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2cf4253710
This implementation is optimized for a little code and data size, not for speed. IMO the code is more readable than in the reference implementation. The biggest advantage of ChaCha over other stream ciphers is the very little data usage with only 64 bytes of context, and its good encryption speed. Also part of this PR is pseudo-random number generator, that just returns the keystream of a randomly initialized ChaCha context.
121 lines
3.7 KiB
C
121 lines
3.7 KiB
C
/*
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* Copyright (C) 2008 D. J. Bernstein (dedicated to the public domain)
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* Copyright (C) 2015 René Kijewski <rene.kijewski@fu-berlin.de>
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*
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* Permission is hereby granted, free of charge, to any person obtaining a copy
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* of this software and associated documentation files (the "Software"), to deal
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* in the Software without restriction, including without limitation the rights
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* to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
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* copies of the Software, and to permit persons to whom the Software is
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* furnished to do so, subject to the following conditions:
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*
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* The above copyright notice and this permission notice shall be included in
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* all copies or substantial portions of the Software.
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*
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* THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
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* IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
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* FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
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* AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
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* LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
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* OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
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* THE SOFTWARE.
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*/
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/*
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* Please notice:
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* - This implementation of the ChaCha stream cipher is very stripped down.
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* - It assumes a little-endian system.
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* - It is implemented for little code and data size, but will likely be
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* slower than the refenrence implementation. Optimized implementation will
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* out-perform the code even more.
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*/
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#include "crypto/chacha.h"
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#include "byteorder.h"
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#if __BYTE_ORDER__ != __ORDER_LITTLE_ENDIAN__
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# error "This code is implementented in a way that it will only work for little-endian systems!"
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#endif
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#include <string.h>
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static void _r(uint32_t *d, uint32_t *a, const uint32_t *b, unsigned c)
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{
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*a += *b;
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uint32_t tmp = *a ^ *d;
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*d = (tmp << c) | (tmp >> (32 - c));
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}
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static void _doubleround(void *output_, const uint32_t input[16], uint8_t rounds)
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{
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uint32_t *output = (uint32_t *) output_;
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memcpy(output, input, 64);
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rounds *= 4;
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for (unsigned i = 0; i < rounds; ++i) {
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uint32_t *a = &output[((i + ((i & 4) ? 0 : 0)) & 3) + (4 * 0)];
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uint32_t *b = &output[((i + ((i & 4) ? 1 : 0)) & 3) + (4 * 1)];
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uint32_t *c = &output[((i + ((i & 4) ? 2 : 0)) & 3) + (4 * 2)];
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uint32_t *d = &output[((i + ((i & 4) ? 3 : 0)) & 3) + (4 * 3)];
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_r(d, a, b, 16);
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_r(b, c, d, 12);
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_r(d, a, b, 8);
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_r(b, c, d, 7);
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}
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for (unsigned i = 0; i < 16; ++i) {
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output[i] += input[i];
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}
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}
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int chacha_init(chacha_ctx *ctx,
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unsigned rounds,
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const uint8_t *key, uint32_t keylen,
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const uint8_t nonce[8])
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{
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if (keylen == 32) {
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memcpy(ctx->state + 0, "expand 32-byte k", 16);
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memcpy(ctx->state + 4, key, 32);
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}
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else if (keylen == 16) {
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memcpy(ctx->state + 0, "expand 16-byte k", 16);
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memcpy(ctx->state + 4, key, 16);
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memcpy(ctx->state + 8, key, 16);
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}
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else {
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return -1;
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}
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if ((rounds == 20) || (rounds == 12) || (rounds == 8)) {
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ctx->rounds = rounds;
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}
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else {
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return -1;
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}
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memset(ctx->state + 12, 0, 8);
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memcpy(ctx->state + 14, nonce, 8);
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return 0;
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}
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void chacha_keystream_bytes(chacha_ctx *ctx, void *x)
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{
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_doubleround(x, ctx->state, ctx->rounds);
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++ctx->state[12];
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if (ctx->state[12] == 0) {
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++ctx->state[13];
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}
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}
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void chacha_encrypt_bytes(chacha_ctx *ctx, const uint8_t *m, uint8_t *c)
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{
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uint8_t x[64];
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chacha_keystream_bytes(ctx, x);
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for (unsigned i = 0 ; i < 64; ++i) {
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c[i] = m[i] ^ x[i];
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}
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}
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