/*

 * Copyright (C) 2009-2012 Free Software Foundation, Inc.

 * Copyright (C) 2013 Nikos Mavrogiannopoulos

 *

 * Authors: Jonathan Bastien-Filiatrault

 *	  Nikos Mavrogiannopoulos

 *

 * This file is part of GNUTLS.

 *

 * The GNUTLS library is free software; you can redistribute it and/or

 * modify it under the terms of the GNU Lesser General Public License

 * as published by the Free Software Foundation; either version 2.1 of

 * the License, or (at your option) any later version.

 *

 * This library is distributed in the hope that it will be useful, but

 * WITHOUT ANY WARRANTY; without even the implied warranty of

 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU

 * Lesser General Public License for more details.

 *

 * You should have received a copy of the GNU Lesser General Public License

 * along with this program.  If not, see <https://www.gnu.org/licenses/>

 *

 */

/* Functions that relate to DTLS retransmission and reassembly.

 */

#include "gnutls_int.h"

#include "errors.h"

#include "debug.h"

#include "dtls.h"

#include "record.h"

#include <mbuffers.h>

#include <buffers.h>

#include <constate.h>

#include <state.h>

#include <gnutls/dtls.h>

#include <algorithms.h>

void _dtls_async_timer_delete(gnutls_session_t session)

{

	if (session->internals.dtls.async_term != 0) {

		_gnutls_dtls_log

		    ("DTLS[%p]: Deinitializing previous handshake state.\n",

		     session);

		session->internals.dtls.async_term = 0;	/* turn off "timer" */

		_dtls_reset_hsk_state(session);

		_gnutls_handshake_io_buffer_clear(session);

		_gnutls_epoch_gc(session);

	}

}

/* This function fragments and transmits a previously buffered

 * outgoing message. It accepts mtu_data which is a buffer to

 * be reused (should be set to NULL initially).

 */

static inline int

transmit_message(gnutls_session_t session,

		 mbuffer_st * bufel, uint8_t ** buf)

{

	uint8_t *data, *mtu_data;

	int ret = 0;

	unsigned int offset, frag_len, data_size;

	unsigned int mtu =

	    gnutls_dtls_get_data_mtu(session);

	if (session->security_parameters.max_record_send_size < mtu)

		mtu = session->security_parameters.max_record_send_size;

	mtu -= DTLS_HANDSHAKE_HEADER_SIZE;

	if (bufel->type == GNUTLS_CHANGE_CIPHER_SPEC) {

		_gnutls_dtls_log

		    ("DTLS[%p]: Sending Packet[%u] fragment %s(%d), mtu %u\n",

		     session, bufel->handshake_sequence,

		     _gnutls_handshake2str(bufel->htype), bufel->htype, mtu);

		return _gnutls_send_int(session, bufel->type, -1,

					bufel->epoch,

					_mbuffer_get_uhead_ptr(bufel),

					_mbuffer_get_uhead_size(bufel), 0);

	}

	if (*buf == NULL)

		*buf = gnutls_malloc(mtu + DTLS_HANDSHAKE_HEADER_SIZE);

	if (*buf == NULL)

		return gnutls_assert_val(GNUTLS_E_MEMORY_ERROR);

	mtu_data = *buf;

	data = _mbuffer_get_udata_ptr(bufel);

	data_size = _mbuffer_get_udata_size(bufel);

	/* Write fixed headers

	 */

	/* Handshake type */

	mtu_data[0] = (uint8_t) bufel->htype;

	/* Total length */

	_gnutls_write_uint24(data_size, &mtu_data[1]);

	/* Handshake sequence */

	_gnutls_write_uint16(bufel->handshake_sequence, &mtu_data[4]);

	/* Chop up and send handshake message into mtu-size pieces. */

	for (offset = 0; offset <= data_size; offset += mtu) {

		/* Calculate fragment length */

		if (offset + mtu > data_size)

			frag_len = data_size - offset;

		else

			frag_len = mtu;

		/* we normally allow fragments of zero length, to allow

		 * the packets which have zero size. On the others don't

		 * send such fragments */

		if (frag_len == 0 && data_size > 0) {

			ret = 0;

			break;

