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 --- crypto/dsa/dsa_ossl.c | 9 +++++---- 1 file changed, 5 insertions(+), 4 deletions(-) diff --git a/crypto/dsa/dsa_ossl.c b/crypto/dsa/dsa_ossl.c index ce1da1cd6f..beb62b2ff0 100644 --- a/crypto/dsa/dsa_ossl.c +++ b/crypto/dsa/dsa_ossl.c @@ -204,10 +204,6 @@ static int dsa_sign_setup(DSA *dsa, BN_CTX *ctx_in, 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, dsa->lock, dsa->p, ctx)) @@ -238,6 +234,11 @@ static int dsa_sign_setup(DSA *dsa, BN_CTX *ctx_in, } 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)) -- 2.34.1