/*

 * Copyright (C) 2016 Red Hat, Inc.

 *

 * Authors: Fridolin Pokorny

 *	  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 sliding window handling.

 */

#ifndef DTLS_SW_NO_INCLUDES

#include "gnutls_int.h"

#include "errors.h"

#include "debug.h"

#include "dtls.h"

#include "record.h"

#endif

/*

 * DTLS sliding window handling

 */

#define DTLS_EPOCH_SHIFT		(6*CHAR_BIT)

#define DTLS_SEQ_NUM_MASK		0x0000FFFFFFFFFFFF

#define DTLS_EMPTY_BITMAP		(0xFFFFFFFFFFFFFFFFULL)

void _dtls_reset_window(struct record_parameters_st *rp)

{

	rp->dtls_sw_have_recv = 0;

}

/* Checks if a sequence number is not replayed. If a replayed

 * packet is detected it returns a negative value (but no sensible error code).

 * Otherwise zero.

 */

int _dtls_record_check(struct record_parameters_st *rp, uint64_t seq_num)

{

	if ((seq_num >> DTLS_EPOCH_SHIFT) != rp->epoch) {

		return gnutls_assert_val(-1);

	}

	seq_num &= DTLS_SEQ_NUM_MASK;

	/*

	 * rp->dtls_sw_next is the next *expected* packet (N), being

	 * the sequence number *after* the latest we have received.

	 *

	 * By definition, therefore, packet N-1 *has* been received.

	 * And thus there's no point wasting a bit in the bitmap for it.

	 *

	 * So the backlog bitmap covers the 64 packets prior to that,

	 * with the LSB representing packet (N - 2), and the MSB

	 * representing (N - 65). A received packet is represented

	 * by a zero bit, and a missing packet is represented by a one.

	 *

	 * Thus we can allow out-of-order reception of packets that are

	 * within a reasonable interval of the latest packet received.

	 */

	if (!rp->dtls_sw_have_recv) {

		rp->dtls_sw_next = seq_num + 1;

		rp->dtls_sw_bits = DTLS_EMPTY_BITMAP;

		rp->dtls_sw_have_recv = 1;

		return 0;

	} else if (seq_num == rp->dtls_sw_next) {

		/* The common case. This is the packet we expected next. */

		rp->dtls_sw_bits <<= 1;

		/* This might reach a value higher than 48-bit DTLS sequence

		 * numbers can actually reach. Which is fine. When that

		 * happens, we'll do the right thing and just not accept

		 * any newer packets. Someone needs to start a new epoch. */

		rp->dtls_sw_next++;

		return 0;

	} else if (seq_num > rp->dtls_sw_next) {

		/* The packet we were expecting has gone missing; this one is newer.

		 * We always advance the window to accommodate it. */

		uint64_t delta = seq_num - rp->dtls_sw_next;

		if (delta >= 64) {

			/* We jumped a long way into the future. We have not seen

			 * any of the previous 32 packets so set the backlog bitmap

			 * to all ones. */

			rp->dtls_sw_bits = DTLS_EMPTY_BITMAP;

		} else if (delta == 63) {

			/* Avoid undefined behaviour that shifting by 64 would incur.

			 * The (clear) top bit represents the packet which is currently

			 * rp->dtls_sw_next, which we know was already received. */

			rp->dtls_sw_bits = DTLS_EMPTY_BITMAP >> 1;

		} else {

			/* We have missed (delta) packets. Shift the backlog by that

			 * amount *plus* the one we would have shifted it anyway if

			 * we'd received the packet we were expecting. The zero bit

			 * representing the packet which is currently rp->dtls_sw_next-1,

			 * which we know has been received, ends up at bit position

			 * (1<

			 * represent the missing packets. */

			rp->dtls_sw_bits <<= delta + 1;

			rp->dtls_sw_bits |= (1ULL << delta) - 1;

		}

		rp->dtls_sw_next = seq_num + 1;

		return 0;

	} else {

		/* This packet is older than the one we were expecting. By how much...? */

		uint64_t delta = rp->dtls_sw_next - seq_num;

		if (delta > 65) {

			/* Too old. We can't know if it's a replay */

			return gnutls_assert_val(-2);

		} else if (delta == 1) {

			/* Not in the bitmask since it is by definition already received. */

			return gnutls_assert_val(-3);

		} else {

			/* Within the sliding window, so we remember whether we've seen it or not */

			uint64_t mask = 1ULL << (rp->dtls_sw_next - seq_num - 2);

			if (!(rp->dtls_sw_bits & mask))

				return gnutls_assert_val(-3);

			rp->dtls_sw_bits &= ~mask;

			return 0;

		}

	}

}