		}

		/* Fragment offset */

		_gnutls_write_uint24(offset, &mtu_data[6]);

		/* Fragment length */

		_gnutls_write_uint24(frag_len, &mtu_data[9]);

		memcpy(&mtu_data[DTLS_HANDSHAKE_HEADER_SIZE],

		       data + offset, frag_len);

		_gnutls_dtls_log

		    ("DTLS[%p]: Sending Packet[%u] fragment %s(%d) with "

		     "length: %u, offset: %u, fragment length: %u, mtu: %u\n",

		     session, bufel->handshake_sequence,

		     _gnutls_handshake2str(bufel->htype), bufel->htype,

		     data_size, offset, frag_len, mtu);

		ret = _gnutls_send_int(session, bufel->type, bufel->htype,

				       bufel->epoch, mtu_data,

				       DTLS_HANDSHAKE_HEADER_SIZE +

				       frag_len, 0);

		if (ret < 0) {

			gnutls_assert();

			break;

		}

	}

	return ret;

}

static int drop_usage_count(gnutls_session_t session,

			    mbuffer_head_st * const send_buffer)

{

	int ret;

	mbuffer_st *cur;

	for (cur = send_buffer->head; cur != NULL; cur = cur->next) {

		ret = _gnutls_epoch_refcount_dec(session, cur->epoch);

		if (ret < 0)

			return gnutls_assert_val(ret);

	}

	return 0;

}

/* Checks whether the received packet contains a handshake

 * packet with sequence higher that the previously received.

 * It must be called only when an actual packet has been

 * received.

 *

 * Returns: 0 if expected, negative value otherwise.

 */

static int is_next_hpacket_expected(gnutls_session_t session)

{

	int ret;

	/* htype is arbitrary */

	ret =

	    _gnutls_recv_in_buffers(session, GNUTLS_HANDSHAKE,

				    GNUTLS_HANDSHAKE_FINISHED, 0);

	if (ret < 0)

		return gnutls_assert_val(ret);

	ret = _gnutls_parse_record_buffered_msgs(session);

	if (ret < 0)

		return gnutls_assert_val(ret);

	if (session->internals.handshake_recv_buffer_size > 0)

		return 0;

	else

		return

		    gnutls_assert_val

		    (GNUTLS_E_UNEXPECTED_HANDSHAKE_PACKET);

}

void _dtls_reset_hsk_state(gnutls_session_t session)

{

	session->internals.dtls.flight_init = 0;

	drop_usage_count(session,

			 &session->internals.handshake_send_buffer);

	_mbuffer_head_clear(&session->internals.handshake_send_buffer);

}

#define UPDATE_TIMER { \

      session->internals.dtls.actual_retrans_timeout_ms *= 2; \

      session->internals.dtls.actual_retrans_timeout_ms %= MAX_DTLS_TIMEOUT; \

    }

#define RESET_TIMER \

      session->internals.dtls.actual_retrans_timeout_ms = session->internals.dtls.retrans_timeout_ms

#define TIMER_WINDOW session->internals.dtls.actual_retrans_timeout_ms

/* This function transmits the flight that has been previously

 * buffered.

 *

 * This function is called from the handshake layer and calls the

 * record layer.

 */

int _dtls_transmit(gnutls_session_t session)

{

	int ret;

	uint8_t *buf = NULL;

	unsigned int timeout;

	/* PREPARING -> SENDING state transition */

	mbuffer_head_st *const send_buffer =

	    &session->internals.handshake_send_buffer;

	mbuffer_st *cur;

	gnutls_handshake_description_t last_type = 0;

	unsigned int diff;

	struct timespec now;

	gnutls_gettime(&now;;

	/* If we have already sent a flight and we are operating in a 

	 * non blocking way, check if it is time to retransmit or just

	 * return.

