view packet.c @ 1715:3974f087d9c0

Disallow leading lines before the ident for server (#102) Per RFC4253 4.2 clients must be able to process other lines of data before the version string, server behavior is not defined neither with MUST/SHOULD nor with MAY. If server process up to 50 lines too - it may cause too long hanging session with invalid/evil client that consume host resources and potentially may lead to DDoS on poor embedded boxes. Let's require first line from client to be version string and fail early if it's not - matches both RFC and real OpenSSH behavior.
author Vladislav Grishenko <themiron@users.noreply.github.com>
date Mon, 15 Jun 2020 18:22:18 +0500
parents 3a97f14c0235
children 3b9b427925a0
line wrap: on
line source

/*
 * Dropbear - a SSH2 server
 * 
 * Copyright (c) 2002,2003 Matt Johnston
 * All rights reserved.
 * 
 * Permission is hereby granted, free of charge, to any person obtaining a copy
 * of this software and associated documentation files (the "Software"), to deal
 * in the Software without restriction, including without limitation the rights
 * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
 * copies of the Software, and to permit persons to whom the Software is
 * furnished to do so, subject to the following conditions:
 * 
 * The above copyright notice and this permission notice shall be included in
 * all copies or substantial portions of the Software.
 * 
 * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
 * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
 * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
 * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
 * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
 * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
 * SOFTWARE. */

#include "includes.h"
#include "packet.h"
#include "session.h"
#include "dbutil.h"
#include "ssh.h"
#include "algo.h"
#include "buffer.h"
#include "kex.h"
#include "dbrandom.h"
#include "service.h"
#include "auth.h"
#include "channel.h"
#include "netio.h"
#include "runopts.h"

static int read_packet_init(void);
static void make_mac(unsigned int seqno, const struct key_context_directional * key_state,
		buffer * clear_buf, unsigned int clear_len, 
		unsigned char *output_mac);
static int checkmac(void);

/* For exact details see http://www.zlib.net/zlib_tech.html
 * 5 bytes per 16kB block, plus 6 bytes for the stream.
 * We might allocate 5 unnecessary bytes here if it's an
 * exact multiple. */
#define ZLIB_COMPRESS_EXPANSION (((RECV_MAX_PAYLOAD_LEN/16384)+1)*5 + 6)
#define ZLIB_DECOMPRESS_INCR 1024
#ifndef DISABLE_ZLIB
static buffer* buf_decompress(const buffer* buf, unsigned int len);
static void buf_compress(buffer * dest, buffer * src, unsigned int len);
#endif

/* non-blocking function writing out a current encrypted packet */
void write_packet() {

	ssize_t written;
#if defined(HAVE_WRITEV) && (defined(IOV_MAX) || defined(UIO_MAXIOV))
	/* 50 is somewhat arbitrary */
	unsigned int iov_count = 50;
	struct iovec iov[50];
#else
	int len;
	buffer* writebuf;
#endif
	
	TRACE2(("enter write_packet"))
	dropbear_assert(!isempty(&ses.writequeue));

#if defined(HAVE_WRITEV) && (defined(IOV_MAX) || defined(UIO_MAXIOV))

	packet_queue_to_iovec(&ses.writequeue, iov, &iov_count);
	/* This may return EAGAIN. The main loop sometimes
	calls write_packet() without bothering to test with select() since
	it's likely to be necessary */
#if DROPBEAR_FUZZ
	if (fuzz.fuzzing) {
		/* pretend to write one packet at a time */
		/* TODO(fuzz): randomise amount written based on the fuzz input */
		written = iov[0].iov_len;
	}
	else
#endif
	{
	written = writev(ses.sock_out, iov, iov_count);
	if (written < 0) {
		if (errno == EINTR || errno == EAGAIN) {
			TRACE2(("leave write_packet: EINTR"))
			return;
		} else {
			dropbear_exit("Error writing: %s", strerror(errno));
		}
	}
	}

