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arc4random.c revision 1.12 
      1 /*	$NetBSD: arc4random.c,v 1.12 2012/03/04 00:36:43 tls 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 1.12 2012/03/04 00:36:43 tls 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 struct arc4_stream {
     48 	mutex_t mtx;
     49 	int initialized;
     50 	uint8_t i;
     51 	uint8_t j;
     52 	uint8_t s[256];
     53 };
     54 
     55 /* XXX lint explodes with an internal error if only mtx is initialized! */
     56 static struct arc4_stream rs = { .i = 0, .mtx = MUTEX_INITIALIZER };
     57 
     58 static inline void arc4_init(struct arc4_stream *);
     59 static inline void arc4_addrandom(struct arc4_stream *, u_char *, int);
     60 static void arc4_stir(struct arc4_stream *);
     61 static inline uint8_t arc4_getbyte(struct arc4_stream *);
     62 static inline uint32_t arc4_getword(struct arc4_stream *);
     63 
     64 static inline void
     65 arc4_init(struct arc4_stream *as)
     66 {
     67 	int     n;
     68 
     69 	for (n = 0; n < 256; n++)
     70 		as->s[n] = n;
     71 	as->i = 0;
     72 	as->j = 0;
     73 
     74 	as->initialized = 1;
     75 	arc4_stir(as);
     76 }
     77 
     78 static inline void
     79 arc4_addrandom(struct arc4_stream *as, u_char *dat, int datlen)
     80 {
     81 	int     n;
     82 	uint8_t si;
     83 
     84 	as->i--;
     85 	for (n = 0; n < 256; n++) {
     86 		as->i = (as->i + 1);
     87 		si = as->s[as->i];
     88 		as->j = (as->j + si + dat[n % datlen]);
     89 		as->s[as->i] = as->s[as->j];
     90 		as->s[as->j] = si;
     91 	}
     92 	as->j = as->i;
     93 }
     94 
     95 static void
     96 arc4_stir(struct arc4_stream *as)
     97 {
     98 	int rdat[128 / sizeof(int)];
     99 	int	n;
    100 	int mib[2];
    101 	unsigned int i;
    102 	size_t len;
    103 
    104 	/*
    105 	 * This code once opened and read /dev/urandom on each
    106 	 * call.  That causes repeated rekeying of the kernel stream
    107 	 * generator, which is very wasteful.  Because of application
    108 	 * behavior, caching the fd doesn't really help.  So we just
    109 	 * fill up the tank from sysctl, which is a tiny bit slower
    110 	 * for us but much friendlier to other entropy consumers.
    111 	 */
    112 
    113 	mib[0] = CTL_KERN;
    114 	mib[1] = KERN_URND;
    115 
    116 	for (i = 0; i < sizeof(rdat) / sizeof(int); i++) {
    117 		len = sizeof(rdat[i]);
    118 		if (sysctl(mib, 2, &rdat[i], &len, NULL, 0) == -1)
    119 			abort();
    120 	}
    121 
    122 	arc4_addrandom(as, (void *) &rdat, sizeof(rdat));
    123 
    124 	/*
    125 	 * Throw away the first N words of output, as suggested in the
    126 	 * paper "Weaknesses in the Key Scheduling Algorithm of RC4"
    127 	 * by Fluher, Mantin, and Shamir.  (N = 256 in our case.)
    128 	 */
    129 	for (n = 0; n < 256 * 4; n++)
    130 		arc4_getbyte(as);
    131 }
    132 
    133 static inline uint8_t
    134 arc4_getbyte(struct arc4_stream *as)
    135 {
    136 	uint8_t si, sj;
    137 
    138 	as->i = (as->i + 1);
    139 	si = as->s[as->i];
    140 	as->j = (as->j + si);
    141 	sj = as->s[as->j];
    142 	as->s[as->i] = sj;
    143 	as->s[as->j] = si;
    144 	return (as->s[(si + sj) & 0xff]);
    145 }
    146 
    147 static inline uint32_t
    148 arc4_getword(struct arc4_stream *as)
    149 {
    150 	uint32_t val;
    151 	val = arc4_getbyte(as) << 24;
    152 	val |= arc4_getbyte(as) << 16;
    153 	val |= arc4_getbyte(as) << 8;
    154 	val |= arc4_getbyte(as);
    155 	return val;
    156 }
    157 
    158 static inline void
    159 _arc4random_stir_unlocked(void)
    160 {
    161 	if (__predict_false(!rs.initialized)) {
    162 		arc4_init(&rs);				/* stirs */
    163 	} else {
    164 		arc4_stir(&rs);
    165 	}
    166 }
    167 
    168 void
    169 arc4random_stir(void)
    170 {
    171 #ifdef _REENTRANT
    172 	if (__isthreaded) {
    173 		mutex_lock(&rs.mtx);
    174                 _arc4random_stir_unlocked();
    175 		mutex_unlock(&rs.mtx);
    176 		return;
    177         }
    178 #endif
    179 	_arc4random_stir_unlocked();
    180 }
    181 
    182 static inline void
