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: src/lib/libc/gen/arc4random.3,v 1.18 2008/07/22 11:33:49 ache Exp $ 32.\" $DragonFly: src/lib/libc/gen/arc4random.3,v 1.6 2007/06/15 10:10:13 swildner Exp $ 33.\" 34.Dd April 15, 1997 35.Dt ARC4RANDOM 3 36.Os 37.Sh NAME 38.Nm arc4random , 39.Nm arc4random_buf , 40.Nm arc4random_uniform , 41.Nm arc4random_stir , 42.Nm arc4random_addrandom 43.Nd arc4 random number generator 44.Sh LIBRARY 45.Lb libc 46.Sh SYNOPSIS 47.In stdlib.h 48.Ft u_int32_t 49.Fn arc4random "void" 50.Ft void 51.Fn arc4random_buf "void *buf" "size_t nbytes" 52.Ft u_int32_t 53.Fn arc4random_uniform "u_int32_t upper_bound" 54.Ft void 55.Fn arc4random_stir "void" 56.Ft void 57.Fn arc4random_addrandom "uint8_t *dat" "size_t datlen" 58.Sh DESCRIPTION 59The 60.Fn arc4random 61function uses the key stream generator employed by the 62arc4 cipher, which uses 8*8 8 bit S-Boxes. 63The S-Boxes 64can be in about 65.if t 2\u\s71700\s10\d 66.if n (2**1700) 67states. 68The 69.Fn arc4random 70function returns pseudo-random numbers in the range of 0 to 71.if t 2\u\s731\s10\d\(mi1, 72.if n (2**32)\(mi1, 73and therefore has twice the range of 74.Xr rand 3 75and 76.Xr random 3 . 77.Pp 78.Fn arc4random_buf 79function fills the region 80.Fa buf 81of length 82.Fa nbytes 83with ARC4-derived random data. 84.Pp 85.Fn arc4random_uniform 86will return a uniformly distributed random number less than 87.Fa upper_bound . 88.Fn arc4random_uniform 89is recommended over constructions like 90.Dq Li arc4random() % upper_bound 91as it avoids "modulo bias" when the upper bound is not a power of two. 92.Pp 93The 94.Fn arc4random_stir 95function reads data from 96.Pa /dev/urandom 97and uses it to permute the S-Boxes via 98.Fn arc4random_addrandom . 99.Pp 100There is no need to call 101.Fn arc4random_stir 102before using 103.Fn arc4random 104functions family, since 105they automatically initialize themselves. 106.Sh EXAMPLES 107The following produces a drop-in replacement for the traditional 108.Fn rand 109and 110.Fn random 111functions using 112.Fn arc4random : 113.Pp 114.Dl "#define foo4random() (arc4random() % ((unsigned)RAND_MAX + 1))" 115.Sh SEE ALSO 116.Xr rand 3 , 117.Xr random 3 , 118.Xr srandomdev 3 , 119.Xr urandom 4 120.Sh HISTORY 121.Tn RC4 122has been designed by RSA Data Security, Inc. 123It was posted anonymously 124to the USENET and was confirmed to be equivalent by several sources who 125had access to the original cipher. 126Since 127.Tn RC4 128used to be a trade secret, the cipher is now referred to as 129.Tn ARC4 . 130