	packet_queue_consume(&ses.writequeue, written);
	ses.writequeue_len -= written;

	if (written == 0) {
		ses.remoteclosed();
	}

#else /* No writev () */
#if DROPBEAR_FUZZ
	_Static_assert(0, "No fuzzing code for no-writev writes");
#endif
	/* Get the next buffer in the queue of encrypted packets to write*/
	writebuf = (buffer*)examine(&ses.writequeue);

	len = writebuf->len - writebuf->pos;
	dropbear_assert(len > 0);
	/* Try to write as much as possible */
	written = write(ses.sock_out, buf_getptr(writebuf, len), len);

	if (written < 0) {
		if (errno == EINTR || errno == EAGAIN) {
			TRACE2(("leave writepacket: EINTR"))
			return;
		} else {
			dropbear_exit("Error writing: %s", strerror(errno));
		}
	} 

	if (written == 0) {
		ses.remoteclosed();
	}

	ses.writequeue_len -= written;

	if (written == len) {
		/* We've finished with the packet, free it */
		dequeue(&ses.writequeue);
		buf_free(writebuf);
		writebuf = NULL;
	} else {
		/* More packet left to write, leave it in the queue for later */
		buf_incrpos(writebuf, written);
	}
#endif /* writev */

	TRACE2(("leave write_packet"))
}

/* Non-blocking function reading available portion of a packet into the
 * ses's buffer, decrypting the length if encrypted, decrypting the
 * full portion if possible */
void read_packet() {

	int len;
	unsigned int maxlen;
	unsigned char blocksize;

	TRACE2(("enter read_packet"))
	blocksize = ses.keys->recv.algo_crypt->blocksize;
	
	if (ses.readbuf == NULL || ses.readbuf->len < blocksize) {
		int ret;
		/* In the first blocksize of a packet */

		/* Read the first blocksize of the packet, so we can decrypt it and
		 * find the length of the whole packet */
		ret = read_packet_init();

		if (ret == DROPBEAR_FAILURE) {
			/* didn't read enough to determine the length */
			TRACE2(("leave read_packet: packetinit done"))
			return;
		}
	}

	/* Attempt to read the remainder of the packet, note that there
	 * mightn't be any available (EAGAIN) */
	maxlen = ses.readbuf->len - ses.readbuf->pos;
	if (maxlen == 0) {
		/* Occurs when the packet is only a single block long and has all
		 * been read in read_packet_init().  Usually means that MAC is disabled
		 */
		len = 0;
	} else {
		len = read(ses.sock_in, buf_getptr(ses.readbuf, maxlen), maxlen);

		if (len == 0) {
			ses.remoteclosed();
		}

		if (len < 0) {
			if (errno == EINTR || errno == EAGAIN) {
				TRACE2(("leave read_packet: EINTR or EAGAIN"))
				return;
			} else {
				dropbear_exit("Error reading: %s", strerror(errno));
			}
		}

		buf_incrpos(ses.readbuf, len);
	}

	if ((unsigned int)len == maxlen) {
		/* The whole packet has been read */
		decrypt_packet();
		/* The main select() loop process_packet() to
		 * handle the packet contents... */
	}
	TRACE2(("leave read_packet"))
}

/* Function used to read the initial portion of a packet, and determine the
 * length. Only called during the first BLOCKSIZE of a packet. */
/* Returns DROPBEAR_SUCCESS if the length is determined, 
 * DROPBEAR_FAILURE otherwise */
static int read_packet_init() {

	unsigned int maxlen;
	int slen;
	unsigned int len, plen;
	unsigned int blocksize;
	unsigned int macsize;


	blocksize = ses.keys->recv.algo_crypt->blocksize;
	macsize = ses.keys->recv.algo_mac->hashsize;

	if (ses.readbuf == NULL) {
		/* start of a new packet */
		ses.readbuf = buf_new(INIT_READBUF);
	}

	maxlen = blocksize - ses.readbuf->pos;
			
