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mbedtls_ecp_gen_privkey: minor refactoring

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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>
This commit is contained in:
Gilles Peskine 2021-03-23 21:58:14 +01:00
parent 84191eab06
commit cce0601485
1 changed files with 20 additions and 15 deletions
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35
library/ecp.c
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@ -3066,36 +3066,39 @@ int mbedtls_ecp_gen_privkey( const mbedtls_ecp_group *grp,
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 ) );
}
@ -3108,18 +3111,20 @@ int mbedtls_ecp_gen_privkey( const mbedtls_ecp_group *grp,
/* 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
@ -3136,7 +3141,7 @@ int mbedtls_ecp_gen_privkey( const mbedtls_ecp_group *grp,
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;