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Improve crypto ordering for Encrypt-then-MAC (EtM) mode MAC algorithms.

Previously we were computing the MAC, decrypting the packet and then
checking the MAC. This gave rise to the possibility of creating a
side-channel oracle in the decryption step, though no such oracle has
been identified.

This adds a mac_check() function that computes and checks the MAC in
one pass, and uses it to advance MAC checking for EtM algorithms to
before payload decryption.

Reported by Jean Paul Degabriele, Kenny Paterson, Torben Hansen and
Martin Albrecht. feedback and ok markus@

/* $OpenBSD: mac.c,v 1.33 2016/07/08 03:44:42 djm 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 <string.h>
#include <stdio.h>

#include "digest.h"
#include "hmac.h"
#include "umac.h"
#include "mac.h"
#include "misc.h"
#include "ssherr.h"
#include "sshbuf.h"

#define SSH_DIGEST	1	/* SSH_DIGEST_XXX */
#define SSH_UMAC	2	/* UMAC (not integrated with OpenSSL) */
#define SSH_UMAC128	3

struct macalg {
	char		*name;
	int		type;
	int		alg;
	int		truncatebits;	/* truncate digest if != 0 */
	int		key_len;	/* just for UMAC */
	int		len;		/* just for UMAC */
	int		etm;		/* Encrypt-then-MAC */
};

static const struct macalg macs[] = {
	/* Encrypt-and-MAC (encrypt-and-authenticate) variants */
	{ "hmac-sha1",				SSH_DIGEST, SSH_DIGEST_SHA1, 0, 0, 0, 0 },
	{ "hmac-sha1-96",			SSH_DIGEST, SSH_DIGEST_SHA1, 96, 0, 0, 0 },
	{ "hmac-sha2-256",			SSH_DIGEST, SSH_DIGEST_SHA256, 0, 0, 0, 0 },
	{ "hmac-sha2-512",			SSH_DIGEST, SSH_DIGEST_SHA512, 0, 0, 0, 0 },
	{ "hmac-md5",				SSH_DIGEST, SSH_DIGEST_MD5, 0, 0, 0, 0 },
	{ "hmac-md5-96",			SSH_DIGEST, SSH_DIGEST_MD5, 96, 0, 0, 0 },
	{ "hmac-ripemd160",			SSH_DIGEST, SSH_DIGEST_RIPEMD160, 0, 0, 0, 0 },
	{ "hmac-ripemd160@openssh.com",		SSH_DIGEST, SSH_DIGEST_RIPEMD160, 0, 0, 0, 0 },
	{ "umac-64@openssh.com",		SSH_UMAC, 0, 0, 128, 64, 0 },
	{ "umac-128@openssh.com",		SSH_UMAC128, 0, 0, 128, 128, 0 },

	/* Encrypt-then-MAC variants */
	{ "hmac-sha1-etm@openssh.com",		SSH_DIGEST, SSH_DIGEST_SHA1, 0, 0, 0, 1 },
	{ "hmac-sha1-96-etm@openssh.com",	SSH_DIGEST, SSH_DIGEST_SHA1, 96, 0, 0, 1 },
	{ "hmac-sha2-256-etm@openssh.com",	SSH_DIGEST, SSH_DIGEST_SHA256, 0, 0, 0, 1 },
	{ "hmac-sha2-512-etm@openssh.com",	SSH_DIGEST, SSH_DIGEST_SHA512, 0, 0, 0, 1 },
	{ "hmac-md5-etm@openssh.com",		SSH_DIGEST, SSH_DIGEST_MD5, 0, 0, 0, 1 },
	{ "hmac-md5-96-etm@openssh.com",	SSH_DIGEST, SSH_DIGEST_MD5, 96, 0, 0, 1 },
	{ "hmac-ripemd160-etm@openssh.com",	SSH_DIGEST, SSH_DIGEST_RIPEMD160, 0, 0, 0, 1 },
	{ "umac-64-etm@openssh.com",		SSH_UMAC, 0, 0, 128, 64, 1 },
	{ "umac-128-etm@openssh.com",		SSH_UMAC128, 0, 0, 128, 128, 1 },

