1.\" $OpenBSD: arc4random.3,v 1.37 2019/09/29 16:30:35 jmc Exp $ 2.\" 3.\" Copyright 1997 Niels Provos <provos@physnet.uni-hamburg.de> 4.\" All rights reserved. 5.\" 6.\" Redistribution and use in source and binary forms, with or without 7.\" modification, are permitted provided that the following conditions 8.\" are met: 9.\" 1. Redistributions of source code must retain the above copyright 10.\" notice, this list of conditions and the following disclaimer. 11.\" 2. Redistributions in binary form must reproduce the above copyright 12.\" notice, this list of conditions and the following disclaimer in the 13.\" documentation and/or other materials provided with the distribution. 14.\" 3. All advertising materials mentioning features or use of this software 15.\" must display the following acknowledgement: 16.\" This product includes software developed by Niels Provos. 17.\" 4. The name of the author may not be used to endorse or promote products 18.\" derived from this software without specific prior written permission. 19.\" 20.\" THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR 21.\" IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES 22.\" OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED. 23.\" IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT, 24.\" INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT 25.\" NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, 26.\" DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY 27.\" THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT 28.\" (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF 29.\" THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE. 30.\" 31.\" Manual page, using -mandoc macros 32.\" 33.Dd $Mdocdate: September 29 2019 $ 34.Dt ARC4RANDOM 3 35.Os 36.Sh NAME 37.Nm arc4random , 38.Nm arc4random_buf , 39.Nm arc4random_uniform 40.Nd random number generator 41.Sh SYNOPSIS 42.In stdlib.h 43.Ft uint32_t 44.Fn arc4random "void" 45.Ft void 46.Fn arc4random_buf "void *buf" "size_t nbytes" 47.Ft uint32_t 48.Fn arc4random_uniform "uint32_t upper_bound" 49.Sh DESCRIPTION 50This family of functions provides higher quality data than those 51described in 52.Xr rand 3 , 53.Xr random 3 , 54and 55.Xr rand48 3 . 56.Pp 57Use of these functions is encouraged for almost all random number 58consumption because the other interfaces are deficient in either 59quality, portability, standardization, or availability. 60These functions can be called in almost all coding environments, 61including 62.Xr pthreads 3 63and 64.Xr chroot 2 . 65.Pp 66High quality 32-bit pseudo-random numbers are generated very quickly. 67On each call, a cryptographic pseudo-random number generator is used 68to generate a new result. 69One data pool is used for all consumers in a process, so that consumption 70under program flow can act as additional stirring. 71The subsystem is re-seeded from the kernel 72.Xr random 4 73subsystem using 74.Xr getentropy 2 75on a regular basis, and also upon 76.Xr fork 2 . 77.Pp 78The 79.Fn arc4random 80function returns a single 32-bit value. 81.Pp 82.Fn arc4random_buf 83fills the region 84.Fa buf 85of length 86.Fa nbytes 87with random data. 88.Pp 89.Fn arc4random_uniform 90will return a single 32-bit value, uniformly distributed but less than 91.Fa upper_bound . 92This is recommended over constructions like 93.Dq Li arc4random() % upper_bound 94as it avoids "modulo bias" when the upper bound is not a power of two. 95In the worst case, this function may consume multiple iterations 96to ensure uniformity; see the source code to understand the problem 97and solution. 98.Sh RETURN VALUES 99These functions are always successful, and no return value is 100reserved to indicate an error. 101.Sh SEE ALSO 102.Xr rand 3 , 103.Xr rand48 3 , 104.Xr random 3 105.Sh HISTORY 106These functions first appeared in 107.Ox 2.1 . 108.Pp 109The original version of this random number generator used the 110RC4 (also known as ARC4) algorithm. 111In 112.Ox 5.5 113it was replaced with the ChaCha20 cipher, and it may be replaced 114again in the future as cryptographic techniques advance. 115A good mnemonic is 116.Dq A Replacement Call for Random . 117