Commit 3afa07f0 authored by Manuel Pégourié-Gonnard's avatar Manuel Pégourié-Gonnard
Browse files

Add coordinate randomization for Curve25519

parent d9ea82e7
......@@ -976,7 +976,7 @@ cleanup:
*
* This countermeasure was first suggested in [2].
*/
static int ecp_randomize_coordinates( const ecp_group *grp, ecp_point *pt,
static int ecp_randomize_jac( const ecp_group *grp, ecp_point *pt,
int (*f_rng)(void *, unsigned char *, size_t), void *p_rng )
{
int ret;
......@@ -1203,7 +1203,7 @@ static int ecp_mul_comb_core( const ecp_group *grp, ecp_point *R,
i = d;
MPI_CHK( ecp_select_comb( grp, R, T, t_len, x[i] ) );
if( f_rng != 0 )
MPI_CHK( ecp_randomize_coordinates( grp, R, f_rng, p_rng ) );
MPI_CHK( ecp_randomize_jac( grp, R, f_rng, p_rng ) );
while( i-- != 0 )
{
......@@ -1363,6 +1363,46 @@ cleanup:
return( ret );
}
/*
* Randomize projective x/z coordinates:
* (X, Z) -> (l X, l Z) for random l
* This is sort of the reverse operation of ecp_normalize_mxz().
*
* This countermeasure was first suggested in [2].
* Cost: 2M
*/
static int ecp_randomize_mxz( const ecp_group *grp, ecp_point *P,
int (*f_rng)(void *, unsigned char *, size_t), void *p_rng )
{
int ret;
mpi l;
size_t p_size = (grp->pbits + 7) / 8;
int count = 0;
mpi_init( &l );
/* Generate l such that 1 < l < p */
do
{
mpi_fill_random( &l, p_size, f_rng, p_rng );
while( mpi_cmp_mpi( &l, &grp->P ) >= 0 )
mpi_shift_r( &l, 1 );
if( count++ > 10 )
return( POLARSSL_ERR_ECP_RANDOM_FAILED );
}
while( mpi_cmp_int( &l, 1 ) <= 0 );
MPI_CHK( mpi_mul_mpi( &P->X, &P->X, &l ) ); MOD_MUL( P->X );
MPI_CHK( mpi_mul_mpi( &P->Z, &P->Z, &l ) ); MOD_MUL( P->Z );
cleanup:
mpi_free( &l );
return( ret );
}
/*
* Double-and-add: R = 2P, S = P + Q, with d = X(P - Q),
* for Montgomery curves in x/z coordinates.
......@@ -1431,13 +1471,13 @@ int ecp_mul_mxz( ecp_group *grp, ecp_point *R,
ecp_point_init( &RP ); mpi_init( &PX );
/* Save PX and read P before writing to R, in case P == R */
/* Save PX and read from P before writing to R, in case P == R */
mpi_copy( &PX, &P->X );
MPI_CHK( ecp_copy( &RP, P ) );
MPI_CHK( ecp_set_zero( R ) );
// TODO: randomize RP coordinates
(void) f_rng; (void) p_rng;
/* Randomize coordinates of the starting point */
MPI_CHK( ecp_randomize_mxz( grp, &RP, f_rng, p_rng ) );
i = mpi_msb( m ) + 1;
while( i-- > 0 )
......
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