1 /* gostdsa-vko.c
2
3 Copyright (C) 2016 Dmitry Eremin-Solenikov
4
5 This file is part of GNU Nettle.
6
7 GNU Nettle is free software: you can redistribute it and/or
8 modify it under the terms of either:
9
10 * the GNU Lesser General Public License as published by the Free
11 Software Foundation; either version 3 of the License, or (at your
12 option) any later version.
13
14 or
15
16 * the GNU General Public License as published by the Free
17 Software Foundation; either version 2 of the License, or (at your
18 option) any later version.
19
20 or both in parallel, as here.
21
22 GNU Nettle is distributed in the hope that it will be useful,
23 but WITHOUT ANY WARRANTY; without even the implied warranty of
24 MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
25 General Public License for more details.
26
27 You should have received copies of the GNU General Public License and
28 the GNU Lesser General Public License along with this program. If
29 not, see http://www.gnu.org/licenses/.
30 */
31
32 #if HAVE_CONFIG_H
33 # include "config.h"
34 #endif
35
36 #include <assert.h>
37 #include <stdlib.h>
38
39 #include "ecc-internal.h"
40 #include "gostdsa.h"
41
42 /*
43 * Shared key derivation/key agreement for GOST DSA algorithm.
44 * It is defined in RFC 4357 Section 5.2 and RFC 7836 Section 4.3.1
45 *
46 * output is 2 * curve size:
47 * 64 bytes for 256 bit curves and 128 bytes for 512 bit ones
48 *
49 * Basically shared key is equal to hash(cofactor * ukm * priv * pub). This
50 * function does multiplication. Caller should do hashing on his own.
51 *
52 * UKM is not a secret value (consider it as a nonce).
53 *
54 * For supported GOST curves cofactor is equal to 1.
55 */
56 void
gostdsa_vko(const struct ecc_scalar * priv,const struct ecc_point * pub,size_t ukm_length,const uint8_t * ukm,uint8_t * out)57 gostdsa_vko (const struct ecc_scalar *priv,
58 const struct ecc_point *pub,
59 size_t ukm_length, const uint8_t *ukm,
60 uint8_t *out)
61 {
62 const struct ecc_curve *ecc = priv->ecc;
63 unsigned bsize = (ecc_bit_size (ecc) + 7) / 8;
64 mp_size_t size = ecc->p.size;
65 mp_size_t itch = 4*size + ecc->mul_itch;
66 mp_limb_t *scratch;
67
68 if (itch < 5*size + ecc->h_to_a_itch)
69 itch = 5*size + ecc->h_to_a_itch;
70
71 assert (pub->ecc == ecc);
72 assert (priv->ecc == ecc);
73 assert (ukm_length <= bsize);
74
75 scratch = gmp_alloc_limbs (itch);
76
77 #define UKM scratch
78 #define TEMP (scratch + 3*size)
79 #define XYZ scratch
80 #define TEMP_Y (scratch + 4*size)
81
82 mpn_set_base256_le (UKM, size, ukm, ukm_length);
83
84 /* If ukm is 0, set it to 1, otherwise the result will be allways equal to 0,
85 * no matter what private and public keys are. See RFC 4357 referencing GOST
86 * R 34.10-2001 (RFC 5832) Section 6.1 step 2. */
87 if (mpn_zero_p (UKM, size))
88 UKM[0] = 1;
89
90 ecc_mod_mul_canonical (&ecc->q, TEMP, priv->p, UKM, TEMP); /* TEMP = UKM * priv */
91 ecc->mul (ecc, XYZ, TEMP, pub->p, scratch + 4*size); /* XYZ = UKM * priv * pub */
92 ecc->h_to_a (ecc, 0, TEMP, XYZ, scratch + 5*size); /* TEMP = XYZ */
93 mpn_get_base256_le (out, bsize, TEMP, size);
94 mpn_get_base256_le (out+bsize, bsize, TEMP_Y, size);
95 gmp_free_limbs (scratch, itch);
96 }
97