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fix: validate ML-DSA s1/s2 coefficient bounds in wc_dilithium_check_key #10254

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fix: validate ML-DSA s1/s2 coefficient bounds in wc_dilithium_check_key #10254

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1b5d119
fix: validate s1/s2 bounds in wc_dilithium_check_key
MarkAtwood Apr 12, 2026
3d992cc
fix: skip NTT when s1/s2 bounds check fails in check_key
MarkAtwood Apr 12, 2026
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77 changes: 48 additions & 29 deletions wolfcrypt/src/dilithium.c
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Original file line number Diff line number Diff line change
Expand Up @@ -11456,39 +11456,58 @@ int wc_dilithium_check_key(dilithium_key* key)
/* Get s1, s2 and t0 from private key. */
dilithium_vec_decode_eta_bits(s1p, params->eta, s1, params->l);
dilithium_vec_decode_eta_bits(s2p, params->eta, s2, params->k);
dilithium_vec_decode_t0(t0p, params->k, t0);

/* Get t1 from public key. */
dilithium_vec_decode_t1(t1p, params->k, t1);
/* Validate s1 and s2 coefficients are within [-eta, eta]. */
{
sword32 eta = (sword32)params->eta;
word32 c;
for (c = 0; c < (word32)(params->l * DILITHIUM_N); c++) {
if (s1[c] < -eta || s1[c] > eta) {
ret = PUBLIC_KEY_E;
break;
}
}
for (c = 0; (ret == 0) && (c < (word32)(params->k * DILITHIUM_N)); c++) {
if (s2[c] < -eta || s2[c] > eta) {
ret = PUBLIC_KEY_E;
break;
}
}
Comment on lines +11463 to +11474
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Copilot AI Apr 17, 2026

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This introduces data-dependent early exits (break) while processing secret-key material (s1/s2). If wc_dilithium_check_key() is used in contexts where timing differences are observable, consider making this check branchless by accumulating a violation flag across all coefficients and setting ret = PUBLIC_KEY_E after the loops complete (same behavior, less timing leakage).

Suggested change
for (c = 0; c < (word32)(params->l * DILITHIUM_N); c++) {
if (s1[c] < -eta || s1[c] > eta) {
ret = PUBLIC_KEY_E;
break;
}
}
for (c = 0; (ret == 0) && (c < (word32)(params->k * DILITHIUM_N)); c++) {
if (s2[c] < -eta || s2[c] > eta) {
ret = PUBLIC_KEY_E;
break;
}
}
word32 bad = 0;
for (c = 0; c < (word32)(params->l * DILITHIUM_N); c++) {
if (s1[c] < -eta || s1[c] > eta) {
bad = 1;
}
}
for (c = 0; c < (word32)(params->k * DILITHIUM_N); c++) {
if (s2[c] < -eta || s2[c] > eta) {
bad = 1;
}
}
if (bad != 0) {
ret = PUBLIC_KEY_E;
}

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}
Comment on lines +11459 to +11475
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Copilot AI Apr 17, 2026

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The new coefficient validation is duplicated for s1 and s2 with slightly different loop bounds. Consider extracting this into a small internal helper (e.g., dilithium_vec_check_eta_bounds(const sword32* v, word32 coeffCount, sword32 eta)) to reduce duplication and make future changes (like constant-time accumulation or different bounds) less error-prone.

Copilot uses AI. Check for mistakes.
if (ret == 0) {
dilithium_vec_decode_t0(t0p, params->k, t0);

/* Calcaluate t = NTT-1(A o NTT(s1)) + s2 */
dilithium_vec_ntt_small_full(s1, params->l);
dilithium_matrix_mul(t, a, s1, params->k, params->l);
#ifdef WOLFSSL_DILITHIUM_SMALL
dilithium_vec_red(t, params->k);
#endif
dilithium_vec_invntt_full(t, params->k);
dilithium_vec_add(t, s2, params->k);
/* Subtract t0 from t. */
dilithium_vec_sub(t, t0, params->k);
/* Make t positive to match t1. */
dilithium_vec_make_pos(t, params->k);
/* Get t1 from public key. */
dilithium_vec_decode_t1(t1p, params->k, t1);

/* Check t - t0 and t1 are the same. */
for (i = 0; i < params->k; i++) {
for (j = 0; j < DILITHIUM_N; j++) {
x |= tt[j] ^ t1[j];
/* Calcaluate t = NTT-1(A o NTT(s1)) + s2 */
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Typo in comment: 'Calcaluate' should be 'Calculate'.

Suggested change
/* Calcaluate t = NTT-1(A o NTT(s1)) + s2 */
/* Calculate t = NTT-1(A o NTT(s1)) + s2 */

Copilot uses AI. Check for mistakes.
dilithium_vec_ntt_small_full(s1, params->l);
dilithium_matrix_mul(t, a, s1, params->k, params->l);
#ifdef WOLFSSL_DILITHIUM_SMALL
dilithium_vec_red(t, params->k);
#endif
dilithium_vec_invntt_full(t, params->k);
dilithium_vec_add(t, s2, params->k);
/* Subtract t0 from t. */
dilithium_vec_sub(t, t0, params->k);
/* Make t positive to match t1. */
dilithium_vec_make_pos(t, params->k);

/* Check t - t0 and t1 are the same. */
for (i = 0; i < params->k; i++) {
for (j = 0; j < DILITHIUM_N; j++) {
x |= tt[j] ^ t1[j];
}
tt += DILITHIUM_N;
t1 += DILITHIUM_N;
}
/* Check the public seed is the same in private and public key. */
for (i = 0; i < DILITHIUM_PUB_SEED_SZ; i++) {
x |= key->p[i] ^ key->k[i];
}
tt += DILITHIUM_N;
t1 += DILITHIUM_N;
}
/* Check the public seed is the same in private and public key. */
for (i = 0; i < DILITHIUM_PUB_SEED_SZ; i++) {
x |= key->p[i] ^ key->k[i];
}

if ((ret == 0) && (x != 0)) {
ret = PUBLIC_KEY_E;
if (x != 0) {
ret = PUBLIC_KEY_E;
}
}
}

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