1.\" 2.\" random.c -- A strong random number generator 3.\" 4.\" Version 0.92, last modified 21-Sep-95 5.\" 6.\" Copyright Theodore Ts'o, 1994, 1995. All rights reserved. 7.\" 8.\" Redistribution and use in source and binary forms, with or without 9.\" modification, are permitted provided that the following conditions 10.\" are met: 11.\" 1. Redistributions of source code must retain the above copyright 12.\" notice, and the entire permission notice in its entirety, 13.\" including the disclaimer of warranties. 14.\" 2. Redistributions in binary form must reproduce the above copyright 15.\" notice, this list of conditions and the following disclaimer in the 16.\" documentation and/or other materials provided with the distribution. 17.\" 3. The name of the author may not be used to endorse or promote 18.\" products derived from this software without specific prior 19.\" written permission. 20.\" 21.\" ALTERNATIVELY, this product may be distributed under the terms of 22.\" the GNU Public License, in which case the provisions of the GPL are 23.\" required INSTEAD OF the above restrictions. (This clause is 24.\" necessary due to a potential bad interaction between the GPL and 25.\" the restrictions contained in a BSD-style copyright.) 26.\" 27.\" THIS SOFTWARE IS PROVIDED ``AS IS'' AND ANY EXPRESS OR IMPLIED 28.\" WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES 29.\" OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE 30.\" DISCLAIMED. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT, 31.\" INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES 32.\" (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR 33.\" SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) 34.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, 35.\" STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) 36.\" ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED 37.\" OF THE POSSIBILITY OF SUCH DAMAGE. 38.\" 39.\" $FreeBSD: src/usr.sbin/rndcontrol/random.4,v 1.9.2.2 2001/11/24 16:14:18 dd Exp $ 40.\" $DragonFly: src/usr.sbin/rndcontrol/random.4,v 1.13 2008/10/17 11:30:24 swildner Exp $ 41.\" 42.Dd October 21, 1995 43.Dt RANDOM 4 i386 44.Os 45.Sh NAME 46.Nm random , 47.Nm urandom 48.Nd random number devices 49.Sh DESCRIPTION 50This device gathers environmental noise from device drivers, etc., 51and returns good random numbers, suitable for cryptographic use. 52Besides the obvious cryptographic uses, these numbers are also good 53for seeding TCP sequence numbers, and other places where it is 54desirable to have numbers which are not only random, but hard to 55predict by an attacker. 56.Ss Theory of operation 57Computers are very predictable devices. Hence it is extremely hard 58to produce truly random numbers on a computer \(em as opposed to 59pseudo-random numbers, which can easily be generated by using an 60algorithm. Unfortunately, it is very easy for attackers to guess 61the sequence of pseudo-random number generators, and for some 62applications this is not acceptable. So instead, we must try to 63gather "environmental noise" from the computer's environment, which 64must be hard for outside attackers to observe, and use that to 65generate random numbers. In a Unix environment, this is best done 66from inside the kernel. 67.Pp 68Previous and contemporary 69.Xr random 4 70implementations typically used 71an "entropy" pool which was processed with a cryptographic hash 72function such as MD5. However, at the time of this writing security 73issues have been discovered in some of these functions 74(MD4, MD5, SHA0, SHA1). 75.Pp 76This implementation uses a CSPRNG (Cryptographically Secure Pseudo 77Random Number Generator) which is continuously reseeded as described above. 78.Pp 79The user interface consists of two character devices 80.Pa /dev/random 81and 82.Pa /dev/urandom . 83The 84.Pa /dev/random 85device is suitable for use when very high quality randomness is desired 86(e.g. for key generation). Previous implementations of this device 87attempted to limit the number of returned bytes based on a guess as to 88the secureness of the pool. However, this resulted in the interface being 89so undependable that most programs just started using /dev/urandom 90instead of /dev/random. 91The current 92.Dx 93implementation will return all requested bytes but the system reserves the 94right in the future to limit the transfer rate to maintain the high quality 95of randomness requested. 96.Pp 97The 98.Pa /dev/urandom 99device uses a different and much faster algorithm, but one which is not 100considered to be as secure (though for all practical purposes it probably 101is good enough). 102.Pp 103Root may write entropy to 104.Pa /dev/random 105to seed the random number generator only if the securelevel is less than 106or equal to zero and the 107.Va kern.seedenable 108sysctl is non-zero. A certain 109degree of entropy is added by RC scripts during the boot sequence. 110.Sh FILES 111.Bl -tag -width Pa -compact 112.It Pa /dev/random 113.It Pa /dev/urandom 114.El 115.Sh "SEE ALSO" 116.Xr arc4random 3 , 117.Xr drand48 3 , 118.Xr rand 3 , 119.Xr RAND_add 3 , 120.Xr RAND_bytes 3 , 121.Xr random 3 , 122.Xr rndcontrol 8 123.Sh HISTORY 124The 125.Pa random , 126.Pa urandom 127files appeared in 128.Fx 2.1.5 . 129.Sh AUTHORS 130.An -nosplit 131The current algorithms are implemented by 132.An Robin Carey 133and were ported to 134.Dx 135by 136.An Matthew Dillon . 137.Pp 138.An Mark Murray 139wrote the 140.Xr rndcontrol 8 141utility for 142.Fx . 143.Pp 144The 145.Em IBAA 146CSPRNG was developed by 147.An Bob Jenkins 148and is used by 149.Pa /dev/random . 150The 151.Em L15 152CSPRNG (based on ARC4) 153was developed by 154.An Robin Carey 155and is used by 156.Pa /dev/urandom . 157