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ecp: avoid heap usage in ecp_mod_koblitz()

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That's one 32-byte buffer and one 40-byte buffer (at most, when the
256-bit "Koblitz" curve is enabled).

Signed-off-by: Manuel Pégourié-Gonnard <manuel.pegourie-gonnard@arm.com>
This commit is contained in:
Manuel Pégourié-Gonnard 2026-05-20 12:10:52 +02:00
parent 37279c1f95
commit 7689f99697
1 changed files with 14 additions and 20 deletions
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34
library/ecp_curves.c
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@ -5615,19 +5615,25 @@ int mbedtls_ecp_mod_p448_raw(mbedtls_mpi_uint *X, size_t X_limbs)
/*
* Fast quasi-reduction modulo P = 2^s - R,
* with R about 33 bits, used by the Koblitz curves.
* with R a 33-bit value, used by the Koblitz curves.
*
* Write X as A0 + 2^224 A1, return A0 + R * A1.
* Write X as A0 + 2^s A1, return A0 + R * A1.
*/
#define P_KOBLITZ_R (8 / sizeof(mbedtls_mpi_uint)) // Limbs in R
#define P_KOBLITZ_R BITS_TO_LIMBS(33)
#if defined(MBEDTLS_ECP_DP_SECP256K1_ENABLED)
#define P_KOBLITZ_MAX_P BITS_TO_LIMBS(256)
#elif defined(MBEDTLS_ECP_DP_SECP224K1_ENABLED)
#define P_KOBLITZ_MAX_P BITS_TO_LIMBS(224)
#else
#define P_KOBLITZ_MAX_P BITS_TO_LIMBS(192)
#endif
static inline int ecp_mod_koblitz(mbedtls_mpi_uint *X,
size_t X_limbs,
mbedtls_mpi_uint *R,
size_t bits)
{
int ret = 0;
/* Determine if A1 is aligned to limb bitsize. If not then the used limbs
* of P, A0 and A1 must be set accordingly and there is a middle limb
* which is shared by A0 and A1 and need to handle accordingly.
@ -5636,19 +5642,11 @@ static inline int ecp_mod_koblitz(mbedtls_mpi_uint *X,
size_t adjust = (shift + biL - 1) / biL;
size_t P_limbs = bits / biL + adjust;
mbedtls_mpi_uint *A1 = mbedtls_calloc(P_limbs, ciL);
if (A1 == NULL) {
return MBEDTLS_ERR_ECP_ALLOC_FAILED;
}
mbedtls_mpi_uint A1[P_KOBLITZ_MAX_P] = { 0 };
/* Create a buffer to store the value of `R * A1` */
size_t R_limbs = P_KOBLITZ_R;
size_t M_limbs = P_limbs + R_limbs;
mbedtls_mpi_uint *M = mbedtls_calloc(M_limbs, ciL);
if (M == NULL) {
ret = MBEDTLS_ERR_ECP_ALLOC_FAILED;
goto cleanup;
}
mbedtls_mpi_uint M[P_KOBLITZ_MAX_P + P_KOBLITZ_R] = { 0 };
mbedtls_mpi_uint mask = 0;
if (adjust != 0) {
@ -5692,11 +5690,7 @@ static inline int ecp_mod_koblitz(mbedtls_mpi_uint *X,
*/
}
cleanup:
mbedtls_free(M);
mbedtls_free(A1);
return ret;
return 0;
}
#endif /* MBEDTLS_ECP_DP_SECP192K1_ENABLED) ||