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      1 /*
      2  * Arc4 random number generator for OpenBSD.
      3  * Copyright 1996 David Mazieres <dm (at) lcs.mit.edu>.
      4  *
      5  * Modification and redistribution in source and binary forms is
      6  * permitted provided that due credit is given to the author and the
      7  * OpenBSD project by leaving this copyright notice intact.
      8  */
      9 
     10 /*
     11  * This code is derived from section 17.1 of Applied Cryptography,
     12  * second edition, which describes a stream cipher allegedly
     13  * compatible with RSA Labs "RC4" cipher (the actual description of
     14  * which is a trade secret).  The same algorithm is used as a stream
     15  * cipher called "arcfour" in Tatu Ylonen's ssh package.
     16  *
     17  * Here the stream cipher has been modified always to include the time
     18  * when initializing the state.  That makes it impossible to
     19  * regenerate the same random sequence twice, so this can't be used
     20  * for encryption, but will generate good random numbers.
     21  *
     22  * RC4 is a registered trademark of RSA Laboratories.
     23  */
     24 
     25 #include <sys/time.h>
     26 
     27 #include <fcntl.h>
     28 #include <stdint.h>
     29 #include <stdlib.h>
     30 #include <unistd.h>
     31 
     32 #include "arc4random.h"
     33 
     34 struct arc4_stream {
     35 	uint8_t i;
     36 	uint8_t j;
     37 	uint8_t s[256];
     38 };
     39 
     40 static int rs_initialized;
     41 static struct arc4_stream rs;
     42 static int arc4_count;
     43 
     44 static void
     45 arc4_init(struct arc4_stream *as)
     46 {
     47 	int n;
     48 
     49 	for (n = 0; n < 256; n++)
     50 		as->s[n] = n;
     51 	as->i = 0;
     52 	as->j = 0;
     53 }
     54 
     55 static void
     56 arc4_addrandom(struct arc4_stream *as, unsigned char *dat, int datlen)
     57 {
     58 	int n;
     59 	uint8_t si;
     60 
     61 	as->i--;
     62 	for (n = 0; n < 256; n++) {
     63 		as->i = (as->i + 1);
     64 		si = as->s[as->i];
     65 		as->j = (as->j + si + dat[n % datlen]);
     66 		as->s[as->i] = as->s[as->j];
     67 		as->s[as->j] = si;
     68 	}
     69 	as->j = as->i;
     70 }
     71 
     72 static uint8_t
     73 arc4_getbyte(struct arc4_stream *as)
     74 {
     75 	uint8_t si, sj;
     76 
     77 	as->i = (as->i + 1);
     78 	si = as->s[as->i];
     79 	as->j = (as->j + si);
     80 	sj = as->s[as->j];
     81 	as->s[as->i] = sj;
     82 	as->s[as->j] = si;
     83 	return (as->s[(si + sj) & 0xff]);
     84 }
     85 
     86 static uint32_t
     87 arc4_getword(struct arc4_stream *as)
     88 {
     89 	uint32_t val;
     90 
     91 	val = arc4_getbyte(as) << 24;
     92 	val |= arc4_getbyte(as) << 16;
     93 	val |= arc4_getbyte(as) << 8;
     94 	val |= arc4_getbyte(as);
     95 	return val;
     96 }
     97 
     98 static void
     99 arc4_stir(struct arc4_stream *as)
    100 {
    101 	int fd;
    102 	struct {
    103 		struct timeval tv;
    104 		unsigned int rnd[(128 - sizeof(struct timeval)) /
    105 			sizeof(unsigned int)];
    106 	}       rdat;
    107 	int n;
    108 
    109 	gettimeofday(&rdat.tv, NULL);
    110 	fd = open("/dev/urandom", O_RDONLY);
    111 	if (fd != -1) {
    112 		n = read(fd, rdat.rnd, sizeof(rdat.rnd));
    113 		close(fd);
    114 	}
    115 
    116 	/* fd < 0?  Ah, what the heck. We'll just take
    117 	 * whatever was on the stack... */
    118 	arc4_addrandom(as, (void *) &rdat, sizeof(rdat));
    119 
    120 	/*
    121 	 * Throw away the first N words of output, as suggested in the
    122 	 * paper "Weaknesses in the Key Scheduling Algorithm of RC4"
    123 	 * by Fluher, Mantin, and Shamir.  (N = 256 in our case.)
    124 	 */
    125 	for (n = 0; n < 256 * 4; n++)
    126 		arc4_getbyte(as);
    127 	arc4_count = 1600000;
    128 }
    129 
    130 void
    131 arc4random_stir()
    132 {
    133 
    134 	if (!rs_initialized) {
    135 		arc4_init(&rs);
    136 		rs_initialized = 1;
    137 	}
    138 	arc4_stir(&rs);
    139 }
    140 
    141 void
    142 arc4random_addrandom(unsigned char *dat, int datlen)
    143 {
    144 
    145 	if (!rs_initialized)
    146 		arc4random_stir();
    147 	arc4_addrandom(&rs, dat, datlen);
    148 }
    149 
    150 uint32_t
    151 arc4random()
    152 {
    153 
    154 	arc4_count -= 4;
    155 	if (!rs_initialized || arc4_count <= 0)
    156 		arc4random_stir();
    157 	return arc4_getword(&rs);
    158 }
    159