source: vendor/FreeBSD-libc/current/gen/arc4random.c

/*
 * 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 (for instance 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/cdefs.h>
__FBSDID("$FreeBSD: src/lib/libc/gen/arc4random.c,v 1.10 2004/03/24 14:44:57 green Exp $");

#include "namespace.h"
#include <sys/types.h>
#include <sys/time.h>
#include <stdlib.h>
#include <fcntl.h>
#include <unistd.h>
#include <pthread.h>

#include "libc_private.h"
#include "un-namespace.h"

struct arc4_stream {
        u_int8_t i;
        u_int8_t j;
        u_int8_t s[256];
};

static pthread_mutex_t  arc4random_mtx = PTHREAD_MUTEX_INITIALIZER;

#define RANDOMDEV       "/dev/urandom"
#define THREAD_LOCK()                                           \
        do {                                                    \
                if (__isthreaded)                               \
                        _pthread_mutex_lock(&arc4random_mtx);   \
        } while (0)

#define THREAD_UNLOCK()                                         \
        do {                                                    \
                if (__isthreaded)                               \
                        _pthread_mutex_unlock(&arc4random_mtx); \
        } while (0)

static struct arc4_stream rs;
static int rs_initialized;
static int rs_stired;

static inline u_int8_t arc4_getbyte(struct arc4_stream *);
static void arc4_stir(struct arc4_stream *);

static inline void
arc4_init(as)
        struct arc4_stream *as;
{
        int     n;

        for (n = 0; n < 256; n++)
                as->s[n] = n;
        as->i = 0;
        as->j = 0;
}

static inline void
arc4_addrandom(as, dat, datlen)
        struct arc4_stream *as;
        u_char *dat;
        int     datlen;
{
        int     n;
        u_int8_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;
        }
}

static void
arc4_stir(as)
        struct arc4_stream *as;
{
        int     fd, n;
        struct {
                struct timeval tv;
                pid_t pid;
                u_int8_t rnd[128 - sizeof(struct timeval) - sizeof(pid_t)];
        }       rdat;

        gettimeofday(&rdat.tv, NULL);
        rdat.pid = getpid();
        fd = _open(RANDOMDEV, O_RDONLY, 0);
        if (fd >= 0) {
                (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... */

        arc4_addrandom(as, (void *) &rdat, sizeof(rdat));

        /*
         * Throw away the first N bytes of output, as suggested in the
         * paper "Weaknesses in the Key Scheduling Algorithm of RC4"
         * by Fluher, Mantin, and Shamir.  N=1024 is based on
         * suggestions in the paper "(Not So) Random Shuffles of RC4"
         * by Ilya Mironov.
         */
        for (n = 0; n < 1024; n++)
                arc4_getbyte(as);
}

static inline u_int8_t
arc4_getbyte(as)
        struct arc4_stream *as;
{
        u_int8_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 inline u_int32_t
arc4_getword(as)
        struct arc4_stream *as;
{
        u_int32_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_check_init(void)
{
        if (!rs_initialized) {
                arc4_init(&rs);
                rs_initialized = 1;
        }
}

static void
arc4_check_stir(void)
{
        if (!rs_stired) {
                arc4_stir(&rs);
                rs_stired = 1;
        }
}

void
arc4random_stir()
{
        THREAD_LOCK();
        arc4_check_init();
        arc4_stir(&rs);
        THREAD_UNLOCK();
}

void
arc4random_addrandom(dat, datlen)
        u_char *dat;
        int     datlen;
{
        THREAD_LOCK();
        arc4_check_init();
        arc4_check_stir();
        arc4_addrandom(&rs, dat, datlen);
        THREAD_UNLOCK();
}

u_int32_t
arc4random()
{
        u_int32_t rnd;

        THREAD_LOCK();
        arc4_check_init();
        arc4_check_stir();
        rnd = arc4_getword(&rs);
        THREAD_UNLOCK();

        return (rnd);
}

#if 0
/*-------- Test code for i386 --------*/
#include <stdio.h>
#include <machine/pctr.h>
int
main(int argc, char **argv)
{
        const int iter = 1000000;
        int     i;
        pctrval v;

        v = rdtsc();
        for (i = 0; i < iter; i++)
                arc4random();
        v = rdtsc() - v;
        v /= iter;

        printf("%qd cycles\n", v);
}
#endif