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

/* Returns a list of supported MACs separated by the specified char. */
char *
mac_alg_list(char sep)
{
	char *ret = NULL, *tmp;
	size_t nlen, rlen = 0;
	const struct macalg *m;

	for (m = macs; m->name != NULL; m++) {
		if (ret != NULL)
			ret[rlen++] = sep;
		nlen = strlen(m->name);
		if ((tmp = realloc(ret, rlen + nlen + 2)) == NULL) {
			free(ret);
			return NULL;
		}
		ret = tmp;
		memcpy(ret + rlen, m->name, nlen + 1);
		rlen += nlen;
	}
	return ret;
}

static int
mac_setup_by_alg(struct sshmac *mac, const struct macalg *macalg)
{
	mac->type = macalg->type;
	if (mac->type == SSH_DIGEST) {
		if ((mac->hmac_ctx = ssh_hmac_start(macalg->alg)) == NULL)
			return SSH_ERR_ALLOC_FAIL;
		mac->key_len = mac->mac_len = ssh_hmac_bytes(macalg->alg);
	} else {
		mac->mac_len = macalg->len / 8;
		mac->key_len = macalg->key_len / 8;
		mac->umac_ctx = NULL;
	}
	if (macalg->truncatebits != 0)
		mac->mac_len = macalg->truncatebits / 8;
	mac->etm = macalg->etm;
	return 0;
}

int
mac_setup(struct sshmac *mac, char *name)
{
	const struct macalg *m;

	for (m = macs; m->name != NULL; m++) {
		if (strcmp(name, m->name) != 0)
			continue;
		if (mac != NULL)
			return mac_setup_by_alg(mac, m);
		return 0;
	}
	return SSH_ERR_INVALID_ARGUMENT;
}

int
mac_init(struct sshmac *mac)
{
	if (mac->key == NULL)
		return SSH_ERR_INVALID_ARGUMENT;
	switch (mac->type) {
	case SSH_DIGEST:
		if (mac->hmac_ctx == NULL ||
		    ssh_hmac_init(mac->hmac_ctx, mac->key, mac->key_len) < 0)
			return SSH_ERR_INVALID_ARGUMENT;
		return 0;
	case SSH_UMAC:
		if ((mac->umac_ctx = umac_new(mac->key)) == NULL)
			return SSH_ERR_ALLOC_FAIL;
		return 0;
	case SSH_UMAC128:
		if ((mac->umac_ctx = umac128_new(mac->key)) == NULL)
			return SSH_ERR_ALLOC_FAIL;
		return 0;
	default:
		return SSH_ERR_INVALID_ARGUMENT;
	}
}

int
mac_compute(struct sshmac *mac, u_int32_t seqno,
    const u_char *data, int datalen,
    u_char *digest, size_t dlen)
{
	static union {
		u_char m[SSH_DIGEST_MAX_LENGTH];
		u_int64_t for_align;
	} u;
	u_char b[4];
	u_char nonce[8];

	if (mac->mac_len > sizeof(u))
		return SSH_ERR_INTERNAL_ERROR;

	switch (mac->type) {
	case SSH_DIGEST:
		put_u32(b, seqno);
		/* reset HMAC context */
		if (ssh_hmac_init(mac->hmac_ctx, NULL, 0) < 0 ||
		    ssh_hmac_update(mac->hmac_ctx, b, sizeof(b)) < 0 ||
		    ssh_hmac_update(mac->hmac_ctx, data, datalen) < 0 ||
		    ssh_hmac_final(mac->hmac_ctx, u.m, sizeof(u.m)) < 0)
			return SSH_ERR_LIBCRYPTO_ERROR;
		break;
	case SSH_UMAC:
		POKE_U64(nonce, seqno);
		umac_update(mac->umac_ctx, data, datalen);
		umac_final(mac->umac_ctx, u.m, nonce);
		break;
	case SSH_UMAC128:
		put_u64(nonce, seqno);
		umac128_update(mac->umac_ctx, data, datalen);
		umac128_final(mac->umac_ctx, u.m, nonce);
		break;
	default:
		return SSH_ERR_INVALID_ARGUMENT;
	}
	if (digest != NULL) {
		if (dlen > mac->mac_len)
			dlen = mac->mac_len;
		memcpy(digest, u.m, dlen);
	}
	return 0;
}

int
mac_check(struct sshmac *mac, u_int32_t seqno,
    const u_char *data, size_t dlen,
    const u_char *theirmac, size_t mlen)
{
	u_char ourmac[SSH_DIGEST_MAX_LENGTH];
	int r;

	if (mac->mac_len > mlen)
		return SSH_ERR_INVALID_ARGUMENT;
	if ((r = mac_compute(mac, seqno, data, dlen,
	    ourmac, sizeof(ourmac))) != 0)
		return r;
	if (timingsafe_bcmp(ourmac, theirmac, mac->mac_len) != 0)
		return SSH_ERR_MAC_INVALID;
	return 0;
}

void
mac_clear(struct sshmac *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->hmac_ctx != NULL)
		ssh_hmac_free(mac->hmac_ctx);
	mac->hmac_ctx = 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;
	if ((maclist = cp = strdup(names)) == NULL)
		return 0;
	for ((p = strsep(&cp, MAC_SEP)); p && *p != '\0';
	    (p = strsep(&cp, MAC_SEP))) {
		if (mac_setup(NULL, p) < 0) {
			free(maclist);
			return 0;
		}
	}
	free(maclist);
	return 1;
}