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arc4random.c revision 1.23 
      1 /*	$NetBSD: arc4random.c,v 1.23 2014/06/12 19:05:37 apb 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 1.23 2014/06/12 19:05:37 apb 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 #define REKEY_BYTES	1600000
     54 
     55 struct arc4_stream {
     56 	bool inited;
     57 	uint8_t i;
     58 	uint8_t j;
     59 	uint8_t s[(uint8_t)~0u + 1u];	/* 256 to you and me */
     60 	size_t count;
     61 	mutex_t mtx;
     62 };
     63 
     64 #ifdef _REENTRANT
     65 #define LOCK(rs)	do { \
     66 				if (__isthreaded) mutex_lock(&(rs)->mtx);
     67 			} while (/*CONSTCOND*/ 0)
     68 #define UNLOCK(rs)	do { \
     69 				if (__isthreaded) mutex_unlock(&(rs)->mtx); \
     70 			} while (/*CONSTCOND*/ 0)
     71 #else
     72 #define LOCK(rs)
     73 #define UNLOCK(rs)
     74 #endif
     75 
     76 #define S(n) (n)
     77 #define S4(n) S(n), S(n + 1), S(n + 2), S(n + 3)
     78 #define S16(n) S4(n), S4(n + 4), S4(n + 8), S4(n + 12)
     79 #define S64(n) S16(n), S16(n + 16), S16(n + 32), S16(n + 48)
     80 #define S256 S64(0), S64(64), S64(128), S64(192)
     81 
     82 static struct arc4_stream rs = { .inited = false,
     83 		.i = 0xff, .j = 0, .s = { S256 },
     84 		.count = 0, .mtx = MUTEX_INITIALIZER };
     85 
     86 #undef S
     87 #undef S4
     88 #undef S16
     89 #undef S64
     90 #undef S256
     91 
     92 static inline void arc4_addrandom(struct arc4_stream *, u_char *, int);
     93 static __noinline void arc4_stir(struct arc4_stream *);
     94 static inline uint8_t arc4_getbyte(struct arc4_stream *);
     95 static inline uint32_t arc4_getword(struct arc4_stream *);
     96 
     97 #ifdef _REENTRANT
     98 static void
     99 arc4_fork_prepare(void)
    100 {
    101 
    102 	LOCK(&rs);
    103 }
    104 
    105 static void
    106 arc4_fork_parent(void)
    107 {
    108 
    109 	UNLOCK(&rs);
    110 }
    111 #else
    112 #define arc4_fork_prepare	NULL
    113 #define arc4_fork_parent	NULL
    114 #endif
    115 
    116 static void
    117 arc4_fork_child(void)
    118 {
    119 
    120 	/* Reset the counter to a force new stir after forking */
    121 	rs.count = 0;
    122 	UNLOCK(&rs);
    123 }
    124 
    125 static inline void
    126 arc4_check_init(struct arc4_stream *as)
    127 {
    128 
    129 	if (__predict_false(!as->inited)) {
    130 		as->inited = true;
    131 		pthread_atfork(arc4_fork_prepare,
    132 		    arc4_fork_parent, arc4_fork_child);
    133 	}
    134 }
    135 
    136 static inline void
    137 arc4_addrandom(struct arc4_stream *as, u_char *dat, int datlen)
    138 {
    139 	uint8_t si;
    140 	size_t n;
    141 
    142 	for (n = 0; n < __arraycount(as->s); n++) {
    143 		as->i = (as->i + 1);
    144 		si = as->s[as->i];
    145 		as->j = (as->j + si + dat[n % datlen]);
    146 		as->s[as->i] = as->s[as->j];
    147 		as->s[as->j] = si;
    148 	}
    149 }
    150 
    151 static __noinline void
    152 arc4_stir(struct arc4_stream *as)
    153 {
    154 	int rdat[32];
    155 	int mib[] = { CTL_KERN, KERN_URND };
    156 	size_t len;
    157 	size_t i, j;
    158 
    159 	arc4_check_init(as);
    160 
    161 	/*
    162 	 * This code once opened and read /dev/urandom on each
    163 	 * call.  That causes repeated rekeying of the kernel stream
    164 	 * generator, which is very wasteful.  Because of application
    165 	 * behavior, caching the fd doesn't really help.  So we just
    166 	 * fill up the tank from sysctl, which is a tiny bit slower
    167 	 * for us but much friendlier to other entropy consumers.
    168 	 */
    169 
    170 	for (i = 0; i < __arraycount(rdat); i++) {
    171 		len = sizeof(rdat[i]);
    172 		if (sysctl(mib, 2, &rdat[i], &len, NULL, 0) == -1)
    173 			abort();
    174 	}
    175 
    176 	arc4_addrandom(as, (void *) &rdat, (int)sizeof(rdat));
    177 
    178 	/*
    179 	 * Throw away the first N words of output, as suggested in the
    180 	 * paper "Weaknesses in the Key Scheduling Algorithm of RC4"
    181 	 * by Fluher, Mantin, and Shamir.  (N = 256 in our case.)