	 */

	if (session->internals.dtls.flight_init != 0

	    && (session->internals.flags & GNUTLS_NONBLOCK)) {

		/* just in case previous run was interrupted */

		ret = _gnutls_io_write_flush(session);

		if (ret < 0) {

			gnutls_assert();

			goto cleanup;

		}

		if (session->internals.dtls.last_flight == 0

		    || !_dtls_is_async(session)) {

			/* check for ACK */

			ret = _gnutls_io_check_recv(session, 0);

			if (ret == GNUTLS_E_TIMEDOUT) {

				/* if no retransmission is required yet just return 

				 */

				if (timespec_sub_ms

				    (&now,

				     &session->internals.dtls.

				     last_retransmit) < TIMER_WINDOW) {

					gnutls_assert();

					goto nb_timeout;

				}

			} else {	/* received something */

				if (ret == 0) {

					ret =

					    is_next_hpacket_expected

					    (session);

					if (ret == GNUTLS_E_AGAIN

					    || ret == GNUTLS_E_INTERRUPTED)

						goto nb_timeout;

					if (ret < 0

					    && ret !=

					    GNUTLS_E_UNEXPECTED_HANDSHAKE_PACKET)

					{

						gnutls_assert();

						goto cleanup;

					}

					if (ret == 0)

						goto end_flight;

					/* if ret == GNUTLS_E_UNEXPECTED_HANDSHAKE_PACKET retransmit */

				} else

					goto nb_timeout;

			}

		}

	}

	do {

		timeout = TIMER_WINDOW;

		diff =

		    timespec_sub_ms(&now,

				    &session->internals.handshake_start_time);

		if (diff >= session->internals.handshake_timeout_ms) {

			_gnutls_dtls_log("Session timeout: %u ms\n", diff);

			ret = gnutls_assert_val(GNUTLS_E_TIMEDOUT);

			goto end_flight;

		}

		diff =

		    timespec_sub_ms(&now,

				    &session->internals.dtls.

				    last_retransmit);

		if (session->internals.dtls.flight_init == 0

		    || diff >= TIMER_WINDOW) {

			_gnutls_dtls_log

			    ("DTLS[%p]: %sStart of flight transmission.\n",

			     session,

			     (session->internals.dtls.flight_init ==

			      0) ? "" : "re-");

			for (cur = send_buffer->head; cur != NULL;

			     cur = cur->next) {

				ret = transmit_message(session, cur, &buf;;

				if (ret < 0) {

					gnutls_assert();

					goto end_flight;

				}

				last_type = cur->htype;

			}

			gnutls_gettime(&session->internals.dtls.last_retransmit);

			if (session->internals.dtls.flight_init == 0) {

				session->internals.dtls.flight_init = 1;

				RESET_TIMER;

				timeout = TIMER_WINDOW;

				if (last_type == GNUTLS_HANDSHAKE_FINISHED) {

					/* On the last flight we cannot ensure retransmission

					 * from here. _dtls_wait_and_retransmit() is being called

					 * by handshake.

					 */

					session->internals.dtls.

					    last_flight = 1;

				} else

					session->internals.dtls.

					    last_flight = 0;

			} else {

				UPDATE_TIMER;

			}

		}

		ret = _gnutls_io_write_flush(session);

		if (ret < 0) {

			ret = gnutls_assert_val(ret);

			goto cleanup;

		}

		/* last message in handshake -> no ack */

		if (session->internals.dtls.last_flight != 0) {

			/* we don't wait here. We just return 0 and

			 * if a retransmission occurs because peer didn't receive it

			 * we rely on the record or handshake

			 * layer calling this function again.

			 */

			ret = 0;

			goto cleanup;

		} else {	/* all other messages -> implicit ack (receive of next flight) */

			if (!(session->internals.flags & GNUTLS_NONBLOCK))

				ret =

				    _gnutls_io_check_recv(session,

							  timeout);

			else {

				ret = _gnutls_io_check_recv(session, 0);

				if (ret == GNUTLS_E_TIMEDOUT) {

					goto nb_timeout;

				}

			}

			if (ret == 0) {

				ret = is_next_hpacket_expected(session);

				if (ret == GNUTLS_E_AGAIN

				    || ret == GNUTLS_E_INTERRUPTED)

					goto nb_timeout;

				if (ret ==

				    GNUTLS_E_UNEXPECTED_HANDSHAKE_PACKET) {

					ret = GNUTLS_E_TIMEDOUT;

					goto keep_up;

