Commit 39994462 authored by Cesar Pereida's avatar Cesar Pereida Committed by Matt Caswell
Browse files

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: default avatarRichard Levitte <levitte@openssl.org>
Reviewed-by: default avatarMatt Caswell <matt@openssl.org>
parent 0a4c87a9

crypto/dsa/dsa_ossl.c

+5 −4
Original line number Diff line number Diff line
@@ -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))