    182 	 */
    183 	for (j = 0; j < __arraycount(as->s) * sizeof(uint32_t); j++)
    184 		arc4_getbyte(as);
    185 
    186 	/* Stir again after REKEY_BYTES bytes, or if the pid changes */
    187 	as->count = REKEY_BYTES;
    188 }
    189 
    190 static inline void
    191 arc4_stir_if_needed(struct arc4_stream *as, size_t len)
    192 {
    193 
    194 	if (__predict_false(as->count <= len))
    195 		arc4_stir(as);
    196 	else
    197 		as->count -= len;
    198 }
    199 
    200 static __inline uint8_t
    201 arc4_getbyte_ij(struct arc4_stream *as, uint8_t *i, uint8_t *j)
    202 {
    203 	uint8_t si, sj;
    204 
    205 	*i = *i + 1;
    206 	si = as->s[*i];
    207 	*j = *j + si;
    208 	sj = as->s[*j];
    209 	as->s[*i] = sj;
    210 	as->s[*j] = si;
    211 	return (as->s[(si + sj) & 0xff]);
    212 }
    213 
    214 static inline uint8_t
    215 arc4_getbyte(struct arc4_stream *as)
    216 {
    217 
    218 	return arc4_getbyte_ij(as, &as->i, &as->j);
    219 }
    220 
    221 static inline uint32_t
    222 arc4_getword(struct arc4_stream *as)
    223 {
    224 	uint32_t val;
    225 
    226 	val = arc4_getbyte(as) << 24;
    227 	val |= arc4_getbyte(as) << 16;
    228 	val |= arc4_getbyte(as) << 8;
    229 	val |= arc4_getbyte(as);
    230 	return val;
    231 }
    232 
    233 void
    234 arc4random_stir(void)
    235 {
    236 
    237 	LOCK(&rs);
    238 	arc4_stir(&rs);
    239 	UNLOCK(&rs);
    240 }
    241 
    242 void
    243 arc4random_addrandom(u_char *dat, int datlen)
    244 {
    245 
    246 	LOCK(&rs);
    247 	arc4_stir_if_needed(&rs, datlen);
    248 	arc4_addrandom(&rs, dat, datlen);
    249 	UNLOCK(&rs);
    250 }
    251 
    252 uint32_t
    253 arc4random(void)
    254 {
    255 	uint32_t v;
    256 
    257 	LOCK(&rs);
    258 	arc4_stir_if_needed(&rs, sizeof(v));
    259 	v = arc4_getword(&rs);
    260 	UNLOCK(&rs);
    261 	return v;
    262 }
    263 
    264 void
    265 arc4random_buf(void *buf, size_t len)
    266 {
    267 	uint8_t *bp = buf;
    268 	uint8_t *ep = bp + len;
    269 	uint8_t i, j;
    270 
    271 	LOCK(&rs);
    272 	arc4_stir_if_needed(&rs, len);
    273 
    274 	/* cache i and j - compiler can't know 'buf' doesn't alias them */
    275 	i = rs.i;
    276 	j = rs.j;
    277 
    278 	while (bp < ep)
    279 		*bp++ = arc4_getbyte_ij(&rs, &i, &j);
    280 	rs.i = i;
    281 	rs.j = j;
    282 
    283 	UNLOCK(&rs);
    284 }
    285 
    286 /*-
    287  * Written by Damien Miller.
    288  * With simplifications by Jinmei Tatuya.
    289  */
    290 
    291 /*
    292  * Calculate a uniformly distributed random number less than
    293  * upper_bound avoiding "modulo bias".
    294  *
    295  * Uniformity is achieved by generating new random numbers
    296  * until the one returned is outside the range
    297  * [0, 2^32 % upper_bound[. This guarantees the selected
    298  * random number will be inside the range
    299  * [2^32 % upper_bound, 2^32[ which maps back to
    300  * [0, upper_bound[ after reduction modulo upper_bound.
    301  */
    302 uint32_t
    303 arc4random_uniform(uint32_t upper_bound)
    304 {
    305 	uint32_t r, min;
    306 
    307 	if (upper_bound < 2)
    308 		return 0;
    309 
    310 	/* calculate (2^32 % upper_bound) avoiding 64-bit math */
    311 	/* ((2^32 - x) % x) == (2^32 % x) when x <= 2^31 */
    312 	min = (0xFFFFFFFFU - upper_bound + 1) % upper_bound;
    313 
    314 	LOCK(&rs);
    315 	arc4_stir_if_needed(&rs, sizeof(r));
    316 
    317 	/*
    318 	 * This could theoretically loop forever but each retry has
    319 	 * p > 0.5 (worst case, usually far better) of selecting a
    320 	 * number inside the range we need, so it should rarely need
    321 	 * to re-roll (at all).
    322 	 */
    323 	do
    324 		r = arc4_getword(&rs);
    325 	while (r < min);
    326 	UNLOCK(&rs);
    327 
    328 	return r % upper_bound;
    329 }
    330