				}

				if (ret < 0) {

					gnutls_assert();

					goto cleanup;

				}

				goto end_flight;

			}

		}

	      keep_up:

		gnutls_gettime(&now;;

	} while (ret == GNUTLS_E_TIMEDOUT);

	if (ret < 0) {

		ret = gnutls_assert_val(ret);

		goto end_flight;

	}

	ret = 0;

      end_flight:

	_gnutls_dtls_log("DTLS[%p]: End of flight transmission.\n",

			 session);

	_dtls_reset_hsk_state(session);

      cleanup:

	if (buf != NULL)

		gnutls_free(buf);

	/* SENDING -> WAITING state transition */

	return ret;

      nb_timeout:

	if (buf != NULL)

		gnutls_free(buf);

	RETURN_DTLS_EAGAIN_OR_TIMEOUT(session, ret);

}

/* Waits for the last flight or retransmits

 * the previous on timeout. Returns 0 on success.

 */

int _dtls_wait_and_retransmit(gnutls_session_t session)

{

	int ret;

	if (!(session->internals.flags & GNUTLS_NONBLOCK))

		ret = _gnutls_io_check_recv(session, TIMER_WINDOW);

	else

		ret = _gnutls_io_check_recv(session, 0);

	if (ret == GNUTLS_E_TIMEDOUT) {

		ret = _dtls_retransmit(session);

		if (ret == 0) {

			RETURN_DTLS_EAGAIN_OR_TIMEOUT(session, 0);

		} else

			return gnutls_assert_val(ret);

	}

	RESET_TIMER;

	return 0;

}

/**

 * gnutls_dtls_set_timeouts:

 * @session: is a #gnutls_session_t type.

 * @retrans_timeout: The time at which a retransmission will occur in milliseconds

 * @total_timeout: The time at which the connection will be aborted, in milliseconds.

 *

 * This function will set the timeouts required for the DTLS handshake

 * protocol. The retransmission timeout is the time after which a

 * message from the peer is not received, the previous messages will

 * be retransmitted. The total timeout is the time after which the

 * handshake will be aborted with %GNUTLS_E_TIMEDOUT.

 *

 * The DTLS protocol recommends the values of 1 sec and 60 seconds

 * respectively, and these are the default values.

 *

 * To disable retransmissions set a @retrans_timeout larger than the @total_timeout.

 *

 * Since: 3.0

 **/

void gnutls_dtls_set_timeouts(gnutls_session_t session,

			      unsigned int retrans_timeout,

			      unsigned int total_timeout)

{

	if (total_timeout == GNUTLS_INDEFINITE_TIMEOUT)

		session->internals.handshake_timeout_ms = 0;

	else

		session->internals.handshake_timeout_ms = total_timeout;

	session->internals.dtls.retrans_timeout_ms = retrans_timeout;

}

/**

 * gnutls_dtls_set_mtu:

 * @session: is a #gnutls_session_t type.

 * @mtu: The maximum transfer unit of the transport

 *

 * This function will set the maximum transfer unit of the transport

 * that DTLS packets are sent over. Note that this should exclude

 * the IP (or IPv6) and UDP headers. So for DTLS over IPv6 on an

 * Ethernet device with MTU 1500, the DTLS MTU set with this function

 * would be 1500 - 40 (IPV6 header) - 8 (UDP header) = 1452.

 *

 * Since: 3.0

 **/

void gnutls_dtls_set_mtu(gnutls_session_t session, unsigned int mtu)

{

	session->internals.dtls.mtu = MIN(mtu, DEFAULT_MAX_RECORD_SIZE);

}

/* when max is non-zero this function will return the maximum

 * overhead that this ciphersuite may introduce, e.g., the maximum

 * amount of padding required */

unsigned _gnutls_record_overhead(const version_entry_st *ver,

				 const cipher_entry_st *cipher,

				 const mac_entry_st *mac,

				 unsigned max)

{

	int total = 0;

	int ret;

	int hash_len = 0;

	if (unlikely(cipher == NULL))

		return 0;