    183 _arc4random_addrandom_unlocked(u_char *dat, int datlen)
    184 {
    185 	if (__predict_false(rs.initialized)) {
    186 		arc4_init(&rs);
    187 	}
    188 	arc4_addrandom(&rs, dat, datlen);
    189 }
    190 
    191 void
    192 arc4random_addrandom(u_char *dat, int datlen)
    193 {
    194 #ifdef _REENTRANT
    195 	if (__isthreaded) {
    196 		mutex_lock(&rs.mtx);
    197 		_arc4random_addrandom_unlocked(dat, datlen);
    198 		mutex_unlock(&rs.mtx);
    199 		return;
    200 	}
    201 #endif
    202 	_arc4random_addrandom_unlocked(dat, datlen);
    203 }
    204 
    205 static inline uint32_t
    206 _arc4random_unlocked(void)
    207 {
    208 	if (__predict_false(!rs.initialized)) {
    209 		arc4_init(&rs);
    210 	}
    211 	return arc4_getword(&rs);
    212 }
    213 
    214 uint32_t
    215 arc4random(void)
    216 {
    217 	uint32_t v;
    218 #ifdef _REENTRANT
    219 	if (__isthreaded) {
    220 		mutex_lock(&rs.mtx);
    221 		v = _arc4random_unlocked();
    222 		mutex_unlock(&rs.mtx);
    223 		return v;
    224 	}
    225 #endif
    226 	v = _arc4random_unlocked();
    227 	return v;
    228 }
    229 
    230 static void
    231 _arc4random_buf_unlocked(void *buf, size_t len)
    232 {
    233 	uint8_t *bp = buf;
    234 	uint8_t *ep = bp + len;
    235 
    236 	if (__predict_false(!rs.initialized)) {
    237 		arc4_init(&rs);
    238 	}
    239 
    240 	bp[0] = arc4_getbyte(&rs) % 3;
    241 	while (bp[0]--)
    242 		(void)arc4_getbyte(&rs);
    243 
    244 	while (bp < ep)
    245 		*bp++ = arc4_getbyte(&rs);
    246 }
    247 
    248 void
    249 arc4random_buf(void *buf, size_t len)
    250 {
    251 #ifdef _REENTRANT
    252 	if (__isthreaded) {
    253 		mutex_lock(&rs.mtx);
    254 		_arc4random_buf_unlocked(buf, len);
    255 		mutex_unlock(&rs.mtx);
    256 		return;
    257 	} else
    258 #endif
    259 	_arc4random_buf_unlocked(buf, len);
    260 }
    261 
    262 /*-
    263  * Written by Damien Miller.
    264  * With simplifications by Jinmei Tatuya.
    265  */
    266 
    267 /*
    268  * Calculate a uniformly distributed random number less than
    269  * upper_bound avoiding "modulo bias".
    270  *
    271  * Uniformity is achieved by generating new random numbers
    272  * until the one returned is outside the range
    273  * [0, 2^32 % upper_bound[. This guarantees the selected
    274  * random number will be inside the range
    275  * [2^32 % upper_bound, 2^32[ which maps back to
    276  * [0, upper_bound[ after reduction modulo upper_bound.
    277  */
    278 static uint32_t
    279 _arc4random_uniform_unlocked(uint32_t upper_bound)
    280 {
    281 	uint32_t r, min;
    282 
    283 	if (upper_bound < 2)
    284 		return 0;
    285 
    286 #if defined(ULONG_MAX) && (ULONG_MAX > 0xFFFFFFFFUL)
    287 	min = 0x100000000UL % upper_bound;
    288 #else
    289 	/* calculate (2^32 % upper_bound) avoiding 64-bit math */
    290 	if (upper_bound > 0x80000000U)
    291 		/* 2^32 - upper_bound (only one "value area") */
    292 		min = 1 + ~upper_bound;
    293 	else
    294 		/* ((2^32 - x) % x) == (2^32 % x) when x <= 2^31 */
    295 		min = (0xFFFFFFFFU - upper_bound + 1) % upper_bound;
    296 #endif
    297 
    298 	/*
    299 	 * This could theoretically loop forever but each retry has
    300 	 * p > 0.5 (worst case, usually far better) of selecting a
    301 	 * number inside the range we need, so it should rarely need
    302 	 * to re-roll (at all).
    303 	 */
    304 	if (__predict_false(!rs.initialized)) {
    305 		arc4_init(&rs);
    306 	}
    307 	if (arc4_getbyte(&rs) & 1)
    308 		(void)arc4_getbyte(&rs);
    309 	do
    310 		r = arc4_getword(&rs);
    311 	while (r < min);
    312 
    313 	return r % upper_bound;
    314 }
    315 
    316 uint32_t
    317 arc4random_uniform(uint32_t upper_bound)
    318 {
    319 	uint32_t v;
    320 #ifdef _REENTRANT
    321 	if (__isthreaded) {
    322 		mutex_lock(&rs.mtx);
    323 		v = _arc4random_uniform_unlocked(upper_bound);
    324 		mutex_unlock(&rs.mtx);
    325 		return v;
    326 	}
    327 #endif
    328 	v = _arc4random_uniform_unlocked(upper_bound);
    329 	return v;
    330 }
    331