/*
* 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];
size_t count;
pid_t stir_pid;
};
#define S(n) (n)
#define S4(n) S(n), S(n + 1), S(n + 2), S(n + 3)
#define S16(n) S4(n), S4(n + 4), S4(n + 8), S4(n + 12)
#define S64(n) S16(n), S16(n + 16), S16(n + 32), S16(n + 48)
#define S256 S64(0), S64(64), S64(128), S64(192)
static struct arc4_stream rs = { .i = 0xff, .j = 0, .s = { S256 },
.count = 0, .stir_pid = 0 };
#undef S
#undef S4
#undef S16
#undef S64
#undef S256
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 = (uint8_t)(as->i + 1);
si = as->s[as->i];
as->j = (uint8_t)(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 = (uint8_t)(as->i + 1);
si = as->s[as->i];
as->j = (uint8_t)(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)
{
int val;
val = (int)((unsigned int)arc4_getbyte(as) << 24);
val |= arc4_getbyte(as) << 16;
val |= arc4_getbyte(as) << 8;
val |= arc4_getbyte(as);
return (uint32_t)val;
}
/* We don't care about any error on read, just use what we have
* on the stack. So mask off this GCC warning. */
#pragma GCC diagnostic ignored "-Wunused-result"
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;
size_t n;
gettimeofday(&rdat.tv, NULL);
fd = open("/dev/urandom", O_RDONLY);
if (fd != -1) {
/* If there is an error reading, just use what is
* on the stack. */
/* coverity[check_return] */
(void)read(fd, rdat.rnd, sizeof(rdat.rnd));
close(fd);
}
/* fd < 0? Ah, what the heck. We'll just take
* whatever was on the stack... */
/* coverity[uninit_use_in_call] */
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 * sizeof(uint32_t); n++)
arc4_getbyte(as);
as->count = 1600000;
}
static void
arc4_stir_if_needed(struct arc4_stream *as)
{
pid_t pid;
pid = getpid();
if (as->count <= sizeof(uint32_t) || as->stir_pid != pid) {
as->stir_pid = pid;
arc4_stir(as);
} else
as->count -= sizeof(uint32_t);
}
uint32_t
arc4random()
{
arc4_stir_if_needed(&rs);
return arc4_getword(&rs);
}
