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verify: Switch to constant-time memcmp #750

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verify: Switch to constant-time memcmp #750

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29 changes: 6 additions & 23 deletions mldsa/src/sign.c
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Original file line number Diff line number Diff line change
Expand Up @@ -874,8 +874,7 @@ int crypto_sign_verify_internal(const uint8_t *sig, size_t siglen,
const uint8_t pk[MLDSA_CRYPTO_PUBLICKEYBYTES],
int externalmu)
{
unsigned int i;
int ret;
int ret, cmp;
MLD_ALLOC(buf, uint8_t, (MLDSA_K * MLDSA_POLYW1_PACKEDBYTES));
MLD_ALLOC(rho, uint8_t, MLDSA_SEEDBYTES);
MLD_ALLOC(mu, uint8_t, MLDSA_CRHBYTES);
Expand Down Expand Up @@ -961,28 +960,12 @@ int crypto_sign_verify_internal(const uint8_t *sig, size_t siglen,
mld_H(c2, MLDSA_CTILDEBYTES, mu, MLDSA_CRHBYTES, buf,
MLDSA_K * MLDSA_POLYW1_PACKEDBYTES, NULL, 0);

/* Constant time: All data in verification is usually considered public.
* However, in our constant-time tests we do not declassify the message and
* context string.
* The following conditional is the only place in verification whose run-time
* depends on the message. As all that can be leakaged here is the output of
* a hash call (that should behave like a random oracle), it is safe to
* declassify here even with a secret message.
*/
MLD_CT_TESTING_DECLASSIFY(c2, MLDSA_CTILDEBYTES);
ret = MLD_ERR_FAIL;
for (i = 0; i < MLDSA_CTILDEBYTES; ++i)
__loop__(
invariant(i <= MLDSA_CTILDEBYTES)
)
{
if (c[i] != c2[i])
{
goto cleanup;
}
}
cmp = mld_ct_memcmp(c, c2, MLDSA_CTILDEBYTES);

ret = 0;
/* Declassify the result of the verification. */
MLD_CT_TESTING_DECLASSIFY(&cmp, sizeof(cmp));

ret = cmp == 0 ? 0 : MLD_ERR_FAIL;

cleanup:
/* @[FIPS204, Section 3.6.3] Destruction of intermediate values. */
Expand Down
1 change: 1 addition & 0 deletions proofs/cbmc/crypto_sign_verify_internal/Makefile
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Original file line number Diff line number Diff line change
Expand Up @@ -39,6 +39,7 @@ USE_FUNCTION_CONTRACTS+=$(MLD_NAMESPACE)polyveck_caddq
USE_FUNCTION_CONTRACTS+=$(MLD_NAMESPACE)polyveck_use_hint
USE_FUNCTION_CONTRACTS+=$(MLD_NAMESPACE)polyveck_pack_w1
USE_FUNCTION_CONTRACTS+=mld_zeroize
USE_FUNCTION_CONTRACTS+=mld_ct_memcmp

APPLY_LOOP_CONTRACTS=on
USE_DYNAMIC_FRAMES=1
Expand Down