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.\"
.\" Copyright (c) 2015
.\" Mark R V Murray
.\" Copyright (c) 2000
.\" The Regents of the University of California. 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.
.\"
.\" THIS SOFTWARE IS PROVIDED BY THE DEVELOPERS ``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 DEVELOPERS 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.
.\"
.\" $FreeBSD$
.\" "
.Dd June 30, 2015
.Dt RANDOM 9
.Os
.Sh NAME
.Nm arc4rand ,
.Nm arc4random ,
.Nm random ,
.Nm read_random ,
.Nm srandom
.Nd supply pseudo-random numbers
.Sh SYNOPSIS
.In sys/libkern.h
.Ft void
.Fn srandom "u_long seed"
.Ft u_long
.Fn random "void"
.Ft void
.Fn arc4rand "void *ptr" "u_int length" "int reseed"
.Ft uint32_t
.Fn arc4random "void"
.Pp
.In sys/random.h
.Ft int
.Fn read_random "void *buffer" "int count"
.Sh DESCRIPTION
The
.Fn random
function will by default produce
a sequence of numbers
that can be duplicated
by calling
.Fn srandom
with some constant
as the
.Fa seed .
The
.Fn srandom
function may be called with any arbitrary
.Fa seed
value to get slightly more unpredictable numbers.
It is important to remember that the
.Fn random
function is entirely predictable,
and is therefore not of use where
knowledge of the sequence of numbers
may be of benefit to an attacker.
.Pp
The
.Fn arc4rand
function will return very good quality random numbers,
better suited
for security-related purposes.
The random numbers from
.Fn arc4rand
are seeded from the entropy device
if it is available.
Automatic reseeds happen
after a certain timeinterval
and after a certain number of bytes
have been delivered.
A forced reseed
can be forced
by passing a non-zero
value in the
.Fa reseed
argument.
.Pp
The
.Fn read_random
function is used to return entropy directly from the entropy device
if it has been loaded.
If the entropy device is not loaded, then
the
.Fa buffer
is ignored
and zero is returned.
The
.Fa buffer
is filled with no more than
.Fa count
bytes.
It is strongly advised that
.Fn read_random
is not used;
instead use
.Fn arc4rand
unless it is
necessary to know
that no entropy
has been returned.
.Pp
All the bits returned by
.Fn random ,
.Fn arc4rand
and
.Fn read_random
are usable.
For example,
.Sq Li random()&01
will produce a random binary value.
.Pp
The
.Fn arc4random
is a convenience function which calls
.Fn arc4rand
to return a 32 bit pseudo-random integer.
.Sh RETURN VALUES
The
.Fn random
function uses
a non-linear additive feedback random number generator
employing a default table
of size 31
containing long integers
to return successive pseudo-random
numbers in the range from 0 to
.if t 2\u\s731\s10\d\(mi1.
.if n (2**31)\(mi1.
The period of this random number generator
is very large,
approximately
.if t 16\(mu(2\u\s731\s10\d\(mi1).
.if n 16*((2**31)\(mi1).
.Pp
The
.Fn arc4rand
function uses the RC4 algorithm
to generate successive pseudo-random bytes.
The
.Fn arc4random
function uses
.Fn arc4rand
to generate pseudo-random numbers
in the range from 0 to
.if t 2\u\s732\s10\d\(mi1.
.if n (2**32)\(mi1.
.Pp
The
.Fn read_random
function returns
the number of bytes placed in
.Fa buffer .
.Sh AUTHORS
.An Dan Moschuk
wrote
.Fn arc4random .
.An Mark R V Murray
wrote
.Fn read_random .
|
