7a0789b485
change here is to ensure that when a process forks after arc4random is seeded, the parent and child don't observe the same random sequence. OpenBSD's fix introduces some additional overhead in the form of a getpid() call. This could be improved upon, e.g., by setting a flag in fork(), if it proves to be a problem. This was discussed with secteam (simon, csjp, rwatson) in 2008, shortly prior to my going out of town and forgetting all about it. The conclusion was that the problem with forks is worrisome, but it doesn't appear to have introduced an actual vulnerability for any known programs. The only significant remaining difference between our arc4random and OpenBSD's is in how we seed the generator in arc4_stir().
296 lines
6.0 KiB
C
296 lines
6.0 KiB
C
/* $OpenBSD: arc4random.c,v 1.22 2010/12/22 08:23:42 otto Exp $ */
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/*
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* Copyright (c) 1996, David Mazieres <dm@uun.org>
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* Copyright (c) 2008, Damien Miller <djm@openbsd.org>
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*
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* Permission to use, copy, modify, and distribute this software for any
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* purpose with or without fee is hereby granted, provided that the above
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* copyright notice and this permission notice appear in all copies.
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*
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* THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
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* WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
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* MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR
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* ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
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* WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN
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* ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF
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* OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
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*/
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/*
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* Arc4 random number generator for OpenBSD.
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*
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* This code is derived from section 17.1 of Applied Cryptography,
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* second edition, which describes a stream cipher allegedly
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* compatible with RSA Labs "RC4" cipher (the actual description of
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* which is a trade secret). The same algorithm is used as a stream
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* cipher called "arcfour" in Tatu Ylonen's ssh package.
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*
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* RC4 is a registered trademark of RSA Laboratories.
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*/
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#include <sys/cdefs.h>
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__FBSDID("$FreeBSD$");
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#include "namespace.h"
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#include <fcntl.h>
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#include <limits.h>
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#include <stdlib.h>
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#include <unistd.h>
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#include <sys/types.h>
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#include <sys/param.h>
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#include <sys/time.h>
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#include <pthread.h>
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#include "libc_private.h"
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#include "un-namespace.h"
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#ifdef __GNUC__
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#define inline __inline
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#else /* !__GNUC__ */
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#define inline
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#endif /* !__GNUC__ */
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struct arc4_stream {
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u_int8_t i;
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u_int8_t j;
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u_int8_t s[256];
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};
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static pthread_mutex_t arc4random_mtx = PTHREAD_MUTEX_INITIALIZER;
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#define RANDOMDEV "/dev/random"
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#define KEYSIZE 128
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#define _ARC4_LOCK() \
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do { \
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if (__isthreaded) \
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_pthread_mutex_lock(&arc4random_mtx); \
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} while (0)
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#define _ARC4_UNLOCK() \
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do { \
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if (__isthreaded) \
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_pthread_mutex_unlock(&arc4random_mtx); \
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} while (0)
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static int rs_initialized;
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static struct arc4_stream rs;
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static pid_t arc4_stir_pid;
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static int arc4_count;
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static inline u_int8_t arc4_getbyte(void);
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static void arc4_stir(void);
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static inline void
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arc4_init(void)
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{
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int n;
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for (n = 0; n < 256; n++)
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rs.s[n] = n;
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rs.i = 0;
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rs.j = 0;
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}
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static inline void
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arc4_addrandom(u_char *dat, int datlen)
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{
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int n;
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u_int8_t si;
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rs.i--;
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for (n = 0; n < 256; n++) {
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rs.i = (rs.i + 1);
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si = rs.s[rs.i];
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rs.j = (rs.j + si + dat[n % datlen]);
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rs.s[rs.i] = rs.s[rs.j];
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rs.s[rs.j] = si;
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}
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rs.j = rs.i;
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}
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static void
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arc4_stir(void)
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{
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int done, fd, i;
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struct {
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struct timeval tv;
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pid_t pid;
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u_char rnd[KEYSIZE];
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} rdat;
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if (!rs_initialized) {
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arc4_init();
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rs_initialized = 1;
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}
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fd = _open(RANDOMDEV, O_RDONLY, 0);
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done = 0;
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if (fd >= 0) {
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if (_read(fd, &rdat, KEYSIZE) == KEYSIZE)
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done = 1;
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(void)_close(fd);
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}
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if (!done) {
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(void)gettimeofday(&rdat.tv, NULL);
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rdat.pid = getpid();
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/* We'll just take whatever was on the stack too... */
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}
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arc4_addrandom((u_char *)&rdat, KEYSIZE);
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/*
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* Discard early keystream, as per recommendations in:
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* "(Not So) Random Shuffles of RC4" by Ilya Mironov.