	/* 1 octet content type in the unencrypted content */

	if (ver->tls13_sem)

		total++;

	if (mac->id == GNUTLS_MAC_AEAD) {

		if (!ver->tls13_sem)

			total += _gnutls_cipher_get_explicit_iv_size(cipher);

		total += _gnutls_cipher_get_tag_size(cipher);

	} else {

		/* STREAM + BLOCK have a MAC appended */

		ret = _gnutls_mac_get_algo_len(mac);

		if (unlikely(ret < 0))

			return 0;

		hash_len = ret;

		total += hash_len;

	}

	/* Block ciphers have padding + IV */

	if (_gnutls_cipher_type(cipher) == CIPHER_BLOCK) {

		int exp_iv;

		exp_iv = _gnutls_cipher_get_explicit_iv_size(cipher);

		if (max)

			total += 2*exp_iv; /* block == iv size */

		else

			total += exp_iv + 1;

	}

	return total;

}

/**

 * gnutls_est_record_overhead_size:

 * @version: is a #gnutls_protocol_t value

 * @cipher: is a #gnutls_cipher_algorithm_t value

 * @mac: is a #gnutls_mac_algorithm_t value

 * @comp: is a #gnutls_compression_method_t value (ignored)

 * @flags: must be zero

 *

 * This function will return the set size in bytes of the overhead

 * due to TLS (or DTLS) per record.

 *

 * Note that this function may provide inacurate values when TLS

 * extensions that modify the record format are negotiated. In these

 * cases a more accurate value can be obtained using gnutls_record_overhead_size() 

 * after a completed handshake.

 *

 * Since: 3.2.2

 **/

size_t gnutls_est_record_overhead_size(gnutls_protocol_t version,

				       gnutls_cipher_algorithm_t cipher,

				       gnutls_mac_algorithm_t mac,

				       gnutls_compression_method_t comp,

				       unsigned int flags)

{

	const cipher_entry_st *c;

	const mac_entry_st *m;

	const version_entry_st *v;

	size_t total = 0;

	c = cipher_to_entry(cipher);

	if (c == NULL)

		return 0;

	m = mac_to_entry(mac);

	if (m == NULL)

		return 0;

	v = version_to_entry(version);

	if (v == NULL)

		return 0;

	if (v->transport == GNUTLS_STREAM)

		total = TLS_RECORD_HEADER_SIZE;

	else

		total = DTLS_RECORD_HEADER_SIZE;

	total += _gnutls_record_overhead(v, c, m, 1);

	return total;

}

/* returns overhead imposed by the record layer (encryption/compression)

 * etc. It does not include the record layer headers, since the caller

 * needs to cope with rounding to multiples of blocksize, and the header

 * is outside that.

 *

 * blocksize: will contain the block size when padding may be required or 1

 *

 * It may return a negative error code on error.

 */

static int record_overhead_rt(gnutls_session_t session)

{

	record_parameters_st *params;

	int ret;

	if (session->internals.initial_negotiation_completed == 0)

		return GNUTLS_E_INVALID_REQUEST;

	ret = _gnutls_epoch_get(session, EPOCH_WRITE_CURRENT, &params);

	if (ret < 0)

		return gnutls_assert_val(ret);

	return _gnutls_record_overhead(get_version(session), params->cipher, params->mac, 1);

}

/**

 * gnutls_record_overhead_size:

 * @session: is #gnutls_session_t

 *

 * This function will return the size in bytes of the overhead

 * due to TLS (or DTLS) per record. On certain occasions

 * (e.g., CBC ciphers) the returned value is the maximum

 * possible overhead.

 *

 * Since: 3.2.2

 **/

size_t gnutls_record_overhead_size(gnutls_session_t session)

{

	const version_entry_st *v = get_version(session);

	int ret;

	size_t total;

	if (v->transport == GNUTLS_STREAM)

		total = TLS_RECORD_HEADER_SIZE;

	else

		total = DTLS_RECORD_HEADER_SIZE;

	ret = record_overhead_rt(session);

	if (ret >= 0)

		total += ret;

	return total;

}

/**

 * gnutls_dtls_get_data_mtu:

 * @session: is a #gnutls_session_t type.

