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add encrypt-then-mac (EtM) modes to openssh by defining new mac algorithms
that change the packet format and compute the MAC over the encrypted
message (including the packet size) instead of the plaintext data;
these EtM modes are considered more secure and used by default.
feedback and ok djm@

/* $OpenBSD: mac.c,v 1.20 2012/12/11 22:31:18 markus Exp $ */
/*
 * Copyright (c) 2001 Markus Friedl.  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.
 *
 * THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``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 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 <sys/types.h>

#include <openssl/hmac.h>

#include <string.h>
#include <signal.h>

#include "xmalloc.h"
#include "log.h"
#include "cipher.h"
#include "buffer.h"
#include "key.h"
#include "kex.h"
#include "mac.h"
#include "misc.h"

#include "umac.h"

#define SSH_EVP		1	/* OpenSSL EVP-based MAC */
#define SSH_UMAC	2	/* UMAC (not integrated with OpenSSL) */
#define SSH_UMAC128	3

struct {
	char		*name;
	int		type;
	const EVP_MD *	(*mdfunc)(void);
	int		truncatebits;	/* truncate digest if != 0 */
	int		key_len;	/* just for UMAC */
	int		len;		/* just for UMAC */
	int		etm;		/* Encrypt-then-MAC */
} macs[] = {
	/* Encrypt-and-MAC (encrypt-and-authenticate) variants */
	{ "hmac-sha1",				SSH_EVP, EVP_sha1, 0, 0, 0, 0 },
	{ "hmac-sha1-96",			SSH_EVP, EVP_sha1, 96, 0, 0, 0 },
	{ "hmac-sha2-256",			SSH_EVP, EVP_sha256, 0, 0, 0, 0 },
	{ "hmac-sha2-512",			SSH_EVP, EVP_sha512, 0, 0, 0, 0 },
	{ "hmac-md5",				SSH_EVP, EVP_md5, 0, 0, 0, 0 },
	{ "hmac-md5-96",			SSH_EVP, EVP_md5, 96, 0, 0, 0 },
	{ "hmac-ripemd160",			SSH_EVP, EVP_ripemd160, 0, 0, 0, 0 },
	{ "hmac-ripemd160@openssh.com",		SSH_EVP, EVP_ripemd160, 0, 0, 0, 0 },
	{ "umac-64@openssh.com",		SSH_UMAC, NULL, 0, 128, 64, 0 },
	{ "umac-128@openssh.com",		SSH_UMAC128, NULL, 0, 128, 128, 0 },

	/* Encrypt-then-MAC variants */
	{ "hmac-sha1-etm@openssh.com",		SSH_EVP, EVP_sha1, 0, 0, 0, 1 },
	{ "hmac-sha1-96-etm@openssh.com",	SSH_EVP, EVP_sha1, 96, 0, 0, 1 },
	{ "hmac-sha2-256-etm@openssh.com",	SSH_EVP, EVP_sha256, 0, 0, 0, 1 },
	{ "hmac-sha2-512-etm@openssh.com",	SSH_EVP, EVP_sha512, 0, 0, 0, 1 },
	{ "hmac-md5-etm@openssh.com",		SSH_EVP, EVP_md5, 0, 0, 0, 1 },
	{ "hmac-md5-96-etm@openssh.com",	SSH_EVP, EVP_md5, 96, 0, 0, 1 },
	{ "hmac-ripemd160-tem@openssh.com",	SSH_EVP, EVP_ripemd160, 0, 0, 0, 1 },
	{ "umac-64-etm@openssh.com",		SSH_UMAC, NULL, 0, 128, 64, 1 },
	{ "umac-128-etm@openssh.com",		SSH_UMAC128, NULL, 0, 128, 128, 1 },