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*/
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for (i = 0; i < 1024; i++)
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(void)arc4_getbyte();
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arc4_count = 1600000;
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}
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static void
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arc4_stir_if_needed(void)
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{
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pid_t pid = getpid();
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if (arc4_count <= 0 || !rs_initialized || arc4_stir_pid != pid)
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{
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arc4_stir_pid = pid;
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arc4_stir();
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}
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}
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static inline u_int8_t
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arc4_getbyte(void)
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{
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u_int8_t si, sj;
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rs.i = (rs.i + 1);
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si = rs.s[rs.i];
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rs.j = (rs.j + si);
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sj = rs.s[rs.j];
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rs.s[rs.i] = sj;
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rs.s[rs.j] = si;
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return (rs.s[(si + sj) & 0xff]);
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}
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static inline u_int32_t
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arc4_getword(void)
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{
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u_int32_t val;
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val = arc4_getbyte() << 24;
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val |= arc4_getbyte() << 16;
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val |= arc4_getbyte() << 8;
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val |= arc4_getbyte();
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return val;
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}
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void
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arc4random_stir(void)
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{
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_ARC4_LOCK();
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arc4_stir();
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_ARC4_UNLOCK();
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}
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void
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arc4random_addrandom(u_char *dat, int datlen)
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{
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_ARC4_LOCK();
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if (!rs_initialized)
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arc4_stir();
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arc4_addrandom(dat, datlen);
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_ARC4_UNLOCK();
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}
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u_int32_t
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arc4random(void)
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{
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u_int32_t val;
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_ARC4_LOCK();
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arc4_count -= 4;
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arc4_stir_if_needed();
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val = arc4_getword();
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_ARC4_UNLOCK();
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return val;
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}
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void
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arc4random_buf(void *_buf, size_t n)
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{
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u_char *buf = (u_char *)_buf;
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_ARC4_LOCK();
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arc4_stir_if_needed();
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while (n--) {
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if (--arc4_count <= 0)
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arc4_stir();
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buf[n] = arc4_getbyte();
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}
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_ARC4_UNLOCK();
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}
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/*
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* Calculate a uniformly distributed random number less than upper_bound
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* avoiding "modulo bias".
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*
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* Uniformity is achieved by generating new random numbers until the one
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* returned is outside the range [0, 2**32 % upper_bound). This
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* guarantees the selected random number will be inside
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* [2**32 % upper_bound, 2**32) which maps back to [0, upper_bound)
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* after reduction modulo upper_bound.
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*/
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u_int32_t
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arc4random_uniform(u_int32_t upper_bound)
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{
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u_int32_t r, min;
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if (upper_bound < 2)
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return 0;
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#if (ULONG_MAX > 0xffffffffUL)
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min = 0x100000000UL % upper_bound;
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#else
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/* Calculate (2**32 % upper_bound) avoiding 64-bit math */
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if (upper_bound > 0x80000000)
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min = 1 + ~upper_bound; /* 2**32 - upper_bound */
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else {
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/* (2**32 - (x * 2)) % x == 2**32 % x when x <= 2**31 */
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min = ((0xffffffff - (upper_bound * 2)) + 1) % upper_bound;
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}
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#endif
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/*
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* This could theoretically loop forever but each retry has
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* p > 0.5 (worst case, usually far better) of selecting a
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* number inside the range we need, so it should rarely need
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* to re-roll.
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*/
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for (;;) {
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r = arc4random();
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if (r >= min)
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break;
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}
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return r % upper_bound;
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}
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#if 0
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/*-------- Test code for i386 --------*/
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#include <stdio.h>
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#include <machine/pctr.h>
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int
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main(int argc, char **argv)
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{
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const int iter = 1000000;
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int i;
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pctrval v;
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v = rdtsc();
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for (i = 0; i < iter; i++)
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arc4random();
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v = rdtsc() - v;
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v /= iter;
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printf("%qd cycles\n", v);
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}
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#endif
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