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FreeBSD/Linux Kernel Cross Reference
sys/netinet/ip_id.c

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    1 /* $OpenBSD: ip_id.c,v 1.2 1999/08/26 13:37:01 provos Exp $ */
    2 
    3 /*-
    4  * Copyright 1998 Niels Provos <provos@citi.umich.edu>
    5  * All rights reserved.
    6  *
    7  * Theo de Raadt <deraadt@openbsd.org> came up with the idea of using
    8  * such a mathematical system to generate more random (yet non-repeating)
    9  * ids to solve the resolver/named problem.  But Niels designed the
   10  * actual system based on the constraints.
   11  *
   12  * Redistribution and use in source and binary forms, with or without
   13  * modification, are permitted provided that the following conditions
   14  * are met:
   15  * 1. Redistributions of source code must retain the above copyright
   16  *    notice, this list of conditions and the following disclaimer.
   17  * 2. Redistributions in binary form must reproduce the above copyright
   18  *    notice, this list of conditions and the following disclaimer in the
   19  *    documentation and/or other materials provided with the distribution.
   20  * 3. All advertising materials mentioning features or use of this software
   21  *    must display the following acknowledgement:
   22  *      This product includes software developed by Niels Provos.
   23  * 4. The name of the author may not be used to endorse or promote products
   24  *    derived from this software without specific prior written permission.
   25  *
   26  * THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR
   27  * IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES
   28  * OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED.
   29  * IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT,
   30  * INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT
   31  * NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
   32  * DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
   33  * THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
   34  * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF
   35  * THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
   36  */
   37 
   38 /*-
   39  * seed = random 15bit
   40  * n = prime, g0 = generator to n,
   41  * j = random so that gcd(j,n-1) == 1
   42  * g = g0^j mod n will be a generator again.
   43  *
   44  * X[0] = random seed.
   45  * X[n] = a*X[n-1]+b mod m is a Linear Congruential Generator
   46  * with a = 7^(even random) mod m,
   47  *      b = random with gcd(b,m) == 1
   48  *      m = 31104 and a maximal period of m-1.
   49  *
   50  * The transaction id is determined by:
   51  * id[n] = seed xor (g^X[n] mod n)
   52  *
   53  * Effectivly the id is restricted to the lower 15 bits, thus yielding two
   54  * different cycles by toggling the msb on and off.  This avoids reuse issues
   55  * caused by reseeding.
   56  */
   57 
   58 #include <sys/cdefs.h>
   59 __FBSDID("$FreeBSD$");
   60 
   61 #include "opt_pf.h"
   62 #include <sys/param.h>
   63 #include <sys/time.h>
   64 #include <sys/kernel.h>
   65 #include <sys/random.h>
   66 
   67 #define RU_OUT  180             /* Time after wich will be reseeded */
   68 #define RU_MAX  30000           /* Uniq cycle, avoid blackjack prediction */
   69 #define RU_GEN  2               /* Starting generator */
   70 #define RU_N    32749           /* RU_N-1 = 2*2*3*2729 */
   71 #define RU_AGEN 7               /* determine ru_a as RU_AGEN^(2*rand) */
   72 #define RU_M    31104           /* RU_M = 2^7*3^5 - don't change */
   73 
   74 #define PFAC_N 3
   75 const static u_int16_t pfacts[PFAC_N] = {
   76         2,
   77         3,
   78         2729
   79 };
   80 
   81 static u_int16_t ru_x;
   82 static u_int16_t ru_seed, ru_seed2;
   83 static u_int16_t ru_a, ru_b;
