.\" $OpenBSD: arc4random.3,v 1.29 2013/06/05 03:39:22 tedu Exp $
.\"
.\" Copyright 1997 Niels Provos <provos@physnet.uni-hamburg.de>
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.\" Manual page, using -mandoc macros
.\"
.Dd $Mdocdate: June 5 2013 $
.Dt ARC4RANDOM 3
.Os
.Sh NAME
.Nm arc4random ,
.Nm arc4random_buf ,
.Nm arc4random_uniform ,
.Nm arc4random_stir ,
.Nm arc4random_addrandom
.Nd arc4 random number generator
.Sh SYNOPSIS
.In stdlib.h
.Ft u_int32_t
.Fn arc4random "void"
.Ft void
.Fn arc4random_buf "void *buf" "size_t nbytes"
.Ft u_int32_t
.Fn arc4random_uniform "u_int32_t upper_bound"
.Ft void
.Fn arc4random_stir "void"
.Ft void
.Fn arc4random_addrandom "u_char *dat" "int datlen"
.Sh DESCRIPTION
The
.Fn arc4random
function provides a high quality 32-bit pseudo-random
number very quickly.
.Fn arc4random
seeds itself on a regular basis from the kernel strong random number
subsystem described in
.Xr random 4 .
On each call, an ARC4 generator is used to generate a new result.
The
.Fn 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.
.Pp
.Fn arc4random
provides a cryptographically secure pseudo-random number generator,
as opposed to the fast but poor quality interfaces described in
.Xr rand 3 ,
.Xr random 3 ,
and
.Xr drand48 3 .
.Pp
.Fn arc4random_buf
fills the region
.Fa buf
of length
.Fa nbytes
with ARC4-derived random data.
.Pp
.Fn arc4random_uniform
will return a uniformly distributed random number less than
.Fa upper_bound .
.Fn arc4random_uniform
is recommended over constructions like
.Dq Li arc4random() % upper_bound
as it avoids "modulo bias" when the upper bound is not a power of two.
.Pp
The
.Fn arc4random_stir
function reads data using
.Xr sysctl 3
from
.Va kern.arandom
and uses it to permute the S-Boxes via
.Fn arc4random_addrandom .
.Pp
There is no need to call
.Fn arc4random_stir
before using
.Fn arc4random ,
since
.Fn arc4random
automatically initializes itself.
.Sh RETURN VALUES
These functions are always successful, and no return value is
reserved to indicate an error.
.Sh SEE ALSO
.Xr rand 3 ,
.Xr rand48 3 ,
.Xr random 3
.Sh HISTORY
An algorithm called
.Pa 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
.Ql Alleged RC4
algorithm.
Since
.Pa RC4
was trademarked, the cipher is now referred to as
.Pa ARC4 .
.Pp
These functions first appeared in
.Ox 2.1 .