/* $OpenBSD: res_random.c,v 1.27 2024/09/03 18:21:55 op Exp $ */ /* * Copyright 1997 Niels Provos <provos@physnet.uni-hamburg.de> * Copyright 2008 Damien Miller <djm@openbsd.org> * All rights reserved. * * Theo de Raadt <deraadt@openbsd.org> came up with the idea of using * such a mathematical system to generate more random (yet non-repeating) * ids to solve the resolver/named problem. But Niels designed the * actual system based on the constraints. * * Later modified by Damien Miller to wrap the LCG output in a 15-bit * permutation generator based on a Luby-Rackoff block cipher. This * ensures the output is non-repeating and preserves the MSB twiddle * trick, but makes it more resistant to LCG prediction. * * Redistribution and use in source and binary forms, with or without * modification, are permitted provided that the following conditions * are met: * 1. Redistributions of source code must retain the above copyright * notice, this list of conditions and the following disclaimer. * 2. Redistributions in binary form must reproduce the above copyright * notice, this list of conditions and the following disclaimer in the * documentation and/or other materials provided with the distribution. * * THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR * IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES * OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED. * IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT, * INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT * NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, * DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY * THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF * THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE. */ /* * seed = random 15bit * n = prime, g0 = generator to n, * j = random so that gcd(j,n-1) == 1 * g = g0^j mod n will be a generator again. * * X[0] = random seed. * X[n] = a*X[n-1]+b mod m is a Linear Congruential Generator * with a = 7^(even random) mod m, * b = random with gcd(b,m) == 1 * m = 31104 and a maximal period of m-1. * * The transaction id is determined by: * id[n] = seed xor (g^X[n] mod n) * * Effectively the id is restricted to the lower 15 bits, thus * yielding two different cycles by toggling the msb on and off. * This avoids reuse issues caused by reseeding. * * The output of this generator is then randomly permuted though a * custom 15 bit Luby-Rackoff block cipher. */ #include <sys/types.h> #include <netinet/in.h> #include <sys/time.h> #include <resolv.h> #include <unistd.h> #include <stdlib.h> #include <string.h> #include "thread_private.h" #define RU_OUT 180 /* Time after which will be reseeded */ #define RU_MAX 30000 /* Uniq cycle, avoid blackjack prediction */ #define RU_GEN 2 /* Starting generator */ #define RU_N 32749 /* RU_N-1 = 2*2*3*2729 */ #define RU_AGEN 7 /* determine ru_a as RU_AGEN^(2*rand) */ #define RU_M 31104 /* RU_M = 2^7*3^5 - don't change */ #define RU_ROUNDS 11 /* Number of rounds for permute (odd) */ struct prf_ctx { /* PRF lookup table for odd rounds (7 bits input to 8 bits output) */ u_char prf7[(RU_ROUNDS / 2) * (1 << 7)]; /* PRF lookup table for even rounds (8 bits input to 7 bits output) */ u_char prf8[((RU_ROUNDS + 1) / 2) * (1 << 8)]; }; #define PFAC_N 3 static const u_int16_t pfacts[PFAC_N] = { 2, 3, 2729 }; static u_int16_t ru_x; static u_int16_t ru_seed, ru_seed2; static u_int16_t