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.\" $OpenBSD: arc4random.9,v 1.10 2014/01/23 08:15:04 jmc Exp $
.\"
.\" Copyright (c) 1996,2000 Michael Shalayeff
.\" 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 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.
.\"
.Dd $Mdocdate: January 23 2014 $
.Dt ARC4RANDOM 9
.Os
.Sh NAME
.Nm arc4random
.Nd kernel random subsystem
.Sh SYNOPSIS
.In dev/rndvar.h
.Ft void
.Fn add_true_randomness "int"
.Ft void
.Fn add_timer_randomness "int"
.Ft void
.Fn add_mouse_randomness "int"
.Ft void
.Fn add_tty_randomness "int"
.Ft void
.Fn add_net_randomness "int"
.Ft void
.Fn add_disk_randomness "int"
.Ft void
.Fn add_audio_randomness "int"
.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"
.Sh DESCRIPTION
The
.Fn add_random
functions below mix input into the system entropy pool, which is then used
to create a key for the ChaCha stream cipher used in the
.Fn arc4random
series of functions.
.Pp
The
.Fn add_mouse_randomness ,
.Fn add_tty_randomness ,
.Fn add_net_randomness ,
.Fn add_disk_randomness
and
.Fn add_audio_randomness
routines are used to
supply data for the random data source device for further processing.
The processing involves calculating inter-event timedelta and inserting
it into a pool which is cryptographically mixed multiple times.
.Pp
.Fn add_true_randomness
does not involve the usual timing calculations,
and causes the supplied data argument to be added to the entropy pool,
increasing the entropy counter by 32 bits.
.Pp
.Fn add_timer_randomness
will not cause the entropy counter to rise.
It is used to change the state of the pool periodically,
mostly by means of timing the random driver's operations.
.Pp
.Fn arc4random
and
.Fn arc4random_buf
provide random numbers and are intended to be called in any
circumstance where random numbers are required.
.Pp
.Fn arc4random_uniform
will return a uniformly distributed random number less than
.Fa upper_bound ,
avoiding "modulo bias" when the upper bound is not a power of two.
In the worst case, this function may consume multiple iterations
to ensure uniformity; see the source code to understand the problem
and solution.
.Pp
.Fn arc4random ,
.Fn arc4random_buf ,
and
.Fn arc4random_uniform
may be called in nearly any kernel context.
.Sh SEE ALSO
.Xr arc4random 3 ,
.Xr pchb 4 ,
.Xr random 4
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