 *

 * This function will return the actual maximum transfer unit for

 * application data. I.e. DTLS headers are subtracted from the

 * actual MTU which is set using gnutls_dtls_set_mtu().

 *

 * Returns: the maximum allowed transfer unit.

 *

 * Since: 3.0

 **/

unsigned int gnutls_dtls_get_data_mtu(gnutls_session_t session)

{

	int mtu = session->internals.dtls.mtu;

	record_parameters_st *params;

	int ret, k, hash_size, block;

	mtu -= RECORD_HEADER_SIZE(session);

	if (session->internals.initial_negotiation_completed == 0)

		return mtu;

	ret = _gnutls_epoch_get(session, EPOCH_WRITE_CURRENT, &params);

	if (ret < 0)

		return mtu;

	if (params->cipher->type == CIPHER_AEAD || params->cipher->type == CIPHER_STREAM)

		return mtu-_gnutls_record_overhead(get_version(session), params->cipher, params->mac, 0);

	/* CIPHER_BLOCK: in CBC ciphers guess the data MTU as it depends on residues

	 */

	hash_size = _gnutls_mac_get_algo_len(params->mac);

	block = _gnutls_cipher_get_explicit_iv_size(params->cipher);

	assert(_gnutls_cipher_get_block_size(params->cipher) == block);

	if (params->etm) {

		/* the maximum data mtu satisfies:

		 * data mtu (mod block) = block-1

		 * or data mtu = (k+1)*(block) - 1

	         *

		 * and data mtu + block + hash size + 1 = link_mtu

		 *     (k+2) * (block) + hash size = link_mtu

		 *

		 *     We try to find k, and thus data mtu

		 */

		k = ((mtu-hash_size)/block) - 2;

		return (k+1)*block - 1;

	} else {

		/* the maximum data mtu satisfies:

		 * data mtu + hash size (mod block) = block-1

		 * or data mtu = (k+1)*(block) - hash size - 1

		 *

		 * and data mtu + block + hash size + 1 = link_mtu

		 *     (k+2) * (block) = link_mtu

		 *

		 *     We try to find k, and thus data mtu

		 */

		k = ((mtu)/block) - 2;

		return (k+1)*block - hash_size - 1;

	}

}

/**

 * gnutls_dtls_set_data_mtu:

 * @session: is a #gnutls_session_t type.

 * @mtu: The maximum unencrypted transfer unit of the session

 *

 * This function will set the maximum size of the *unencrypted* records

 * which will be sent over a DTLS session. It is equivalent to calculating

 * the DTLS packet overhead with the current encryption parameters, and

 * calling gnutls_dtls_set_mtu() with that value. In particular, this means

 * that you may need to call this function again after any negotiation or

 * renegotiation, in order to ensure that the MTU is still sufficient to

 * account for the new protocol overhead.

 *

 * In most cases you only need to call gnutls_dtls_set_mtu() with

 * the maximum MTU of your transport layer.

 *

 * Returns: %GNUTLS_E_SUCCESS (0) on success, or a negative error code.

 *

 * Since: 3.1

 **/

int gnutls_dtls_set_data_mtu(gnutls_session_t session, unsigned int mtu)

{

	int overhead;

	overhead = record_overhead_rt(session);

	/* You can't call this until the session is actually running */

	if (overhead < 0)

		return GNUTLS_E_INVALID_SESSION;

	/* Add the overhead inside the encrypted part */

	mtu += overhead;

	/* Add the *unencrypted header size */

	mtu += RECORD_HEADER_SIZE(session);

	gnutls_dtls_set_mtu(session, mtu);

	return GNUTLS_E_SUCCESS;

}

/**

 * gnutls_dtls_get_mtu:

 * @session: is a #gnutls_session_t type.

 *

 * This function will return the MTU size as set with

 * gnutls_dtls_set_mtu(). This is not the actual MTU

 * of data you can transmit. Use gnutls_dtls_get_data_mtu()

 * for that reason.

 *

 * Returns: the set maximum transfer unit.