	{ NULL,					0, NULL, 0, 0, 0, 0 }
};

static void
mac_setup_by_id(Mac *mac, int which)
{
	int evp_len;
	mac->type = macs[which].type;
	if (mac->type == SSH_EVP) {
		mac->evp_md = (*macs[which].mdfunc)();
		if ((evp_len = EVP_MD_size(mac->evp_md)) <= 0)
			fatal("mac %s len %d", mac->name, evp_len);
		mac->key_len = mac->mac_len = (u_int)evp_len;
	} else {
		mac->mac_len = macs[which].len / 8;
		mac->key_len = macs[which].key_len / 8;
		mac->umac_ctx = NULL;
	}
	if (macs[which].truncatebits != 0)
		mac->mac_len = macs[which].truncatebits / 8;
	mac->etm = macs[which].etm;
}

int
mac_setup(Mac *mac, char *name)
{
	int i;

	for (i = 0; macs[i].name; i++) {
		if (strcmp(name, macs[i].name) == 0) {
			if (mac != NULL)
				mac_setup_by_id(mac, i);
			debug2("mac_setup: found %s", name);
			return (0);
		}
	}
	debug2("mac_setup: unknown %s", name);
	return (-1);
}

int
mac_init(Mac *mac)
{
	if (mac->key == NULL)
		fatal("mac_init: no key");
	switch (mac->type) {
	case SSH_EVP:
		if (mac->evp_md == NULL)
			return -1;
		HMAC_CTX_init(&mac->evp_ctx);
		HMAC_Init(&mac->evp_ctx, mac->key, mac->key_len, mac->evp_md);
		return 0;
	case SSH_UMAC:
		mac->umac_ctx = umac_new(mac->key);
		return 0;
	case SSH_UMAC128:
		mac->umac_ctx = umac128_new(mac->key);
		return 0;
	default:
		return -1;
	}
}

u_char *
mac_compute(Mac *mac, u_int32_t seqno, u_char *data, int datalen)
{
	static u_char m[EVP_MAX_MD_SIZE];
	u_char b[4], nonce[8];

	if (mac->mac_len > sizeof(m))
		fatal("mac_compute: mac too long %u %lu",
		    mac->mac_len, (u_long)sizeof(m));

	switch (mac->type) {
	case SSH_EVP:
		put_u32(b, seqno);
		/* reset HMAC context */
		HMAC_Init(&mac->evp_ctx, NULL, 0, NULL);
		HMAC_Update(&mac->evp_ctx, b, sizeof(b));
		HMAC_Update(&mac->evp_ctx, data, datalen);
		HMAC_Final(&mac->evp_ctx, m, NULL);
		break;
	case SSH_UMAC:
		put_u64(nonce, seqno);
		umac_update(mac->umac_ctx, data, datalen);
		umac_final(mac->umac_ctx, m, nonce);
		break;
	case SSH_UMAC128:
		put_u64(nonce, seqno);
		umac128_update(mac->umac_ctx, data, datalen);
		umac128_final(mac->umac_ctx, m, nonce);
		break;
	default:
		fatal("mac_compute: unknown MAC type");
	}
	return (m);
}

void
mac_clear(Mac *mac)
{
	if (mac->type == SSH_UMAC) {
		if (mac->umac_ctx != NULL)
			umac_delete(mac->umac_ctx);
	} else if (mac->type == SSH_UMAC128) {
		if (mac->umac_ctx != NULL)
			umac128_delete(mac->umac_ctx);
	} else if (mac->evp_md != NULL)
		HMAC_cleanup(&mac->evp_ctx);
	mac->evp_md = NULL;
	mac->umac_ctx = NULL;
}

/* XXX copied from ciphers_valid */
#define	MAC_SEP	","
int
mac_valid(const char *names)
{
	char *maclist, *cp, *p;

	if (names == NULL || strcmp(names, "") == 0)
		return (0);
	maclist = cp = xstrdup(names);
	for ((p = strsep(&cp, MAC_SEP)); p && *p != '\0';
	    (p = strsep(&cp, MAC_SEP))) {
		if (mac_setup(NULL, p) < 0) {
			debug("bad mac %s [%s]", p, names);
			xfree(maclist);
			return (0);
		} else {
			debug3("mac ok: %s [%s]", p, names);
		}
	}
	debug3("macs ok: [%s]", names);
	xfree(maclist);
	return (1);
}