	/* read the rest of the packet if possible */
	slen = read(ses.sock_in, buf_getwriteptr(ses.readbuf, maxlen),
			maxlen);
	if (slen == 0) {
		ses.remoteclosed();
	}
	if (slen < 0) {
		if (errno == EINTR || errno == EAGAIN) {
			TRACE2(("leave read_packet_init: EINTR"))
			return DROPBEAR_FAILURE;
		}
		dropbear_exit("Error reading: %s", strerror(errno));
	}

	buf_incrwritepos(ses.readbuf, slen);

	if ((unsigned int)slen != maxlen) {
		/* don't have enough bytes to determine length, get next time */
		return DROPBEAR_FAILURE;
	}

	/* now we have the first block, need to get packet length, so we decrypt
	 * the first block (only need first 4 bytes) */
	buf_setpos(ses.readbuf, 0);
#if DROPBEAR_AEAD_MODE
	if (ses.keys->recv.crypt_mode->aead_crypt) {
		if (ses.keys->recv.crypt_mode->aead_getlength(ses.recvseq,
					buf_getptr(ses.readbuf, blocksize), &plen,
					blocksize,
					&ses.keys->recv.cipher_state) != CRYPT_OK) {
			dropbear_exit("Error decrypting");
		}
		len = plen + 4 + macsize;
	} else
#endif
	{
		if (ses.keys->recv.crypt_mode->decrypt(buf_getptr(ses.readbuf, blocksize), 
					buf_getwriteptr(ses.readbuf, blocksize),
					blocksize,
					&ses.keys->recv.cipher_state) != CRYPT_OK) {
			dropbear_exit("Error decrypting");
		}
		plen = buf_getint(ses.readbuf) + 4;
		len = plen + macsize;
	}

	TRACE2(("packet size is %u, block %u mac %u", len, blocksize, macsize))


	/* check packet length */
	if ((len > RECV_MAX_PACKET_LEN) ||
		(plen < blocksize) ||
		(plen % blocksize != 0)) {
		dropbear_exit("Integrity error (bad packet size %u)", len);
	}

	if (len > ses.readbuf->size) {
		ses.readbuf = buf_resize(ses.readbuf, len);		
	}
	buf_setlen(ses.readbuf, len);
	buf_setpos(ses.readbuf, blocksize);
	return DROPBEAR_SUCCESS;
}

/* handle the received packet */
void decrypt_packet() {

	unsigned char blocksize;
	unsigned char macsize;
	unsigned int padlen;
	unsigned int len;

	TRACE2(("enter decrypt_packet"))
	blocksize = ses.keys->recv.algo_crypt->blocksize;
	macsize = ses.keys->recv.algo_mac->hashsize;

	ses.kexstate.datarecv += ses.readbuf->len;

#if DROPBEAR_AEAD_MODE
	if (ses.keys->recv.crypt_mode->aead_crypt) {
		/* first blocksize is not decrypted yet */
		buf_setpos(ses.readbuf, 0);

		/* decrypt it in-place */
		len = ses.readbuf->len - macsize - ses.readbuf->pos;
		if (ses.keys->recv.crypt_mode->aead_crypt(ses.recvseq,
					buf_getptr(ses.readbuf, len + macsize),
					buf_getwriteptr(ses.readbuf, len),
					len, macsize,
					&ses.keys->recv.cipher_state, LTC_DECRYPT) != CRYPT_OK) {
			dropbear_exit("Error decrypting");
		}
		buf_incrpos(ses.readbuf, len);
	} else
#endif
	{
		/* we've already decrypted the first blocksize in read_packet_init */
		buf_setpos(ses.readbuf, blocksize);