   84 static u_int16_t ru_g;
   85 static u_int16_t ru_counter = 0;
   86 static u_int16_t ru_msb = 0;
   87 static long ru_reseed;
   88 static u_int32_t tmp;           /* Storage for unused random */
   89 
   90 static u_int16_t        pmod(u_int16_t, u_int16_t, u_int16_t);
   91 static void             ip_initid(void);
   92 u_int16_t               ip_randomid(void);
   93 
   94 /*
   95  * Do a fast modular exponation, returned value will be in the range of 0 -
   96  * (mod-1).
   97  */
   98 
   99 static u_int16_t
  100 pmod(u_int16_t gen, u_int16_t exp, u_int16_t mod)
  101 {
  102         u_int16_t s, t, u;
  103 
  104         s = 1;
  105         t = gen;
  106         u = exp;
  107 
  108         while (u) {
  109                 if (u & 1)
  110                         s = (s*t) % mod;
  111                 u >>= 1;
  112                 t = (t*t) % mod;
  113         }
  114         return (s);
  115 }
  116 
  117 /*
  118  * Initalizes the seed and chooses a suitable generator.  Also toggles the
  119  * msb flag.  The msb flag is used to generate two distinct cycles of random
  120  * numbers and thus avoiding reuse of ids.
  121  *
  122  * This function is called from id_randomid() when needed, an application
  123  * does not have to worry about it.
  124  */
  125 static void
  126 ip_initid(void)
  127 {
  128         u_int16_t j, i;
  129         int noprime = 1;
  130         struct timeval time;
  131 
  132         getmicrotime(&time);
  133         read_random((void *) &tmp, sizeof(tmp));
  134         ru_x = (tmp & 0xFFFF) % RU_M;
  135 
  136         /* 15 bits of random seed. */
  137         ru_seed = (tmp >> 16) & 0x7FFF;
  138         read_random((void *) &tmp, sizeof(tmp));
  139         ru_seed2 = tmp & 0x7FFF;
  140 
  141         read_random((void *) &tmp, sizeof(tmp));
  142 
  143         /* Determine the LCG we use. */
  144         ru_b = (tmp & 0xfffe) | 1;
  145         ru_a = pmod(RU_AGEN, (tmp >> 16) & 0xfffe, RU_M);
  146         while (ru_b % 3 == 0)
  147           ru_b += 2;
  148 
  149         read_random((void *) &tmp, sizeof(tmp));
  150         j = tmp % RU_N;
  151         tmp = tmp >> 16;
  152 
  153         /*
  154          * Do a fast gcd(j,RU_N-1), so we can find a j with gcd(j, RU_N-1) ==
  155          * 1, giving a new generator for RU_GEN^j mod RU_N.
  156          */
  157         while (noprime) {
  158                 for (i=0; i<PFAC_N; i++)
  159                         if (j%pfacts[i] == 0)
  160                                 break;
  161 
  162                 if (i>=PFAC_N)
  163                         noprime = 0;
  164                 else
  165                         j = (j+1) % RU_N;
  166         }
  167 
  168         ru_g = pmod(RU_GEN,j,RU_N);
  169         ru_counter = 0;
  170 
  171         ru_reseed = time.tv_sec + RU_OUT;
  172         ru_msb = ru_msb == 0x8000 ? 0 : 0x8000;
  173 }
  174 
  175 u_int16_t
  176 ip_randomid(void)
  177 {
  178         int i, n;
  179         struct timeval time;
  180 
  181         /* XXX not reentrant */
  182         getmicrotime(&time);
  183         if (ru_counter >= RU_MAX || time.tv_sec > ru_reseed)
  184                 ip_initid();
  185 
  186         if (!tmp)
  187                 read_random((void *) &tmp, sizeof(tmp));
  188 
  189         /* Skip a random number of ids. */
  190         n = tmp & 0x3; tmp = tmp >> 2;
  191         if (ru_counter + n >= RU_MAX)
  192                 ip_initid();
  193 
  194         for (i = 0; i <= n; i++)
  195                 /* Linear Congruential Generator. */
  196                 ru_x = (ru_a*ru_x + ru_b) % RU_M;
  197 
  198         ru_counter += i;
  199 
  200         return (ru_seed ^ pmod(ru_g,ru_seed2 ^ ru_x,RU_N)) | ru_msb;
  201 }

Cache object: 510e90ea428a084ec25848a6b624aad0


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