Added AESCTR_DRBG implementation (beta).
[BearSSL] / src / rand / aesctr_drbg.c
1 /*
2 * Copyright (c) 2018 Thomas Pornin <pornin@bolet.org>
3 *
4 * Permission is hereby granted, free of charge, to any person obtaining
5 * a copy of this software and associated documentation files (the
6 * "Software"), to deal in the Software without restriction, including
7 * without limitation the rights to use, copy, modify, merge, publish,
8 * distribute, sublicense, and/or sell copies of the Software, and to
9 * permit persons to whom the Software is furnished to do so, subject to
10 * the following conditions:
11 *
12 * The above copyright notice and this permission notice shall be
13 * included in all copies or substantial portions of the Software.
14 *
15 * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND,
16 * EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF
17 * MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND
18 * NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS
19 * BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN
20 * ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN
21 * CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
22 * SOFTWARE.
23 */
24
25 #include "inner.h"
26
27 /* see bearssl_rand.h */
28 void
29 br_aesctr_drbg_init(br_aesctr_drbg_context *ctx,
30 const br_block_ctr_class *aesctr,
31 const void *seed, size_t len)
32 {
33 unsigned char tmp[16];
34
35 ctx->vtable = &br_aesctr_drbg_vtable;
36 memset(tmp, 0, sizeof tmp);
37 aesctr->init(&ctx->sk.vtable, tmp, 16);
38 ctx->cc = 0;
39 br_aesctr_drbg_update(ctx, seed, len);
40 }
41
42 /* see bearssl_rand.h */
43 void
44 br_aesctr_drbg_generate(br_aesctr_drbg_context *ctx, void *out, size_t len)
45 {
46 unsigned char *buf;
47 unsigned char iv[12];
48
49 buf = out;
50 memset(iv, 0, sizeof iv);
51 while (len > 0) {
52 size_t clen;
53
54 /*
55 * We generate data by blocks of at most 65280 bytes. This
56 * allows for unambiguously testing the counter overflow
57 * condition; also, it should work on 16-bit architectures
58 * (where 'size_t' is 16 bits only).
59 */
60 clen = len;
61 if (clen > 65280) {
62 clen = 65280;
63 }
64
65 /*
66 * We make sure that the counter won't exceed the configured
67 * limit.
68 */
69 if ((uint32_t)(ctx->cc + ((clen + 15) >> 4)) > 32768) {
70 clen = (32768 - ctx->cc) << 4;
71 if (clen > len) {
72 clen = len;
73 }
74 }
75
76 /*
77 * Run CTR.
78 */
79 memset(buf, 0, clen);
80 ctx->cc = ctx->sk.vtable->run(&ctx->sk.vtable,
81 iv, ctx->cc, buf, clen);
82 buf += clen;
83 len -= clen;
84
85 /*
86 * Every 32768 blocks, we force a state update.
87 */
88 if (ctx->cc >= 32768) {
89 br_aesctr_drbg_update(ctx, NULL, 0);
90 }
91 }
92 }
93
94 /* see bearssl_rand.h */
95 void
96 br_aesctr_drbg_update(br_aesctr_drbg_context *ctx, const void *seed, size_t len)
97 {
98 /*
99 * We use a Hirose construction on AES-256 to make a hash function.
100 * Function definition:
101 * - running state consists in two 16-byte blocks G and H
102 * - initial values of G and H are conventional
103 * - there is a fixed block-sized constant C
104 * - for next data block m:
105 * set AES key to H||m
106 * G' = E(G) xor G
107 * H' = E(G xor C) xor G xor C
108 * G <- G', H <- H'
109 * - once all blocks have been processed, output is H||G
110 *
111 * Constants:
112 * G_init = B6 B6 ... B6
113 * H_init = A5 A5 ... A5
114 * C = 01 00 ... 00
115 *
116 * With this hash function h(), we compute the new state as
117 * follows:
118 * - produce a state-dependent value s as encryption of an
119 * all-one block with AES and the current key
120 * - compute the new key as the first 128 bits of h(s||seed)
121 *
122 * Original Hirose article:
123 * https://www.iacr.org/archive/fse2006/40470213/40470213.pdf
124 */
125
126 unsigned char s[16], iv[12];
127 unsigned char G[16], H[16];
128 int first;
129
130 /*
131 * Use an all-one IV to get a fresh output block that depends on the
132 * current seed.
133 */
134 memset(iv, 0xFF, sizeof iv);
135 memset(s, 0, 16);
136 ctx->sk.vtable->run(&ctx->sk.vtable, iv, 0xFFFFFFFF, s, 16);
137
138 /*
139 * Set G[] and H[] to conventional start values.
140 */
141 memset(G, 0xB6, sizeof G);
142 memset(H, 0x5A, sizeof H);
143
144 /*
145 * Process the concatenation of the current state and the seed
146 * with the custom hash function.
147 */
148 first = 1;
149 for (;;) {
150 unsigned char tmp[32];
151 unsigned char newG[16];
152
153 /*
154 * Assemble new key H||m into tmp[].
155 */
156 memcpy(tmp, H, 16);
157 if (first) {
158 memcpy(tmp + 16, s, 16);
159 first = 0;
160 } else {
161 size_t clen;
162
163 if (len == 0) {
164 break;
165 }
166 clen = len < 16 ? len : 16;
167 memcpy(tmp + 16, seed, clen);
168 memset(tmp + 16 + clen, 0, 16 - clen);
169 seed = (const unsigned char *)seed + clen;
170 len -= clen;
171 }
172 ctx->sk.vtable->init(&ctx->sk.vtable, tmp, 32);
173
174 /*
175 * Compute new G and H values.
176 */
177 memcpy(iv, G, 12);
178 memcpy(newG, G, 16);
179 ctx->sk.vtable->run(&ctx->sk.vtable, iv,
180 br_dec32be(G + 12), newG, 16);
181 iv[0] ^= 0x01;
182 memcpy(H, G, 16);
183 H[0] ^= 0x01;
184 ctx->sk.vtable->run(&ctx->sk.vtable, iv,
185 br_dec32be(G + 12), H, 16);
186 memcpy(G, newG, 16);
187 }
188
189 /*
190 * Output hash value is H||G. We truncate it to its first 128 bits,
191 * i.e. H; that's our new AES key.
192 */
193 ctx->sk.vtable->init(&ctx->sk.vtable, H, 16);
194 ctx->cc = 0;
195 }
196
197 /* see bearssl_rand.h */
198 const br_prng_class br_aesctr_drbg_vtable = {
199 sizeof(br_aesctr_drbg_context),
200 (void (*)(const br_prng_class **, const void *, const void *, size_t))
201 &br_aesctr_drbg_init,
202 (void (*)(const br_prng_class **, void *, size_t))
203 &br_aesctr_drbg_generate,
204 (void (*)(const br_prng_class **, const void *, size_t))
205 &br_aesctr_drbg_update
206 };