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.\" $OpenBSD: arc4random.3,v 1.2 1997/04/27 22:40:25 angelos Exp $
.\" Copyright 1997 Niels Provos <provos@physnet.uni-hamburg.de>
.\" All rights reserved.
.\"
.\" Redistribution and use in source and binary forms, with or without
.\" modification, are permitted provided that the following conditions
.\" are met:
.\" 1. Redistributions of source code must retain the above copyright
.\" notice, this list of conditions and the following disclaimer.
.\" 2. Redistributions in binary form must reproduce the above copyright
.\" notice, this list of conditions and the following disclaimer in the
.\" documentation and/or other materials provided with the distribution.
.\" 3. All advertising materials mentioning features or use of this software
.\" must display the following acknowledgement:
.\" This product includes software developed by Niels Provos.
.\" 4. The name of the author may not be used to endorse or promote products
.\" derived from this software without specific prior written permission.
.\"
.\" THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR
.\" IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES
.\" OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED.
.\" IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT,
.\" INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT
.\" NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
.\" DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
.\" THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
.\" (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF
.\" THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
.\"
.\" Manual page, using -mandoc macros
.\" $FreeBSD$
.\"
.Dd April 15, 1997
.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 LIBRARY
.ds str-Lb-libbsd Utility functions from BSD systems (libbsd, \-lbsd)
.Lb libbsd
.Sh SYNOPSIS
.In bsd/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 "unsigned char *dat" "int datlen"
.Sh DESCRIPTION
The
.Fn arc4random
function uses the key stream generator employed by the
arc4 cipher, which uses 8*8 8 bit S-Boxes.
The S-Boxes
can be in about
.if t 2\u\s71700\s10\d
.if n (2**1700)
states.
The
.Fn arc4random
function returns pseudo-random numbers in the range of 0 to
.if t 2\u\s731\s10\d\(mi1,
.if n (2**32)\(mi1,
and therefore has twice the range of
.Xr rand 3
and
.Xr random 3 .
.Pp
.Fn arc4random_buf
function 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 from
.Pa /dev/urandom
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
functions family, since
they automatically initialize themselves.
.Sh EXAMPLES
The following produces a drop-in replacement for the traditional
.Fn rand
and
.Fn random
functions using
.Fn arc4random :
.Pp
.Dl "#define foo4random() (arc4random() % ((unsigned)RAND_MAX + 1))"
.Sh SEE ALSO
.Xr rand 3 ,
.Xr random 3 ,
.Xr srandomdev 3
.Sh HISTORY
.Pa 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
.Pa RC4
used to be a trade secret, the cipher is now referred to as
.Pa ARC4 .
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