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Optimize ecp_modp()

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Makes it 22% faster, for a 5% gain on ecp_mul()
This commit is contained in:
Manuel Pégourié-Gonnard 2013-10-17 17:15:35 +02:00
parent d1e7a45fdd
commit c9e387ca9e
1 changed files with 17 additions and 6 deletions
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23
library/ecp.c
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@ -451,6 +451,8 @@ int ecp_tls_write_point( const ecp_group *grp, const ecp_point *pt,
/* /*
* Wrapper around fast quasi-modp functions, with fall-back to mpi_mod_mpi. * Wrapper around fast quasi-modp functions, with fall-back to mpi_mod_mpi.
* See the documentation of struct ecp_group. * See the documentation of struct ecp_group.
*
* This function is in the critial loop for ecp_mul, so pay attention to perf.
*/ */
static int ecp_modp( mpi *N, const ecp_group *grp ) static int ecp_modp( mpi *N, const ecp_group *grp )
{ {
@ -459,16 +461,22 @@ static int ecp_modp( mpi *N, const ecp_group *grp )
if( grp->modp == NULL ) if( grp->modp == NULL )
return( mpi_mod_mpi( N, N, &grp->P ) ); return( mpi_mod_mpi( N, N, &grp->P ) );
if( mpi_cmp_int( N, 0 ) < 0 || mpi_msb( N ) > 2 * grp->pbits ) /* N->s < 0 is a much faster test, which fails only if N is 0 */
if( ( N->s < 0 && mpi_cmp_int( N, 0 ) != 0 ) ||
mpi_msb( N ) > 2 * grp->pbits )
{
return( POLARSSL_ERR_ECP_BAD_INPUT_DATA ); return( POLARSSL_ERR_ECP_BAD_INPUT_DATA );
}
MPI_CHK( grp->modp( N ) ); MPI_CHK( grp->modp( N ) );
while( mpi_cmp_int( N, 0 ) < 0 ) /* N->s < 0 is a much faster test, which fails only if N is 0 */
while( N->s < 0 && mpi_cmp_int( N, 0 ) != 0 )
MPI_CHK( mpi_add_mpi( N, N, &grp->P ) ); MPI_CHK( mpi_add_mpi( N, N, &grp->P ) );
while( mpi_cmp_mpi( N, &grp->P ) >= 0 ) while( mpi_cmp_mpi( N, &grp->P ) >= 0 )
MPI_CHK( mpi_sub_mpi( N, N, &grp->P ) ); /* we known P, N and the result are positive */
MPI_CHK( mpi_sub_abs( N, N, &grp->P ) );
cleanup: cleanup:
return( ret ); return( ret );
@ -915,17 +923,20 @@ const ecp_curve_info *ecp_curve_info_from_grp_id( ecp_group_id grp_id )
/* /*
* Reduce a mpi mod p in-place, to use after mpi_sub_mpi * Reduce a mpi mod p in-place, to use after mpi_sub_mpi
* N->s < 0 is a very fast test, which fails only if N is 0
*/ */
#define MOD_SUB( N ) \ #define MOD_SUB( N ) \
while( mpi_cmp_int( &N, 0 ) < 0 ) \ while( N.s < 0 && mpi_cmp_int( &N, 0 ) != 0 ) \
MPI_CHK( mpi_add_mpi( &N, &N, &grp->P ) ) MPI_CHK( mpi_add_mpi( &N, &N, &grp->P ) )
/* /*
* Reduce a mpi mod p in-place, to use after mpi_add_mpi and mpi_mul_int * Reduce a mpi mod p in-place, to use after mpi_add_mpi and mpi_mul_int.
* We known P, N and the result are positive, so sub_abs is correct, and
* a bit faster.
*/ */
#define MOD_ADD( N ) \ #define MOD_ADD( N ) \
while( mpi_cmp_mpi( &N, &grp->P ) >= 0 ) \ while( mpi_cmp_mpi( &N, &grp->P ) >= 0 ) \
MPI_CHK( mpi_sub_mpi( &N, &N, &grp->P ) ) MPI_CHK( mpi_sub_abs( &N, &N, &grp->P ) )
/* /*
* Normalize jacobian coordinates so that Z == 0 || Z == 1 (GECC 3.2.1) * Normalize jacobian coordinates so that Z == 0 || Z == 1 (GECC 3.2.1)