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.
@@ -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 );
