sys/netinet/ip_id.c

   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  * $FreeBSD: src/sys/netinet/ip_id.c,v 1.1.2.1 2001/07/19 06:37:26 kris Exp $
  38  * $DragonFly: src/sys/netinet/ip_id.c,v 1.6 2006/01/14 11:33:50 swildner Exp $
  39  */
  40
  41 /*
  42  * seed = random 15bit
  43  * n = prime, g0 = generator to n,
  44  * j = random so that gcd(j,n-1) == 1
  45  * g = g0^j mod n will be a generator again.
  46  *
  47  * X[0] = random seed.
  48  * X[n] = a*X[n-1]+b mod m is a Linear Congruential Generator
  49  * with a = 7^(even random) mod m,
  50  *      b = random with gcd(b,m) == 1
  51  *      m = 31104 and a maximal period of m-1.
  52  *
  53  * The transaction id is determined by:
  54  * id[n] = seed xor (g^X[n] mod n)
  55  *
  56  * Effectivly the id is restricted to the lower 15 bits, thus
  57  * yielding two different cycles by toggling the msb on and off.
  58  * This avoids reuse issues caused by reseeding.
  59  */
  60
  61 #include <sys/param.h>
  62 #include <sys/time.h>
  63 #include <sys/kernel.h>
  64 #include <sys/random.h>
  65
  66 #define RU_OUT  180             /* Time after wich will be reseeded */
  67 #define RU_MAX  30000           /* Uniq cycle, avoid blackjack prediction */
  68 #define RU_GEN  2               /* Starting generator */
  69 #define RU_N    32749           /* RU_N-1 = 2*2*3*2729 */
  70 #define RU_AGEN 7               /* determine ru_a as RU_AGEN^(2*rand) */
  71 #define RU_M    31104           /* RU_M = 2^7*3^5 - don't change */
  72
  73 #define PFAC_N 3
  74 const static u_int16_t pfacts[PFAC_N] = {
  75         2,
  76         3,
  77         2729
  78 };
  79
  80 static u_int16_t ru_x;
  81 static u_int16_t ru_seed, ru_seed2;
  82 static u_int16_t ru_a, ru_b;
  83 static u_int16_t ru_g;
  84 static u_int16_t ru_counter = 0;
  85 static u_int16_t ru_msb = 0;
  86 static long ru_reseed;
  87 static u_int32_t tmp;           /* Storage for unused random */
  88
  89 static u_int16_t pmod (u_int16_t, u_int16_t, u_int16_t);
  90 static void ip_initid (void);
  91 u_int16_t ip_randomid (void);
  92
  93 /*
  94  * Do a fast modular exponation, returned value will be in the range
  95  * of 0 - (mod-1)
  96  */
  97 static u_int16_t
  98 pmod(u_int16_t gen, u_int16_t exp, u_int16_t mod)
  99 {
 100         u_int16_t s, t, u;
 101
 102         s = 1;
 103         t = gen;
 104         u = exp;
 105
 106         while (u) {
 107                 if (u & 1)
 108                         s = (s*t) % mod;
 109                 u >>= 1;
 110                 t = (t*t) % mod;
 111         }
 112         return (s);
 113 }
 114
 115 /*
 116  * Initalizes the seed and chooses a suitable generator. Also toggles
 117  * the msb flag. The msb flag is used to generate two distinct
 118  * cycles of random numbers and thus avoiding reuse of ids.
 119  *
 120  * This function is called from id_randomid() when needed, an
 121  * application does not have to worry about it.
 122  */
 123 static void
 124 ip_initid(void)
 125 {
 126         u_int16_t j, i;
 127         int noprime = 1;
 128         struct timeval time;
 129
 130         getmicrotime(&time);
 131         read_random((void *) &tmp, sizeof(tmp));
 132         ru_x = (tmp & 0xFFFF) % RU_M;
 133
 134         /* 15 bits of random seed */
 135         ru_seed = (tmp >> 16) & 0x7FFF;
 136         read_random((void *) &tmp, sizeof(tmp));
 137         ru_seed2 = tmp & 0x7FFF;
 138
 139         read_random((void *) &tmp, sizeof(tmp));
 140
 141         /* Determine the LCG we use */
 142         ru_b = (tmp & 0xfffe) | 1;
 143         ru_a = pmod(RU_AGEN, (tmp >> 16) & 0xfffe, RU_M);
 144         while (ru_b % 3 == 0)
 145           ru_b += 2;
 146
 147         read_random((void *) &tmp, sizeof(tmp));
 148         j = tmp % RU_N;
 149         tmp = tmp >> 16;
 150
 151         /*
 152          * Do a fast gcd(j,RU_N-1), so we can find a j with
 153          * gcd(j, RU_N-1) == 1, giving a new generator for
 154          * RU_GEN^j mod RU_N
 155          */
 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         getmicrotime(&time);
 182         if (ru_counter >= RU_MAX || time.tv_sec > ru_reseed)
 183                 ip_initid();
 184
 185         if (!tmp)
 186                 read_random((void *) &tmp, sizeof(tmp));
 187
 188         /* Skip a random number of ids */
 189         n = tmp & 0x3; tmp = tmp >> 2;
 190         if (ru_counter + n >= RU_MAX)
 191                 ip_initid();
 192
 193         for (i = 0; i <= n; i++)
 194                 /* Linear Congruential Generator */
 195                 ru_x = (ru_a*ru_x + ru_b) % RU_M;
 196
 197         ru_counter += i;
 198
 199         return (ru_seed ^ pmod(ru_g,ru_seed2 ^ ru_x,RU_N)) | ru_msb;
 200 }