1.\" $OpenBSD: random.4,v 1.22 2008/10/10 20:13:29 deraadt Exp $ 2.\" 3.\" Copyright (c) 1996, 1997 Michael Shalayeff 4.\" 5.\" Redistribution and use in source and binary forms, with or without 6.\" modification, are permitted provided that the following conditions 7.\" are met: 8.\" 1. Redistributions of source code must retain the above copyright 9.\" notice, this list of conditions and the following disclaimer. 10.\" 2. Redistributions in binary form must reproduce the above copyright 11.\" notice, this list of conditions and the following disclaimer in the 12.\" documentation and/or other materials provided with the distribution. 13.\" 14.\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND 15.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE 16.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE 17.\" ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE 18.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL 19.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS 20.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) 21.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT 22.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY 23.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF 24.\" SUCH DAMAGE. 25.\" 26.Dd $Mdocdate: October 10 2008 $ 27.Dt RANDOM 4 28.Os 29.Sh NAME 30.Nm random , 31.Nm srandom , 32.Nm urandom , 33.Nm arandom 34.Nd random data source devices 35.Sh SYNOPSIS 36.Fd #include <sys/types.h> 37.Fd #include <dev/rndvar.h> 38.Fd #include <dev/rndioctl.h> 39.Sh DESCRIPTION 40The various 41.Nm 42devices produce random output data with different random qualities. 43Entropy data is collected from system activity (like disk and 44network device interrupts and such), and then run through various 45hash or message digest functions to generate the output. 46.Bl -hang -width /dev/srandomX 47.It Pa /dev/random 48This device is reserved for future support of hardware 49random generators. 50.It Pa /dev/srandom 51Strong random data. 52This device returns reliable random data. 53If sufficient entropy is not currently available (i.e., the entropy 54pool quality starts to run low), the driver pauses while more of 55such data is collected. 56The entropy pool data is converted into output data using MD5. 57.It Pa /dev/urandom 58Same as above, but does not guarantee the data to be strong. 59The entropy pool data is converted into output data using MD5. 60When the entropy pool quality runs low, the driver will continue 61to output data. 62.It Pa /dev/arandom 63As required, entropy pool data re-seeds an ARC4 generator, 64which then generates high-quality pseudo-random output data. 65.Pp 66The 67.Xr arc4random 3 68function in userland libraries seeds itself from this device, 69providing a second level of ARC4 hashed data. 70.El 71.Sh FILES 72.Bl -tag -width /dev/srandom -compact 73.It Pa /dev/random 74.It Pa /dev/srandom 75.It Pa /dev/urandom 76.It Pa /dev/arandom 77.El 78.Sh SEE ALSO 79.Xr arc4random 3 , 80.Xr md5 3 , 81.Xr random 3 , 82.Xr amdpm 4 , 83.Xr glxsb 4 , 84.Xr pchb 4 , 85.Xr md5 9 , 86.Xr random 9 87.Sh HISTORY 88A 89.Nm 90device first appeared in the Linux operating system. 91This is a cloned interface. 92.Pp 93ARC4 routines added by David Mazieres. 94.Sh BUGS 95No randomness testing suite provided. 96