1.\" $OpenBSD: arc4random.3,v 1.35 2014/11/25 16:45:24 millert 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.\" $FreeBSD$ 33.\" 34.Dd March 21, 2019 35.Dt ARC4RANDOM 3 36.Os 37.Sh NAME 38.Nm arc4random , 39.Nm arc4random_buf , 40.Nm arc4random_uniform 41.Nd random number generator 42.Sh LIBRARY 43.Lb libc 44.Sh SYNOPSIS 45.In stdlib.h 46.Ft uint32_t 47.Fn arc4random "void" 48.Ft void 49.Fn arc4random_buf "void *buf" "size_t nbytes" 50.Ft uint32_t 51.Fn arc4random_uniform "uint32_t upper_bound" 52.Sh DESCRIPTION 53This family of functions provides higher quality data than those 54described in 55.Xr rand 3 , 56.Xr random 3 , 57and 58.Xr rand48 3 . 59.Pp 60Use of these functions is encouraged for almost all random number 61consumption because the other interfaces are deficient in either 62quality, portability, standardization, or availability. 63These functions can be called in almost all coding environments, 64including 65.Xr pthreads 3 66and 67.Xr chroot 2 . 68.Pp 69High quality 32-bit pseudo-random numbers are generated very quickly. 70On each call, a cryptographic pseudo-random number generator is used 71to generate a new result. 72One data pool is used for all consumers in a process, so that consumption 73under program flow can act as additional stirring. 74The subsystem is re-seeded from the kernel random number subsystem using 75.Xr getentropy 2 76on a regular basis, and also upon 77.Xr fork 2 . 78.Pp 79The 80.Fn arc4random 81function returns a single 32-bit value. 82The 83.Fn arc4random 84function returns pseudo-random numbers in the range of 0 to 85.if t 2\u\s731\s10\d\(mi1, 86.if n (2**32)\(mi1, 87and therefore has twice the range of 88.Xr rand 3 89and 90.Xr random 3 . 91.Pp 92.Fn arc4random_buf 93fills the region 94.Fa buf 95of length 96.Fa nbytes 97with random data. 98.Pp 99.Fn arc4random_uniform 100will return a single 32-bit value, uniformly distributed but less than 101.Fa upper_bound . 102This is recommended over constructions like 103.Dq Li arc4random() % upper_bound 104as it avoids "modulo bias" when the upper bound is not a power of two. 105In the worst case, this function may consume multiple iterations 106to ensure uniformity; see the source code to understand the problem 107and solution. 108.Sh RETURN VALUES 109These functions are always successful, and no return value is 110reserved to indicate an error. 111.Sh EXAMPLES 112The following produces a drop-in replacement for the traditional 113.Fn rand 114and 115.Fn random 116functions using 117.Fn arc4random : 118.Pp 119.Dl "#define foo4random() (arc4random_uniform(RAND_MAX + 1))" 120.Sh SEE ALSO 121.Xr rand 3 , 122.Xr rand48 3 , 123.Xr random 3 124.Sh HISTORY 125These functions first appeared in 126.Ox 2.1 . 127.Pp 128The original version of this random number generator used the 129RC4 (also known as ARC4) algorithm. 130In 131.Ox 5.5 132it was replaced with the ChaCha20 cipher, and it may be replaced 133again in the future as cryptographic techniques advance. 134A good mnemonic is 135.Dq A Replacement Call for Random . 136.Pp 137The 138.Fn arc4random 139random number generator was first introduced in 140.Fx 2.2.6 . 141The ChaCha20 based implementation was introduced in 142.Fx 12.0 , 143with obsolete stir and addrandom interfaces removed at the same time. 144