Avoid a side-channel cache-timing attack that can leak the ECDSA private

keys when signing. This is due to BN_mod_inverse() being used without the
constant time flag being set.

This issue was reported by Cesar Pereida Garcia and Billy Brumley
(Tampere University of Technology). The fix was developed by Cesar Pereida
Garcia.

1 files changed, 3 insertions, 1 deletions

diff --git a/src/lib/libcrypto/ecdsa/ecs_ossl.c b/src/lib/libcrypto/ecdsa/ecs_ossl.c
index 31102138c0..26158a001b 100644
--- a/src/lib/libcrypto/ecdsa/ecs_ossl.c
+++ b/src/lib/libcrypto/ecdsa/ecs_ossl.c
@@ -1,4 +1,4 @@
-/* $OpenBSD: ecs_ossl.c,v 1.6 2015/02/08 13:35:07 jsing Exp $ */
+/* $OpenBSD: ecs_ossl.c,v 1.7 2017/01/05 13:25:52 jsing Exp $ */
 /*
  * Written by Nils Larsch for the OpenSSL project
  */
@@ -142,6 +142,8 @@ ecdsa_sign_setup(EC_KEY *eckey, BN_CTX *ctx_in, BIGNUM **kinvp, BIGNUM **rp)
                         if (!BN_add(k, k, order))
                                 goto err;
 
+                BN_set_flags(k, BN_FLG_CONSTTIME);
+
                 /* compute r the x-coordinate of generator * k */
                 if (!EC_POINT_mul(group, tmp_point, k, NULL, NULL, ctx)) {
                         ECDSAerr(ECDSA_F_ECDSA_SIGN_SETUP, ERR_R_EC_LIB);