OpenBSD-4.6/share/man/man4/random.4

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.\"	$OpenBSD: random.4,v 1.22 2008/10/10 20:13:29 deraadt Exp $
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.Dd $Mdocdate: October 10 2008 $
.Dt RANDOM 4
.Os
.Sh NAME
.Nm random ,
.Nm srandom ,
.Nm urandom ,
.Nm arandom
.Nd random data source devices
.Sh SYNOPSIS
.Fd #include <sys/types.h>
.Fd #include <dev/rndvar.h>
.Fd #include <dev/rndioctl.h>
.Sh DESCRIPTION
The various
.Nm
devices produce random output data with different random qualities.
Entropy data is collected from system activity (like disk and
network device interrupts and such), and then run through various
hash or message digest functions to generate the output.
.Bl -hang -width /dev/srandomX
.It Pa /dev/random
This device is reserved for future support of hardware
random generators.
.It Pa /dev/srandom
Strong random data.
This device returns reliable random data.
If sufficient entropy is not currently available (i.e., the entropy
pool quality starts to run low), the driver pauses while more of
such data is collected.
The entropy pool data is converted into output data using MD5.
.It Pa /dev/urandom
Same as above, but does not guarantee the data to be strong.
The entropy pool data is converted into output data using MD5.
When the entropy pool quality runs low, the driver will continue
to output data.
.It Pa /dev/arandom
As required, entropy pool data re-seeds an ARC4 generator,
which then generates high-quality pseudo-random output data.
.Pp
The
.Xr arc4random 3
function in userland libraries seeds itself from this device,
providing a second level of ARC4 hashed data.
.El
.Sh FILES
.Bl -tag -width /dev/srandom -compact
.It Pa /dev/random
.It Pa /dev/srandom
.It Pa /dev/urandom
.It Pa /dev/arandom
.El
.Sh SEE ALSO
.Xr arc4random 3 ,
.Xr md5 3 ,
.Xr random 3 ,
.Xr amdpm 4 ,
.Xr glxsb 4 ,
.Xr pchb 4 ,
.Xr md5 9 ,
.Xr random 9
.Sh HISTORY
A
.Nm
device first appeared in the Linux operating system.
This is a cloned interface.
.Pp
ARC4 routines added by David Mazieres.
.Sh BUGS
No randomness testing suite provided.