Annotation of sys/netinet/ip_id.c, Revision 1.1.1.1
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: }
CVSweb
![[BACK]](/icons/back.gif){kind=icon}
Return to ip_id.c CVS log ![[TXT]](/icons/text.gif){kind=icon}
![[DIR]](/icons/dir.gif){kind=icon}
Up to [local] / sys / netinet