1 //==========================================================================
3 // src/sys/netinet/ip_id.c
5 //==========================================================================
6 //####BSDCOPYRIGHTBEGIN####
8 // -------------------------------------------
10 // Portions of this software may have been derived from OpenBSD,
11 // FreeBSD or other sources, and are covered by the appropriate
12 // copyright disclaimers included herein.
14 // Portions created by Red Hat are
15 // Copyright (C) 2002 Red Hat, Inc. All Rights Reserved.
17 // -------------------------------------------
19 //####BSDCOPYRIGHTEND####
20 //==========================================================================
22 /* $OpenBSD: ip_id.c,v 1.2 1999/08/26 13:37:01 provos Exp $ */
25 * Copyright 1998 Niels Provos <provos@citi.umich.edu>
26 * All rights reserved.
28 * Theo de Raadt <deraadt@openbsd.org> came up with the idea of using
29 * such a mathematical system to generate more random (yet non-repeating)
30 * ids to solve the resolver/named problem. But Niels designed the
31 * actual system based on the constraints.
33 * Redistribution and use in source and binary forms, with or without
34 * modification, are permitted provided that the following conditions
36 * 1. Redistributions of source code must retain the above copyright
37 * notice, this list of conditions and the following disclaimer.
38 * 2. Redistributions in binary form must reproduce the above copyright
39 * notice, this list of conditions and the following disclaimer in the
40 * documentation and/or other materials provided with the distribution.
41 * 3. All advertising materials mentioning features or use of this software
42 * must display the following acknowledgement:
43 * This product includes software developed by Niels Provos.
44 * 4. The name of the author may not be used to endorse or promote products
45 * derived from this software without specific prior written permission.
47 * THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR
48 * IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES
49 * OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED.
50 * IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT,
51 * INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT
52 * NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
53 * DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
54 * THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
55 * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF
56 * THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
58 * $FreeBSD: src/sys/netinet/ip_id.c,v 1.1.2.1 2001/07/19 06:37:26 kris Exp $
63 * n = prime, g0 = generator to n,
64 * j = random so that gcd(j,n-1) == 1
65 * g = g0^j mod n will be a generator again.
68 * X[n] = a*X[n-1]+b mod m is a Linear Congruential Generator
69 * with a = 7^(even random) mod m,
70 * b = random with gcd(b,m) == 1
71 * m = 31104 and a maximal period of m-1.
73 * The transaction id is determined by:
74 * id[n] = seed xor (g^X[n] mod n)
76 * Effectivly the id is restricted to the lower 15 bits, thus
77 * yielding two different cycles by toggling the msb on and off.
78 * This avoids reuse issues caused by reseeding.
81 #include <sys/param.h>
84 #define RU_OUT 180 /* Time after wich will be reseeded */
85 #define RU_MAX 30000 /* Uniq cycle, avoid blackjack prediction */
86 #define RU_GEN 2 /* Starting generator */
87 #define RU_N 32749 /* RU_N-1 = 2*2*3*2729 */
88 #define RU_AGEN 7 /* determine ru_a as RU_AGEN^(2*rand) */
89 #define RU_M 31104 /* RU_M = 2^7*3^5 - don't change */
92 const static u_int16_t pfacts[PFAC_N] = {
98 static u_int16_t ru_x;
99 static u_int16_t ru_seed, ru_seed2;
100 static u_int16_t ru_a, ru_b;
101 static u_int16_t ru_g;
102 static u_int16_t ru_counter = 0;
103 static u_int16_t ru_msb = 0;
104 static long ru_reseed;
105 static u_int32_t tmp; /* Storage for unused random */
107 static u_int16_t pmod __P((u_int16_t, u_int16_t, u_int16_t));
108 static void ip_initid __P((void));
109 u_int16_t ip_randomid __P((void));
112 * Do a fast modular exponation, returned value will be in the range
118 pmod(u_int16_t gen, u_int16_t exp, u_int16_t mod)
122 u_int16_t gen, exp, mod;
141 * Initalizes the seed and chooses a suitable generator. Also toggles
142 * the msb flag. The msb flag is used to generate two distinct
143 * cycles of random numbers and thus avoiding reuse of ids.
145 * This function is called from id_randomid() when needed, an
146 * application does not have to worry about it.
156 read_random((void *) &tmp, sizeof(tmp));
157 ru_x = (tmp & 0xFFFF) % RU_M;
159 /* 15 bits of random seed */
160 ru_seed = (tmp >> 16) & 0x7FFF;
161 read_random((void *) &tmp, sizeof(tmp));
162 ru_seed2 = tmp & 0x7FFF;
164 read_random((void *) &tmp, sizeof(tmp));
166 /* Determine the LCG we use */
167 ru_b = (tmp & 0xfffe) | 1;
168 ru_a = pmod(RU_AGEN, (tmp >> 16) & 0xfffe, RU_M);
169 while (ru_b % 3 == 0)
172 read_random((void *) &tmp, sizeof(tmp));
177 * Do a fast gcd(j,RU_N-1), so we can find a j with
178 * gcd(j, RU_N-1) == 1, giving a new generator for
183 for (i=0; i<PFAC_N; i++)
184 if (j%pfacts[i] == 0)
193 ru_g = pmod(RU_GEN,j,RU_N);
196 ru_reseed = time.tv_sec + RU_OUT;
197 ru_msb = ru_msb == 0x8000 ? 0 : 0x8000;
207 if (ru_counter >= RU_MAX || time.tv_sec > ru_reseed)
211 read_random((void *) &tmp, sizeof(tmp));
213 /* Skip a random number of ids */
214 n = tmp & 0x3; tmp = tmp >> 2;
215 if (ru_counter + n >= RU_MAX)
218 for (i = 0; i <= n; i++)
219 /* Linear Congruential Generator */
220 ru_x = (ru_a*ru_x + ru_b) % RU_M;
224 return (ru_seed ^ pmod(ru_g,ru_seed2 ^ ru_x,RU_N)) | ru_msb;
227 #endif /* RANDOM_IP_ID */