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Rm subtraction from ecp_add_mixed()

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Manuel Pégourié-Gonnard 2013-11-21 18:20:43 +01:00
parent 01fca5e882
commit 469a209334
1 changed files with 19 additions and 39 deletions
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58
library/ecp.c
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@ -1067,21 +1067,15 @@ cleanup:
}
/*
* Addition or subtraction: R = P + Q or R = P - Q,
* mixed affine-Jacobian coordinates (GECC 3.22)
* Addition: R = P + Q, mixed affine-Jacobian coordinates (GECC 3.22)
*
* The coordinates of Q must be normalized (= affine),
* but those of P don't need to. R is not normalized.
*
* If sign >= 0, perform addition, otherwise perform subtraction,
* taking advantage of the fact that, for Q != 0, we have
* -Q = (Q.X, -Q.Y, Q.Z)
*
* Cost: 1A := 8M + 3S
*/
static int ecp_add_mixed( const ecp_group *grp, ecp_point *R,
const ecp_point *P, const ecp_point *Q,
signed char sign )
const ecp_point *P, const ecp_point *Q )
{
int ret;
mpi T1, T2, T3, T4, X, Y, Z;
@ -1092,26 +1086,14 @@ static int ecp_add_mixed( const ecp_group *grp, ecp_point *R,
/*
* Trivial cases: P == 0 or Q == 0
* (Check Q first, so that we know Q != 0 when we compute -Q.)
* This will never happen during ecp_mul() so we don't mind the branches.
*/
if( mpi_cmp_int( &P->Z, 0 ) == 0 )
return( ecp_copy( R, Q ) );
if( mpi_cmp_int( &Q->Z, 0 ) == 0 )
return( ecp_copy( R, P ) );
if( mpi_cmp_int( &P->Z, 0 ) == 0 )
{
ret = ecp_copy( R, Q );
/*
* -R.Y mod P = P - R.Y unless R.Y == 0
*/
if( ret == 0 && sign < 0)
if( mpi_cmp_int( &R->Y, 0 ) != 0 )
ret = mpi_sub_mpi( &R->Y, &grp->P, &R->Y );
return( ret );
}
/*
* Make sure Q coordinates are normalized
*/
@ -1125,18 +1107,6 @@ static int ecp_add_mixed( const ecp_group *grp, ecp_point *R,
MPI_CHK( mpi_mul_mpi( &T2, &T1, &P->Z ) ); MOD_MUL( T2 );
MPI_CHK( mpi_mul_mpi( &T1, &T1, &Q->X ) ); MOD_MUL( T1 );
MPI_CHK( mpi_mul_mpi( &T2, &T2, &Q->Y ) ); MOD_MUL( T2 );
/*
* For subtraction, -Q.Y should have been used instead of Q.Y,
* so we replace T2 by -T2, which is P - T2 mod P
* (Again, not used by ecp_mul(), so not worry about the branch.)
*/
if( sign < 0 )
{
MPI_CHK( mpi_sub_mpi( &T2, &grp->P, &T2 ) );
MOD_SUB( T2 );
}
MPI_CHK( mpi_sub_mpi( &T1, &T1, &P->X ) ); MOD_SUB( T1 );
MPI_CHK( mpi_sub_mpi( &T2, &T2, &P->Y ) ); MOD_SUB( T2 );
@ -1189,7 +1159,7 @@ int ecp_add( const ecp_group *grp, ecp_point *R,
{
int ret;
MPI_CHK( ecp_add_mixed( grp, R, P, Q , 1 ) );
MPI_CHK( ecp_add_mixed( grp, R, P, Q ) );
MPI_CHK( ecp_normalize( grp, R ) );
cleanup:
@ -1204,11 +1174,21 @@ int ecp_sub( const ecp_group *grp, ecp_point *R,
const ecp_point *P, const ecp_point *Q )
{
int ret;
ecp_point mQ;
MPI_CHK( ecp_add_mixed( grp, R, P, Q, -1 ) );
ecp_point_init( &mQ );
/* mQ = - Q */
ecp_copy( &mQ, Q );
if( mpi_cmp_int( &mQ.Y, 0 ) != 0 )
MPI_CHK( mpi_sub_mpi( &mQ.Y, &grp->P, &mQ.Y ) );
MPI_CHK( ecp_add_mixed( grp, R, P, &mQ ) );
MPI_CHK( ecp_normalize( grp, R ) );
cleanup:
ecp_point_free( &mQ );
return( ret );
}
@ -1370,7 +1350,7 @@ static int ecp_precompute_comb( const ecp_group *grp,
j = i;
while( j-- )
{
ecp_add_mixed( grp, &T[i + j], &T[j], &T[i], +1 );
ecp_add_mixed( grp, &T[i + j], &T[j], &T[i] );
TT[k++] = &T[i + j];
}
}
@ -1443,7 +1423,7 @@ static int ecp_mul_comb_core( const ecp_group *grp, ecp_point *R,
{
MPI_CHK( ecp_double_jac( grp, R, R ) );
MPI_CHK( ecp_select_comb( grp, &Txi, T, x[i] ) );
MPI_CHK( ecp_add_mixed( grp, R, R, &Txi, +1 ) );
MPI_CHK( ecp_add_mixed( grp, R, R, &Txi ) );
}
cleanup: