/*
- * Copyright 1995-2016 The OpenSSL Project Authors. All Rights Reserved.
+ * Copyright 1995-2018 The OpenSSL Project Authors. All Rights Reserved.
*
- * Licensed under the OpenSSL license (the "License"). You may not use
+ * Licensed under the Apache License 2.0 (the "License"). You may not use
* this file except in compliance with the License. You can obtain a copy
* in the file LICENSE in the source distribution or at
* https://www.openssl.org/source/license.html
#include <stdio.h>
#include "internal/cryptlib.h"
+#include "internal/bn_int.h"
#include <openssl/bn.h>
#include <openssl/sha.h>
#include "dsa_locl.h"
DSA_SIG *sig, DSA *dsa);
static int dsa_init(DSA *dsa);
static int dsa_finish(DSA *dsa);
+static BIGNUM *dsa_mod_inverse_fermat(const BIGNUM *k, const BIGNUM *q,
+ BN_CTX *ctx);
static DSA_METHOD openssl_dsa_meth = {
"OpenSSL DSA method",
static DSA_SIG *dsa_do_sign(const unsigned char *dgst, int dlen, DSA *dsa)
{
BIGNUM *kinv = NULL;
- BIGNUM *m;
- BIGNUM *xr;
+ BIGNUM *m, *blind, *blindm, *tmp;
BN_CTX *ctx = NULL;
int reason = ERR_R_BN_LIB;
DSA_SIG *ret = NULL;
int rv = 0;
- m = BN_new();
- xr = BN_new();
- if (m == NULL || xr == NULL)
- goto err;
-
- if (!dsa->p || !dsa->q || !dsa->g) {
+ if (dsa->p == NULL || dsa->q == NULL || dsa->g == NULL) {
reason = DSA_R_MISSING_PARAMETERS;
goto err;
}
ctx = BN_CTX_new();
if (ctx == NULL)
goto err;
+ m = BN_CTX_get(ctx);
+ blind = BN_CTX_get(ctx);
+ blindm = BN_CTX_get(ctx);
+ tmp = BN_CTX_get(ctx);
+ if (tmp == NULL)
+ goto err;
+
redo:
if (!dsa_sign_setup(dsa, ctx, &kinv, &ret->r, dgst, dlen))
goto err;
if (BN_bin2bn(dgst, dlen, m) == NULL)
goto err;
- /* Compute s = inv(k) (m + xr) mod q */
- if (!BN_mod_mul(xr, dsa->priv_key, ret->r, dsa->q, ctx))
- goto err; /* s = xr */
- if (!BN_add(ret->s, xr, m))
- goto err; /* s = m + xr */
- if (BN_cmp(ret->s, dsa->q) > 0)
- if (!BN_sub(ret->s, ret->s, dsa->q))
+ /*
+ * The normal signature calculation is:
+ *
+ * s := k^-1 * (m + r * priv_key) mod q
+ *
+ * We will blind this to protect against side channel attacks
+ *
+ * s := blind^-1 * k^-1 * (blind * m + blind * r * priv_key) mod q
+ */
+
+ /* Generate a blinding value */
+ do {
+ if (!BN_priv_rand(blind, BN_num_bits(dsa->q) - 1,
+ BN_RAND_TOP_ANY, BN_RAND_BOTTOM_ANY))
goto err;
+ } while (BN_is_zero(blind));
+ BN_set_flags(blind, BN_FLG_CONSTTIME);
+ BN_set_flags(blindm, BN_FLG_CONSTTIME);
+ BN_set_flags(tmp, BN_FLG_CONSTTIME);
+
+ /* tmp := blind * priv_key * r mod q */
+ if (!BN_mod_mul(tmp, blind, dsa->priv_key, dsa->q, ctx))
+ goto err;
+ if (!BN_mod_mul(tmp, tmp, ret->r, dsa->q, ctx))
+ goto err;
+
+ /* blindm := blind * m mod q */
+ if (!BN_mod_mul(blindm, blind, m, dsa->q, ctx))
+ goto err;
+
+ /* s : = (blind * priv_key * r) + (blind * m) mod q */
+ if (!BN_mod_add_quick(ret->s, tmp, blindm, dsa->q))
+ goto err;
+
+ /* s := s * k^-1 mod q */
if (!BN_mod_mul(ret->s, ret->s, kinv, dsa->q, ctx))
goto err;
+ /* s:= s * blind^-1 mod q */
+ if (BN_mod_inverse(blind, blind, dsa->q, ctx) == NULL)
+ goto err;
+ if (!BN_mod_mul(ret->s, ret->s, blind, dsa->q, ctx))
+ goto err;
+
/*
* Redo if r or s is zero as required by FIPS 186-3: this is very
* unlikely.
