1 /* $OpenBSD: ip_id.c,v 1.2 1999/08/26 13:37:01 provos Exp $ */
4 * Copyright 1998 Niels Provos <provos@citi.umich.edu>
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.
12 * Redistribution and use in source and binary forms, with or without
13 * modification, are permitted provided that the following conditions
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.
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.
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 1.3 2003/08/23 11:18:00 rob Exp $
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.
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.
53 * The transaction id is determined by:
54 * id[n] = seed xor (g^X[n] mod n)
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.
61 #include "opt_random_ip_id.h"
62 #include <sys/param.h>
64 #include <sys/kernel.h>
65 #include <sys/random.h>
68 #define RU_OUT 180 /* Time after wich will be reseeded */
69 #define RU_MAX 30000 /* Uniq cycle, avoid blackjack prediction */
70 #define RU_GEN 2 /* Starting generator */
71 #define RU_N 32749 /* RU_N-1 = 2*2*3*2729 */
72 #define RU_AGEN 7 /* determine ru_a as RU_AGEN^(2*rand) */
73 #define RU_M 31104 /* RU_M = 2^7*3^5 - don't change */
76 const static u_int16_t pfacts[PFAC_N] = {
82 static u_int16_t ru_x;
83 static u_int16_t ru_seed, ru_seed2;
84 static u_int16_t ru_a, ru_b;
85 static u_int16_t ru_g;
86 static u_int16_t ru_counter = 0;
87 static u_int16_t ru_msb = 0;
88 static long ru_reseed;
89 static u_int32_t tmp; /* Storage for unused random */
91 static u_int16_t pmod (u_int16_t, u_int16_t, u_int16_t);
92 static void ip_initid (void);
93 u_int16_t ip_randomid (void);
96 * Do a fast modular exponation, returned value will be in the range
102 pmod(u_int16_t gen, u_int16_t exp, u_int16_t mod)
106 u_int16_t gen, exp, mod;
125 * Initalizes the seed and chooses a suitable generator. Also toggles
126 * the msb flag. The msb flag is used to generate two distinct
127 * cycles of random numbers and thus avoiding reuse of ids.
129 * This function is called from id_randomid() when needed, an
130 * application does not have to worry about it.
140 read_random((void *) &tmp, sizeof(tmp));
141 ru_x = (tmp & 0xFFFF) % RU_M;
143 /* 15 bits of random seed */
144 ru_seed = (tmp >> 16) & 0x7FFF;
145 read_random((void *) &tmp, sizeof(tmp));
146 ru_seed2 = tmp & 0x7FFF;
148 read_random((void *) &tmp, sizeof(tmp));
150 /* Determine the LCG we use */
151 ru_b = (tmp & 0xfffe) | 1;
152 ru_a = pmod(RU_AGEN, (tmp >> 16) & 0xfffe, RU_M);
153 while (ru_b % 3 == 0)
156 read_random((void *) &tmp, sizeof(tmp));
161 * Do a fast gcd(j,RU_N-1), so we can find a j with
162 * gcd(j, RU_N-1) == 1, giving a new generator for
167 for (i=0; i<PFAC_N; i++)
168 if (j%pfacts[i] == 0)
177 ru_g = pmod(RU_GEN,j,RU_N);
180 ru_reseed = time.tv_sec + RU_OUT;
181 ru_msb = ru_msb == 0x8000 ? 0 : 0x8000;
191 if (ru_counter >= RU_MAX || time.tv_sec > ru_reseed)
195 read_random((void *) &tmp, sizeof(tmp));
197 /* Skip a random number of ids */
198 n = tmp & 0x3; tmp = tmp >> 2;
199 if (ru_counter + n >= RU_MAX)
202 for (i = 0; i <= n; i++)
203 /* Linear Congruential Generator */
204 ru_x = (ru_a*ru_x + ru_b) % RU_M;
208 return (ru_seed ^ pmod(ru_g,ru_seed2 ^ ru_x,RU_N)) | ru_msb;
211 #endif /* RANDOM_IP_ID */