1 /* eddsa-verify.c
2
3 Copyright (C) 2014 Niels Möller
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
38 #include "eddsa.h"
39 #include "eddsa-internal.h"
40
41 #include <nettle/ecc.h>
42 #include "ecc-internal.h"
43 #include <nettle/nettle-meta.h>
44
45 /* Checks if x1/z1 == x2/z2 (mod p). Assumes z1 and z2 are
46 non-zero. */
47 static int
equal_h(const struct ecc_modulo * p,const mp_limb_t * x1,const mp_limb_t * z1,const mp_limb_t * x2,const mp_limb_t * z2,mp_limb_t * scratch)48 equal_h (const struct ecc_modulo *p,
49 const mp_limb_t *x1, const mp_limb_t *z1,
50 const mp_limb_t *x2, const mp_limb_t *z2,
51 mp_limb_t *scratch)
52 {
53 #define t0 scratch
54 #define t1 (scratch + p->size)
55
56 ecc_mod_mul (p, t0, x1, z2);
57 if (mpn_cmp (t0, p->m, p->size) >= 0)
58 mpn_sub_n (t0, t0, p->m, p->size);
59
60 ecc_mod_mul (p, t1, x2, z1);
61 if (mpn_cmp (t1, p->m, p->size) >= 0)
62 mpn_sub_n (t1, t1, p->m, p->size);
63
64 return mpn_cmp (t0, t1, p->size) == 0;
65
66 #undef t0
67 #undef t1
68 }
69
70 mp_size_t
_eddsa_verify_itch(const struct ecc_curve * ecc)71 _eddsa_verify_itch (const struct ecc_curve *ecc)
72 {
73 assert (_eddsa_decompress_itch (ecc) <= ecc->mul_itch);
74 return 8*ecc->p.size + ecc->mul_itch;
75 }
76
77 int
_eddsa_verify(const struct ecc_curve * ecc,const struct ecc_eddsa * eddsa,const uint8_t * pub,const mp_limb_t * A,void * ctx,size_t length,const uint8_t * msg,const uint8_t * signature,mp_limb_t * scratch)78 _eddsa_verify (const struct ecc_curve *ecc,
79 const struct ecc_eddsa *eddsa,
80 const uint8_t *pub,
81 const mp_limb_t *A,
82 void *ctx,
83 size_t length,
84 const uint8_t *msg,
85 const uint8_t *signature,
86 mp_limb_t *scratch)
87 {
88 size_t nbytes;
89 #define R scratch
90 #define sp (scratch + 2*ecc->p.size)
91 #define hp (scratch + 3*ecc->p.size)
92 #define P (scratch + 5*ecc->p.size)
93 #define scratch_out (scratch + 8*ecc->p.size)
94 #define S R
95 #define hash ((uint8_t *) P)
96
97 nbytes = 1 + ecc->p.bit_size / 8;
98
99 /* Could maybe save some storage by delaying the R and S operations,
100 but it makes sense to check them for validity up front. */
101 if (!_eddsa_decompress (ecc, R, signature, R+2*ecc->p.size))
102 return 0;
103
104 mpn_set_base256_le (sp, ecc->q.size, signature + nbytes, nbytes);
105 /* Check that s < q */
106 if (mpn_cmp (sp, ecc->q.m, ecc->q.size) >= 0)
107 return 0;
108
109 eddsa->dom (ctx);
110 eddsa->update (ctx, nbytes, signature);
111 eddsa->update (ctx, nbytes, pub);
112 eddsa->update (ctx, length, msg);
113 eddsa->digest (ctx, 2*nbytes, hash);
114 _eddsa_hash (&ecc->q, hp, 2*nbytes, hash);
115
116 /* Compute h A + R - s G, which should be the neutral point */
117 ecc->mul (ecc, P, hp, A, scratch_out);
118 ecc->add_hh (ecc, P, P, R, scratch_out);
119 /* Move out of the way. */
120 mpn_copyi (hp, sp, ecc->q.size);
121 ecc->mul_g (ecc, S, hp, scratch_out);
122
123 return equal_h (&ecc->p,
124 P, P + 2*ecc->p.size,
125 S, S + 2*ecc->p.size, scratch_out)
126 && equal_h (&ecc->p,
127 P + ecc->p.size, P + 2*ecc->p.size,
128 S + ecc->p.size, S + 2*ecc->p.size, scratch_out);
129
130 #undef R
131 #undef sp
132 #undef hp
133 #undef P
134 #undef S
135 }
136