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arc4random.c revision 1.17 
      1 /*	$NetBSD: arc4random.c,v 1.17 2012/08/18 15:55:07 dsl Exp $	*/
      2 /*	$OpenBSD: arc4random.c,v 1.6 2001/06/05 05:05:38 pvalchev Exp $	*/
      3 
      4 /*
      5  * Arc4 random number generator for OpenBSD.
      6  * Copyright 1996 David Mazieres <dm (at) lcs.mit.edu>.
      7  *
      8  * Modification and redistribution in source and binary forms is
      9  * permitted provided that due credit is given to the author and the
     10  * OpenBSD project by leaving this copyright notice intact.
     11  */
     12 
     13 /*
     14  * This code is derived from section 17.1 of Applied Cryptography,
     15  * second edition, which describes a stream cipher allegedly
     16  * compatible with RSA Labs "RC4" cipher (the actual description of
     17  * which is a trade secret).  The same algorithm is used as a stream
     18  * cipher called "arcfour" in Tatu Ylonen's ssh package.
     19  *
     20  * Here the stream cipher has been modified always to include the time
     21  * when initializing the state.  That makes it impossible to
     22  * regenerate the same random sequence twice, so this can't be used
     23  * for encryption, but will generate good random numbers.
     24  *
     25  * RC4 is a registered trademark of RSA Laboratories.
     26  */
     27 
     28 #include <sys/cdefs.h>
     29 #if defined(LIBC_SCCS) && !defined(lint)
     30 __RCSID("$NetBSD: arc4random.c,v 1.17 2012/08/18 15:55:07 dsl Exp $");
     31 #endif /* LIBC_SCCS and not lint */
     32 
     33 #include "namespace.h"
     34 #include "reentrant.h"
     35 #include <fcntl.h>
     36 #include <stdlib.h>
     37 #include <unistd.h>
     38 #include <sys/types.h>
     39 #include <sys/param.h>
     40 #include <sys/time.h>
     41 #include <sys/sysctl.h>
     42 
     43 #ifdef __weak_alias
     44 __weak_alias(arc4random,_arc4random)
     45 #endif
     46 
     47 #define RSIZE 256
     48 struct arc4_stream {
     49 	mutex_t mtx;
     50 	int initialized;
     51 	uint8_t i;
     52 	uint8_t j;
     53 	uint8_t s[RSIZE];
     54 };
     55 
     56 #ifdef _REENTRANT
     57 #define LOCK(rs) { \
     58 		int isthreaded = __isthreaded; \
     59 		if (isthreaded)        \
     60 			mutex_lock(&(rs)->mtx);
     61 #define UNLOCK(rs) \
     62 		if (isthreaded)        \
     63 			mutex_unlock(&(rs)->mtx);      \
     64 	}
     65 #else
     66 #define LOCK(rs)
     67 #define UNLOCK(rs)
     68 #endif
     69 
     70 
     71 /* XXX lint explodes with an internal error if only mtx is initialized! */
     72 static struct arc4_stream rs = { .i = 0, .mtx = MUTEX_INITIALIZER };
     73 
     74 static inline void arc4_addrandom(struct arc4_stream *, u_char *, int);
     75 static void arc4_stir(struct arc4_stream *);
     76 static inline uint8_t arc4_getbyte(struct arc4_stream *);
     77 static inline uint32_t arc4_getword(struct arc4_stream *);
     78 
     79 static __noinline void
     80 arc4_init(struct arc4_stream *as)
     81 {
     82 	int n;
     83 	for (n = 0; n < RSIZE; n++)
     84 		as->s[n] = n;
     85 	as->i = 0;
     86 	as->j = 0;
     87 
     88 	as->initialized = 1;
     89 	arc4_stir(as);
     90 }
     91 
     92 static inline int
     93 arc4_check_init(struct arc4_stream *as)
     94 {
     95 	if (__predict_true(rs.initialized))
     96 		return 0;
     97 
     98 	arc4_init(as);
     99 	return 1;
    100 }
    101 
    102 static inline void
    103 arc4_addrandom(struct arc4_stream *as, u_char *dat, int datlen)
    104 {
    105 	uint8_t si;
    106 	int n;
    107 
    108 	as->i--;
    109 	for (n = 0; n < RSIZE; n++) {
    110 		as->i = (as->i + 1);
    111 		si = as->s[as->i];
    112 		as->j = (as->j + si + dat[n % datlen]);
    113 		as->s[as->i] = as->s[as->j];
    114 		as->s[as->j] = si;
    115 	}
    116 	as->j = as->i;
    117 }
    118 
    119 static void
    120 arc4_stir(struct arc4_stream *as)
    121 {
    122 	int rdat[32];
    123 	int mib[] = { CTL_KERN, KERN_URND };
    124 	size_t len;
    125 	size_t i, j;
    126 
    127 	/*
    128 	 * This code once opened and read /dev/urandom on each
    129 	 * call.  That causes repeated rekeying of the kernel stream
    130 	 * generator, which is very wasteful.  Because of application
    131 	 * behavior, caching the fd doesn't really help.  So we just
    132 	 * fill up the tank from sysctl, which is a tiny bit slower
    133 	 * for us but much friendlier to other entropy consumers.