		/* decrypt it in-place */
		len = ses.readbuf->len - macsize - ses.readbuf->pos;
		if (ses.keys->recv.crypt_mode->decrypt(
					buf_getptr(ses.readbuf, len), 
					buf_getwriteptr(ses.readbuf, len),
					len,
					&ses.keys->recv.cipher_state) != CRYPT_OK) {
			dropbear_exit("Error decrypting");
		}
		buf_incrpos(ses.readbuf, len);

		/* check the hmac */
		if (checkmac() != DROPBEAR_SUCCESS) {
			dropbear_exit("Integrity error");
		}
	}

	/* get padding length */
	buf_setpos(ses.readbuf, PACKET_PADDING_OFF);
	padlen = buf_getbyte(ses.readbuf);
		
	/* payload length */
	/* - 4 - 1 is for LEN and PADLEN values */
	len = ses.readbuf->len - padlen - 4 - 1 - macsize;
	if ((len > RECV_MAX_PAYLOAD_LEN+ZLIB_COMPRESS_EXPANSION) || (len < 1)) {
		dropbear_exit("Bad packet size %u", len);
	}

	buf_setpos(ses.readbuf, PACKET_PAYLOAD_OFF);

#ifndef DISABLE_ZLIB
	if (is_compress_recv()) {
		/* decompress */
		ses.payload = buf_decompress(ses.readbuf, len);
		buf_setpos(ses.payload, 0);
		ses.payload_beginning = 0;
		buf_free(ses.readbuf);
	} else 
#endif
	{
		ses.payload = ses.readbuf;
		ses.payload_beginning = ses.payload->pos;
		buf_setlen(ses.payload, ses.payload->pos + len);
	}
	ses.readbuf = NULL;

	ses.recvseq++;

	TRACE2(("leave decrypt_packet"))
}

/* Checks the mac at the end of a decrypted readbuf.
 * Returns DROPBEAR_SUCCESS or DROPBEAR_FAILURE */
static int checkmac() {

	unsigned char mac_bytes[MAX_MAC_LEN];
	unsigned int mac_size, contents_len;
	
	mac_size = ses.keys->recv.algo_mac->hashsize;
	contents_len = ses.readbuf->len - mac_size;

	buf_setpos(ses.readbuf, 0);
	make_mac(ses.recvseq, &ses.keys->recv, ses.readbuf, contents_len, mac_bytes);

#if DROPBEAR_FUZZ
	if (fuzz.fuzzing) {
	 	/* fail 1 in 2000 times to test error path. */
		unsigned int value = 0;
		if (mac_size > sizeof(value)) {
			memcpy(&value, mac_bytes, sizeof(value));
		}
		if (value % 2000 == 99) {
			return DROPBEAR_FAILURE;
		}
		return DROPBEAR_SUCCESS;
	}
#endif

	/* compare the hash */
	buf_setpos(ses.readbuf, contents_len);
	if (constant_time_memcmp(mac_bytes, buf_getptr(ses.readbuf, mac_size), mac_size) != 0) {
		return DROPBEAR_FAILURE;
	} else {
		return DROPBEAR_SUCCESS;
	}
}

#ifndef DISABLE_ZLIB
/* returns a pointer to a newly created buffer */
static buffer* buf_decompress(const buffer* buf, unsigned int len) {

	int result;
	buffer * ret;
	z_streamp zstream;

	zstream = ses.keys->recv.zstream;
	ret = buf_new(len);

	zstream->avail_in = len;
	zstream->next_in = buf_getptr(buf, len);

