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Preserve in-place message on signing failure #1230

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3 changes: 3 additions & 0 deletions mldsa/mldsa_native.h
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Original file line number Diff line number Diff line change
Expand Up @@ -444,6 +444,9 @@ int MLD_API_NAMESPACE(signature_extmu)(
* MLD_CONFIG_CONTEXT_PARAMETER is defined; type set by
* MLD_CONFIG_CONTEXT_PARAMETER_TYPE.
*
* On failure, *smlen is set to 0. No partial signed-message output is
* preserved, but if sm == m, the input message bytes remain unchanged.
*
* @retval 0 Success.
* @retval MLD_ERR_OUT_OF_MEMORY MLD_CONFIG_CUSTOM_ALLOC_FREE was
* used and an allocation via
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21 changes: 15 additions & 6 deletions mldsa/src/sign.c
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Expand Up @@ -1151,8 +1151,17 @@ int mld_sign(uint8_t *sm, size_t *smlen, const uint8_t *m, size_t mlen,
MLD_CONFIG_CONTEXT_PARAMETER_TYPE context)
{
int ret;
uint8_t sig[MLDSA_CRYPTO_BYTES];
size_t siglen;
size_t i;

ret = mld_sign_signature(sig, &siglen, m, mlen, ctx, ctxlen, sk, context);
if (ret != 0)
{
*smlen = 0;
goto cleanup;
}

Comment on lines +1154 to +1164

@hanno-becker hanno-becker Jun 27, 2026

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Hmm, I'm not too happy with this additional stack allocation (NB: large allocations should come from MLD_ALLOC/FREE, but that is a simple change).

I agree, though, that's unintuitive that the message is corrupted upon error.

Alternative: If signing fails and sm == m (the only interesting case), copy the message from the tail of sm to m. This isn't pretty, but it does the job, because that tail of sm is guaranteed not to be touched.

Leveling up, though: Why do we need this API in the first place? @mkannwischer can you share some perspectives?

for (i = 0; i < mlen; ++i)
__loop__(
assigns(i, object_whole(sm))
Expand All @@ -1162,12 +1171,12 @@ int mld_sign(uint8_t *sm, size_t *smlen, const uint8_t *m, size_t mlen,
{
sm[MLDSA_CRYPTO_BYTES + mlen - 1 - i] = m[mlen - 1 - i];
}
ret = mld_sign_signature(sm, smlen, sm + MLDSA_CRYPTO_BYTES, mlen, ctx,
ctxlen, sk, context);
if (ret == 0)
{
*smlen += mlen;
}

mld_memcpy(sm, sig, MLDSA_CRYPTO_BYTES);
*smlen = siglen + mlen;

cleanup:
mld_zeroize(sig, sizeof(sig));
return ret;
}
#endif /* !MLD_CONFIG_NO_RANDOMIZED_API */
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3 changes: 3 additions & 0 deletions mldsa/src/sign.h
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Expand Up @@ -259,6 +259,9 @@ __contract__(
* MLD_CONFIG_CONTEXT_PARAMETER is defined; type set by
* MLD_CONFIG_CONTEXT_PARAMETER_TYPE.
*
* On failure, *smlen is set to 0. No partial signed-message output is

@hanno-becker hanno-becker Jun 27, 2026

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The parameter is called sig here. Can we align this across sign.h, api.h, and sign.c, while at it?

I think sig and siglen are more intuitive than sm/smlen. @mkannwischer wdyt

* preserved, but if sm == m, the input message bytes remain unchanged.
*
* @retval 0 Success.
* @retval MLD_ERR_OUT_OF_MEMORY MLD_CONFIG_CUSTOM_ALLOC_FREE was
* used and an allocation via
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69 changes: 69 additions & 0 deletions test/src/test_mldsa.c
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Expand Up @@ -134,6 +134,73 @@ static int test_sign_unaligned(void)
return test_sign_core(pk + 1, sk + 1, sm + 1, m + 1, m2 + 1, ctx + 1);
}

static int test_sign_inplace_success(void)
{
uint8_t pk[CRYPTO_PUBLICKEYBYTES];
uint8_t sk[CRYPTO_SECRETKEYBYTES];
uint8_t sm[MLEN + CRYPTO_BYTES];
uint8_t expected[MLEN];
uint8_t m2[MLEN + CRYPTO_BYTES];
uint8_t ctx[CTXLEN];
size_t smlen;
size_t mlen;
int rc;

CHECK(crypto_sign_keypair(pk, sk) == 0);
CHECK(randombytes(ctx, sizeof(ctx)) == 0);
MLD_CT_TESTING_SECRET(ctx, sizeof(ctx));
CHECK(randombytes(sm, MLEN) == 0);
MLD_CT_TESTING_SECRET(sm, MLEN);
memcpy(expected, sm, MLEN);
MLD_CT_TESTING_SECRET(expected, sizeof(expected));

