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mbedtls_mpi_random: avoid local allocation

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Rewrite the minimum bound comparison to avoid a local allocation. This costs
a bit of code size, but saves RAM. This is in preparation for moving the
bulk of the function to the bignum_core module where allocation is not
permitted.

Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>
This commit is contained in:
Gilles Peskine
2022-09-09 23:19:04 +02:00
parent 6f949ea67b
commit 8a32a75aa2
1 changed files with 4 additions and 10 deletions
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14
library/bignum.c
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@ -1968,10 +1968,9 @@ int mbedtls_mpi_random( mbedtls_mpi *X,
{
int ret = MBEDTLS_ERR_MPI_BAD_INPUT_DATA;
int count;
unsigned lt_lower = 1, lt_upper = 0;
unsigned ge_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 );
@ -1997,14 +1996,10 @@ int mbedtls_mpi_random( mbedtls_mpi *X,
*/
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 ) );
/*
* Match the procedure given in RFC 6979 §3.3 (deterministic ECDSA)
@ -2027,13 +2022,12 @@ int mbedtls_mpi_random( mbedtls_mpi *X,
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 ) );
ge_lower = mbedtls_mpi_core_uint_le_mpi( min, X->p, X->n );
lt_upper = mbedtls_mpi_core_lt_ct( X->p, N->p, N->n );
}
while( lt_lower != 0 || lt_upper == 0 );
while( ge_lower == 0 || lt_upper == 0 );
cleanup:
mbedtls_mpi_free( &lower_bound );
return( ret );
}