ru_a, ru_b; static u_int16_t ru_g; static u_int16_t ru_counter = 0; static u_int16_t ru_msb = 0; static struct prf_ctx *ru_prf = NULL; static time_t ru_reseed; static pid_t ru_pid; static u_int16_t pmod(u_int16_t, u_int16_t, u_int16_t); static void res_initid(void); /* * Do a fast modular exponation, returned value will be in the range * of 0 - (mod-1) */ static u_int16_t pmod(u_int16_t gen, u_int16_t exp, u_int16_t mod) { u_int16_t s, t, u; s = 1; t = gen; u = exp; while (u) { if (u & 1) s = (s * t) % mod; u >>= 1; t = (t * t) % mod; } return (s); } /* * 15-bit permutation based on Luby-Rackoff block cipher */ static u_int permute15(u_int in) { int i; u_int left, right, tmp; if (ru_prf == NULL) return in; left = (in >> 8) & 0x7f; right = in & 0xff; /* * Each round swaps the width of left and right. Even rounds have * a 7-bit left, odd rounds have an 8-bit left. Since this uses an * odd number of rounds, left is always 8 bits wide at the end. */ for (i = 0; i < RU_ROUNDS; i++) { if ((i & 1) == 0) tmp = ru_prf->prf8[(i << (8 - 1)) | right] & 0x7f; else tmp = ru_prf->prf7[((i - 1) << (7 - 1)) | right]; tmp ^= left; left = right; right = tmp; } return (right << 8) | left; } /* * Initializes the seed and chooses a suitable generator. Also toggles * the msb flag. The msb flag is used to generate two distinct * cycles of random numbers and thus avoiding reuse of ids. * * This function is called from res_randomid() when needed, an * application does not have to worry about it. */ static void res_initid(void) { u_int16_t j, i; u_int32_t tmp; int noprime = 1; struct timespec ts; ru_x = arc4random_uniform(RU_M); /* 15 bits of random seed */ tmp = arc4random(); ru_seed = (tmp >> 16) & 0x7FFF; ru_seed2 = tmp & 0x7FFF; /* Determine the LCG we use */ tmp = arc4random(); ru_b = (tmp & 0xfffe) | 1; ru_a = pmod(RU_AGEN, (tmp >> 16) & 0xfffe, RU_M); while (ru_b % 3 == 0) ru_b += 2; j = arc4random_uniform(RU_N); /* * Do a fast gcd(j,RU_N-1), so we can find a j with * gcd(j, RU_N-1) == 1, giving a new generator for * RU_GEN^j mod RU_N */ while (noprime) { for (i = 0; i < PFAC_N; i++) if (j % pfacts[i] == 0) break; if (i >= PFAC_N) noprime = 0; else j = (j + 1) % RU_N; } ru_g = pmod(RU_GEN, j, RU_N); ru_counter = 0; /* Initialise PRF for Luby-Rackoff permutation */ if (ru_prf == NULL) ru_prf = malloc(sizeof(*ru_prf)); if (ru_prf != NULL) arc4random_buf(ru_prf, sizeof(*ru_prf)); WRAP(clock_gettime)(CLOCK_MONOTONIC, &ts); ru_reseed = ts.tv_sec + RU_OUT; ru_msb = ru_msb == 0x8000 ? 0 : 0x8000; } u_int __res_randomid(void) { struct timespec ts; pid_t pid; u_int r; static void *randomid_mutex; WRAP(clock_gettime)(CLOCK_MONOTONIC, &ts); pid = getpid(); _MUTEX_LOCK(&randomid_mutex); if (ru_counter >= RU_MAX || ts.tv_sec > ru_reseed || pid != ru_pid) { res_initid(); ru_pid = pid; } /* Linear Congruential Generator */ ru_x = (ru_a * ru_x + ru_b) % RU_M; ru_counter++; r = permute15(ru_seed ^ pmod(ru_g, ru_seed2 + ru_x, RU_N)) | ru_msb; _MUTEX_UNLOCK(&randomid_mutex); return (r); } DEF_STRONG(__res_randomid); #if 0 int main(int argc, char **argv) { int i, n; u_int16_t wert; res_initid(); printf("Generator: %u\n", ru_g); printf("Seed: %u\n", ru_seed); printf("Reseed at %ld\n", ru_reseed); printf("Ru_X: %u\n", ru_x); printf("Ru_A: %u\n", ru_a); printf("Ru_B: %u\n", ru_b); n = argc > 1 ? atoi(argv[1]) : 60001; for (i=0;i<n;i++) { wert = res_randomid(); printf("%u\n", wert); } return 0; } #endif
