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arc4random.c revision 1.20 
      1 /*	$NetBSD: arc4random.c,v 1.20 2012/08/20 21:38:09 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 1.20 2012/08/20 21:38:09 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 __weak_alias(arc4random_addrandom,_arc4random_addrandom)
     46 __weak_alias(arc4random_buf,_arc4random_buf)
     47 __weak_alias(arc4random_stir,_arc4random_stir)
     48 __weak_alias(arc4random_uniform,_arc4random_uniform)
     49 #endif
     50 
     51 struct arc4_stream {
     52 	uint8_t stirred;
     53 	uint8_t pad;
     54 	uint8_t i;
     55 	uint8_t j;
     56 	uint8_t s[(uint8_t)~0u + 1u];	/* 256 to you and me */
     57 	mutex_t mtx;
     58 };
     59 
     60 #ifdef _REENTRANT
     61 #define LOCK(rs) { \
     62 		int isthreaded = __isthreaded; \
     63 		if (isthreaded)        \
     64 			mutex_lock(&(rs)->mtx);
     65 #define UNLOCK(rs) \
     66 		if (isthreaded)        \
     67 			mutex_unlock(&(rs)->mtx);      \
     68 	}
     69 #else
     70 #define LOCK(rs)
     71 #define UNLOCK(rs)
     72 #endif
     73 
     74 #define S(n) (n)
     75 #define S4(n) S(n), S(n + 1), S(n + 2), S(n + 3)
     76 #define S16(n) S4(n), S4(n + 4), S4(n + 8), S4(n + 12)
     77 #define S64(n) S16(n), S16(n + 16), S16(n + 32), S16(n + 48)
     78 #define S256 S64(0), S64(64), S64(128), S64(192)
     79 
     80 static struct arc4_stream rs = { .i = 0xff, .j = 0, .s = { S256 },
     81 		.stirred = 0, .mtx = MUTEX_INITIALIZER };
     82 
     83 #undef S
     84 #undef S4
     85 #undef S16
     86 #undef S64
     87 #undef S256
     88 
     89 static inline void arc4_addrandom(struct arc4_stream *, u_char *, int);
     90 static __noinline void arc4_stir(struct arc4_stream *);
     91 static inline uint8_t arc4_getbyte(struct arc4_stream *);
     92 static inline uint32_t arc4_getword(struct arc4_stream *);
     93 
     94 static inline int
     95 arc4_check_init(struct arc4_stream *as)
     96 {
     97 	if (__predict_true(rs.stirred))
     98 		return 0;
     99 
    100 	arc4_stir(as);
    101 	return 1;
    102 }
    103 
    104 static inline void
    105 arc4_addrandom(struct arc4_stream *as, u_char *dat, int datlen)
    106 {
    107 	uint8_t si;
    108 	size_t n;
    109 
    110 	for (n = 0; n < __arraycount(as->s); n++) {
    111 		as->i = (as->i + 1);
    112 		si = as->s[as->i];
    113 		as->j = (as->j + si + dat[n % datlen]);
    114 		as->s[as->i] = as->s[as->j];
    115 		as->s[as->j] = si;
    116 	}
    117 }
    118 
    119 static __noinline void
    120 arc4_stir(struct arc4_stream *as)
    121 {
    122 	int rdat[32];
    123 	int mib[] = { CTL_KERN, KERN_URND };
    124 	size_t len;
    125 	size_t i, j;
    126 
    127 	/*
    128 	 * This code once opened and read /dev/urandom on each
    129 	 * call.  That causes repeated rekeying of the kernel stream
    130 	 * generator, which is very wasteful.  Because of application
    131 	 * behavior, caching the fd doesn't really help.  So we just
    132 	 * fill up the tank from sysctl, which is a tiny bit slower
    133 	 * for us but much friendlier to other entropy consumers.
    134 	 */
    135 
    136 	for (i = 0; i < __arraycount(rdat); i++) {
    137 		len = sizeof(rdat[i]);
    138 		if (sysctl(mib, 2, &rdat[i], &len, NULL, 0) == -1)
    139 			abort();
    140 	}
    141 
    142 	arc4_addrandom(as, (void *) &rdat, (int)sizeof(rdat));
    143 
    144 	/*
    145 	 * Throw away the first N words of output, as suggested in the
    146 	 * paper "Weaknesses in the Key Scheduling Algorithm of RC4"
    147 	 * by Fluher, Mantin, and Shamir.  (N = 256 in our case.)