	/* decompress the payload, incrementally resizing the output buffer */
	while (1) {

		zstream->avail_out = ret->size - ret->pos;
		zstream->next_out = buf_getwriteptr(ret, zstream->avail_out);

		result = inflate(zstream, Z_SYNC_FLUSH);

		buf_setlen(ret, ret->size - zstream->avail_out);
		buf_setpos(ret, ret->len);

		if (result != Z_BUF_ERROR && result != Z_OK) {
			dropbear_exit("zlib error");
		}

		if (zstream->avail_in == 0 &&
		   		(zstream->avail_out != 0 || result == Z_BUF_ERROR)) {
			/* we can only exit if avail_out hasn't all been used,
			 * and there's no remaining input */
			return ret;
		}

		if (zstream->avail_out == 0) {
			int new_size = 0;
			if (ret->size >= RECV_MAX_PAYLOAD_LEN) {
				/* Already been increased as large as it can go,
				 * yet didn't finish up the decompression */
				dropbear_exit("bad packet, oversized decompressed");
			}
			new_size = MIN(RECV_MAX_PAYLOAD_LEN, ret->size + ZLIB_DECOMPRESS_INCR);
			ret = buf_resize(ret, new_size);
		}
	}
}
#endif


/* returns 1 if the packet is a valid type during kex (see 7.1 of rfc4253) */
static int packet_is_okay_kex(unsigned char type) {
	if (type >= SSH_MSG_USERAUTH_REQUEST) {
		return 0;
	}
	if (type == SSH_MSG_SERVICE_REQUEST || type == SSH_MSG_SERVICE_ACCEPT) {
		return 0;
	}
	if (type == SSH_MSG_KEXINIT) {
		/* XXX should this die horribly if !dataallowed ?? */
		return 0;
	}
	return 1;
}

static void enqueue_reply_packet() {
	struct packetlist * new_item = NULL;
	new_item = m_malloc(sizeof(struct packetlist));
	new_item->next = NULL;
	
	new_item->payload = buf_newcopy(ses.writepayload);
	buf_setpos(ses.writepayload, 0);
	buf_setlen(ses.writepayload, 0);
	
	if (ses.reply_queue_tail) {
		ses.reply_queue_tail->next = new_item;
	} else {
		ses.reply_queue_head = new_item;
	}
	ses.reply_queue_tail = new_item;
}

void maybe_flush_reply_queue() {
	struct packetlist *tmp_item = NULL, *curr_item = NULL;
	if (!ses.dataallowed)
	{
		TRACE(("maybe_empty_reply_queue - no data allowed"))
		return;
	}
		
	for (curr_item = ses.reply_queue_head; curr_item; ) {
		CHECKCLEARTOWRITE();
		buf_putbytes(ses.writepayload,
			curr_item->payload->data, curr_item->payload->len);
			
		buf_free(curr_item->payload);
		tmp_item = curr_item;
		curr_item = curr_item->next;
		m_free(tmp_item);
		encrypt_packet();
	}
	ses.reply_queue_head = ses.reply_queue_tail = NULL;
}
	
/* encrypt the writepayload, putting into writebuf, ready for write_packet()
 * to put on the wire */
void encrypt_packet() {

	unsigned char padlen;
	unsigned char blocksize, mac_size;
	buffer * writebuf; /* the packet which will go on the wire. This is 
	                      encrypted in-place. */
	unsigned char packet_type;
	unsigned int len, encrypt_buf_size;
	unsigned char mac_bytes[MAX_MAC_LEN];

	time_t now;
	
	TRACE2(("enter encrypt_packet()"))

	buf_setpos(ses.writepayload, 0);
	packet_type = buf_getbyte(ses.writepayload);
	buf_setpos(ses.writepayload, 0);

	TRACE2(("encrypt_packet type is %d", packet_type))
	
	if ((!ses.dataallowed && !packet_is_okay_kex(packet_type))) {
		/* During key exchange only particular packets are allowed.
			Since this packet_type isn't OK we just enqueue it to send 
			after the KEX, see maybe_flush_reply_queue */
		enqueue_reply_packet();
		return;
	}
		
	blocksize = ses.keys->trans.algo_crypt->blocksize;
	mac_size = ses.keys->trans.algo_mac->hashsize;

