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/*
* Copyright (C) 2019 Red Hat, Inc.
*
* Author: Daiki Ueno
*
* This file is part of GNUTLS.
*
* The GNUTLS library is free software; you can redistribute it and/or
* modify it under the terms of the GNU Lesser General Public License
* as published by the Free Software Foundation; either version 2.1 of
* the License, or (at your option) any later version.
*
* This library is distributed in the hope that it will be useful, but
* WITHOUT ANY WARRANTY; without even the implied warranty of
* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
* Lesser General Public License for more details.
*
* You should have received a copy of the GNU Lesser General Public License
* along with this program. If not, see <https://www.gnu.org/licenses/>
*
*/
#if HAVE_CONFIG_H
# include "config.h"
#endif
#include "dsa-compute-k.h"
#include "gnutls_int.h"
#include "mem.h"
#include "mpn-base256.h"
#include <string.h>
#define BITS_TO_LIMBS(bits) (((bits) + GMP_NUMB_BITS - 1) / GMP_NUMB_BITS)
/* The maximum size of q, choosen from the fact that we support
* 521-bit elliptic curve generator and 512-bit DSA subgroup at
* maximum. */
#define MAX_Q_BITS 521
#define MAX_Q_SIZE ((MAX_Q_BITS + 7) / 8)
#define MAX_Q_LIMBS BITS_TO_LIMBS(MAX_Q_BITS)
#define MAX_HASH_BITS (MAX_HASH_SIZE * 8)
#define MAX_HASH_LIMBS BITS_TO_LIMBS(MAX_HASH_BITS)
int
_gnutls_dsa_compute_k(mpz_t k,
const mpz_t q,
const mpz_t x,
gnutls_mac_algorithm_t mac,
const uint8_t *digest,
size_t length)
{
uint8_t V[MAX_HASH_SIZE];
uint8_t K[MAX_HASH_SIZE];
uint8_t xp[MAX_Q_SIZE];
uint8_t tp[MAX_Q_SIZE];
mp_limb_t h[MAX(MAX_Q_LIMBS, MAX_HASH_LIMBS)];
mp_bitcnt_t q_bits = mpz_sizeinbase (q, 2);
mp_size_t qn = mpz_size(q);
mp_bitcnt_t h_bits = length * 8;
mp_size_t hn = BITS_TO_LIMBS(h_bits);
size_t nbytes = (q_bits + 7) / 8;
const uint8_t c0 = 0x00;
const uint8_t c1 = 0x01;
mp_limb_t cy;
gnutls_hmac_hd_t hd;
int ret = 0;
if (unlikely(q_bits > MAX_Q_BITS))
return gnutls_assert_val(GNUTLS_E_INVALID_REQUEST);
if (unlikely(length > MAX_HASH_SIZE))
return gnutls_assert_val(GNUTLS_E_INVALID_REQUEST);
/* int2octets(x) */
mpn_get_base256(xp, nbytes, mpz_limbs_read(x), qn);
/* bits2octets(h) */
mpn_set_base256(h, hn, digest, length);
if (hn < qn)
/* qlen > blen: add zero bits to the left */
mpn_zero(&h[hn], qn - hn);
else if (h_bits > q_bits) {
/* qlen < blen: keep the leftmost qlen bits. We do this in 2
* steps because mpn_rshift only accepts shift count in the
* range 1 to mp_bits_per_limb-1.
*/
mp_bitcnt_t shift = h_bits - q_bits;
if (shift / GMP_NUMB_BITS > 0) {
mpn_copyi(h, &h[shift / GMP_NUMB_BITS], qn);
hn -= shift / GMP_NUMB_BITS;
}
if (shift % GMP_NUMB_BITS > 0)
mpn_rshift(h, h, hn, shift % GMP_NUMB_BITS);
}
cy = mpn_sub_n(h, h, mpz_limbs_read(q), qn);
/* Fall back to addmul_1, if nettle is linked with mini-gmp. */
#ifdef mpn_cnd_add_n
mpn_cnd_add_n(cy, h, h, mpz_limbs_read(q), qn);
#else
mpn_addmul_1(h, mpz_limbs_read(q), qn, cy != 0);
#endif
mpn_get_base256(tp, nbytes, h, qn);
/* Step b */
memset(V, c1, length);
/* Step c */
memset(K, c0, length);
/* Step d */
ret = gnutls_hmac_init(&hd, mac, K, length);
if (ret < 0)
goto out;
ret = gnutls_hmac(hd, V, length);
if (ret < 0)
goto out;
ret = gnutls_hmac(hd, &c0, 1);
if (ret < 0)
goto out;
ret = gnutls_hmac(hd, xp, nbytes);
if (ret < 0)
goto out;
ret = gnutls_hmac(hd, tp, nbytes);
if (ret < 0)
goto out;
gnutls_hmac_deinit(hd, K);
/* Step e */
ret = gnutls_hmac_fast(mac, K, length, V, length, V);
if (ret < 0)
goto out;
/* Step f */
ret = gnutls_hmac_init(&hd, mac, K, length);
if (ret < 0)
goto out;
ret = gnutls_hmac(hd, V, length);
if (ret < 0)
goto out;
ret = gnutls_hmac(hd, &c1, 1);
if (ret < 0)
goto out;
ret = gnutls_hmac(hd, xp, nbytes);
if (ret < 0)
goto out;
ret = gnutls_hmac(hd, tp, nbytes);
if (ret < 0)
goto out;
gnutls_hmac_deinit(hd, K);
/* Step g */
ret = gnutls_hmac_fast(mac, K, length, V, length, V);
if (ret < 0)
goto out;
/* Step h */
for (;;) {
/* Step 1 */
size_t tlen = 0;
/* Step 2 */
while (tlen < nbytes) {
size_t remaining = MIN(nbytes - tlen, length);
ret = gnutls_hmac_fast(mac, K, length, V, length, V);
if (ret < 0)
goto out;
memcpy (&tp[tlen], V, remaining);
tlen += remaining;
}
/* Step 3 */
mpn_set_base256 (h, qn, tp, tlen);
if (tlen * 8 > q_bits)
mpn_rshift (h, h, qn, tlen * 8 - q_bits);
/* Check if k is in [1,q-1] */
if (!mpn_zero_p (h, qn) &&
mpn_cmp (h, mpz_limbs_read(q), qn) < 0) {
mpn_copyi(mpz_limbs_write(k, qn), h, qn);
mpz_limbs_finish(k, qn);
break;
}
ret = gnutls_hmac_init(&hd, mac, K, length);
if (ret < 0)
goto out;
ret = gnutls_hmac(hd, V, length);
if (ret < 0)
goto out;
ret = gnutls_hmac(hd, &c0, 1);
if (ret < 0)
goto out;
gnutls_hmac_deinit(hd, K);
ret = gnutls_hmac_fast(mac, K, length, V, length, V);
if (ret < 0)
goto out;
}
out:
zeroize_key(xp, sizeof(xp));
zeroize_key(tp, sizeof(tp));
return ret;
}
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