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      1 /*	$NetBSD: cipher-chachapoly.c,v 1.7 2023/10/25 20:19:57 christos Exp $	*/
      2 /* $OpenBSD: cipher-chachapoly.c,v 1.10 2023/07/17 05:26:38 djm Exp $ */
      3 
      4 /*
      5  * Copyright (c) 2013 Damien Miller <djm (at) mindrot.org>
      6  *
      7  * Permission to use, copy, modify, and distribute this software for any
      8  * purpose with or without fee is hereby granted, provided that the above
      9  * copyright notice and this permission notice appear in all copies.
     10  *
     11  * THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
     12  * WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
     13  * MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR
     14  * ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
     15  * WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN
     16  * ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF
     17  * OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
     18  */
     19 
     20 #include "includes.h"
     21 __RCSID("$NetBSD: cipher-chachapoly.c,v 1.7 2023/10/25 20:19:57 christos Exp $");
     22 
     23 #include <sys/types.h>
     24 #include <stdarg.h> /* needed for log.h */
     25 #include <string.h>
     26 #include <stdio.h>  /* needed for misc.h */
     27 
     28 #include "log.h"
     29 #include "sshbuf.h"
     30 #include "ssherr.h"
     31 #include "cipher-chachapoly.h"
     32 
     33 struct chachapoly_ctx {
     34 	struct chacha_ctx main_ctx, header_ctx;
     35 };
     36 
     37 struct chachapoly_ctx *
     38 chachapoly_new(const u_char *key, u_int keylen)
     39 {
     40 	struct chachapoly_ctx *ctx;
     41 
     42 	if (keylen != (32 + 32)) /* 2 x 256 bit keys */
     43 		return NULL;
     44 	if ((ctx = calloc(1, sizeof(*ctx))) == NULL)
     45 		return NULL;
     46 	chacha_keysetup(&ctx->main_ctx, key, 256);
     47 	chacha_keysetup(&ctx->header_ctx, key + 32, 256);
     48 	return ctx;
     49 }
     50 
     51 void
     52 chachapoly_free(struct chachapoly_ctx *cpctx)
     53 {
     54 	freezero(cpctx, sizeof(*cpctx));
     55 }
     56 
     57 /*
     58  * chachapoly_crypt() operates as following:
     59  * En/decrypt with header key 'aadlen' bytes from 'src', storing result
     60  * to 'dest'. The ciphertext here is treated as additional authenticated
     61  * data for MAC calculation.
     62  * En/decrypt 'len' bytes at offset 'aadlen' from 'src' to 'dest'. Use
     63  * POLY1305_TAGLEN bytes at offset 'len'+'aadlen' as the authentication
     64  * tag. This tag is written on encryption and verified on decryption.
     65  */
     66 int
     67 chachapoly_crypt(struct chachapoly_ctx *ctx, u_int seqnr, u_char *dest,
     68     const u_char *src, u_int len, u_int aadlen, u_int authlen, int do_encrypt)
     69 {
     70 	u_char seqbuf[8];
     71 	const u_char one[8] = { 1, 0, 0, 0, 0, 0, 0, 0 }; /* NB little-endian */
     72 	u_char expected_tag[POLY1305_TAGLEN], poly_key[POLY1305_KEYLEN];
     73 	int r = SSH_ERR_INTERNAL_ERROR;
     74 
     75 	/*
     76 	 * Run ChaCha20 once to generate the Poly1305 key. The IV is the
     77 	 * packet sequence number.
     78 	 */
     79 	memset(poly_key, 0, sizeof(poly_key));
     80 	POKE_U64(seqbuf, seqnr);
     81 	chacha_ivsetup(&ctx->main_ctx, seqbuf, NULL);
     82 	chacha_encrypt_bytes(&ctx->main_ctx,
     83 	    poly_key, poly_key, sizeof(poly_key));
     84 
     85 	/* If decrypting, check tag before anything else */
     86 	if (!do_encrypt) {
     87 		const u_char *tag = src + aadlen + len;
     88 
     89 		poly1305_auth(expected_tag, src, aadlen + len, poly_key);
     90 		if (timingsafe_bcmp(expected_tag, tag, POLY1305_TAGLEN) != 0) {
     91 			r = SSH_ERR_MAC_INVALID;
     92 			goto out;
     93 		}
     94 	}
     95 
     96 	/* Crypt additional data */
     97 	if (aadlen) {
     98 		chacha_ivsetup(&ctx->header_ctx, seqbuf, NULL);
     99 		chacha_encrypt_bytes(&ctx->header_ctx, src, dest, aadlen);
    100 	}
    101 
    102 	/* Set Chacha's block counter to 1 */
    103 	chacha_ivsetup(&ctx->main_ctx, seqbuf, one);
    104 	chacha_encrypt_bytes(&ctx->main_ctx, src + aadlen,
    105 	    dest + aadlen, len);
    106 
    107 	/* If encrypting, calculate and append tag */
    108 	if (do_encrypt) {
    109 		poly1305_auth(dest + aadlen + len, dest, aadlen + len,
    110 		    poly_key);
    111 	}
    112 	r = 0;
    113  out:
    114 	explicit_bzero(expected_tag, sizeof(expected_tag));
    115 	explicit_bzero(seqbuf, sizeof(seqbuf));
    116 	explicit_bzero(poly_key, sizeof(poly_key));
    117 	return r;
    118 }
    119 
    120 /* Decrypt and extract the encrypted packet length */
    121 int
    122 chachapoly_get_length(struct chachapoly_ctx *ctx,
    123     u_int *plenp, u_int seqnr, const u_char *cp, u_int len)
    124 {
    125 	u_char buf[4], seqbuf[8];
    126 
    127 	if (len < 4)
    128 		return SSH_ERR_MESSAGE_INCOMPLETE;
    129 	POKE_U64(seqbuf, seqnr);
    130 	chacha_ivsetup(&ctx->header_ctx, seqbuf, NULL);
    131 	chacha_encrypt_bytes(&ctx->header_ctx, cp, buf, 4);
    132 	*plenp = PEEK_U32(buf);
    133 	return 0;
    134 }
    135