 *

 * Since: 3.0

 **/

unsigned int gnutls_dtls_get_mtu(gnutls_session_t session)

{

	return session->internals.dtls.mtu;

}

/**

 * gnutls_dtls_get_timeout:

 * @session: is a #gnutls_session_t type.

 *

 * This function will return the milliseconds remaining

 * for a retransmission of the previously sent handshake

 * message. This function is useful when DTLS is used in

 * non-blocking mode, to estimate when to call gnutls_handshake()

 * if no packets have been received.

 *

 * Returns: the remaining time in milliseconds.

 *

 * Since: 3.0

 **/

unsigned int gnutls_dtls_get_timeout(gnutls_session_t session)

{

	struct timespec now;

	unsigned int diff;

	gnutls_gettime(&now;;

	diff =

	    timespec_sub_ms(&now,

			    &session->internals.dtls.last_retransmit);

	if (diff >= TIMER_WINDOW)

		return 0;

	else

		return TIMER_WINDOW - diff;

}

#define COOKIE_SIZE 16

#define COOKIE_MAC_SIZE 16

/*   MAC

 * 16 bytes

 *

 * total 19 bytes

 */

#define C_HASH GNUTLS_MAC_SHA1

#define C_HASH_SIZE 20

/**

 * gnutls_dtls_cookie_send:

 * @key: is a random key to be used at cookie generation

 * @client_data: contains data identifying the client (i.e. address)

 * @client_data_size: The size of client's data

 * @prestate: The previous cookie returned by gnutls_dtls_cookie_verify()

 * @ptr: A transport pointer to be used by @push_func

 * @push_func: A function that will be used to reply

 *

 * This function can be used to prevent denial of service

 * attacks to a DTLS server by requiring the client to

 * reply using a cookie sent by this function. That way

 * it can be ensured that a client we allocated resources

 * for (i.e. #gnutls_session_t) is the one that the 

 * original incoming packet was originated from.

 *

 * This function must be called at the first incoming packet,

 * prior to allocating any resources and must be succeeded

 * by gnutls_dtls_cookie_verify().

 *

 * Returns: the number of bytes sent, or a negative error code.  

 *

 * Since: 3.0

 **/

int gnutls_dtls_cookie_send(gnutls_datum_t * key, void *client_data,

			    size_t client_data_size,

			    gnutls_dtls_prestate_st * prestate,

			    gnutls_transport_ptr_t ptr,

			    gnutls_push_func push_func)

{

	uint8_t hvr[20 + DTLS_HANDSHAKE_HEADER_SIZE + COOKIE_SIZE];

	int hvr_size = 0, ret;

	uint8_t digest[C_HASH_SIZE];

	if (key == NULL || key->data == NULL || key->size == 0)

		return gnutls_assert_val(GNUTLS_E_INVALID_REQUEST);

/* send

 *  struct {

 *    ContentType type - 1 byte GNUTLS_HANDSHAKE;

 *    ProtocolVersion version; - 2 bytes (254,255)

 *    uint16 epoch; - 2 bytes (0, 0)

 *    uint48 sequence_number; - 4 bytes (0,0,0,0)

 *    uint16 length; - 2 bytes (COOKIE_SIZE+1+2)+DTLS_HANDSHAKE_HEADER_SIZE

 *    uint8_t fragment[DTLSPlaintext.length];

 *  } DTLSPlaintext;

 *

 *

 * struct {

 *    HandshakeType msg_type; 1 byte - GNUTLS_HANDSHAKE_HELLO_VERIFY_REQUEST

 *    uint24 length; - COOKIE_SIZE+3

 *    uint16 message_seq; - 2 bytes (0,0)

 *    uint24 fragment_offset; - 3 bytes (0,0,0)

 *    uint24 fragment_length; - same as length

 * }

 *

 * struct {

 *   ProtocolVersion server_version;

 *   uint8_t cookie<0..32>;

 * } HelloVerifyRequest;

 */

	hvr[hvr_size++] = GNUTLS_HANDSHAKE;

	/* version */

	hvr[hvr_size++] = 254;

	hvr[hvr_size++] = 255;