NAME
arc4random, arc4random_buf, arc4random_uniform, arc4random_stir,
arc4random_addrandom - arc4 random number generator
LIBRARY
Utility functions from BSD systems (libbsd, -lbsd)
SYNOPSIS
#include <stdlib.h>
u_int32_t
arc4random(void);
void
arc4random_buf(void *buf, size_t nbytes);
u_int32_t
arc4random_uniform(u_int32_t upper_bound);
void
arc4random_stir(void);
void
arc4random_addrandom(unsigned char *dat, int datlen);
DESCRIPTION
The arc4random() function uses the key stream generator employed by the
arc4 cipher, which uses 8*8 8 bit S-Boxes. The S-Boxes can be in about
(2**1700) states. The arc4random() function returns pseudo-random
numbers in the range of 0 to (2**32)−1, and therefore has twice the range
of rand(3) and random(3).
arc4random_buf() function fills the region buf of length nbytes with
ARC4-derived random data.
arc4random_uniform() will return a uniformly distributed random number
less than upper_bound. arc4random_uniform() is recommended over
constructions like “arc4random() % upper_bound” as it avoids "modulo
bias" when the upper bound is not a power of two.
The arc4random_stir() function reads data from /dev/urandom and uses it
to permute the S-Boxes via arc4random_addrandom().
There is no need to call arc4random_stir() before using arc4random()
functions family, since they automatically initialize themselves.
EXAMPLES
The following produces a drop-in replacement for the traditional rand()
and random() functions using arc4random():
#define foo4random() (arc4random() % ((unsigned)RAND_MAX + 1))
SEE ALSO
rand(3), random(3), srandomdev(3)
HISTORY
RC4 has been designed by RSA Data Security, Inc. It was posted
anonymously to the USENET and was confirmed to be equivalent by several
sources who had access to the original cipher. Since RC4 used to be a
trade secret, the cipher is now referred to as ARC4.