Commit ef11e19d authored by Pauli's avatar Pauli
Browse files

Timing vulnerability in DSA signature generation (CVE-2018-0734). 



Avoid a timing attack that leaks information via a side channel that
triggers when a BN is resized.  Increasing the size of the BNs
prior to doing anything with them suppresses the attack.

Thanks due to Samuel Weiser for finding and locating this.

Reviewed-by: default avatarBernd Edlinger <bernd.edlinger@hotmail.de>
(Merged from https://github.com/openssl/openssl/pull/7486)

(cherry picked from commit a9cfb8c2)
parent 84862c09

crypto/dsa/dsa_ossl.c

+15 −13
Original line number Diff line number Diff line
@@ -11,6 +11,7 @@ 


#include <stdio.h>

#include "internal/cryptlib.h"

#include "internal/bn_int.h"

#include <openssl/bn.h>

#include <openssl/sha.h>

#include "dsa_locl.h"
@@ -182,9 +183,9 @@ static int dsa_sign_setup(DSA *dsa, BN_CTX *ctx_in, 
{

    BN_CTX *ctx = NULL;

    BIGNUM *k, *kinv = NULL, *r = *rp;

    BIGNUM *l, *m;

    BIGNUM *l;

    int ret = 0;

    int q_bits;

    int q_bits, q_words;



    if (!dsa->p || !dsa->q || !dsa->g) {

        DSAerr(DSA_F_DSA_SIGN_SETUP, DSA_R_MISSING_PARAMETERS);
@@ -193,8 +194,7 @@ static int dsa_sign_setup(DSA *dsa, BN_CTX *ctx_in, 


    k = BN_new();

    l = BN_new();

    m = BN_new();

    if (k == NULL || l == NULL || m == NULL)

    if (k == NULL || l == NULL)

        goto err;



    if (ctx_in == NULL) {
@@ -205,9 +205,9 @@ static int dsa_sign_setup(DSA *dsa, BN_CTX *ctx_in, 


    /* Preallocate space */

    q_bits = BN_num_bits(dsa->q);

    if (!BN_set_bit(k, q_bits)

        || !BN_set_bit(l, q_bits)

        || !BN_set_bit(m, q_bits))

    q_words = bn_get_top(dsa->q);

    if (!bn_wexpand(k, q_words + 2)

        || !bn_wexpand(l, q_words + 2))

        goto err;



    /* Get random k */
@@ -242,14 +242,17 @@ static int dsa_sign_setup(DSA *dsa, BN_CTX *ctx_in, 
     * small timing information leakage.  We then choose the sum that is

     * one bit longer than the modulus.

     *

     * TODO: revisit the BN_copy aiming for a memory access agnostic

     * conditional copy.

     * There are some concerns about the efficacy of doing this.  More

     * specificly refer to the discussion starting with:

     *     https://github.com/openssl/openssl/pull/7486#discussion_r228323705

     * The fix is to rework BN so these gymnastics aren't required.

     */

    if (!BN_add(l, k, dsa->q)

        || !BN_add(m, l, dsa->q)

        || !BN_copy(k, BN_num_bits(l) > q_bits ? l : m))

        || !BN_add(k, l, dsa->q))

        goto err;



    BN_consttime_swap(BN_is_bit_set(l, q_bits), k, l, q_words + 2);



    if ((dsa)->meth->bn_mod_exp != NULL) {

            if (!dsa->meth->bn_mod_exp(dsa, r, dsa->g, k, dsa->p, ctx,

                                       dsa->method_mont_p))
@@ -277,7 +280,6 @@ static int dsa_sign_setup(DSA *dsa, BN_CTX *ctx_in, 
        BN_CTX_free(ctx);

    BN_clear_free(k);

    BN_clear_free(l);

    BN_clear_free(m);

    return ret;

}