_ Git - BearSSL/blob - 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 };