Use bit operations for mpi_safe_cond_assign()
- copied limbs
- sign
- cleared limbs
Signed-off-by: Manuel Pégourié-Gonnard <manuel.pegourie-gonnard@arm.com>
diff --git a/library/bignum.c b/library/bignum.c
index 808de17..343ce48 100644
--- a/library/bignum.c
+++ b/library/bignum.c
@@ -249,8 +249,23 @@
unsigned char assign )
{
size_t i;
+
+ /* MSVC has a warning about unary minus on unsigned integer types,
+ * but this is well-defined and precisely what we want to do here. */
+#if defined(_MSC_VER)
+#pragma warning( push )
+#pragma warning( disable : 4146 )
+#endif
+
+ /* all-bits 1 if assign is 1, all-bits 0 if assign is 0 */
+ const mbedtls_mpi_uint mask = -assign;
+
+#if defined(_MSC_VER)
+#pragma warning( pop )
+#endif
+
for( i = 0; i < n; i++ )
- dest[i] = dest[i] * ( 1 - assign ) + src[i] * assign;
+ dest[i] = ( src[i] & mask ) | ( dest[i] & ~mask );
}
/*
@@ -262,20 +277,36 @@
{
int ret = 0;
size_t i;
+ unsigned int mask;
+ mbedtls_mpi_uint limb_mask;
MPI_VALIDATE_RET( X != NULL );
MPI_VALIDATE_RET( Y != NULL );
+ /* MSVC has a warning about unary minus on unsigned integer types,
+ * but this is well-defined and precisely what we want to do here. */
+#if defined(_MSC_VER)
+#pragma warning( push )
+#pragma warning( disable : 4146 )
+#endif
+
/* make sure assign is 0 or 1 in a time-constant manner */
assign = (assign | (unsigned char)-assign) >> 7;
+ /* all-bits 1 if assign is 1, all-bits 0 if assign is 0 */
+ mask = -assign;
+ limb_mask = -assign;
+
+#if defined(_MSC_VER)
+#pragma warning( pop )
+#endif
MBEDTLS_MPI_CHK( mbedtls_mpi_grow( X, Y->n ) );
- X->s = X->s * ( 1 - assign ) + Y->s * assign;
+ X->s = ( X->s & ~mask ) | ( Y->s & mask );
mpi_safe_cond_assign( Y->n, X->p, Y->p, assign );
for( i = Y->n; i < X->n; i++ )
- X->p[i] *= ( 1 - assign );
+ X->p[i] &= ~limb_mask;
cleanup:
return( ret );
