[PATCH v2 2/6] crypto: lib/sha256 - Don't clear temporary variables

Arvind Sankar
2020-10-20T20:40:08Z

The assignments to clear a through h and t1/t2 are optimized out by the
compiler because they are unused after the assignments.

These variables shouldn't be very sensitive: t1/t2 can be calculated
from a through h, so they don't reveal any additional information.
Knowing a through h is equivalent to knowing one 64-byte block's SHA256
hash (with non-standard initial value) which, assuming SHA256 is secure,
doesn't reveal any information about the input.

Signed-off-by: Arvind Sankar <nivedita@alum.mit.edu>
---
lib/crypto/sha256.c | 1 -
1 file changed, 1 deletion(-)

diff --git a/lib/crypto/sha256.c b/lib/crypto/sha256.c
index d43bc39ab05e..099cd11f83c1 100644
--- a/lib/crypto/sha256.c
+++ b/lib/crypto/sha256.c
@@ -202,7 +202,6 @@ static void sha256_transform(u32 *state, const u8 *input)
state[4] += e; state[5] += f; state[6] += g; state[7] += h;

/* clear any sensitive info... */
- a = b = c = d = e = f = g = h = t1 = t2 = 0;
memzero_explicit(W, 64 * sizeof(u32));
}

--
2.26.2