ret = NULL;
}
BN_CTX_free(ctx);
- BN_clear_free(m);
- BN_clear_free(xr);
BN_clear_free(kinv);
return ret;
}
{
BN_CTX *ctx = NULL;
BIGNUM *k, *kinv = NULL, *r = *rp;
+ BIGNUM *l;
int ret = 0;
+ int q_bits, q_words;
if (!dsa->p || !dsa->q || !dsa->g) {
DSAerr(DSA_F_DSA_SIGN_SETUP, DSA_R_MISSING_PARAMETERS);
}
k = BN_new();
- if (k == NULL)
+ l = BN_new();
+ if (k == NULL || l == NULL)
goto err;
if (ctx_in == NULL) {
} else
ctx = ctx_in;
+ /* Preallocate space */
+ q_bits = BN_num_bits(dsa->q);
+ 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 */
do {
if (dgst != NULL) {
} while (BN_is_zero(k));
BN_set_flags(k, BN_FLG_CONSTTIME);
+ BN_set_flags(l, BN_FLG_CONSTTIME);
if (dsa->flags & DSA_FLAG_CACHE_MONT_P) {
if (!BN_MONT_CTX_set_locked(&dsa->method_mont_p,
/*
* 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
- * is a kludge that we need because the BN_mod_exp_mont() does not
- * let us specify the desired timing behaviour.)
+ * compute G^k using an equivalent scalar of fixed bit-length.
+ *
+ * We unconditionally perform both of these additions to prevent a
+ * small timing information leakage. We then choose the sum that is
+ * one bit longer than the modulus.
+ *
+ * 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(k, k, dsa->q))
+ if (!BN_add(l, k, dsa->q)
+ || !BN_add(k, l, dsa->q))
goto err;
- if (BN_num_bits(k) <= BN_num_bits(dsa->q)) {
- if (!BN_add(k, k, 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,
if (!BN_mod(r, r, dsa->q, ctx))
goto err;
- /* Compute part of 's = inv(k) (m + xr) mod q' */
- if ((kinv = BN_mod_inverse(NULL, k, dsa->q, ctx)) == NULL)
+ /* Compute part of 's = inv(k) (m + xr) mod q' */
+ if ((kinv = dsa_mod_inverse_fermat(k, dsa->q, ctx)) == NULL)
goto err;
BN_clear_free(*kinvp);
if (ctx != ctx_in)
BN_CTX_free(ctx);
BN_clear_free(k);
+ BN_clear_free(l);
return ret;
}
BN_MONT_CTX_free(dsa->method_mont_p);
return 1;
}
+
+/*
+ * Compute the inverse of k modulo q.
+ * Since q is prime, Fermat's Little Theorem applies, which reduces this to
+ * mod-exp operation. Both the exponent and modulus are public information
+ * so a mod-exp that doesn't leak the base is sufficient. A newly allocated
+ * BIGNUM is returned which the caller must free.
+ */
+static BIGNUM *dsa_mod_inverse_fermat(const BIGNUM *k, const BIGNUM *q,
+ BN_CTX *ctx)
+{
+ BIGNUM *res = NULL;
+ BIGNUM *r, *e;
+
+ if ((r = BN_new()) == NULL)
+ return NULL;
+
+ BN_CTX_start(ctx);
+ if ((e = BN_CTX_get(ctx)) != NULL
+ && BN_set_word(r, 2)
+ && BN_sub(e, q, r)
+ && BN_mod_exp_mont(r, k, e, q, ctx, NULL))
+ res = r;
+ else
+ BN_free(r);
+ BN_CTX_end(ctx);
+ return res;
+}
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