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bignum: use CT modinv when A is odd (any range)

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Signed-off-by: Manuel Pégourié-Gonnard <manuel.pegourie-gonnard@arm.com>
This commit is contained in:
Manuel Pégourié-Gonnard
2025-08-08 09:25:28 +02:00
parent e41709c17e
commit 1ac0a1e071
1 changed files with 38 additions and 4 deletions
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42
library/bignum.c
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@@ -2045,6 +2045,42 @@ cleanup:
return ret; return ret;
} }
/*
* Compute X = A^-1 mod N with N even and A odd (but in any range).
*
* Return MBEDTLS_ERR_MPI_NOT_ACCEPTABLE if the inverse doesn't exist.
*/
static int mbedtls_mpi_inv_mod_even(mbedtls_mpi *X,
mbedtls_mpi const *A,
mbedtls_mpi const *N)
{
int ret;
mbedtls_mpi AA;
mbedtls_mpi_init(&AA);
/* Bring A in the range [0, N). */
MBEDTLS_MPI_CHK(mbedtls_mpi_mod_mpi(&AA, A, N));
/* We know A >= 0 but the next functions wants A > 1 */
int cmp = mbedtls_mpi_cmp_int(&AA, 1);
if (cmp < 0) { // AA == 0
ret = MBEDTLS_ERR_MPI_NOT_ACCEPTABLE;
goto cleanup;
}
if (cmp == 0) { // AA = 1
MBEDTLS_MPI_CHK(mbedtls_mpi_lset(X, 1));
goto cleanup;
}
/* Now we know 1 < A < N, N is even and AA is still odd */
MBEDTLS_MPI_CHK(mbedtls_mpi_inv_mod_even_in_range(X, &AA, N));
cleanup:
mbedtls_mpi_free(&AA);
return ret;
}
/* /*
* Modular inverse: X = A^-1 mod N (HAC 14.61 / 14.64) * Modular inverse: X = A^-1 mod N (HAC 14.61 / 14.64)
*/ */
@@ -2061,10 +2097,8 @@ int mbedtls_mpi_inv_mod(mbedtls_mpi *X, const mbedtls_mpi *A, const mbedtls_mpi
return mbedtls_mpi_inv_mod_odd(X, A, N); return mbedtls_mpi_inv_mod_odd(X, A, N);
} }
if (mbedtls_mpi_get_bit(A, 0) == 1 && if (mbedtls_mpi_get_bit(A, 0) == 1) {
mbedtls_mpi_cmp_int(A, 1) > 0 && return mbedtls_mpi_inv_mod_even(X, A, N);
mbedtls_mpi_cmp_mpi(A, N) < 0) {
return mbedtls_mpi_inv_mod_even_in_range(X, A, N);
} }
mbedtls_mpi_init(&TA); mbedtls_mpi_init(&TU); mbedtls_mpi_init(&U1); mbedtls_mpi_init(&U2); mbedtls_mpi_init(&TA); mbedtls_mpi_init(&TU); mbedtls_mpi_init(&U1); mbedtls_mpi_init(&U2);