mbedtls_ecp_gen_privkey: minor refactoring
Prepare to isolate the Montgomery and short Weierstrass
implementations of mbedtls_ecp_gen_privkey into their own function.
Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>
diff --git a/library/ecp.c b/library/ecp.c
index 6b29415..2b6bcc8 100644
--- a/library/ecp.c
+++ b/library/ecp.c
@@ -3049,36 +3049,39 @@
void *p_rng )
{
int ret = MBEDTLS_ERR_ECP_BAD_INPUT_DATA;
- size_t n_size;
+ size_t n_bits;
+ const mbedtls_mpi *N = NULL;
ECP_VALIDATE_RET( grp != NULL );
ECP_VALIDATE_RET( d != NULL );
ECP_VALIDATE_RET( f_rng != NULL );
- n_size = ( grp->nbits + 7 ) / 8;
+ N = &grp->N;
+ n_bits = grp->nbits;
#if defined(MBEDTLS_ECP_MONTGOMERY_ENABLED)
if( mbedtls_ecp_get_type( grp ) == MBEDTLS_ECP_TYPE_MONTGOMERY )
{
- /* [M225] page 5 */
size_t b;
+ size_t n_bytes = ( n_bits + 7 ) / 8;
+ /* [Curve25519] page 5 */
do {
- MBEDTLS_MPI_CHK( mbedtls_mpi_fill_random( d, n_size, f_rng, p_rng ) );
+ MBEDTLS_MPI_CHK( mbedtls_mpi_fill_random( d, n_bytes, f_rng, p_rng ) );
} while( mbedtls_mpi_bitlen( d ) == 0);
- /* Make sure the most significant bit is nbits */
+ /* Make sure the most significant bit is n_bits */
b = mbedtls_mpi_bitlen( d ) - 1; /* mbedtls_mpi_bitlen is one-based */
- if( b > grp->nbits )
- MBEDTLS_MPI_CHK( mbedtls_mpi_shift_r( d, b - grp->nbits ) );
+ if( b > n_bits )
+ MBEDTLS_MPI_CHK( mbedtls_mpi_shift_r( d, b - n_bits ) );
else
- MBEDTLS_MPI_CHK( mbedtls_mpi_set_bit( d, grp->nbits, 1 ) );
+ MBEDTLS_MPI_CHK( mbedtls_mpi_set_bit( d, n_bits, 1 ) );
/* Make sure the last two bits are unset for Curve448, three bits for
Curve25519 */
MBEDTLS_MPI_CHK( mbedtls_mpi_set_bit( d, 0, 0 ) );
MBEDTLS_MPI_CHK( mbedtls_mpi_set_bit( d, 1, 0 ) );
- if( grp->nbits == 254 )
+ if( n_bits == 254 )
{
MBEDTLS_MPI_CHK( mbedtls_mpi_set_bit( d, 2, 0 ) );
}
@@ -3091,18 +3094,20 @@
/* SEC1 3.2.1: Generate d such that 1 <= n < N */
int count = 0;
unsigned cmp = 0;
+ size_t n_bytes = ( n_bits + 7 ) / 8;
/*
- * Match the procedure given in RFC 6979 (deterministic ECDSA):
+ * 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 nbits bits of the generated octet string;
+ * - keep the leftmost n_bits bits of the generated octet string;
* - try until result is in the desired range.
- * This also avoids any biais, which is especially important for ECDSA.
+ * This also avoids any bias, which is especially important for ECDSA.
*/
do
{
- MBEDTLS_MPI_CHK( mbedtls_mpi_fill_random( d, n_size, f_rng, p_rng ) );
- MBEDTLS_MPI_CHK( mbedtls_mpi_shift_r( d, 8 * n_size - grp->nbits ) );
+ MBEDTLS_MPI_CHK( mbedtls_mpi_fill_random( d, n_bytes, f_rng, p_rng ) );
+ MBEDTLS_MPI_CHK( mbedtls_mpi_shift_r( d, 8 * n_bytes - n_bits ) );
/*
* Each try has at worst a probability 1/2 of failing (the msb has
@@ -3119,7 +3124,7 @@
goto cleanup;
}
- ret = mbedtls_mpi_lt_mpi_ct( d, &grp->N, &cmp );
+ ret = mbedtls_mpi_lt_mpi_ct( d, N, &cmp );
if( ret != 0 )
{
goto cleanup;