Home | History | Annotate | Line # | Download | only in gen
arc4random.c revision 1.25 
      1 /*	$NetBSD: arc4random.c,v 1.25 2014/07/19 14:53:22 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 1.25 2014/07/19 14:53:22 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 #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 	 * pthread_atfork(3) only allows async-signal-safe functions in
    130 	 * the child handler.
    131 	 * NetBSD's mutex_unlock is async-signal safe, other implementations
    132 	 * may not be.
    133 	 */
    134 
    135 	if (__predict_false(!as->inited)) {
    136 		as->inited = true;
    137 		pthread_atfork(arc4_fork_prepare,
    138 		    arc4_fork_parent, arc4_fork_child);
    139 	}
    140 }
    141 
    142 static inline void
    143 arc4_addrandom(struct arc4_stream *as, u_char *dat, int datlen)
    144 {
    145 	uint8_t si;
    146 	size_t n;
    147 
    148 	for (n = 0; n < __arraycount(as->s); n++) {
    149 		as->i = (as->i + 1);
    150 		si = as->s[as->i];
    151 		as->j = (as->j + si + dat[n % datlen]);
    152 		as->s[as->i] = as->s[as->j];
    153 		as->s[as->j] = si;
    154 	}
    155 }
    156 
    157 static __noinline void
    158 arc4_stir(struct arc4_stream *as)
    159 {
    160 	int rdat[32];
    161 	int mib[] = { CTL_KERN, KERN_URND };
    162 	size_t len;
    163 	size_t i, j;
    164 
    165 	arc4_check_init(as);
    166 
    167 	/*
    168 	 * This code once opened and read /dev/urandom on each
    169 	 * call.  That causes repeated rekeying of the kernel stream
    170 	 * generator, which is very wasteful.  Because of application
    171 	 * behavior, caching the fd doesn't really help.  So we just
    172 	 * fill up the tank from sysctl, which is a tiny bit slower
    173 	 * for us but much friendlier to other entropy consumers.
    174 	 */
    175 
    176 	for (i = 0; i < __arraycount(rdat); i++) {
    177 		len = sizeof(rdat[i]);
    178 		if (sysctl(mib, 2, &rdat[i], &len, NULL, 0) == -1)
    179 			abort();
    180 	}
    181 
    182 	arc4_addrandom(as, (void *) &rdat, (int)sizeof(rdat));
    183 
    184 	/*
    185 	 * Throw away the first N words of output, as suggested in the
    186 	 * paper "Weaknesses in the Key Scheduling Algorithm of RC4"
    187 	 * by Fluher, Mantin, and Shamir.  (N = 256 in our case.)
    188 	 */
    189 	for (j = 0; j < __arraycount(as->s) * sizeof(uint32_t); j++)
    190 		arc4_getbyte(as);
    191 
    192 	/* Stir again after REKEY_BYTES bytes, or if the pid changes */
    193 	as->count = REKEY_BYTES;
    194 }
    195 
    196 static inline void
    197 arc4_stir_if_needed(struct arc4_stream *as, size_t len)
    198 {
    199 
    200 	if (__predict_false(as->count <= len))
    201 		arc4_stir(as);
    202 	else
    203 		as->count -= len;
    204 }
    205 
    206 static __inline uint8_t
    207 arc4_getbyte_ij(struct arc4_stream *as, uint8_t *i, uint8_t *j)
    208 {
    209 	uint8_t si, sj;
    210 
    211 	*i = *i + 1;
    212 	si = as->s[*i];
    213 	*j = *j + si;
    214 	sj = as->s[*j];
    215 	as->s[*i] = sj;
    216 	as->s[*j] = si;
    217 	return (as->s[(si + sj) & 0xff]);
    218 }
    219 
    220 static inline uint8_t
    221 arc4_getbyte(struct arc4_stream *as)
    222 {
    223 
    224 	return arc4_getbyte_ij(as, &as->i, &as->j);
    225 }
    226 
    227 static inline uint32_t
    228 arc4_getword(struct arc4_stream *as)
    229 {
    230 	uint32_t val;
    231 
    232 	val = arc4_getbyte(as) << 24;
    233 	val |= arc4_getbyte(as) << 16;
    234 	val |= arc4_getbyte(as) << 8;
    235 	val |= arc4_getbyte(as);
    236 	return val;
    237 }
    238 
    239 void
    240 arc4random_stir(void)
    241 {
    242 
    243 	LOCK(&rs);
    244 	arc4_stir(&rs);
    245 	UNLOCK(&rs);
    246 }
    247 
    248 void
    249 arc4random_addrandom(u_char *dat, int datlen)
    250 {
    251 
    252 	LOCK(&rs);
    253 	arc4_stir_if_needed(&rs, datlen);
    254 	arc4_addrandom(&rs, dat, datlen);
    255 	UNLOCK(&rs);
    256 }
    257 
    258 uint32_t
    259 arc4random(void)
    260 {
    261 	uint32_t v;
    262 
    263 	LOCK(&rs);
    264 	arc4_stir_if_needed(&rs, sizeof(v));
    265 	v = arc4_getword(&rs);
    266 	UNLOCK(&rs);
    267 	return v;
    268 }
    269 
    270 void
    271 arc4random_buf(void *buf, size_t len)
    272 {
    273 	uint8_t *bp = buf;
    274 	uint8_t *ep = bp + len;
    275 	uint8_t i, j;
    276 
    277 	LOCK(&rs);
    278 	arc4_stir_if_needed(&rs, len);
    279 
    280 	/* cache i and j - compiler can't know 'buf' doesn't alias them */
    281 	i = rs.i;
    282 	j = rs.j;
    283 
    284 	while (bp < ep)
    285 		*bp++ = arc4_getbyte_ij(&rs, &i, &j);
    286 	rs.i = i;
    287 	rs.j = j;
    288 
    289 	UNLOCK(&rs);
    290 }
    291 
    292 /*-
    293  * Written by Damien Miller.
    294  * With simplifications by Jinmei Tatuya.
    295  */
    296 
    297 /*
    298  * Calculate a uniformly distributed random number less than
    299  * upper_bound avoiding "modulo bias".
    300  *
    301  * Uniformity is achieved by generating new random numbers
    302  * until the one returned is outside the range
    303  * [0, 2^32 % upper_bound[. This guarantees the selected
    304  * random number will be inside the range
    305  * [2^32 % upper_bound, 2^32[ which maps back to
    306  * [0, upper_bound[ after reduction modulo upper_bound.
    307  */
    308 uint32_t
    309 arc4random_uniform(uint32_t upper_bound)
    310 {
    311 	uint32_t r, min;
    312 
    313 	if (upper_bound < 2)
    314 		return 0;
    315 
    316 	/* calculate (2^32 % upper_bound) avoiding 64-bit math */
    317 	/* ((2^32 - x) % x) == (2^32 % x) when x <= 2^31 */
    318 	min = (0xFFFFFFFFU - upper_bound + 1) % upper_bound;
    319 
    320 	LOCK(&rs);
    321 	arc4_stir_if_needed(&rs, sizeof(r));
    322 
    323 	/*
    324 	 * This could theoretically loop forever but each retry has
    325 	 * p > 0.5 (worst case, usually far better) of selecting a
    326 	 * number inside the range we need, so it should rarely need
    327 	 * to re-roll (at all).
    328 	 */
    329 	do
    330 		r = arc4_getword(&rs);
    331 	while (r < min);
    332 	UNLOCK(&rs);
    333 
    334 	return r % upper_bound;
    335 }
    336