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arc4random.c revision 1.14 
      1 /*	$NetBSD: arc4random.c,v 1.14 2012/07/29 14:44:13 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 1.14 2012/07/29 14:44:13 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 /* XXX lint explodes with an internal error if only mtx is initialized! */
     57 static struct arc4_stream rs = { .i = 0, .mtx = MUTEX_INITIALIZER };
     58 
     59 static inline void arc4_init(struct arc4_stream *);
     60 static inline void arc4_addrandom(struct arc4_stream *, u_char *, int);
     61 static void arc4_stir(struct arc4_stream *);
     62 static inline uint8_t arc4_getbyte(struct arc4_stream *);
     63 static inline uint32_t arc4_getword(struct arc4_stream *);
     64 
     65 static inline void
     66 arc4_init(struct arc4_stream *as)
     67 {
     68 	int n;
     69 	for (n = 0; n < RSIZE; 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 	uint8_t si;
     82 	int n;
     83 
     84 	as->i--;
     85 	for (n = 0; n < RSIZE; 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[32];
     99 	int mib[] = { CTL_KERN, KERN_URND };
    100 	size_t len;
    101 	size_t i, j;
    102 
    103 	/*
    104 	 * This code once opened and read /dev/urandom on each
    105 	 * call.  That causes repeated rekeying of the kernel stream
    106 	 * generator, which is very wasteful.  Because of application
    107 	 * behavior, caching the fd doesn't really help.  So we just
    108 	 * fill up the tank from sysctl, which is a tiny bit slower
    109 	 * for us but much friendlier to other entropy consumers.
    110 	 */
    111 
    112 	for (i = 0; i < __arraycount(rdat); i++) {
    113 		len = sizeof(rdat[i]);
    114 		if (sysctl(mib, 2, &rdat[i], &len, NULL, 0) == -1)
    115 			abort();
    116 	}
    117 
    118 	arc4_addrandom(as, (void *) &rdat, (int)sizeof(rdat));
    119 
    120 	/*
    121 	 * Throw away the first N words of output, as suggested in the
    122 	 * paper "Weaknesses in the Key Scheduling Algorithm of RC4"
    123 	 * by Fluher, Mantin, and Shamir.  (N = 256 in our case.)
    124 	 */
    125 	for (j = 0; j < RSIZE * 4; j++)
    126 		arc4_getbyte(as);
    127 }
    128 
    129 static inline uint8_t
    130 arc4_getbyte(struct arc4_stream *as)
    131 {
    132 	uint8_t si, sj;
    133 
    134 	as->i = (as->i + 1);
    135 	si = as->s[as->i];
    136 	as->j = (as->j + si);
    137 	sj = as->s[as->j];
    138 	as->s[as->i] = sj;
    139 	as->s[as->j] = si;
    140 	return (as->s[(si + sj) & 0xff]);
    141 }
    142 
    143 static inline uint32_t
    144 arc4_getword(struct arc4_stream *as)
    145 {
    146 	uint32_t val;
    147 	val = arc4_getbyte(as) << 24;
    148 	val |= arc4_getbyte(as) << 16;
    149 	val |= arc4_getbyte(as) << 8;
    150 	val |= arc4_getbyte(as);
    151 	return val;
    152 }
    153 
    154 static inline void
    155 _arc4random_stir_unlocked(void)
    156 {
    157 	if (__predict_false(!rs.initialized)) {
    158 		arc4_init(&rs);				/* stirs */
    159 	} else {
    160 		arc4_stir(&rs);
    161 	}
    162 }
    163 
    164 void
    165 arc4random_stir(void)
    166 {
    167 #ifdef _REENTRANT
    168 	if (__isthreaded) {
    169 		mutex_lock(&rs.mtx);
    170                 _arc4random_stir_unlocked();
    171 		mutex_unlock(&rs.mtx);
    172 		return;
    173         }
    174 #endif
    175 	_arc4random_stir_unlocked();
    176 }
    177 
    178 static inline void
    179 _arc4random_addrandom_unlocked(u_char *dat, int datlen)
    180 {
    181 	if (__predict_false(rs.initialized)) {
    182 		arc4_init(&rs);
    183 	}
    184 	arc4_addrandom(&rs, dat, datlen);
    185 }
    186 
    187 void
    188 arc4random_addrandom(u_char *dat, int datlen)
    189 {
    190 #ifdef _REENTRANT
    191 	if (__isthreaded) {
    192 		mutex_lock(&rs.mtx);
