Annotation of sys/netinet/ip_id.c, Revision 1.1
1.1 ! nbrk 1: /* $OpenBSD: ip_id.c,v 1.14 2007/05/27 19:59:11 dlg 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: *
! 21: * THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR
! 22: * IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES
! 23: * OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED.
! 24: * IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT,
! 25: * INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT
! 26: * NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
! 27: * DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
! 28: * THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
! 29: * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF
! 30: * THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
! 31: */
! 32:
! 33: /*
! 34: * seed = random 15bit
! 35: * n = prime, g0 = generator to n,
! 36: * j = random so that gcd(j,n-1) == 1
! 37: * g = g0^j mod n will be a generator again.
! 38: *
! 39: * X[0] = random seed.
! 40: * X[n] = a*X[n-1]+b mod m is a Linear Congruential Generator
! 41: * with a = 7^(even random) mod m,
! 42: * b = random with gcd(b,m) == 1
! 43: * m = 31104 and a maximal period of m-1.
! 44: *
! 45: * The transaction id is determined by:
! 46: * id[n] = seed xor (g^X[n] mod n)
! 47: *
! 48: * Effectively the id is restricted to the lower 15 bits, thus
! 49: * yielding two different cycles by toggling the msb on and off.
! 50: * This avoids reuse issues caused by reseeding.
! 51: */
! 52:
! 53: #include <sys/param.h>
! 54: #include <sys/kernel.h>
! 55:
! 56: #include <dev/rndvar.h>
! 57:
! 58: #define RU_OUT 180 /* Time after wich will be reseeded */
! 59: #define RU_MAX 30000 /* Uniq cycle, avoid blackjack prediction */
! 60: #define RU_GEN 2 /* Starting generator */
! 61: #define RU_N 32749 /* RU_N-1 = 2*2*3*2729 */
! 62: #define RU_AGEN 7 /* determine ru_a as RU_AGEN^(2*rand) */
! 63: #define RU_M 31104 /* RU_M = 2^7*3^5 - don't change */
! 64:
! 65: #define PFAC_N 3
! 66: const static u_int16_t pfacts[PFAC_N] = {
! 67: 2,
! 68: 3,
! 69: 2729
! 70: };
! 71:
! 72: static u_int16_t ru_x;
! 73: static u_int16_t ru_seed, ru_seed2;
! 74: static u_int16_t ru_a, ru_b;
! 75: static u_int16_t ru_g;
! 76: static u_int16_t ru_counter = 0;
! 77: static u_int16_t ru_msb = 0;
! 78: static long ru_reseed;
! 79: static u_int32_t tmp; /* Storage for unused random */
! 80:
! 81: u_int16_t pmod(u_int16_t, u_int16_t, u_int16_t);
! 82: void ip_initid(void);
! 83: u_int16_t ip_randomid(void);
! 84:
! 85: /*
! 86: * Do a fast modular exponation, returned value will be in the range
! 87: * of 0 - (mod-1)
! 88: */
! 89:
! 90: u_int16_t
! 91: pmod(u_int16_t gen, u_int16_t expo, u_int16_t mod)
! 92: {
! 93: u_int16_t s, t, u;
! 94:
! 95: s = 1;
! 96: t = gen;
! 97: u = expo;
! 98:
! 99: while (u) {
! 100: if (u & 1)
! 101: s = (s*t) % mod;
! 102: u >>= 1;
! 103: t = (t*t) % mod;
! 104: }
! 105: return (s);
! 106: }
! 107:
! 108: /*
! 109: * Initalizes the seed and chooses a suitable generator. Also toggles
! 110: * the msb flag. The msb flag is used to generate two distinct
! 111: * cycles of random numbers and thus avoiding reuse of ids.
! 112: *
! 113: * This function is called from id_randomid() when needed, an
! 114: * application does not have to worry about it.
! 115: */
! 116: void
! 117: ip_initid(void)
! 118: {
! 119: u_int16_t j, i;
! 120: int noprime = 1;
! 121:
! 122: ru_x = ((tmp = arc4random()) & 0xFFFF) % RU_M;
! 123:
! 124: /* 15 bits of random seed */
! 125: ru_seed = (tmp >> 16) & 0x7FFF;
! 126: ru_seed2 = arc4random() & 0x7FFF;
! 127:
! 128: /* Determine the LCG we use */
! 129: ru_b = ((tmp = arc4random()) & 0xfffe) | 1;
! 130: ru_a = pmod(RU_AGEN, (tmp >> 16) & 0xfffe, RU_M);
! 131: while (ru_b % 3 == 0)
! 132: ru_b += 2;
! 133:
! 134: j = (tmp = arc4random()) % RU_N;
! 135: tmp = tmp >> 16;
! 136:
! 137: /*
! 138: * Do a fast gcd(j,RU_N-1), so we can find a j with
! 139: * gcd(j, RU_N-1) == 1, giving a new generator for
! 140: * RU_GEN^j mod RU_N
! 141: */
! 142:
! 143: while (noprime) {
! 144: for (i = 0; i < PFAC_N; i++)
! 145: if (j % pfacts[i] == 0)
! 146: break;
! 147:
! 148: if (i >= PFAC_N)
! 149: noprime = 0;
! 150: else
! 151: j = (j+1) % RU_N;
! 152: }
! 153:
! 154: ru_g = pmod(RU_GEN,j,RU_N);
! 155: ru_counter = 0;
! 156:
! 157: ru_reseed = time_second + RU_OUT;
! 158: ru_msb = ru_msb == 0x8000 ? 0 : 0x8000;
! 159: }
! 160:
! 161: u_int16_t
! 162: ip_randomid(void)
! 163: {
! 164: int i, n;
! 165:
! 166: if (ru_counter >= RU_MAX || time_second > ru_reseed)
! 167: ip_initid();
! 168:
! 169: #if 0
! 170: if (!tmp)
! 171: tmp = arc4random();
! 172:
! 173: /* Skip a random number of ids */
! 174: n = tmp & 0x3; tmp = tmp >> 2;
! 175: if (ru_counter + n >= RU_MAX)
! 176: ip_initid();
! 177: #else
! 178: n = 0;
! 179: #endif
! 180:
! 181: for (i = 0; i <= n; i++)
! 182: /* Linear Congruential Generator */
! 183: ru_x = (ru_a * ru_x + ru_b) % RU_M;
! 184:
! 185: ru_counter += i;
! 186:
! 187: return (ru_seed ^ pmod(ru_g,ru_seed2 + ru_x, RU_N)) | ru_msb;
! 188: }
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