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ecp: avoid heap usage in mbedtls_ecp_mod_p448_raw()

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With 64-bit limbs, we're using two 64-byte arrays on the stack.
With 32-bit limbs, that's two 60-byte arrays.

This is entirely acceptable, and much better than allocating all the
time in a performance-critical function.

Signed-off-by: Manuel Pégourié-Gonnard <manuel.pegourie-gonnard@arm.com>
This commit is contained in:
Manuel Pégourié-Gonnard 2026-04-10 10:11:43 +02:00
parent 1b531ff7e5
commit 581f773c99
2 changed files with 6 additions and 31 deletions
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35
library/ecp_curves.c
View File

@ -5533,32 +5533,15 @@ static void ecp_shift_l_224(mbedtls_mpi_uint *X)
MBEDTLS_STATIC_TESTABLE
int mbedtls_ecp_mod_p448_raw(mbedtls_mpi_uint *X, size_t X_limbs)
{
size_t round;
int ret = MBEDTLS_ERR_ERROR_CORRUPTION_DETECTED;
if (X_limbs != BITS_TO_LIMBS(448) * 2) {
return MBEDTLS_ERR_ECP_BAD_INPUT_DATA;
}
/* Both M and Q require an extra limb to catch carries. */
size_t M_limbs = BITS_TO_LIMBS(448) + 1;
const size_t Q_limbs = M_limbs;
mbedtls_mpi_uint *M = NULL;
mbedtls_mpi_uint *Q = NULL;
M = mbedtls_calloc(M_limbs, ciL);
if (M == NULL) {
return MBEDTLS_ERR_ECP_ALLOC_FAILED;
}
Q = mbedtls_calloc(Q_limbs, ciL);
if (Q == NULL) {
ret = MBEDTLS_ERR_ECP_ALLOC_FAILED;
goto cleanup;
}
mbedtls_mpi_uint M[P448_WIDTH + 1];
mbedtls_mpi_uint Q[P448_WIDTH + 1];
const size_t M_limbs = P448_WIDTH + 1;
const size_t Q_limbs = P448_WIDTH + 1;
/* M = A1 */
memset(M, 0, (M_limbs * ciL));
@ -5602,7 +5585,7 @@ int mbedtls_ecp_mod_p448_raw(mbedtls_mpi_uint *X, size_t X_limbs)
* B1=0.
* Using this we need to calculate:
* A0 + A1 + B1 + (B0 + B1) * 2^224 = A0 + A1 + B0 * 2^224. */
for (round = 0; round < 2; ++round) {
for (size_t round = 0; round < 2; ++round) {
/* M = A1 */
memset(M, 0, (M_limbs * ciL));
@ -5626,13 +5609,7 @@ int mbedtls_ecp_mod_p448_raw(mbedtls_mpi_uint *X, size_t X_limbs)
(void) mbedtls_mpi_core_add(X, X, M, M_limbs);
}
ret = 0;
cleanup:
mbedtls_free(M);
mbedtls_free(Q);
return ret;
return 0;
}
#endif /* MBEDTLS_ECP_DP_CURVE448_ENABLED */

2
library/ecp_invasive.h
View File

@ -277,8 +277,6 @@ int mbedtls_ecp_mod_p255_raw(mbedtls_mpi_uint *X, size_t X_limbs);
* \return \c 0 on Success.
* \return #MBEDTLS_ERR_ECP_BAD_INPUT_DATA if \p X does not have
* twice as many limbs as the modulus.
* \return #MBEDTLS_ERR_ECP_ALLOC_FAILED if memory allocation
* failed.
*/
MBEDTLS_STATIC_TESTABLE
int mbedtls_ecp_mod_p448_raw(mbedtls_mpi_uint *X, size_t X_limbs);