1/* $OpenBSD: cipher-chachapoly.c,v 1.10 2023/07/17 05:26:38 djm Exp $ */
2/*
3 * Copyright (c) 2013 Damien Miller <djm@mindrot.org>
4 *
5 * Permission to use, copy, modify, and distribute this software for any
6 * purpose with or without fee is hereby granted, provided that the above
7 * copyright notice and this permission notice appear in all copies.
8 *
9 * THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
10 * WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
11 * MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR
12 * ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
13 * WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN
14 * ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF
15 * OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
16 */
17
18#include "includes.h"
19#ifdef WITH_OPENSSL
20#include "openbsd-compat/openssl-compat.h"
21#endif
22
23#if !defined(HAVE_EVP_CHACHA20) || defined(HAVE_BROKEN_CHACHA20)
24
25#include <sys/types.h>
26#include <stdarg.h> /* needed for log.h */
27#include <string.h>
28#include <stdio.h>  /* needed for misc.h */
29
30#include "log.h"
31#include "sshbuf.h"
32#include "ssherr.h"
33#include "cipher-chachapoly.h"
34
35struct chachapoly_ctx {
36	struct chacha_ctx main_ctx, header_ctx;
37};
38
39struct chachapoly_ctx *
40chachapoly_new(const u_char *key, u_int keylen)
41{
42	struct chachapoly_ctx *ctx;
43
44	if (keylen != (32 + 32)) /* 2 x 256 bit keys */
45		return NULL;
46	if ((ctx = calloc(1, sizeof(*ctx))) == NULL)
47		return NULL;
48	chacha_keysetup(&ctx->main_ctx, key, 256);
49	chacha_keysetup(&ctx->header_ctx, key + 32, 256);
50	return ctx;
51}
52
53void
54chachapoly_free(struct chachapoly_ctx *cpctx)
55{
56	freezero(cpctx, sizeof(*cpctx));
57}
58
59/*
60 * chachapoly_crypt() operates as following:
61 * En/decrypt with header key 'aadlen' bytes from 'src', storing result
62 * to 'dest'. The ciphertext here is treated as additional authenticated
63 * data for MAC calculation.
64 * En/decrypt 'len' bytes at offset 'aadlen' from 'src' to 'dest'. Use
65 * POLY1305_TAGLEN bytes at offset 'len'+'aadlen' as the authentication
66 * tag. This tag is written on encryption and verified on decryption.
67 */
68int
69chachapoly_crypt(struct chachapoly_ctx *ctx, u_int seqnr, u_char *dest,
70    const u_char *src, u_int len, u_int aadlen, u_int authlen, int do_encrypt)
71{
72	u_char seqbuf[8];
73	const u_char one[8] = { 1, 0, 0, 0, 0, 0, 0, 0 }; /* NB little-endian */
74	u_char expected_tag[POLY1305_TAGLEN], poly_key[POLY1305_KEYLEN];
75	int r = SSH_ERR_INTERNAL_ERROR;
76
77	/*
78	 * Run ChaCha20 once to generate the Poly1305 key. The IV is the
79	 * packet sequence number.
80	 */
81	memset(poly_key, 0, sizeof(poly_key));
82	POKE_U64(seqbuf, seqnr);
83	chacha_ivsetup(&ctx->main_ctx, seqbuf, NULL);
84	chacha_encrypt_bytes(&ctx->main_ctx,
85	    poly_key, poly_key, sizeof(poly_key));
86
87	/* If decrypting, check tag before anything else */
88	if (!do_encrypt) {
89		const u_char *tag = src + aadlen + len;
90
91		poly1305_auth(expected_tag, src, aadlen + len, poly_key);
92		if (timingsafe_bcmp(expected_tag, tag, POLY1305_TAGLEN) != 0) {
93			r = SSH_ERR_MAC_INVALID;
94			goto out;
95		}
96	}
97
98	/* Crypt additional data */
99	if (aadlen) {
100		chacha_ivsetup(&ctx->header_ctx, seqbuf, NULL);
101		chacha_encrypt_bytes(&ctx->header_ctx, src, dest, aadlen);
102	}
103
104	/* Set Chacha's block counter to 1 */
105	chacha_ivsetup(&ctx->main_ctx, seqbuf, one);
106	chacha_encrypt_bytes(&ctx->main_ctx, src + aadlen,
107	    dest + aadlen, len);
108
109	/* If encrypting, calculate and append tag */
110	if (do_encrypt) {
111		poly1305_auth(dest + aadlen + len, dest, aadlen + len,
112		    poly_key);
113	}
114	r = 0;
115 out:
116	explicit_bzero(expected_tag, sizeof(expected_tag));
117	explicit_bzero(seqbuf, sizeof(seqbuf));
118	explicit_bzero(poly_key, sizeof(poly_key));
119	return r;
120}
121
122/* Decrypt and extract the encrypted packet length */
123int
124chachapoly_get_length(struct chachapoly_ctx *ctx,
125    u_int *plenp, u_int seqnr, const u_char *cp, u_int len)
126{
127	u_char buf[4], seqbuf[8];
128
129	if (len < 4)
130		return SSH_ERR_MESSAGE_INCOMPLETE;
131	POKE_U64(seqbuf, seqnr);
132	chacha_ivsetup(&ctx->header_ctx, seqbuf, NULL);
133	chacha_encrypt_bytes(&ctx->header_ctx, cp, buf, 4);
134	*plenp = PEEK_U32(buf);
135	return 0;
136}
137
138#endif /* !defined(HAVE_EVP_CHACHA20) || defined(HAVE_BROKEN_CHACHA20) */
139