Mailing List Archive

* src/const-time.c (_gcry_ct_memmov_cond): Use dual mask + AND/OR
instead of single mask + XOR.
--

Signed-off-by: Jussi Kivilinna <jussi.kivilinna@iki.fi>
---
src/const-time.c | 9 +++++----
1 file changed, 5 insertions(+), 4 deletions(-)

diff --git a/src/const-time.c b/src/const-time.c
index 2066d48d..73bf8b40 100644
--- a/src/const-time.c
+++ b/src/const-time.c
@@ -74,12 +74,13 @@ void
_gcry_ct_memmov_cond (void *dst, const void *src, size_t len,
unsigned long op_enable)
{
- size_t i;
- unsigned char mask;
+ /* Note: dual mask with AND/OR used for EM leakage mitigation */
+ unsigned char mask1 = _gcry_ct_vzero - op_enable;
+ unsigned char mask2 = op_enable - _gcry_ct_vone;
unsigned char *b_dst = dst;
const unsigned char *b_src = src;
+ size_t i;

- mask = -(unsigned char)op_enable;
for (i = 0; i < len; i++)
- b_dst[i] ^= mask & (b_dst[i] ^ b_src[i]);
+ b_dst[i] = (b_dst[i] & mask2) | (b_src[i] & mask1);
}
--
2.40.1


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