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arc4random.c revision 1.22 
      1 /*	$NetBSD: arc4random.c,v 1.22 2014/06/07 20:55:47 roy 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 1.22 2014/06/07 20:55:47 roy Exp $");
     31 #endif /* LIBC_SCCS and not lint */
     32 
     33 #include "namespace.h"
     34 #include "reentrant.h"
     35 #include <fcntl.h>
     36 #include <pthread.h>
     37 #include <stdbool.h>
     38 #include <stdlib.h>
     39 #include <unistd.h>
     40 #include <sys/types.h>
     41 #include <sys/param.h>
     42 #include <sys/time.h>
     43 #include <sys/sysctl.h>
     44 
     45 #ifdef __weak_alias
     46 __weak_alias(arc4random,_arc4random)
     47 __weak_alias(arc4random_addrandom,_arc4random_addrandom)
     48 __weak_alias(arc4random_buf,_arc4random_buf)
     49 __weak_alias(arc4random_stir,_arc4random_stir)
     50 __weak_alias(arc4random_uniform,_arc4random_uniform)
     51 #endif
     52 
     53 struct arc4_stream {
     54 	bool inited;
     55 	uint8_t i;
     56 	uint8_t j;
     57 	uint8_t s[(uint8_t)~0u + 1u];	/* 256 to you and me */
     58 	size_t count;
     59 	mutex_t mtx;
     60 };
     61 
     62 #ifdef _REENTRANT
     63 #define LOCK(rs)	if (__isthreaded) mutex_lock(&(rs)->mtx);
     64 #define UNLOCK(rs)	if (__isthreaded) mutex_unlock(&(rs)->mtx);
     65 #else
     66 #define LOCK(rs)
     67 #define UNLOCK(rs)
     68 #endif
     69 
     70 #define S(n) (n)
     71 #define S4(n) S(n), S(n + 1), S(n + 2), S(n + 3)
     72 #define S16(n) S4(n), S4(n + 4), S4(n + 8), S4(n + 12)
     73 #define S64(n) S16(n), S16(n + 16), S16(n + 32), S16(n + 48)
     74 #define S256 S64(0), S64(64), S64(128), S64(192)
     75 
     76 static struct arc4_stream rs = { .inited = false,
     77 		.i = 0xff, .j = 0, .s = { S256 },
     78 		.count = 0, .mtx = MUTEX_INITIALIZER };
     79 
     80 #undef S
     81 #undef S4
     82 #undef S16
     83 #undef S64
     84 #undef S256
     85 
     86 static inline void arc4_addrandom(struct arc4_stream *, u_char *, int);
     87 static __noinline void arc4_stir(struct arc4_stream *);
     88 static inline uint8_t arc4_getbyte(struct arc4_stream *);
     89 static inline uint32_t arc4_getword(struct arc4_stream *);
     90 
     91 #ifdef _REENTRANT
     92 static void
     93 arc4_fork_prepare(void)
     94 {
     95 
     96 	LOCK(&rs);
     97 }
     98 
     99 static void
    100 arc4_fork_parent(void)
    101 {
    102 
    103 	UNLOCK(&rs);
    104 }
    105 #else
    106 #define arc4_fork_prepare	NULL
    107 #define arc4_fork_parent	NULL
    108 #endif
    109 
    110 static void
    111 arc4_fork_child(void)
    112 {
    113 
    114 	/* Reset the counter to a force new stir after forking */
    115 	rs.count = 0;
    116 	UNLOCK(&rs);
    117 }
    118 
    119 static inline void
    120 arc4_check_init(struct arc4_stream *as)
    121 {
    122 
    123 	if (__predict_false(!as->inited)) {
    124 		as->inited = true;
    125 		pthread_atfork(arc4_fork_prepare,
    126 		    arc4_fork_parent, arc4_fork_child);
    127 	}
    128 }
    129 
    130 static inline void
    131 arc4_addrandom(struct arc4_stream *as, u_char *dat, int datlen)
    132 {
    133 	uint8_t si;
    134 	size_t n;
    135 
    136 	for (n = 0; n < __arraycount(as->s); n++) {
    137 		as->i = (as->i + 1);
    138 		si = as->s[as->i];
    139 		as->j = (as->j + si + dat[n % datlen]);
    140 		as->s[as->i] = as->s[as->j];
    141 		as->s[as->j] = si;
    142 	}
    143 }
    144 
    145 static __noinline void
    146 arc4_stir(struct arc4_stream *as)
    147 {
    148 	int rdat[32];
    149 	int mib[] = { CTL_KERN, KERN_URND };
    150 	size_t len;
    151 	size_t i, j;
    152 
    153 	arc4_check_init(as);
    154 
    155 	/*
    156 	 * This code once opened and read /dev/urandom on each
    157 	 * call.  That causes repeated rekeying of the kernel stream
    158 	 * generator, which is very wasteful.  Because of application
    159 	 * behavior, caching the fd doesn't really help.  So we just
    160 	 * fill up the tank from sysctl, which is a tiny bit slower
    161 	 * for us but much friendlier to other entropy consumers.
