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//==========================================================================

//

//      src/sys/netinet/ip_id.c

//

//==========================================================================

//####BSDCOPYRIGHTBEGIN####

//

// -------------------------------------------

//

// Portions of this software may have been derived from OpenBSD, 

// FreeBSD or other sources, and are covered by the appropriate

// copyright disclaimers included herein.

//

// Portions created by Red Hat are

// Copyright (C) 2002 Red Hat, Inc. All Rights Reserved.

//

// -------------------------------------------

//

//####BSDCOPYRIGHTEND####

//==========================================================================

/* $OpenBSD: ip_id.c,v 1.2 1999/08/26 13:37:01 provos Exp $ */

/*

 * Copyright 1998 Niels Provos <provos@citi.umich.edu>

 * All rights reserved.

 *

 * Theo de Raadt <deraadt@openbsd.org> came up with the idea of using

 * such a mathematical system to generate more random (yet non-repeating)

 * ids to solve the resolver/named problem.  But Niels designed the

 * actual system based on the constraints.

 *

 * 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.

 * 3. All advertising materials mentioning features or use of this software

 *    must display the following acknowledgement:

 *      This product includes software developed by Niels Provos.

 * 4. The name of the author may not be used to endorse or promote products

 *    derived from this software without specific prior written permission.

 *

 * 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.

 *

 * $FreeBSD: src/sys/netinet/ip_id.c,v 1.1.2.1 2001/07/19 06:37:26 kris Exp $

 */

/*

 * seed = random 15bit

 * n = prime, g0 = generator to n,

 * j = random so that gcd(j,n-1) == 1

 * g = g0^j mod n will be a generator again.

 *

 * X[0] = random seed.

 * X[n] = a*X[n-1]+b mod m is a Linear Congruential Generator

 * with a = 7^(even random) mod m,

 *      b = random with gcd(b,m) == 1

 *      m = 31104 and a maximal period of m-1.

 *

 * The transaction id is determined by:

 * id[n] = seed xor (g^X[n] mod n)

 *

 * Effectivly the id is restricted to the lower 15 bits, thus

 * yielding two different cycles by toggling the msb on and off.

 * This avoids reuse issues caused by reseeding.

 */

#include <sys/param.h>

#ifdef RANDOM_IP_ID

#define RU_OUT  180             /* Time after wich will be reseeded */

#define RU_MAX  30000           /* Uniq cycle, avoid blackjack prediction */

#define RU_GEN  2               /* Starting generator */

#define RU_N    32749           /* RU_N-1 = 2*2*3*2729 */

#define RU_AGEN 7               /* determine ru_a as RU_AGEN^(2*rand) */

#define RU_M    31104           /* RU_M = 2^7*3^5 - don't change */

#define PFAC_N 3

const static u_int16_t pfacts[PFAC_N] = {

        2,

        3,

        2729

};

static u_int16_t ru_x;

static u_int16_t ru_seed, ru_seed2;

static u_int16_t ru_a, ru_b;

static u_int16_t ru_g;

static u_int16_t ru_counter = 0;

static u_int16_t ru_msb = 0;

static long ru_reseed;

static u_int32_t tmp;           /* Storage for unused random */

static u_int16_t pmod __P((u_int16_t, u_int16_t, u_int16_t));

static void ip_initid __P((void));

u_int16_t ip_randomid __P((void));

/*

 * Do a fast modular exponation, returned value will be in the range

 * of 0 - (mod-1)

 */

#ifdef __STDC__

static u_int16_t

pmod(u_int16_t gen, u_int16_t exp, u_int16_t mod)

#else

static u_int16_t

pmod(gen, exp, mod)

        u_int16_t gen, exp, mod;

#endif

{

        u_int16_t s, t, u;

        s = 1;

        t = gen;

        u = exp;

        while (u) {

                if (u & 1)

                        s = (s*t) % mod;

                u >>= 1;

                t = (t*t) % mod;

        }

        return (s);

}

/*

 * Initalizes the seed and chooses a suitable generator. Also toggles

 * the msb flag. The msb flag is used to generate two distinct

 * cycles of random numbers and thus avoiding reuse of ids.

 *

 * This function is called from id_randomid() when needed, an

 * application does not have to worry about it.

 */

static void

ip_initid(void)

{

        u_int16_t j, i;

        int noprime = 1;

        struct timeval time;

        getmicrotime(&time);

        read_random((void *) &tmp, sizeof(tmp));

        ru_x = (tmp & 0xFFFF) % RU_M;

        /* 15 bits of random seed */

        ru_seed = (tmp >> 16) & 0x7FFF;

        read_random((void *) &tmp, sizeof(tmp));

        ru_seed2 = tmp & 0x7FFF;

        read_random((void *) &tmp, sizeof(tmp));

        /* Determine the LCG we use */

        ru_b = (tmp & 0xfffe) | 1;

        ru_a = pmod(RU_AGEN, (tmp >> 16) & 0xfffe, RU_M);

        while (ru_b % 3 == 0)

          ru_b += 2;

        read_random((void *) &tmp, sizeof(tmp));

        j = tmp % RU_N;

        tmp = tmp >> 16;

        /*

         * Do a fast gcd(j,RU_N-1), so we can find a j with

         * gcd(j, RU_N-1) == 1, giving a new generator for

         * RU_GEN^j mod RU_N

         */

        while (noprime) {

                for (i=0; i<PFAC_N; i++)

                        if (j%pfacts[i] == 0)

                                break;

                if (i>=PFAC_N)

                        noprime = 0;

                else

                        j = (j+1) % RU_N;

        }

        ru_g = pmod(RU_GEN,j,RU_N);

        ru_counter = 0;

        ru_reseed = time.tv_sec + RU_OUT;

        ru_msb = ru_msb == 0x8000 ? 0 : 0x8000;

}

u_int16_t

ip_randomid(void)

{

        int i, n;

        struct timeval time;

        getmicrotime(&time);

        if (ru_counter >= RU_MAX || time.tv_sec > ru_reseed)

                ip_initid();

        if (!tmp)

                read_random((void *) &tmp, sizeof(tmp));

        /* Skip a random number of ids */

        n = tmp & 0x3; tmp = tmp >> 2;

        if (ru_counter + n >= RU_MAX)

                ip_initid();

        for (i = 0; i <= n; i++)

                /* Linear Congruential Generator */

                ru_x = (ru_a*ru_x + ru_b) % RU_M;

        ru_counter += i;

        return (ru_seed ^ pmod(ru_g,ru_seed2 ^ ru_x,RU_N)) | ru_msb;

}

#endif /* RANDOM_IP_ID */