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Merge pull request #6303 from gilles-peskine-arm/bignum-core-random

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Bignum: Implement mbedtls_mpi_core_random
This commit is contained in:
Manuel Pégourié-Gonnard
2022-12-16 09:58:07 +01:00
committed by GitHub
parent d1dd41f3fc acdefdd51a
commit 5bf8629b2c
9 changed files with 823 additions and 440 deletions
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66
library/bignum.c
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@ -2032,75 +2032,19 @@ int mbedtls_mpi_random( mbedtls_mpi *X,
int (*f_rng)(void *, unsigned char *, size_t),
void *p_rng )
{
int ret = MBEDTLS_ERR_MPI_BAD_INPUT_DATA;
int count;
unsigned lt_lower = 1, lt_upper = 0;
size_t n_bits = mbedtls_mpi_bitlen( N );
size_t n_bytes = ( n_bits + 7 ) / 8;
mbedtls_mpi lower_bound;
if( min < 0 )
return( MBEDTLS_ERR_MPI_BAD_INPUT_DATA );
if( mbedtls_mpi_cmp_int( N, min ) <= 0 )
return( MBEDTLS_ERR_MPI_BAD_INPUT_DATA );
/*
* When min == 0, each try has at worst a probability 1/2 of failing
* (the msb has a probability 1/2 of being 0, and then the result will
* be < N), so after 30 tries failure probability is a most 2**(-30).
*
* When N is just below a power of 2, as is the case when generating
* a random scalar on most elliptic curves, 1 try is enough with
* overwhelming probability. When N is just above a power of 2,
* as when generating a random scalar on secp224k1, each try has
* a probability of failing that is almost 1/2.
*
* The probabilities are almost the same if min is nonzero but negligible
* compared to N. This is always the case when N is crypto-sized, but
* it's convenient to support small N for testing purposes. When N
* is small, use a higher repeat count, otherwise the probability of
* failure is macroscopic.
*/
count = ( n_bytes > 4 ? 30 : 250 );
mbedtls_mpi_init( &lower_bound );
/* Ensure that target MPI has exactly the same number of limbs
* as the upper bound, even if the upper bound has leading zeros.
* This is necessary for the mbedtls_mpi_lt_mpi_ct() check. */
MBEDTLS_MPI_CHK( mbedtls_mpi_resize_clear( X, N->n ) );
MBEDTLS_MPI_CHK( mbedtls_mpi_grow( &lower_bound, N->n ) );
MBEDTLS_MPI_CHK( mbedtls_mpi_lset( &lower_bound, min ) );
* This is necessary for mbedtls_mpi_core_random. */
int ret = mbedtls_mpi_resize_clear( X, N->n );
if( ret != 0 )
return( ret );
/*
* Match the procedure given in RFC 6979 §3.3 (deterministic ECDSA)
* when f_rng is a suitably parametrized instance of HMAC_DRBG:
* - use the same byte ordering;
* - keep the leftmost n_bits bits of the generated octet string;
* - try until result is in the desired range.
* This also avoids any bias, which is especially important for ECDSA.
*/
do
{
MBEDTLS_MPI_CHK( mbedtls_mpi_core_fill_random( X->p, X->n,
n_bytes,
f_rng, p_rng ) );
MBEDTLS_MPI_CHK( mbedtls_mpi_shift_r( X, 8 * n_bytes - n_bits ) );
if( --count == 0 )
{
ret = MBEDTLS_ERR_MPI_NOT_ACCEPTABLE;
goto cleanup;
}
MBEDTLS_MPI_CHK( mbedtls_mpi_lt_mpi_ct( X, &lower_bound, &lt_lower ) );
MBEDTLS_MPI_CHK( mbedtls_mpi_lt_mpi_ct( X, N, &lt_upper ) );
}
while( lt_lower != 0 || lt_upper == 0 );
cleanup:
mbedtls_mpi_free( &lower_bound );
return( ret );
return( mbedtls_mpi_core_random( X->p, min, N->p, X->n, f_rng, p_rng ) );
}
/*