_obj_new_str_from_vstr(&mp_type_bytes, &commitment);
return MP_OBJ_FROM_PTR(tuple);
}
STATIC MP_DEFINE_CONST_FUN_OBJ_0(mod_trezorcrypto_ed25519_cosi_commit_obj,
mod_trezorcrypto_ed25519_cosi_commit);
/// def cosi_sign(
/// secret_key: bytes,
/// message: bytes,
@ -272,12 +291,15 @@ STATIC mp_obj_t mod_trezorcrypto_ed25519_cosi_sign(size_t n_args,
}
vstr_t sig = {0};
vstr_init_len(&sig, sizeof(ed25519_cosi_signature));
;
ed25519_cosi_sign(msg.buf, msg.len, *(const ed25519_secret_key *)sk.buf,
*(const ed25519_secret_key *)nonce.buf,
*(const ed25519_public_key *)sigR.buf,
*(const ed25519_secret_key *)pk.buf,
*(ed25519_cosi_signature *)sig.buf);
if (0 != ed25519_cosi_sign(msg.buf, msg.len,
*(const ed25519_secret_key *)sk.buf,
*(const ed25519_secret_key *)nonce.buf,
*(const ed25519_public_key *)sigR.buf,
*(const ed25519_secret_key *)pk.buf,
*(ed25519_cosi_signature *)sig.buf)) {
vstr_clear(&sig);
mp_raise_ValueError("Signing failed");
}
return mp_obj_new_str_from_vstr(&mp_type_bytes, &sig);
}
STATIC MP_DEFINE_CONST_FUN_OBJ_VAR_BETWEEN(
@ -301,6 +323,8 @@ STATIC const mp_rom_map_elem_t mod_trezorcrypto_ed25519_globals_table[] = {
MP_ROM_PTR(&mod_trezorcrypto_ed25519_cosi_combine_publickeys_obj)},
{MP_ROM_QSTR(MP_QSTR_cosi_combine_signatures),
MP_ROM_PTR(&mod_trezorcrypto_ed25519_cosi_combine_signatures_obj)},
{MP_ROM_QSTR(MP_QSTR_cosi_commit),
MP_ROM_PTR(&mod_trezorcrypto_ed25519_cosi_commit_obj)},
{MP_ROM_QSTR(MP_QSTR_cosi_sign),
MP_ROM_PTR(&mod_trezorcrypto_ed25519_cosi_sign_obj)},
};

7
core/mocks/generated/trezorcrypto/ed25519.pyi
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@ -53,6 +53,13 @@ def cosi_combine_signatures(R: bytes, signatures: list[bytes]) -> bytes:
"""
# extmod/modtrezorcrypto/modtrezorcrypto-ed25519.h
def cosi_commit() -> tuple[bytes, bytes]:
"""
Generate a nonce and commitment for the CoSi cosigning scheme.
"""
# extmod/modtrezorcrypto/modtrezorcrypto-ed25519.h
def cosi_sign(
secret_key: bytes,

3
core/tests/test_trezor.crypto.curve.ed25519_cosi.py
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@ -26,8 +26,7 @@ class TestCryptoEd25519Cosi(unittest.TestCase):
nonces = [None] * N
Rs = [None] * N
for j in range(N):
nonces[j] = ed25519.generate_secret()
Rs[j] = ed25519.publickey(nonces[j])
nonces[j], Rs[j] = ed25519.cosi_commit()
R = ed25519.cosi_combine_publickeys(Rs)

33
crypto/ed25519-donna/ed25519.c
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@ -18,6 +18,7 @@
#include "ed25519.h"
#include "ed25519-hash-custom.h"
#include "rand.h"
#include "memzero.h"
/*
@ -50,16 +51,34 @@ ED25519_FN(ed25519_publickey) (const ed25519_secret_key sk, ed25519_public_key p
}
void
ED25519_FN(ed25519_cosi_commit) (ed25519_secret_key nonce, ed25519_public_key commitment) {
bignum256modm r = {0};
ge25519 ALIGN(16) R;
unsigned char extnonce[64] = {0};
/* r = random512 mod L */
random_buffer(extnonce, sizeof(extnonce));
expand256_modm(r, extnonce, sizeof(extnonce));
memzero(&extnonce, sizeof(extnonce));
contract256_modm(nonce, r);
/* R = rB */
ge25519_scalarmult_base_niels(&R, ge25519_niels_base_multiples, r);
memzero(&r, sizeof(r));
ge25519_pack(commitment, &R);
}
int
ED25519_FN(ed25519_cosi_sign) (const unsigned char *m, size_t mlen, const ed25519_secret_key sk, const ed25519_secret_key nonce, const ed25519_public_key R, const ed25519_public_key pk, ed25519_cosi_signature sig) {
bignum256modm r = {0}, S = {0}, a = {0};
hash_512bits extsk = {0}, extnonce = {0}, hram = {0};
hash_512bits extsk = {0}, hram = {0};
ed25519_extsk(extsk, sk);
ed25519_extsk(extnonce, nonce);
/* r = nonce */
expand256_modm(r, extnonce, 32);
memzero(&extnonce, sizeof(extnonce));
/* r */
expand_raw256_modm(r, nonce);
if (!is_reduced256_modm(r))
return -1;
/* S = H(R,A,m).. */
ed25519_hram(hram, R, pk, m, mlen);
@ -77,6 +96,8 @@ ED25519_FN(ed25519_cosi_sign) (const unsigned char *m, size_t mlen, const ed2551
/* S = (r + H(R,A,m)a) mod L */
contract256_modm(sig, S);
return 0;
}
void
@ -155,7 +176,7 @@ ED25519_FN(ed25519_sign_open) (const unsigned char *m, size_t mlen, const ed2551
/* S */
expand_raw256_modm(S, RS + 32);
if (!is_reduced256_modm(S))
return -1;
return -1;
/* SB - H(R,A,m)A */
ge25519_double_scalarmult_vartime(&R, &A, hram, S);

