NAME
arc4random,
arc4random_buf,
arc4random_uniform,
arc4random_stir,
arc4random_addrandom —
arc4 random number generator
LIBRARY
library “libc”
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(uint8_t
*dat, size_t
datlen);
DESCRIPTION
Thearc4random()
function uses the key stream generator employed by the arc4 cipher, which uses
256 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
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.