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/*
* Copyright (c) 2018 Thomas Pornin <pornin@bolet.org>
*
* Permission is hereby granted, free of charge, to any person obtaining
* a copy of this software and associated documentation files (the
* "Software"), to deal in the Software without restriction, including
* without limitation the rights to use, copy, modify, merge, publish,
* distribute, sublicense, and/or sell copies of the Software, and to
* permit persons to whom the Software is furnished to do so, subject to
* the following conditions:
*
* The above copyright notice and this permission notice shall be
* included in all copies or substantial portions of the Software.
*
* THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND,
* EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF
* MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND
* NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS
* BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN
* ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN
* CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
* SOFTWARE.
*/
#include "inner.h"
/* see bearssl_rand.h */
void
br_aesctr_drbg_init(br_aesctr_drbg_context *ctx,
const br_block_ctr_class *aesctr,
const void *seed, size_t len)
{
unsigned char tmp[16];
ctx->vtable = &br_aesctr_drbg_vtable;
memset(tmp, 0, sizeof tmp);
aesctr->init(&ctx->sk.vtable, tmp, 16);
ctx->cc = 0;
br_aesctr_drbg_update(ctx, seed, len);
}
/* see bearssl_rand.h */
void
br_aesctr_drbg_generate(br_aesctr_drbg_context *ctx, void *out, size_t len)
{
unsigned char *buf;
unsigned char iv[12];
buf = out;
memset(iv, 0, sizeof iv);
while (len > 0) {
size_t clen;
/*
* We generate data by blocks of at most 65280 bytes. This
* allows for unambiguously testing the counter overflow
* condition; also, it should work on 16-bit architectures
* (where 'size_t' is 16 bits only).
*/
clen = len;
if (clen > 65280) {
clen = 65280;
}
/*
* We make sure that the counter won't exceed the configured
* limit.
*/
if ((uint32_t)(ctx->cc + ((clen + 15) >> 4)) > 32768) {
clen = (32768 - ctx->cc) << 4;
if (clen > len) {
clen = len;
}
}
/*
* Run CTR.
*/
memset(buf, 0, clen);
ctx->cc = ctx->sk.vtable->run(&ctx->sk.vtable,
iv, ctx->cc, buf, clen);
buf += clen;
len -= clen;
/*
* Every 32768 blocks, we force a state update.
*/
if (ctx->cc >= 32768) {
br_aesctr_drbg_update(ctx, NULL, 0);
}
}
}
/* see bearssl_rand.h */
void
br_aesctr_drbg_update(br_aesctr_drbg_context *ctx, const void *seed, size_t len)
{
/*
* We use a Hirose construction on AES-256 to make a hash function.
* Function definition:
* - running state consists in two 16-byte blocks G and H
* - initial values of G and H are conventional
* - there is a fixed block-sized constant C
* - for next data block m:
* set AES key to H||m
* G' = E(G) xor G
* H' = E(G xor C) xor G xor C
* G <- G', H <- H'
* - once all blocks have been processed, output is H||G
*
* Constants:
* G_init = B6 B6 ... B6
* H_init = A5 A5 ... A5
* C = 01 00 ... 00
*
* With this hash function h(), we compute the new state as
* follows:
* - produce a state-dependent value s as encryption of an
* all-one block with AES and the current key
* - compute the new key as the first 128 bits of h(s||seed)
*
* Original Hirose article:
* https://www.iacr.org/archive/fse2006/40470213/40470213.pdf
*/
unsigned char s[16], iv[12];
unsigned char G[16], H[16];
int first;
/*
* Use an all-one IV to get a fresh output block that depends on the
* current seed.
*/
memset(iv, 0xFF, sizeof iv);
memset(s, 0, 16);
ctx->sk.vtable->run(&ctx->sk.vtable, iv, 0xFFFFFFFF, s, 16);
/*
* Set G[] and H[] to conventional start values.
*/
memset(G, 0xB6, sizeof G);
memset(H, 0x5A, sizeof H);
/*
* Process the concatenation of the current state and the seed
* with the custom hash function.
*/
first = 1;
for (;;) {
unsigned char tmp[32];
unsigned char newG[16];
/*
* Assemble new key H||m into tmp[].
*/
memcpy(tmp, H, 16);
if (first) {
memcpy(tmp + 16, s, 16);
first = 0;
} else {
size_t clen;
if (len == 0) {
break;
}
clen = len < 16 ? len : 16;
memcpy(tmp + 16, seed, clen);
memset(tmp + 16 + clen, 0, 16 - clen);
seed = (const unsigned char *)seed + clen;
len -= clen;
}
ctx->sk.vtable->init(&ctx->sk.vtable, tmp, 32);
/*
* Compute new G and H values.
*/
memcpy(iv, G, 12);
memcpy(newG, G, 16);
ctx->sk.vtable->run(&ctx->sk.vtable, iv,
br_dec32be(G + 12), newG, 16);
iv[0] ^= 0x01;
memcpy(H, G, 16);
H[0] ^= 0x01;
ctx->sk.vtable->run(&ctx->sk.vtable, iv,
br_dec32be(G + 12), H, 16);
memcpy(G, newG, 16);
}
/*
* Output hash value is H||G. We truncate it to its first 128 bits,
* i.e. H; that's our new AES key.
*/
ctx->sk.vtable->init(&ctx->sk.vtable, H, 16);
ctx->cc = 0;
}
/* see bearssl_rand.h */
const br_prng_class br_aesctr_drbg_vtable = {
sizeof(br_aesctr_drbg_context),
(void (*)(const br_prng_class **, const void *, const void *, size_t))
&br_aesctr_drbg_init,
(void (*)(const br_prng_class **, void *, size_t))
&br_aesctr_drbg_generate,
(void (*)(const br_prng_class **, const void *, size_t))
&br_aesctr_drbg_update
};
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