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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>
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.\" Manual page, using -mandoc macros
.\" $FreeBSD: release/9.1.0/lib/libc/gen/arc4random.3 180688 2008-07-22 11:33:49Z ache $
.\"
.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
.Lb libc
.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 "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 .