    134 	 */
    135 
    136 	for (i = 0; i < __arraycount(rdat); i++) {
    137 		len = sizeof(rdat[i]);
    138 		if (sysctl(mib, 2, &rdat[i], &len, NULL, 0) == -1)
    139 			abort();
    140 	}
    141 
    142 	arc4_addrandom(as, (void *) &rdat, (int)sizeof(rdat));
    143 
    144 	/*
    145 	 * Throw away the first N words of output, as suggested in the
    146 	 * paper "Weaknesses in the Key Scheduling Algorithm of RC4"
    147 	 * by Fluher, Mantin, and Shamir.  (N = 256 in our case.)
    148 	 */
    149 	for (j = 0; j < RSIZE * 4; j++)
    150 		arc4_getbyte(as);
    151 }
    152 
    153 static __always_inline uint8_t
    154 arc4_getbyte_ij(struct arc4_stream *as, uint8_t *i, uint8_t *j)
    155 {
    156 	uint8_t si, sj;
    157 
    158 	*i = *i + 1;
    159 	si = as->s[*i];
    160 	*j = *j + si;
    161 	sj = as->s[*j];
    162 	as->s[*i] = sj;
    163 	as->s[*j] = si;
    164 	return (as->s[(si + sj) & 0xff]);
    165 }
    166 
    167 static inline uint8_t
    168 arc4_getbyte(struct arc4_stream *as)
    169 {
    170 	return arc4_getbyte_ij(as, &as->i, &as->j);
    171 }
    172 
    173 static inline uint32_t
    174 arc4_getword(struct arc4_stream *as)
    175 {
    176 	uint32_t val;
    177 	val = arc4_getbyte(as) << 24;
    178 	val |= arc4_getbyte(as) << 16;
    179 	val |= arc4_getbyte(as) << 8;
    180 	val |= arc4_getbyte(as);
    181 	return val;
    182 }
    183 
    184 void
    185 arc4random_stir(void)
    186 {
    187 	LOCK(&rs);
    188 	if (__predict_false(!arc4_check_init(&rs)))	/* init() stirs */
    189 		arc4_stir(&rs);
    190 	UNLOCK(&rs);
    191 }
    192 
    193 void
    194 arc4random_addrandom(u_char *dat, int datlen)
    195 {
    196 	LOCK(&rs);
    197 	arc4_check_init(&rs);
    198 	arc4_addrandom(&rs, dat, datlen);
    199 	UNLOCK(&rs);
    200 }
    201 
    202 uint32_t
    203 arc4random(void)
    204 {
    205 	uint32_t v;
    206 
    207 	LOCK(&rs);
    208 	arc4_check_init(&rs);
    209 	v = arc4_getword(&rs);
    210 	UNLOCK(&rs);
    211 	return v;
    212 }
    213 
    214 void
    215 arc4random_buf(void *buf, size_t len)
    216 {
    217 	uint8_t *bp = buf;
    218 	uint8_t *ep = bp + len;
    219 	uint8_t i, j;
    220 	int skip;
    221 
    222 	LOCK(&rs);
    223 	arc4_check_init(&rs);
    224 
    225 	/* cache i and j - compiler can't know 'buf' doesn't alias them */
    226 	i = rs.i;
    227 	j = rs.j;
    228 
    229 	skip = arc4_getbyte_ij(&rs, &i, &j) % 3;
    230 	while (skip--)
    231 		(void)arc4_getbyte_ij(&rs, &i, &j);
    232 
    233 	while (bp < ep)
    234 		*bp++ = arc4_getbyte_ij(&rs, &i, &j);
    235 	rs.i = i;
    236 	rs.j = j;
    237 
    238 	UNLOCK(&rs);
    239 }
    240 
    241 /*-
    242  * Written by Damien Miller.
    243  * With simplifications by Jinmei Tatuya.
    244  */
    245 
    246 /*
    247  * Calculate a uniformly distributed random number less than
    248  * upper_bound avoiding "modulo bias".
    249  *
    250  * Uniformity is achieved by generating new random numbers
    251  * until the one returned is outside the range
    252  * [0, 2^32 % upper_bound[. This guarantees the selected
    253  * random number will be inside the range
    254  * [2^32 % upper_bound, 2^32[ which maps back to
    255  * [0, upper_bound[ after reduction modulo upper_bound.
    256  */
    257 uint32_t
    258 arc4random_uniform(uint32_t upper_bound)
    259 {
    260 	uint32_t r, min;
    261 
    262 	if (upper_bound < 2)
    263 		return 0;
    264 
    265 	/* calculate (2^32 % upper_bound) avoiding 64-bit math */
    266 	/* ((2^32 - x) % x) == (2^32 % x) when x <= 2^31 */
    267 	min = (0xFFFFFFFFU - upper_bound + 1) % upper_bound;
    268 
    269 	LOCK(&rs);
    270 	arc4_check_init(&rs);
    271 
    272 	if (arc4_getbyte(&rs) & 1)
    273 		(void)arc4_getbyte(&rs);
    274 
    275 	/*
    276 	 * This could theoretically loop forever but each retry has
    277 	 * p > 0.5 (worst case, usually far better) of selecting a
    278 	 * number inside the range we need, so it should rarely need
    279 	 * to re-roll (at all).
    280 	 */
    281 	do
    282 		r = arc4_getword(&rs);
    283 	while (r < min);
    284 	UNLOCK(&rs);
    285 
    286 	return r % upper_bound;
    287 }
    288