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Make mod_p{224,256,384] a bit faster

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Speedup is roughly 25%, giving a 6% speedup on ecp_mul() for these curves.
This commit is contained in:
Manuel Pégourié-Gonnard
2013-10-23 20:17:00 +02:00
parent c04c530a98
commit 5779cbe582
2 changed files with 19 additions and 7 deletions
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24
library/ecp.c
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@ -663,22 +663,32 @@ static inline void sub32( uint32_t *dst, uint32_t src, signed char *carry )
if( c < 0 ) fix_negative( N, c, bits );
/*
* If the result is negative, we get it in the form c * 2^192 + N,
* with c negative and N positive (the c >= 0 case is handled by LAST).
* If the result is negative, we get it in the form
* c * 2^(bits + 32) + N, with c negative and N positive shorter than 'bits'
*/
static inline int fix_negative( mpi *N, signed char c, size_t bits )
{
int ret;
mpi C;
t_uint Cp[ 384 / 8 / sizeof( t_uint) + 1 ];
mpi_init( &C );
/* C = - c * 2^(bits + 32) */
C.s = 1;
C.n = bits / 8 / sizeof( t_uint ) + 1;
C.p = Cp;
memset( Cp, 0, C.n * sizeof( t_uint ) );
#if defined(POLARSSL_HAVE_INT64)
if( bits == 224 )
Cp[ C.n - 1 ] = ((t_uint) -c) << 32;
else
#endif
Cp[ C.n - 1 ] = (t_uint) -c;
MPI_CHK( mpi_lset( &C, c ) );
MPI_CHK( mpi_shift_l( &C, bits ) );
MPI_CHK( mpi_add_mpi( N, N, &C ) );
/* N = - ( C - N ) */
MPI_CHK( mpi_sub_abs( N, &C, N ) );
N->s = -1;
cleanup:
mpi_free( &C );
return( ret );
}