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Avoid potential leak in ecp_mul_mxz()
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@ -1473,6 +1473,7 @@ static int ecp_mul_mxz( ecp_group *grp, ecp_point *R,
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{
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int ret;
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size_t i;
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unsigned char b;
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ecp_point RP;
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mpi PX;
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@ -1491,14 +1492,23 @@ static int ecp_mul_mxz( ecp_group *grp, ecp_point *R,
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if( f_rng != NULL )
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MPI_CHK( ecp_randomize_mxz( grp, &RP, f_rng, p_rng ) );
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/* Loop invariant: R = result so far, RP = R + P */
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i = mpi_msb( m ); /* one past the (zero-based) most significant bit */
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while( i-- > 0 )
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{
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// TODO: no branch, and constant memory-access pattern
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if( mpi_get_bit( m, i ) )
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MPI_CHK( ecp_double_add_mxz( grp, &RP, R, &RP, R, &PX ) );
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else
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MPI_CHK( ecp_double_add_mxz( grp, R, &RP, R, &RP, &PX ) );
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b = mpi_get_bit( m, i );
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/*
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* if (b) R = 2R + P else R = 2R,
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* which is:
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* if (b) double_add( RP, R, RP, R )
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* else double_add( R, RP, R, RP )
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* but using safe conditional swaps to avoid leaks
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*/
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MPI_CHK( mpi_safe_cond_swap( &R->X, &RP.X, b ) );
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MPI_CHK( mpi_safe_cond_swap( &R->Z, &RP.Z, b ) );
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MPI_CHK( ecp_double_add_mxz( grp, R, &RP, R, &RP, &PX ) );
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MPI_CHK( mpi_safe_cond_swap( &R->X, &RP.X, b ) );
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MPI_CHK( mpi_safe_cond_swap( &R->Z, &RP.Z, b ) );
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}
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MPI_CHK( ecp_normalize_mxz( grp, R ) );
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