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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: Bernd Edlinger <bernd.edlinger@hotmail.de> (Merged from #7486) (cherry picked from commit a9cfb8c)
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Diff for: crypto/dsa/dsa_ossl.c

+15-13
Original file line numberDiff line numberDiff line change
@@ -9,6 +9,7 @@
99

1010
#include <stdio.h>
1111
#include "internal/cryptlib.h"
12+
#include "internal/bn_int.h"
1213
#include <openssl/bn.h>
1314
#include <openssl/sha.h>
1415
#include "dsa_locl.h"
@@ -180,9 +181,9 @@ static int dsa_sign_setup(DSA *dsa, BN_CTX *ctx_in,
180181
{
181182
BN_CTX *ctx = NULL;
182183
BIGNUM *k, *kinv = NULL, *r = *rp;
183-
BIGNUM *l, *m;
184+
BIGNUM *l;
184185
int ret = 0;
185-
int q_bits;
186+
int q_bits, q_words;
186187

187188
if (!dsa->p || !dsa->q || !dsa->g) {
188189
DSAerr(DSA_F_DSA_SIGN_SETUP, DSA_R_MISSING_PARAMETERS);
@@ -191,8 +192,7 @@ static int dsa_sign_setup(DSA *dsa, BN_CTX *ctx_in,
191192

192193
k = BN_new();
193194
l = BN_new();
194-
m = BN_new();
195-
if (k == NULL || l == NULL || m == NULL)
195+
if (k == NULL || l == NULL)
196196
goto err;
197197

198198
if (ctx_in == NULL) {
@@ -203,9 +203,9 @@ static int dsa_sign_setup(DSA *dsa, BN_CTX *ctx_in,
203203

204204
/* Preallocate space */
205205
q_bits = BN_num_bits(dsa->q);
206-
if (!BN_set_bit(k, q_bits)
207-
|| !BN_set_bit(l, q_bits)
208-
|| !BN_set_bit(m, q_bits))
206+
q_words = bn_get_top(dsa->q);
207+
if (!bn_wexpand(k, q_words + 2)
208+
|| !bn_wexpand(l, q_words + 2))
209209
goto err;
210210

211211
/* Get random k */
@@ -240,14 +240,17 @@ static int dsa_sign_setup(DSA *dsa, BN_CTX *ctx_in,
240240
* small timing information leakage. We then choose the sum that is
241241
* one bit longer than the modulus.
242242
*
243-
* TODO: revisit the BN_copy aiming for a memory access agnostic
244-
* conditional copy.
243+
* There are some concerns about the efficacy of doing this. More
244+
* specificly refer to the discussion starting with:
245+
* https://github.com/openssl/openssl/pull/7486#discussion_r228323705
246+
* The fix is to rework BN so these gymnastics aren't required.
245247
*/
246248
if (!BN_add(l, k, dsa->q)
247-
|| !BN_add(m, l, dsa->q)
248-
|| !BN_copy(k, BN_num_bits(l) > q_bits ? l : m))
249+
|| !BN_add(k, l, dsa->q))
249250
goto err;
250251

252+
BN_consttime_swap(BN_is_bit_set(l, q_bits), k, l, q_words + 2);
253+
251254
if ((dsa)->meth->bn_mod_exp != NULL) {
252255
if (!dsa->meth->bn_mod_exp(dsa, r, dsa->g, k, dsa->p, ctx,
253256
dsa->method_mont_p))
@@ -260,7 +263,7 @@ static int dsa_sign_setup(DSA *dsa, BN_CTX *ctx_in,
260263
if (!BN_mod(r, r, dsa->q, ctx))
261264
goto err;
262265

263-
/* Compute part of 's = inv(k) (m + xr) mod q' */
266+
/* Compute part of 's = inv(k) (m + xr) mod q' */
264267
if ((kinv = dsa_mod_inverse_fermat(k, dsa->q, ctx)) == NULL)
265268
goto err;
266269

@@ -275,7 +278,6 @@ static int dsa_sign_setup(DSA *dsa, BN_CTX *ctx_in,
275278
BN_CTX_free(ctx);
276279
BN_clear_free(k);
277280
BN_clear_free(l);
278-
BN_clear_free(m);
279281
return ret;
280282
}
281283

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