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Fix DSA, preserve BN_FLG_CONSTTIME

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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>
This commit is contained in:
Cesar Pereida 2016-05-23 12:45:25 +03:00 committed by Matt Caswell
parent 0a4c87a90c
commit 399944622d
1 changed files with 5 additions and 4 deletions
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9
crypto/dsa/dsa_ossl.c
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@ -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))