    162 	 */
    163 
    164 	for (i = 0; i < __arraycount(rdat); i++) {
    165 		len = sizeof(rdat[i]);
    166 		if (sysctl(mib, 2, &rdat[i], &len, NULL, 0) == -1)
    167 			abort();
    168 	}
    169 
    170 	arc4_addrandom(as, (void *) &rdat, (int)sizeof(rdat));
    171 
    172 	/*
    173 	 * Throw away the first N words of output, as suggested in the
    174 	 * paper "Weaknesses in the Key Scheduling Algorithm of RC4"
    175 	 * by Fluher, Mantin, and Shamir.  (N = 256 in our case.)
    176 	 */
    177 	for (j = 0; j < __arraycount(as->s) * sizeof(uint32_t); j++)
    178 		arc4_getbyte(as);
    179 
    180 	/* Stir again after swallowing 1600000 bytes or if the pid changes */
    181 	as->count = 1600000;
    182 }
    183 
    184 static inline void
    185 arc4_stir_if_needed(struct arc4_stream *as, size_t len)
    186 {
    187 
    188 	if (__predict_false(as->count <= len))
    189 		arc4_stir(as);
    190 	else
    191 		as->count -= len;
    192 }
    193 
    194 static __inline uint8_t
    195 arc4_getbyte_ij(struct arc4_stream *as, uint8_t *i, uint8_t *j)
    196 {
    197 	uint8_t si, sj;
    198 
    199 	*i = *i + 1;
    200 	si = as->s[*i];
    201 	*j = *j + si;
    202 	sj = as->s[*j];
    203 	as->s[*i] = sj;
    204 	as->s[*j] = si;
    205 	return (as->s[(si + sj) & 0xff]);
    206 }
    207 
    208 static inline uint8_t
    209 arc4_getbyte(struct arc4_stream *as)
    210 {
    211 
    212 	return arc4_getbyte_ij(as, &as->i, &as->j);
    213 }
    214 
    215 static inline uint32_t
    216 arc4_getword(struct arc4_stream *as)
    217 {
    218 	uint32_t val;
    219 
    220 	val = arc4_getbyte(as) << 24;
    221 	val |= arc4_getbyte(as) << 16;
    222 	val |= arc4_getbyte(as) << 8;
    223 	val |= arc4_getbyte(as);
    224 	return val;
    225 }
    226 
    227 void
    228 arc4random_stir(void)
    229 {
    230 
    231 	LOCK(&rs);
    232 	arc4_stir(&rs);
    233 	UNLOCK(&rs);
    234 }
    235 
    236 void
    237 arc4random_addrandom(u_char *dat, int datlen)
    238 {
    239 
    240 	LOCK(&rs);
    241 	arc4_stir_if_needed(&rs, datlen);
    242 	arc4_addrandom(&rs, dat, datlen);
    243 	UNLOCK(&rs);
    244 }
    245 
    246 uint32_t
    247 arc4random(void)
    248 {
    249 	uint32_t v;
    250 
    251 	LOCK(&rs);
    252 	arc4_stir_if_needed(&rs, sizeof(v));
    253 	v = arc4_getword(&rs);
    254 	UNLOCK(&rs);
    255 	return v;
    256 }
    257 
    258 void
    259 arc4random_buf(void *buf, size_t len)
    260 {
    261 	uint8_t *bp = buf;
    262 	uint8_t *ep = bp + len;
    263 	uint8_t i, j;
    264 
    265 	LOCK(&rs);
    266 	arc4_stir_if_needed(&rs, len);
    267 
    268 	/* cache i and j - compiler can't know 'buf' doesn't alias them */
    269 	i = rs.i;
    270 	j = rs.j;
    271 
    272 	while (bp < ep)
    273 		*bp++ = arc4_getbyte_ij(&rs, &i, &j);
    274 	rs.i = i;
    275 	rs.j = j;
    276 
    277 	UNLOCK(&rs);
    278 }
    279 
    280 /*-
    281  * Written by Damien Miller.
    282  * With simplifications by Jinmei Tatuya.
    283  */
    284 
    285 /*
    286  * Calculate a uniformly distributed random number less than
    287  * upper_bound avoiding "modulo bias".
    288  *
    289  * Uniformity is achieved by generating new random numbers
    290  * until the one returned is outside the range
    291  * [0, 2^32 % upper_bound[. This guarantees the selected
    292  * random number will be inside the range
    293  * [2^32 % upper_bound, 2^32[ which maps back to
    294  * [0, upper_bound[ after reduction modulo upper_bound.
    295  */
    296 uint32_t
    297 arc4random_uniform(uint32_t upper_bound)
    298 {
    299 	uint32_t r, min;
    300 
    301 	if (upper_bound < 2)
    302 		return 0;
    303 
    304 	/* calculate (2^32 % upper_bound) avoiding 64-bit math */
    305 	/* ((2^32 - x) % x) == (2^32 % x) when x <= 2^31 */
    306 	min = (0xFFFFFFFFU - upper_bound + 1) % upper_bound;
    307 
    308 	LOCK(&rs);
    309 	arc4_stir_if_needed(&rs, sizeof(r));
    310 
    311 	/*
    312 	 * This could theoretically loop forever but each retry has
    313 	 * p > 0.5 (worst case, usually far better) of selecting a
    314 	 * number inside the range we need, so it should rarely need
    315 	 * to re-roll (at all).
    316 	 */
    317 	do
    318 		r = arc4_getword(&rs);
    319 	while (r < min);
    320 	UNLOCK(&rs);
    321 
    322 	return r % upper_bound;
    323 }
    324