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From 621eaf49a289bfac26d4cbcdb7396e796784c534 Mon Sep 17 00:00:00 2001
From: Cesar Pereida <cesar.pereida@aalto.fi>
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 <levitte@openssl.org>
Reviewed-by: Matt Caswell <matt@openssl.org>
---
crypto/dsa/dsa_ossl.c | 6 +++---
1 file changed, 3 insertions(+), 3 deletions(-)
diff --git a/crypto/dsa/dsa_ossl.c b/crypto/dsa/dsa_ossl.c
index efc4f1b..b29eb4b 100644
--- a/crypto/dsa/dsa_ossl.c
+++ b/crypto/dsa/dsa_ossl.c
@@ -248,9 +248,6 @@ static int dsa_sign_setup(DSA *dsa, BN_CTX *ctx_in, BIGNUM **kinvp,
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,
@@ -279,9 +276,12 @@ static int dsa_sign_setup(DSA *dsa, BN_CTX *ctx_in, BIGNUM **kinvp,
}
K = &kq;
+
+ BN_set_flags(K, BN_FLG_CONSTTIME);
} else {
K = &k;
}
+
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))
--
1.9.1
From b7d0f2834e139a20560d64c73e2565e93715ce2b Mon Sep 17 00:00:00 2001
From: Matt Caswell <matt@openssl.org>
Date: Tue, 7 Jun 2016 09:12:51 +0100
Subject: [PATCH] More fix DSA, preserve BN_FLG_CONSTTIME
The previous "fix" still left "k" exposed to constant time problems in
the later BN_mod_inverse() call. Ensure both k and kq have the
BN_FLG_CONSTTIME flag set at the earliest opportunity after creation.
CVE-2016-2178
Reviewed-by: Rich Salz <rsalz@openssl.org>
---
crypto/dsa/dsa_ossl.c | 11 ++++++++---
1 file changed, 8 insertions(+), 3 deletions(-)
diff --git a/crypto/dsa/dsa_ossl.c b/crypto/dsa/dsa_ossl.c
index b29eb4b..58013a4 100644
--- a/crypto/dsa/dsa_ossl.c
+++ b/crypto/dsa/dsa_ossl.c
@@ -247,7 +247,12 @@ static int dsa_sign_setup(DSA *dsa, BN_CTX *ctx_in, BIGNUM **kinvp,
do
if (!BN_rand_range(&k, dsa->q))
goto err;
- while (BN_is_zero(&k)) ;
+ 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,
@@ -261,6 +266,8 @@ static int dsa_sign_setup(DSA *dsa, BN_CTX *ctx_in, BIGNUM **kinvp,
if (!BN_copy(&kq, &k))
goto err;
+ BN_set_flags(&kq, BN_FLG_CONSTTIME);
+
/*
* We do not want timing information to leak the length of k, so we
* compute g^k using an equivalent exponent of fixed length. (This
@@ -276,8 +283,6 @@ static int dsa_sign_setup(DSA *dsa, BN_CTX *ctx_in, BIGNUM **kinvp,
}
K = &kq;
-
- BN_set_flags(K, BN_FLG_CONSTTIME);
} else {
K = &k;
}
--
1.9.1
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