NAME
arc4random,
arc4random_buf,
arc4random_uniform,
arc4random_stir,
arc4random_addrandom —
arc4 random number generator
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(u_char
*dat, int
datlen);
DESCRIPTION
The
arc4random()
function provides a high quality 32-bit pseudo-random number very quickly.
arc4random() seeds itself on a regular basis from
the kernel strong random number subsystem described in
random(4). On each call, an ARC4 generator is used to generate a new
result. The arc4random() function uses the ARC4
cipher key stream generator, which uses 8*8 8-bit S-Boxes. The S-Boxes can
be in about (2**1700) states.
arc4random()
fits into a middle ground not covered by other subsystems such as the
strong, slow, and resource expensive random devices described in
random(4) versus the fast but poor quality interfaces described in
rand(3),
random(3), and
drand48(3).
arc4random_buf()
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 using
sysctl(3) from kern.arandom and uses it to
permute the S-Boxes via
arc4random_addrandom().
There is no need to call
arc4random_stir()
before using arc4random(), since
arc4random() automatically initializes itself.
RETURN VALUES
These functions are always successful, and no return value is reserved to indicate an error.
SEE ALSO
HISTORY
An algorithm called RC4 was designed by
RSA Data Security, Inc. It was considered a trade secret. Because it was a
trade secret, it obviously could not be patented. A clone of this was posted
anonymously to USENET and confirmed to be equivalent by several sources who
had access to the original cipher. Because of the trade secret situation,
RSA Data Security, Inc. could do nothing about the release of the
‘Alleged RC4’ algorithm. Since
RC4 was trademarked, the cipher is now referred to
as ARC4.
These functions first appeared in OpenBSD 2.1.