/*
* Arc4 random number generator for OpenBSD.
* Copyright 1996 David Mazieres <dm@lcs.mit.edu>.
*
* Modification and redistribution in source and binary forms is
* permitted provided that due credit is given to the author and the
* OpenBSD project by leaving this copyright notice intact.
*/
/*
* This code is derived from section 17.1 of Applied Cryptography,
* second edition, which describes a stream cipher allegedly
* compatible with RSA Labs "RC4" cipher (the actual description of
* which is a trade secret). The same algorithm is used as a stream
* cipher called "arcfour" in Tatu Ylonen's ssh package.
*
* Here the stream cipher has been modified always to include the time
* when initializing the state. That makes it impossible to
* regenerate the same random sequence twice, so this can't be used
* for encryption, but will generate good random numbers.
*
* RC4 is a registered trademark of RSA Laboratories.
*/
#include <sys/time.h>
#include <fcntl.h>
#include <stdint.h>
#include <stdlib.h>
#include <unistd.h>
#include "arc4random.h"
struct arc4_stream {
uint8_t i;
uint8_t j;
uint8_t s[256];
};
static int rs_initialized;
static struct arc4_stream rs;
static int arc4_count;
static void
arc4_init(struct arc4_stream *as)
{
int n;
for (n = 0; n < 256; n++)
as->s[n] = n;
as->i = 0;
as->j = 0;
}
static void
arc4_addrandom(struct arc4_stream *as, unsigned char *dat, int datlen)
{
int n;
uint8_t si;
as->i--;
for (n = 0; n < 256; n++) {
as->i = (as->i + 1);
si = as->s[as->i];
as->j = (as->j + si + dat[n % datlen]);
as->s[as->i] = as->s[as->j];
as->s[as->j] = si;
}
as->j = as->i;
}
static uint8_t
arc4_getbyte(struct arc4_stream *as)
{
uint8_t si, sj;
as->i = (as->i + 1);
si = as->s[as->i];
as->j = (as->j + si);
sj = as->s[as->j];
as->s[as->i] = sj;
as->s[as->j] = si;
return (as->s[(si + sj) & 0xff]);
}
static uint32_t
arc4_getword(struct arc4_stream *as)
{
uint32_t val;
val = arc4_getbyte(as) << 24;
val |= arc4_getbyte(as) << 16;
val |= arc4_getbyte(as) << 8;
val |= arc4_getbyte(as);
return val;
}
static void
arc4_stir(struct arc4_stream *as)
{
int fd;
struct {
struct timeval tv;
unsigned int rnd[(128 - sizeof(struct timeval)) /
sizeof(unsigned int)];
} rdat;
int n;
gettimeofday(&rdat.tv, NULL);
fd = open("/dev/urandom", O_RDONLY);
if (fd != -1) {
n = read(fd, rdat.rnd, sizeof(rdat.rnd));
close(fd);
}
/* fd < 0? Ah, what the heck. We'll just take
* whatever was on the stack... */
arc4_addrandom(as, (void *) &rdat, sizeof(rdat));
/*
* Throw away the first N words of output, as suggested in the
* paper "Weaknesses in the Key Scheduling Algorithm of RC4"
* by Fluher, Mantin, and Shamir. (N = 256 in our case.)
*/
for (n = 0; n < 256 * 4; n++)
arc4_getbyte(as);
arc4_count = 1600000;
}
void
arc4random_stir()
{
if (!rs_initialized) {
arc4_init(&rs);
rs_initialized = 1;
}
arc4_stir(&rs);
}
void
arc4random_addrandom(unsigned char *dat, int datlen)
{
if (!rs_initialized)
arc4random_stir();
arc4_addrandom(&rs, dat, datlen);
}
uint32_t
arc4random()
{
arc4_count -= 4;
if (!rs_initialized || arc4_count <= 0)
arc4random_stir();
return arc4_getword(&rs);
}
