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arc4random.c revision 1.15 
      1 /*	$NetBSD: arc4random.c,v 1.15 2012/08/18 14:42:46 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 1.15 2012/08/18 14:42:46 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_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 __noinline void
     65 arc4_init(struct arc4_stream *as)
     66 {
     67 	int n;
     68 	for (n = 0; n < RSIZE; n++)
     69 		as->s[n] = n;
     70 	as->i = 0;
     71 	as->j = 0;
     72 
     73 	as->initialized = 1;
     74 	arc4_stir(as);
     75 }
     76 
     77 static inline int
     78 arc4_check_init(struct arc4_stream *as)
     79 {
     80 	if (__predict_true(rs.initialized))
     81 		return 0;
     82 
     83 	arc4_init(as);
     84 	return 1;
     85 }
     86 
     87 static inline void
     88 arc4_addrandom(struct arc4_stream *as, u_char *dat, int datlen)
     89 {
     90 	uint8_t si;
     91 	int n;
     92 
     93 	as->i--;
     94 	for (n = 0; n < RSIZE; n++) {
     95 		as->i = (as->i + 1);
     96 		si = as->s[as->i];
     97 		as->j = (as->j + si + dat[n % datlen]);
     98 		as->s[as->i] = as->s[as->j];
     99 		as->s[as->j] = si;
    100 	}
    101 	as->j = as->i;
    102 }
    103 
    104 static void
    105 arc4_stir(struct arc4_stream *as)
    106 {
    107 	int rdat[32];
    108 	int mib[] = { CTL_KERN, KERN_URND };
    109 	size_t len;
    110 	size_t i, j;
    111 
    112 	/*
    113 	 * This code once opened and read /dev/urandom on each
    114 	 * call.  That causes repeated rekeying of the kernel stream
    115 	 * generator, which is very wasteful.  Because of application
    116 	 * behavior, caching the fd doesn't really help.  So we just
    117 	 * fill up the tank from sysctl, which is a tiny bit slower
    118 	 * for us but much friendlier to other entropy consumers.
    119 	 */
    120 
    121 	for (i = 0; i < __arraycount(rdat); i++) {
    122 		len = sizeof(rdat[i]);
    123 		if (sysctl(mib, 2, &rdat[i], &len, NULL, 0) == -1)
    124 			abort();
    125 	}
    126 
    127 	arc4_addrandom(as, (void *) &rdat, (int)sizeof(rdat));
    128 
    129 	/*
    130 	 * Throw away the first N words of output, as suggested in the
    131 	 * paper "Weaknesses in the Key Scheduling Algorithm of RC4"
    132 	 * by Fluher, Mantin, and Shamir.  (N = 256 in our case.)
    133 	 */
    134 	for (j = 0; j < RSIZE * 4; j++)
    135 		arc4_getbyte(as);
    136 }
    137 
    138 static inline uint8_t
    139 arc4_getbyte(struct arc4_stream *as)
    140 {
    141 	uint8_t si, sj;
    142 
    143 	as->i = (as->i + 1);
    144 	si = as->s[as->i];
    145 	as->j = (as->j + si);
    146 	sj = as->s[as->j];
    147 	as->s[as->i] = sj;
    148 	as->s[as->j] = si;
    149 	return (as->s[(si + sj) & 0xff]);
    150 }
    151 
    152 static inline uint32_t
    153 arc4_getword(struct arc4_stream *as)
    154 {
    155 	uint32_t val;
    156 	val = arc4_getbyte(as) << 24;
    157 	val |= arc4_getbyte(as) << 16;
    158 	val |= arc4_getbyte(as) << 8;
    159 	val |= arc4_getbyte(as);
    160 	return val;
    161 }
    162 
    163 static inline void
    164 _arc4random_stir_unlocked(void)
    165 {
    166 	if (__predict_false(!arc4_check_init(&rs)))	/* init() stirs */
    167 		arc4_stir(&rs);
    168 }
    169 
    170 void
    171 arc4random_stir(void)
    172 {
    173 #ifdef _REENTRANT
    174 	if (__isthreaded) {
    175 		mutex_lock(&rs.mtx);
    176                 _arc4random_stir_unlocked();
    177 		mutex_unlock(&rs.mtx);
    178 		return;
    179         }
    180 #endif
    181 	_arc4random_stir_unlocked();
    182 }
    183 
    184 static inline void
    185 _arc4random_addrandom_unlocked(u_char *dat, int datlen)
    186 {
    187 	arc4_check_init(&rs);
    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 	arc4_check_init(&rs);
    209 	return arc4_getword(&rs);
    210 }
    211 
    212 uint32_t
    213 arc4random(void)
    214 {
    215 	uint32_t v;
    216 #ifdef _REENTRANT
    217 	if (__isthreaded) {
    218 		mutex_lock(&rs.mtx);
    219 		v = _arc4random_unlocked();
    220 		mutex_unlock(&rs.mtx);
    221 		return v;
    222 	}
    223 #endif
    224 	v = _arc4random_unlocked();
    225 	return v;
    226 }
    227 
    228 static void
    229 _arc4random_buf_unlocked(void *buf, size_t len)
    230 {
    231 	uint8_t *bp = buf;
    232 	uint8_t *ep = bp + len;
    233 
    234 	arc4_check_init(&rs);
    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 	arc4_check_init(&rs);
    301 	if (arc4_getbyte(&rs) & 1)
    302 		(void)arc4_getbyte(&rs);
    303 	do
    304 		r = arc4_getword(&rs);
    305 	while (r < min);
    306 
    307 	return r % upper_bound;
    308 }
    309 
    310 uint32_t
    311 arc4random_uniform(uint32_t upper_bound)
    312 {
    313 	uint32_t v;
    314 #ifdef _REENTRANT
    315 	if (__isthreaded) {
    316 		mutex_lock(&rs.mtx);
    317 		v = _arc4random_uniform_unlocked(upper_bound);
    318 		mutex_unlock(&rs.mtx);
    319 		return v;
    320 	}
    321 #endif
    322 	v = _arc4random_uniform_unlocked(upper_bound);
    323 	return v;
    324 }
    325