	/* Encrypted packet len is payload+5. We need to then make sure
	 * there is enough space for padding or MIN_PACKET_LEN. 
	 * Add extra 3 since we need at least 4 bytes of padding */
	encrypt_buf_size = (ses.writepayload->len+4+1) 
		+ MAX(MIN_PACKET_LEN, blocksize) + 3
	/* add space for the MAC at the end */
				+ mac_size
#ifndef DISABLE_ZLIB
	/* some extra in case 'compression' makes it larger */
				+ ZLIB_COMPRESS_EXPANSION
#endif
	/* and an extra cleartext (stripped before transmission) byte for the
	 * packet type */
				+ 1;

	writebuf = buf_new(encrypt_buf_size);
	buf_setlen(writebuf, PACKET_PAYLOAD_OFF);
	buf_setpos(writebuf, PACKET_PAYLOAD_OFF);

#ifndef DISABLE_ZLIB
	/* compression */
	if (is_compress_trans()) {
		buf_compress(writebuf, ses.writepayload, ses.writepayload->len);
	} else
#endif
	{
		memcpy(buf_getwriteptr(writebuf, ses.writepayload->len),
				buf_getptr(ses.writepayload, ses.writepayload->len),
				ses.writepayload->len);
		buf_incrwritepos(writebuf, ses.writepayload->len);
	}

	/* finished with payload */
	buf_setpos(ses.writepayload, 0);
	buf_setlen(ses.writepayload, 0);

	/* length of padding - packet length excluding the packetlength uint32
	 * field in aead mode must be a multiple of blocksize, with a minimum of
	 * 4 bytes of padding */
	len = writebuf->len;
#if DROPBEAR_AEAD_MODE
	if (ses.keys->trans.crypt_mode->aead_crypt) {
		len -= 4;
	}
#endif
	padlen = blocksize - len % blocksize;
	if (padlen < 4) {
		padlen += blocksize;
	}
	/* check for min packet length */
	if (writebuf->len + padlen < MIN_PACKET_LEN) {
		padlen += blocksize;
	}

	buf_setpos(writebuf, 0);
	/* packet length excluding the packetlength uint32 */
	buf_putint(writebuf, writebuf->len + padlen - 4);

	/* padding len */
	buf_putbyte(writebuf, padlen);
	/* actual padding */
	buf_setpos(writebuf, writebuf->len);
	buf_incrlen(writebuf, padlen);
	genrandom(buf_getptr(writebuf, padlen), padlen);

#if DROPBEAR_AEAD_MODE
	if (ses.keys->trans.crypt_mode->aead_crypt) {
		/* do the actual encryption, in-place */
		buf_setpos(writebuf, 0);
		/* encrypt it in-place*/
		len = writebuf->len;
		buf_incrlen(writebuf, mac_size);
		if (ses.keys->trans.crypt_mode->aead_crypt(ses.transseq,
					buf_getptr(writebuf, len),
					buf_getwriteptr(writebuf, len + mac_size),
					len, mac_size,
					&ses.keys->trans.cipher_state, LTC_ENCRYPT) != CRYPT_OK) {
			dropbear_exit("Error encrypting");
		}
		buf_incrpos(writebuf, len + mac_size);
	} else
#endif
	{
		make_mac(ses.transseq, &ses.keys->trans, writebuf, writebuf->len, mac_bytes);

		/* do the actual encryption, in-place */
		buf_setpos(writebuf, 0);
		/* encrypt it in-place*/
		len = writebuf->len;
		if (ses.keys->trans.crypt_mode->encrypt(
					buf_getptr(writebuf, len),
					buf_getwriteptr(writebuf, len),
					len,
					&ses.keys->trans.cipher_state) != CRYPT_OK) {
			dropbear_exit("Error encrypting");
		}
		buf_incrpos(writebuf, len);

		/* stick the MAC on it */
		buf_putbytes(writebuf, mac_bytes, mac_size);
	}

	/* Update counts */
	ses.kexstate.datatrans += writebuf->len;

	writebuf_enqueue(writebuf);