    148 	 */
    149 	for (j = 0; j < __arraycount(as->s) * 4; j++)
    150 		arc4_getbyte(as);
    151 
    152 	as->stirred = 1;
    153 }
    154 
    155 static __always_inline uint8_t
    156 arc4_getbyte_ij(struct arc4_stream *as, uint8_t *i, uint8_t *j)
    157 {
    158 	uint8_t si, sj;
    159 
    160 	*i = *i + 1;
    161 	si = as->s[*i];
    162 	*j = *j + si;
    163 	sj = as->s[*j];
    164 	as->s[*i] = sj;
    165 	as->s[*j] = si;
    166 	return (as->s[(si + sj) & 0xff]);
    167 }
    168 
    169 static inline uint8_t
    170 arc4_getbyte(struct arc4_stream *as)
    171 {
    172 	return arc4_getbyte_ij(as, &as->i, &as->j);
    173 }
    174 
    175 static inline uint32_t
    176 arc4_getword(struct arc4_stream *as)
    177 {
    178 	uint32_t val;
    179 	val = arc4_getbyte(as) << 24;
    180 	val |= arc4_getbyte(as) << 16;
    181 	val |= arc4_getbyte(as) << 8;
    182 	val |= arc4_getbyte(as);
    183 	return val;
    184 }
    185 
    186 void
    187 arc4random_stir(void)
    188 {
    189 	LOCK(&rs);
    190 	arc4_stir(&rs);
    191 	UNLOCK(&rs);
    192 }
    193 
    194 void
    195 arc4random_addrandom(u_char *dat, int datlen)
    196 {
    197 	LOCK(&rs);
    198 	arc4_check_init(&rs);
    199 	arc4_addrandom(&rs, dat, datlen);
    200 	UNLOCK(&rs);
    201 }
    202 
    203 uint32_t
    204 arc4random(void)
    205 {
    206 	uint32_t v;
    207 
    208 	LOCK(&rs);
    209 	arc4_check_init(&rs);
    210 	v = arc4_getword(&rs);
    211 	UNLOCK(&rs);
    212 	return v;
    213 }
    214 
    215 void
    216 arc4random_buf(void *buf, size_t len)
    217 {
    218 	uint8_t *bp = buf;
    219 	uint8_t *ep = bp + len;
    220 	uint8_t i, j;
    221 
    222 	LOCK(&rs);
    223 	arc4_check_init(&rs);
    224 
    225 	/* cache i and j - compiler can't know 'buf' doesn't alias them */
    226 	i = rs.i;
    227 	j = rs.j;
    228 
    229 	while (bp < ep)
    230 		*bp++ = arc4_getbyte_ij(&rs, &i, &j);
    231 	rs.i = i;
    232 	rs.j = j;
    233 
    234 	UNLOCK(&rs);
    235 }
    236 
    237 /*-
    238  * Written by Damien Miller.
    239  * With simplifications by Jinmei Tatuya.
    240  */
    241 
    242 /*
    243  * Calculate a uniformly distributed random number less than
    244  * upper_bound avoiding "modulo bias".
    245  *
    246  * Uniformity is achieved by generating new random numbers
    247  * until the one returned is outside the range
    248  * [0, 2^32 % upper_bound[. This guarantees the selected
    249  * random number will be inside the range
    250  * [2^32 % upper_bound, 2^32[ which maps back to
    251  * [0, upper_bound[ after reduction modulo upper_bound.
    252  */
    253 uint32_t
    254 arc4random_uniform(uint32_t upper_bound)
    255 {
    256 	uint32_t r, min;
    257 
    258 	if (upper_bound < 2)
    259 		return 0;
    260 
    261 	/* calculate (2^32 % upper_bound) avoiding 64-bit math */
    262 	/* ((2^32 - x) % x) == (2^32 % x) when x <= 2^31 */
    263 	min = (0xFFFFFFFFU - upper_bound + 1) % upper_bound;
    264 
    265 	LOCK(&rs);
    266 	arc4_check_init(&rs);
    267 
    268 	/*
    269 	 * This could theoretically loop forever but each retry has
    270 	 * p > 0.5 (worst case, usually far better) of selecting a
    271 	 * number inside the range we need, so it should rarely need
    272 	 * to re-roll (at all).
    273 	 */
    274 	do
    275 		r = arc4_getword(&rs);
    276 	while (r < min);
    277 	UNLOCK(&rs);
    278 
    279 	return r % upper_bound;
    280 }
    281