/*  Copyright (C) 2018 CZ.NIC, z.s.p.o. <knot-dns@labs.nic.cz>

    This program is free software: you can redistribute it and/or modify
    it under the terms of the GNU General Public License as published by
    the Free Software Foundation, either version 3 of the License, or
    (at your option) any later version.

    This program 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 General Public License for more details.

    You should have received a copy of the GNU General Public License
    along with this program.  If not, see <https://www.gnu.org/licenses/>.
 */

#include <assert.h>
#include <stdint.h>
#include <string.h>

#include "libdnssec/binary.h"
#include "libdnssec/error.h"
#include "libdnssec/key/dnskey.h"
#include "libdnssec/shared/shared.h"

/*!
 * Compute keytag for RSA/MD5 key.
 *
 * \see RFC 2537 (section 2), RFC 4034 (appendix B.1)
 */
static uint16_t keytag_compat(const dnssec_binary_t *rdata)
{
	assert(rdata);
	assert(rdata->data);

	if (rdata->size < 9) { // in fact, the condition could be stricter
		return 0;
	}

	uint8_t msb = rdata->data[rdata->size - 3];
	uint8_t lsb = rdata->data[rdata->size - 2];

	return (msb << 8) + lsb;
}

/*!
 * Compute keytag for other than RSA/MD5 key.
 *
 * \see RFC 4034 (appendix B)
 */
static uint16_t keytag_current(const dnssec_binary_t *rdata)
{
	assert(rdata);
	assert(rdata->data);

	uint32_t ac = 0;
	for (int i = 0; i < rdata->size; i++) {
		ac += (i & 1) ? rdata->data[i] : rdata->data[i] << 8;
	}

	return (ac >> 16) + ac;
}

/* -- public API ----------------------------------------------------------- */

/*!
 * Compute keytag for a DNSSEC key.
 */
_public_
int dnssec_keytag(const dnssec_binary_t *rdata, uint16_t *keytag)
{
	if (!rdata || !keytag) {
		return DNSSEC_EINVAL;
	}

	if (!rdata->data || rdata->size < DNSKEY_RDATA_OFFSET_PUBKEY) {
		return DNSSEC_MALFORMED_DATA;
	}

	uint8_t algorithm = rdata->data[DNSKEY_RDATA_OFFSET_ALGORITHM];
	if (algorithm == 1) {
		*keytag = keytag_compat(rdata);
	} else {
		*keytag = keytag_current(rdata);
	}

	return DNSSEC_EOK;
}