/*-
 * Copyright (c) 2017 Dag-Erling Smørgrav
 * All rights reserved.
 *
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions
 * are met:
 * 1. Redistributions of source code must retain the above copyright
 *    notice, this list of conditions and the following disclaimer.
 * 2. Redistributions in binary form must reproduce the above copyright
 *    notice, this list of conditions and the following disclaimer in the
 *    documentation and/or other materials provided with the distribution.
 * 3. The name of the author may not be used to endorse or promote
 *    products derived from this software without specific prior written
 *    permission.
 *
 * THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS ``AS IS'' AND
 * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
 * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
 * ARE DISCLAIMED.  IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE
 * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
 * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
 * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
 * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
 * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
 * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
 * SUCH DAMAGE.
 */

#include "cryb/impl.h"

#include <stdint.h>
#include <string.h>

#include <cryb/assert.h>
#include <cryb/bitwise.h>
#include <cryb/endian.h>
#include <cryb/memset_s.h>

#include <cryb/salsa.h>

#define SALSA_QR(x, a, b, c, d)						\
	do {								\
		x[a] ^= rol32(x[d] + x[c],  7);				\
		x[b] ^= rol32(x[a] + x[d],  9);				\
		x[c] ^= rol32(x[b] + x[a], 13);				\
		x[d] ^= rol32(x[c] + x[b], 18);				\
	} while (0)

static const char magic128[] = "expand 16-byte k";
static const char magic256[] = "expand 32-byte k";

/*
 * Fill the state array with 16 bytes of magic and 32 bytes of key,
 * repeating the key if necessary.  The 8-byte stream position and the
 * 8-byte nonce are initialized to all-zeroes.  The number of rounds is
 * set to 20, the most commonly used value.
 */
void
salsa_init(salsa_ctx *ctx, cipher_mode mode CRYB_UNUSED,
    const uint8_t *key, size_t keylen)
{

	assert(mode == CIPHER_MODE_ENCRYPT || mode == CIPHER_MODE_DECRYPT);
	assert(keylen == 16 || keylen == 32);
	memset(ctx, 0, sizeof *ctx);
	if (keylen == 32) {
		/* magic */
		ctx->state[ 0] = le32dec(magic256 + 0);
		ctx->state[ 5] = le32dec(magic256 + 4);
		ctx->state[10] = le32dec(magic256 + 8);
		ctx->state[15] = le32dec(magic256 + 12);
		/* first half of key */
		ctx->state[ 1] = le32dec(key + 0);
		ctx->state[ 2] = le32dec(key + 4);
		ctx->state[ 3] = le32dec(key + 8);
		ctx->state[ 4] = le32dec(key + 12);
		/* second half of key */
		ctx->state[11] = le32dec(key + 16);
		ctx->state[12] = le32dec(key + 20);
		ctx->state[13] = le32dec(key + 24);
		ctx->state[14] = le32dec(key + 28);
	} else {
		/* magic */
		ctx->state[ 0] = le32dec(magic128 + 0);
		ctx->state[ 5] = le32dec(magic128 + 4);
		ctx->state[10] = le32dec(magic128 + 8);
		ctx->state[15] = le32dec(magic128 + 12);
		/* first half of key */
		ctx->state[ 1] = le32dec(key + 0);
		ctx->state[ 2] = le32dec(key + 4);
		ctx->state[ 3] = le32dec(key + 8);
		ctx->state[ 4] = le32dec(key + 12);
		/* repeat first half of key */
		ctx->state[11] = le32dec(key + 0);
		ctx->state[12] = le32dec(key + 4);
		ctx->state[13] = le32dec(key + 8);
		ctx->state[14] = le32dec(key + 12);
	}
	ctx->rounds = 20;
}

/*
 * Reset the stream position, load a new nonce, and change the number of
 * rounds if requested.
 */
void
salsa_reset(salsa_ctx *ctx, const uint8_t *nonce, unsigned int rounds)
{

	/* reset stream counter */
	ctx->state[8] = 0;
	ctx->state[9] = 0;
	/* copy nonce */
	ctx->state[6] = le32dec(nonce + 0);
	ctx->state[7] = le32dec(nonce + 4);
	/* set rounds if specified */
	if (rounds != 0)
		ctx->rounds = rounds;
}

/*
 * Generate a block of keystream.
 */
size_t
salsa_keystream(salsa_ctx *ctx, uint8_t *ks, size_t len)
{

	return (salsa_encrypt(ctx, NULL, ks, len));
}

/*
 * Encryption: generate a block of keystream, xor it with the plaintext to
 * produce the ciphertext, and increment the stream position.
 */
size_t
salsa_encrypt(salsa_ctx *ctx, const void *vpt, uint8_t *ct, size_t len)
{
	const uint8_t *pt = vpt;
	uint64_t ctr;
	uint32_t mix[16];
	uint8_t ks[64];
	unsigned int b, i;

	len -= len % sizeof ks;
	for (b = 0; b < len; b += sizeof ks) {
		memcpy(mix, ctx->state, sizeof mix);
		for (i = 0; i < ctx->rounds; i += 2) {
			SALSA_QR(mix,  4,  8, 12,  0);
			SALSA_QR(mix,  9, 13,  1,  5);
			SALSA_QR(mix, 14,  2,  6, 10);
			SALSA_QR(mix,  3,  7, 11, 15);
			SALSA_QR(mix,  1,  2,  3,  0);
			SALSA_QR(mix,  6,  7,  4,  5);
			SALSA_QR(mix, 11,  8,  9, 10);
			SALSA_QR(mix, 12, 13, 14, 15);
		}
		for (i = 0; i < 16; ++i)
			le32enc(ks + i * 4, ctx->state[i] + mix[i]);
		if (pt == NULL) {
			memcpy(ct, ks, sizeof ks);
			ct += sizeof ks;
		} else {
			for (i = 0; i < 64 && i < len; ++i)
				*ct++ = *pt++ ^ ks[i];
		}
		ctr = le64dec(ctx->state + 12);
		le64enc(ctx->state + 12, ++ctr);
	}
	return (len);
}

/*
 * Decryption: identical to encryption.
 */
size_t
salsa_decrypt(salsa_ctx *ctx, const uint8_t *ct, void *vpt, size_t len)
{

	return (salsa_encrypt(ctx, ct, vpt, len));
}

/*
 * Wipe our state.
 */
void
salsa_finish(salsa_ctx *ctx)
{

	(void)memset_s(ctx, 0, sizeof *ctx, sizeof *ctx);
}