1.\" $OpenBSD: arc4random.3,v 1.2 1997/04/27 22:40:25 angelos Exp $ 2.\" Copyright 1997 Niels Provos <provos@physnet.uni-hamburg.de> 3.\" All rights reserved. 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.\" 3. All advertising materials mentioning features or use of this software 14.\" must display the following acknowledgement: 15.\" This product includes software developed by Niels Provos. 16.\" 4. The name of the author may not be used to endorse or promote products 17.\" derived from this software without specific prior written permission. 18.\" 19.\" THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR 20.\" IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES 21.\" OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED. 22.\" IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT, 23.\" INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT 24.\" NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, 25.\" DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY 26.\" THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT 27.\" (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF 28.\" THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE. 29.\" 30.\" Manual page, using -mandoc macros 31.\" $FreeBSD$ 32.\" 33.Dd April 15, 1997 34.Dt ARC4RANDOM 3 35.Os 36.Sh NAME 37.Nm arc4random , 38.Nm arc4random_buf , 39.Nm arc4random_uniform , 40.Nm arc4random_stir , 41.Nm arc4random_addrandom 42.Nd arc4 random number generator 43.Sh LIBRARY 44.Lb libc 45.Sh SYNOPSIS 46.In stdlib.h 47.Ft uint32_t 48.Fn arc4random "void" 49.Ft void 50.Fn arc4random_buf "void *buf" "size_t nbytes" 51.Ft uint32_t 52.Fn arc4random_uniform "uint32_t upper_bound" 53.Ft void 54.Fn arc4random_stir "void" 55.Ft void 56.Fn arc4random_addrandom "unsigned char *dat" "int datlen" 57.Sh DESCRIPTION 58The 59.Fn arc4random 60function uses the key stream generator employed by the 61arc4 cipher, which uses 8*8 8 bit S-Boxes. 62The S-Boxes 63can be in about 64.if t 2\u\s71700\s10\d 65.if n (2**1700) 66states. 67The 68.Fn arc4random 69function returns pseudo-random numbers in the range of 0 to 70.if t 2\u\s731\s10\d\(mi1, 71.if n (2**32)\(mi1, 72and therefore has twice the range of 73.Xr rand 3 74and 75.Xr random 3 . 76.Pp 77.Fn arc4random_buf 78function fills the region 79.Fa buf 80of length 81.Fa nbytes 82with ARC4-derived random data. 83.Pp 84.Fn arc4random_uniform 85will return a uniformly distributed random number less than 86.Fa upper_bound . 87.Fn arc4random_uniform 88is recommended over constructions like 89.Dq Li arc4random() % upper_bound 90as it avoids "modulo bias" when the upper bound is not a power of two. 91.Pp 92The 93.Fn arc4random_stir 94function reads data from 95.Pa /dev/urandom 96and uses it to permute the S-Boxes via 97.Fn arc4random_addrandom . 98.Pp 99There is no need to call 100.Fn arc4random_stir 101before using 102.Fn arc4random 103functions family, since 104they automatically initialize themselves. 105.Sh EXAMPLES 106The following produces a drop-in replacement for the traditional 107.Fn rand 108and 109.Fn random 110functions using 111.Fn arc4random : 112.Pp 113.Dl "#define foo4random() (arc4random() % ((unsigned)RAND_MAX + 1))" 114.Sh SEE ALSO 115.Xr rand 3 , 116.Xr random 3 , 117.Xr srandomdev 3 118.Sh HISTORY 119.Pa RC4 120has been designed by RSA Data Security, Inc. 121It was posted anonymously 122to the USENET and was confirmed to be equivalent by several sources who 123had access to the original cipher. 124Since 125.Pa RC4 126used to be a trade secret, the cipher is now referred to as 127.Pa ARC4 . 128