From 399944622df7bd81af62e67ea967c470534090e2 Mon Sep 17 00:00:00 2001 From: Cesar Pereida Date: Mon, 23 May 2016 12:45:25 +0300 Subject: [PATCH] Fix DSA, preserve BN_FLG_CONSTTIME Operations in the DSA signing algorithm should run in constant time in order to avoid side channel attacks. A flaw in the OpenSSL DSA implementation means that a non-constant time codepath is followed for certain operations. This has been demonstrated through a cache-timing attack to be sufficient for an attacker to recover the private DSA key. CVE-2016-2178 Reviewed-by: Richard Levitte Reviewed-by: Matt Caswell Upstream-Status: Backport CVE: CVE-2016-2178 Signed-off-by: Armin Kuster --- crypto/dsa/dsa_ossl.c | 9 +++++---- 1 file changed, 5 insertions(+), 4 deletions(-) Index: openssl-1.0.2h/crypto/dsa/dsa_ossl.c =================================================================== --- openssl-1.0.2h.orig/crypto/dsa/dsa_ossl.c +++ openssl-1.0.2h/crypto/dsa/dsa_ossl.c @@ -248,9 +248,6 @@ static int dsa_sign_setup(DSA *dsa, BN_C if (!BN_rand_range(&k, dsa->q)) goto err; while (BN_is_zero(&k)) ; - if ((dsa->flags & DSA_FLAG_NO_EXP_CONSTTIME) == 0) { - BN_set_flags(&k, BN_FLG_CONSTTIME); - } if (dsa->flags & DSA_FLAG_CACHE_MONT_P) { if (!BN_MONT_CTX_set_locked(&dsa->method_mont_p, @@ -282,6 +279,11 @@ static int dsa_sign_setup(DSA *dsa, BN_C } else { K = &k; } + + if ((dsa->flags & DSA_FLAG_NO_EXP_CONSTTIME) == 0) { + BN_set_flags(K, BN_FLG_CONSTTIME); + } + DSA_BN_MOD_EXP(goto err, dsa, r, dsa->g, K, dsa->p, ctx, dsa->method_mont_p); if (!BN_mod(r, r, dsa->q, ctx))