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Made choice of w safer and more optimal

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Manuel Pégourié-Gonnard 2012-11-21 18:49:45 +01:00 committed by Paul Bakker
parent cdd44324e9
commit 3680c82c5a
1 changed files with 16 additions and 6 deletions
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22
library/ecp.c
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@ -507,11 +507,12 @@ cleanup:
/*
* Normalize jacobian coordinates of an array of points,
* using Montgomery's trick to perform only one division.
* using Montgomery's trick to perform only one inversion mod P.
* (See for example Cohen's "A Course in Computational Algebraic Number
* Theory", Algorithm 10.3.4.)
*
* FIXME: assumes all points are non zero
* Warning: fails if one of the points is zero!
* This should never happen, see choice of w in ecp_mul().
*/
static int ecp_normalize_many( const ecp_group *grp,
ecp_point T[], size_t t_len )
@ -868,7 +869,7 @@ static int ecp_precompute( const ecp_group *grp,
/*
* T[0] = P already has normalized coordinates
*/
ecp_normalize_many( grp, T + 1, t_len - 1 );
MPI_CHK( ecp_normalize_many( grp, T + 1, t_len - 1 ) );
cleanup:
@ -911,12 +912,20 @@ int ecp_mul( const ecp_group *grp, ecp_point *R,
if( mpi_cmp_int( m, 0 ) < 0 || mpi_msb( m ) > grp->nbits )
return( POLARSSL_ERR_ECP_GENERIC );
w = 5; // TODO: find optimal values once precompute() is optimized
w = grp->nbits >= 521 ? 6 :
grp->nbits >= 224 ? 5 :
4;
if( w < 2 )
w = 2;
/*
* Make sure w is within the limits.
* The last test ensures that none of the precomputed points is zero,
* which wouldn't be handled correctly by ecp_normalize_many().
* It is only useful for small curves, as used in the test suite.
*/
if( w > POLARSSL_ECP_WINDOW_SIZE )
w = POLARSSL_ECP_WINDOW_SIZE;
if( w < 2 || w >= grp->nbits )
w = 2;
pre_len = 1 << ( w - 1 );
naf_len = grp->nbits / w + 1;
@ -980,6 +989,7 @@ int ecp_mul( const ecp_group *grp, ecp_point *R,
MPI_CHK( ecp_add( grp, &T[1], P, P ) );
MPI_CHK( ecp_sub( grp, R, &Q, &T[m_is_odd] ) );
cleanup:
mpi_free( &M );