.\" $OpenBSD: arc4random.3,v 1.22 2007/05/31 19:19:27 jmc Exp $
.\"
.\" Copyright 1997 Niels Provos <provos@physnet.uni-hamburg.de>
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.\"      This product includes software developed by Niels Provos.
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.\" Manual page, using -mandoc macros
.\"
.Dd $Mdocdate: May 31 2007 $
.Dt ARC4RANDOM 3
.Os
.Sh NAME
.Nm arc4random ,
.Nm arc4random_stir ,
.Nm arc4random_addrandom
.Nd arc4 random number generator
.Sh SYNOPSIS
.Fd #include <stdlib.h>
.Ft u_int32_t
.Fn arc4random "void"
.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
fits into a middle ground not covered by other subsystems such as
the strong, slow, and resource expensive random
devices described in
.Xr random 4
versus the fast but poor quality interfaces described in
.Xr rand 3 ,
.Xr random 3 ,
and
.Xr drand48 3 .
.Pp
The
.Fn arc4random_stir
function reads data from
.Pa /dev/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. can do
nothing about the release of the ARC4 algorithm.
Since
.Pa RC4
used to be a trade secret, the cipher is now referred to as
.Pa ARC4 .
.Pp
These functions first appeared in
.Ox 2.1 .