CHECK_SIGN_RC(crypto_sign(sm, &smlen, sm, MLEN, ctx, sizeof(ctx), sk));
rc = crypto_sign_open(m2, &mlen, sm, smlen, ctx, sizeof(ctx), pk);

MLD_CT_TESTING_DECLASSIFY(&rc, sizeof(rc));
MLD_CT_TESTING_DECLASSIFY(expected, sizeof(expected));
MLD_CT_TESTING_DECLASSIFY(m2, sizeof(m2));

CHECK(rc == 0);
CHECK(smlen == MLEN + CRYPTO_BYTES);
CHECK(mlen == MLEN);
CHECK(memcmp(expected, m2, MLEN) == 0);

return 0;
}

static int test_sign_inplace_context_failure_preserves_message(void)
{
uint8_t pk[CRYPTO_PUBLICKEYBYTES];
uint8_t sk[CRYPTO_SECRETKEYBYTES];
uint8_t sm[MLEN + CRYPTO_BYTES];
uint8_t expected[MLEN];
uint8_t ctx[256];
size_t smlen = MLEN + CRYPTO_BYTES;
int rc;

CHECK(crypto_sign_keypair(pk, sk) == 0);
CHECK(randombytes(ctx, sizeof(ctx)) == 0);
MLD_CT_TESTING_SECRET(ctx, sizeof(ctx));
CHECK(randombytes(sm, MLEN) == 0);
MLD_CT_TESTING_SECRET(sm, MLEN);
memcpy(expected, sm, MLEN);
MLD_CT_TESTING_SECRET(expected, sizeof(expected));

rc = crypto_sign(sm, &smlen, sm, MLEN, ctx, sizeof(ctx), sk);

MLD_CT_TESTING_DECLASSIFY(&rc, sizeof(rc));
MLD_CT_TESTING_DECLASSIFY(&smlen, sizeof(smlen));
MLD_CT_TESTING_DECLASSIFY(sm, MLEN);
MLD_CT_TESTING_DECLASSIFY(expected, sizeof(expected));

CHECK(rc == MLD_ERR_FAIL);
CHECK(smlen == 0);
CHECK(memcmp(sm, expected, MLEN) == 0);

return 0;
}

static int test_sign_extmu(void)
{
uint8_t pk[CRYPTO_PUBLICKEYBYTES];
Expand Down Expand Up @@ -509,6 +576,8 @@ int main(void)
!defined(MLD_CONFIG_NO_VERIFY_API)
r |= test_sign();
r |= test_sign_unaligned();
r |= test_sign_inplace_success();
r |= test_sign_inplace_context_failure_preserves_message();
r |= test_wrong_pk();
r |= test_wrong_sig();
r |= test_wrong_ctx();
Expand Down
41 changes: 41 additions & 0 deletions test/src/test_rng_fail.c
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Expand Up @@ -158,6 +158,46 @@ static int test_sign_combined_rng_failure(void)
return 0;
}

static int test_sign_combined_inplace_rng_failure_preserves_message(void)
{
uint8_t sm[CRYPTO_BYTES + TEST_VECTOR_MSG_LEN];
uint8_t expected[TEST_VECTOR_MSG_LEN];
size_t smlen = CRYPTO_BYTES + TEST_VECTOR_MSG_LEN;
int rc;

memcpy(sm, TEST_VECTOR_MSG, TEST_VECTOR_MSG_LEN);
memcpy(expected, TEST_VECTOR_MSG, TEST_VECTOR_MSG_LEN);

reset_all();
randombytes_fail_on_counter = 0;
rc = crypto_sign(sm, &smlen, sm, TEST_VECTOR_MSG_LEN,
(const uint8_t *)TEST_VECTOR_CTX, TEST_VECTOR_CTX_LEN,
test_vector_sk);
if (rc != MLD_ERR_RNG_FAIL)
{
fprintf(stderr,
"ERROR: crypto_sign returned %d on in-place RNG failure "
"(expected %d)\n",
rc, MLD_ERR_RNG_FAIL);
return 1;
}
if (smlen != 0)
{
fprintf(stderr,
"ERROR: crypto_sign returned smlen=%zu on in-place RNG failure\n",
smlen);
return 1;
}
if (memcmp(sm, expected, TEST_VECTOR_MSG_LEN) != 0)
{
fprintf(stderr,
"ERROR: crypto_sign changed the in-place message on RNG failure\n");
return 1;
}

return 0;
}

static int test_signature_extmu_rng_failure(void)
{
uint8_t sig[CRYPTO_BYTES];
Expand Down Expand Up @@ -249,6 +289,7 @@ int main(void)
#if !defined(MLD_CONFIG_NO_SIGN_API)
r |= test_sign_rng_failure();
r |= test_sign_combined_rng_failure();
r |= test_sign_combined_inplace_rng_failure_preserves_message();
r |= test_signature_extmu_rng_failure();
r |= test_signature_pre_hash_shake256_rng_failure();
#endif /* !MLD_CONFIG_NO_SIGN_API */
Expand Down
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