From 1fc97594da2394cc32649eb015df317c35f8513e Mon Sep 17 00:00:00 2001 From: Janos Follath Date: Fri, 11 Oct 2019 10:43:40 +0100 Subject: [PATCH] mbedtls_mpi_cmp_mpi_ct: remove multiplications Multiplication is known to have measurable timing variations based on the operands. For example it typically is much faster if one of the operands is zero. Remove them from constant time code. --- library/bignum.c | 16 ++++++++++------ 1 file changed, 10 insertions(+), 6 deletions(-) diff --git a/library/bignum.c b/library/bignum.c index ff8f8296d..b90404512 100644 --- a/library/bignum.c +++ b/library/bignum.c @@ -1175,6 +1175,11 @@ static int ct_lt_mpi_uint( const mbedtls_mpi_uint x, const mbedtls_mpi_uint y ) return ret; } +static int ct_bool_get_mask( unsigned int b ) +{ + return ~( b - 1 ); +} + /* * Compare signed values in constant time */ @@ -1206,7 +1211,7 @@ int mbedtls_mpi_cmp_mpi_ct( const mbedtls_mpi *X, const mbedtls_mpi *Y, sign_X = X->s; sign_Y = Y->s; cond = ( ( sign_X ^ sign_Y ) >> ( sizeof( unsigned int ) * 8 - 1 ) ); - *ret = cond * X->s; + *ret = ct_bool_get_mask( cond ) & X->s; done = cond; for( i = X->n; i > 0; i-- ) @@ -1219,8 +1224,8 @@ int mbedtls_mpi_cmp_mpi_ct( const mbedtls_mpi *X, const mbedtls_mpi *Y, * } */ cond = ct_lt_mpi_uint( Y->p[i - 1], X->p[i - 1] ); - *ret |= ( cond * ( 1 - done ) ) * X->s; - done |= cond * ( 1 - done ); + *ret |= ct_bool_get_mask( cond & ( 1 - done ) ) & X->s; + done |= cond & ( 1 - done ); /* * if( ( X->p[i - 1] < Y->p[i - 1] ) && !done ) @@ -1230,9 +1235,8 @@ int mbedtls_mpi_cmp_mpi_ct( const mbedtls_mpi *X, const mbedtls_mpi *Y, * } */ cond = ct_lt_mpi_uint( X->p[i - 1], Y->p[i - 1] ); - *ret |= ( cond * ( 1 - done ) ) * -X->s; - done |= cond * ( 1 - done ); - + *ret |= ct_bool_get_mask( cond & ( 1 - done ) ) & -X->s; + done |= cond & ( 1 - done ); } return( 0 );