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.\" 41.Dd December 31, 2021 42.Dt RANDOM 4 43.Os 44.Sh NAME 45.Nm random , 46.Nm urandom 47.Nd random number devices 48.Sh DESCRIPTION 49This device gathers environmental noise from device drivers, etc., 50and returns good random numbers, suitable for cryptographic use. 51Besides the obvious cryptographic uses, these numbers are also good 52for seeding TCP sequence numbers, and other places where it is 53desirable to have numbers which are not only random, but hard to 54predict by an attacker. 55.Ss Theory of operation 56Computers are very predictable devices. Hence it is extremely hard 57to produce truly random numbers on a computer \(em as opposed to 58pseudo-random numbers, which can easily be generated by using an 59algorithm. Unfortunately, it is very easy for attackers to guess 60the sequence of pseudo-random number generators, and for some 61applications this is not acceptable. So instead, we must try to 62gather "environmental noise" from the computer's environment, which 63must be hard for outside attackers to observe, and use that to 64generate random numbers. In a Unix environment, this is best done 65from inside the kernel. 66.Pp 67Previous and contemporary 68.Nm 69implementations typically used 70an "entropy" pool which was processed with a cryptographic hash 71function such as MD5. However, at the time of this writing security 72issues have been discovered in some of these functions 73(MD4, MD5, SHA0, SHA1). 74.Pp 75This implementation uses a CSPRNG (Cryptographically Secure Pseudo 76Random Number Generator) which is continuously reseeded as described above. 77.Pp 78The user interface consists of two character devices 79.Pa /dev/random 80and 81.Pa /dev/urandom . 82The 83.Pa /dev/random 84device is suitable for use when very high quality randomness is desired 85(e.g. for key generation). Previous implementations of this device 86attempted to limit the number of returned bytes based on a guess as to 87the secureness of the pool. However, this resulted in the interface being 88so undependable that most programs just started using /dev/urandom 89instead of /dev/random. 90The current 91.Dx 92implementation will return all requested bytes but the system reserves the 93right in the future to limit the transfer rate to maintain the high quality 94of randomness requested. 95.Pp 96The 97.Pa /dev/urandom 98device uses a different and much faster algorithm, but one which is not 99considered to be as secure (though for all practical purposes it probably 100is good enough). 101.Pp 102Root may write entropy to 103.Pa /dev/random 104to seed the random number generator only if the securelevel is less than 105or equal to zero and the 106.Va kern.seedenable 107sysctl is non-zero. A certain 108degree of entropy is added by RC scripts during the boot sequence. 109.Sh FILES 110.Bl -tag -width Pa -compact 111.It Pa /dev/random 112.It Pa /dev/urandom 113.El 114.Sh SEE ALSO 115.Xr arc4random 3 , 116.Xr drand48 3 , 117.Xr getentropy 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 131.An Mark Murray 132wrote the 133.Xr rndcontrol 8 134utility for 135.Fx . 136.Pp 137The 138.Em IBAA 139CSPRNG was developed by 140.An Bob Jenkins 141and is used by 142.Pa /dev/urandom . 143The 144.Dx 145implementation was ported by 146.An Matthew Dillon 147from initial work done by 148.An Robin Carey . 149The 150.Pa /dev/random 151generator uses a choice of 152.Em IBAA 153and a 154.Em Fortuna 155-based CSPRNG implemented by 156.An Alex Hornung . 157