ARC4RANDOM(3) BSD Library Functions Manual ARC4RANDOM(3)NAME
arc4random, arc4random_buf, arc4random_uniform, arc4random_stir,
arc4random_addrandom — arc4 random number generator
Standard C Library (libc, -lc)
arc4random_buf(void *buf, size_t nbytes);
arc4random_addrandom(unsigned char *dat, int datlen);
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 num‐
bers 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 con‐
structions 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.
The following produces a drop-in replacement for the traditional rand()
and random() functions using arc4random():
#define foo4random() (arc4random() % ((unsigned)RAND_MAX + 1))
SEE ALSOrand(3), random(3), srandomdev(3)HISTORY
RC4 has been designed by RSA Data Security, Inc. It was posted anony‐
mously 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.
BSD April 15, 1997 BSD