    193 		_arc4random_addrandom_unlocked(dat, datlen);
    194 		mutex_unlock(&rs.mtx);
    195 		return;
    196 	}
    197 #endif
    198 	_arc4random_addrandom_unlocked(dat, datlen);
    199 }
    200 
    201 static inline uint32_t
    202 _arc4random_unlocked(void)
    203 {
    204 	if (__predict_false(!rs.initialized)) {
    205 		arc4_init(&rs);
    206 	}
    207 	return arc4_getword(&rs);
    208 }
    209 
    210 uint32_t
    211 arc4random(void)
    212 {
    213 	uint32_t v;
    214 #ifdef _REENTRANT
    215 	if (__isthreaded) {
    216 		mutex_lock(&rs.mtx);
    217 		v = _arc4random_unlocked();
    218 		mutex_unlock(&rs.mtx);
    219 		return v;
    220 	}
    221 #endif
    222 	v = _arc4random_unlocked();
    223 	return v;
    224 }
    225 
    226 static void
    227 _arc4random_buf_unlocked(void *buf, size_t len)
    228 {
    229 	uint8_t *bp = buf;
    230 	uint8_t *ep = bp + len;
    231 
    232 	if (__predict_false(!rs.initialized)) {
    233 		arc4_init(&rs);
    234 	}
    235 
    236 	bp[0] = arc4_getbyte(&rs) % 3;
    237 	while (bp[0]--)
    238 		(void)arc4_getbyte(&rs);
    239 
    240 	while (bp < ep)
    241 		*bp++ = arc4_getbyte(&rs);
    242 }
    243 
    244 void
    245 arc4random_buf(void *buf, size_t len)
    246 {
    247 #ifdef _REENTRANT
    248 	if (__isthreaded) {
    249 		mutex_lock(&rs.mtx);
    250 		_arc4random_buf_unlocked(buf, len);
    251 		mutex_unlock(&rs.mtx);
    252 		return;
    253 	} else
    254 #endif
    255 	_arc4random_buf_unlocked(buf, len);
    256 }
    257 
    258 /*-
    259  * Written by Damien Miller.
    260  * With simplifications by Jinmei Tatuya.
    261  */
    262 
    263 /*
    264  * Calculate a uniformly distributed random number less than
    265  * upper_bound avoiding "modulo bias".
    266  *
    267  * Uniformity is achieved by generating new random numbers
    268  * until the one returned is outside the range
    269  * [0, 2^32 % upper_bound[. This guarantees the selected
    270  * random number will be inside the range
    271  * [2^32 % upper_bound, 2^32[ which maps back to
    272  * [0, upper_bound[ after reduction modulo upper_bound.
    273  */
    274 static uint32_t
    275 _arc4random_uniform_unlocked(uint32_t upper_bound)
    276 {
    277 	uint32_t r, min;
    278 
    279 	if (upper_bound < 2)
    280 		return 0;
    281 
    282 #if defined(ULONG_MAX) && (ULONG_MAX > 0xFFFFFFFFUL)
    283 	min = (uint32_t)(0x100000000U % upper_bound);
    284 #else
    285 	/* calculate (2^32 % upper_bound) avoiding 64-bit math */
    286 	if (upper_bound > 0x80000000U)
    287 		/* 2^32 - upper_bound (only one "value area") */
    288 		min = 1 + ~upper_bound;
    289 	else
    290 		/* ((2^32 - x) % x) == (2^32 % x) when x <= 2^31 */
    291 		min = (0xFFFFFFFFU - upper_bound + 1) % upper_bound;
    292 #endif
    293 
    294 	/*
    295 	 * This could theoretically loop forever but each retry has
    296 	 * p > 0.5 (worst case, usually far better) of selecting a
    297 	 * number inside the range we need, so it should rarely need
    298 	 * to re-roll (at all).
    299 	 */
    300 	if (__predict_false(!rs.initialized)) {
    301 		arc4_init(&rs);
    302 	}
    303 	if (arc4_getbyte(&rs) & 1)
    304 		(void)arc4_getbyte(&rs);
    305 	do
    306 		r = arc4_getword(&rs);
    307 	while (r < min);
    308 
    309 	return r % upper_bound;
    310 }
    311 
    312 uint32_t
    313 arc4random_uniform(uint32_t upper_bound)
    314 {
    315 	uint32_t v;
    316 #ifdef _REENTRANT
    317 	if (__isthreaded) {
    318 		mutex_lock(&rs.mtx);
    319 		v = _arc4random_uniform_unlocked(upper_bound);
    320 		mutex_unlock(&rs.mtx);
    321 		return v;
    322 	}
    323 #endif
    324 	v = _arc4random_uniform_unlocked(upper_bound);
    325 	return v;
    326 }
    327