	/* Update counts */
	ses.transseq++;

	now = monotonic_now();
	ses.last_packet_time_any_sent = now;
	/* idle timeout shouldn't be affected by responses to keepalives.
	send_msg_keepalive() itself also does tricks with 
	ses.last_packet_idle_time - read that if modifying this code */
	if (packet_type != SSH_MSG_REQUEST_FAILURE
		&& packet_type != SSH_MSG_UNIMPLEMENTED
		&& packet_type != SSH_MSG_IGNORE) {
		ses.last_packet_time_idle = now;

	}

	TRACE2(("leave encrypt_packet()"))
}

void writebuf_enqueue(buffer * writebuf) {
	/* enqueue the packet for sending. It will get freed after transmission. */
	buf_setpos(writebuf, 0);
	enqueue(&ses.writequeue, (void*)writebuf);
	ses.writequeue_len += writebuf->len;
}


/* Create the packet mac, and append H(seqno|clearbuf) to the output */
/* output_mac must have ses.keys->trans.algo_mac->hashsize bytes. */
static void make_mac(unsigned int seqno, const struct key_context_directional * key_state,
		buffer * clear_buf, unsigned int clear_len, 
		unsigned char *output_mac) {
	unsigned char seqbuf[4];
	unsigned long bufsize;
	hmac_state hmac;

	if (key_state->algo_mac->hashsize > 0) {
		/* calculate the mac */
		if (hmac_init(&hmac, 
					key_state->hash_index,
					key_state->mackey,
					key_state->algo_mac->keysize) != CRYPT_OK) {
			dropbear_exit("HMAC error");
		}
	
		/* sequence number */
		STORE32H(seqno, seqbuf);
		if (hmac_process(&hmac, seqbuf, 4) != CRYPT_OK) {
			dropbear_exit("HMAC error");
		}
	
		/* the actual contents */
		buf_setpos(clear_buf, 0);
		if (hmac_process(&hmac, 
					buf_getptr(clear_buf, clear_len),
					clear_len) != CRYPT_OK) {
			dropbear_exit("HMAC error");
		}
	
		bufsize = MAX_MAC_LEN;
		if (hmac_done(&hmac, output_mac, &bufsize) != CRYPT_OK) {
			dropbear_exit("HMAC error");
		}
	}
	TRACE2(("leave writemac"))
}

#ifndef DISABLE_ZLIB
/* compresses len bytes from src, outputting to dest (starting from the
 * respective current positions. dest must have sufficient space,
 * len+ZLIB_COMPRESS_EXPANSION */
static void buf_compress(buffer * dest, buffer * src, unsigned int len) {

	unsigned int endpos = src->pos + len;
	int result;

	TRACE2(("enter buf_compress"))

	dropbear_assert(dest->size - dest->pos >= len+ZLIB_COMPRESS_EXPANSION);

	ses.keys->trans.zstream->avail_in = endpos - src->pos;
	ses.keys->trans.zstream->next_in = 
		buf_getptr(src, ses.keys->trans.zstream->avail_in);

	ses.keys->trans.zstream->avail_out = dest->size - dest->pos;
	ses.keys->trans.zstream->next_out =
		buf_getwriteptr(dest, ses.keys->trans.zstream->avail_out);

	result = deflate(ses.keys->trans.zstream, Z_SYNC_FLUSH);

	buf_setpos(src, endpos - ses.keys->trans.zstream->avail_in);
	buf_setlen(dest, dest->size - ses.keys->trans.zstream->avail_out);
	buf_setpos(dest, dest->len);

	if (result != Z_OK) {
		dropbear_exit("zlib error");
	}

	/* fails if destination buffer wasn't large enough */
	dropbear_assert(ses.keys->trans.zstream->avail_in == 0);
	TRACE2(("leave buf_compress"))
}
#endif