ARC4RANDOM(3) OpenBSD Programmer's Manual ARC4RANDOM(3)NAME
arc4random, arc4random_buf, arc4random_uniform, arc4random_stir,
arc4random_addrandom - arc4 random number generator
arc4random_buf(void *buf, size_t nbytes);
arc4random_addrandom(u_char *dat, int datlen);
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
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
There is no need to call arc4random_stir() before using arc4random(),
since arc4random() automatically initializes itself.
These functions are always successful, and no return value is reserved to
indicate an error.
SEE ALSOrand(3), rand48(3), random(3)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
These functions first appeared in OpenBSD 2.1.
OpenBSD 4.9 December 23, 2008 OpenBSD 4.9