3
crypto/ed25519-donna/ed25519.h
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@ -35,7 +35,8 @@ void curve25519_scalarmult_basepoint(curve25519_key mypublic, const curve25519_k
int ed25519_cosi_combine_publickeys(ed25519_public_key res, CONST ed25519_public_key *pks, size_t n);
void ed25519_cosi_combine_signatures(ed25519_signature res, const ed25519_public_key R, CONST ed25519_cosi_signature *sigs, size_t n);
void ed25519_cosi_sign(const unsigned char *m, size_t mlen, const ed25519_secret_key key, const ed25519_secret_key nonce, const ed25519_public_key R, const ed25519_public_key pk, ed25519_cosi_signature sig);
void ed25519_cosi_commit(ed25519_secret_key nonce, ed25519_public_key commitment);
int ed25519_cosi_sign(const unsigned char *m, size_t mlen, const ed25519_secret_key key, const ed25519_secret_key nonce, const ed25519_public_key R, const ed25519_public_key pk, ed25519_cosi_signature sig);
#if defined(__cplusplus)
}

7
crypto/tests/test_check.c
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@ -6926,8 +6926,7 @@ START_TEST(test_ed25519_cosi) {
/* phase 1: create nonces, commitments (R values) and combine commitments */
for (int j = 0; j < N; j++) {
generate_rfc6979(nonces[j], &rng);
ed25519_publickey(nonces[j], Rs[j]);
ed25519_cosi_commit(nonces[j], Rs[j]);
}
res = ed25519_cosi_combine_publickeys(R, Rs, N);
ck_assert_int_eq(res, 0);
@ -6935,7 +6934,9 @@ START_TEST(test_ed25519_cosi) {
MARK_SECRET_DATA(keys, sizeof(keys));
/* phase 2: sign and combine signatures */
for (int j = 0; j < N; j++) {
ed25519_cosi_sign(msg, sizeof(msg), keys[j], nonces[j], R, pk, sigs[j]);
res = ed25519_cosi_sign(msg, sizeof(msg), keys[j], nonces[j], R, pk,
sigs[j]);
ck_assert_int_eq(res, 0);
}
UNMARK_SECRET_DATA(sigs, sizeof(sigs));

1
legacy/firmware/.changelog.d/noissue.security.2 Normal file
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@ -0,0 +1 @@
Fix nonce bias in CoSi signing.

17
legacy/firmware/fsm_msg_crypto.h
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@ -18,6 +18,7 @@
*/
static uint8_t cosi_nonce[32] = {0};
static uint8_t cosi_commitment[32] = {0};
static bool cosi_nonce_is_set = false;
void fsm_msgCipherKeyValue(const CipherKeyValue *msg) {
@ -271,7 +272,7 @@ void fsm_msgCosiCommit(const CosiCommit *msg) {
if (!node) return;
if (!cosi_nonce_is_set) {
random_buffer(cosi_nonce, sizeof(cosi_nonce));
ed25519_cosi_commit(cosi_nonce, cosi_commitment);
cosi_nonce_is_set = true;
}
@ -280,7 +281,7 @@ void fsm_msgCosiCommit(const CosiCommit *msg) {
resp->commitment.size = 32;
resp->pubkey.size = 32;
ed25519_publickey(cosi_nonce, resp->commitment.bytes);
memcpy(resp->commitment.bytes, cosi_commitment, sizeof(cosi_commitment));
ed25519_publickey(node->private_key, resp->pubkey.bytes);
msg_write(MessageType_MessageType_CosiCommitment, resp);
@ -326,11 +327,13 @@ void fsm_msgCosiSign(const CosiSign *msg) {
resp->signature.size = 32;
cosi_nonce_is_set = false;
ed25519_cosi_sign(msg->data.bytes, msg->data.size, node->private_key,
cosi_nonce, msg->global_commitment.bytes,
msg->global_pubkey.bytes, resp->signature.bytes);
if (ed25519_cosi_sign(msg->data.bytes, msg->data.size, node->private_key,
cosi_nonce, msg->global_commitment.bytes,
msg->global_pubkey.bytes, resp->signature.bytes) == 0) {
msg_write(MessageType_MessageType_CosiSignature, resp);
} else {
fsm_sendFailure(FailureType_Failure_FirmwareError, NULL);
}
memzero(cosi_nonce, sizeof(cosi_nonce));
msg_write(MessageType_MessageType_CosiSignature, resp);
layoutHome();
}