1.\" $OpenBSD: arc4random.9,v 1.14 2018/04/28 20:25:07 jmc Exp $ 2.\" 3.\" Copyright (c) 1996,2000 Michael Shalayeff 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.\" 15.\" THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR 16.\" IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES 17.\" OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED. 18.\" IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT, 19.\" INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT 20.\" NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, 21.\" DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY 22.\" THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT 23.\" (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF 24.\" THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE. 25.\" 26.Dd $Mdocdate: April 28 2018 $ 27.Dt ARC4RANDOM 9 28.Os 29.Sh NAME 30.Nm arc4random , 31.Nm arc4random_buf , 32.Nm arc4random_uniform , 33.Nm enqueue_randomness 34.Nd kernel random subsystem 35.Sh SYNOPSIS 36.In sys/systm.h 37.Ft u_int32_t 38.Fn arc4random "void" 39.Ft void 40.Fn arc4random_buf "void *buf" "size_t nbytes" 41.Ft u_int32_t 42.Fn arc4random_uniform "u_int32_t upper_bound" 43.In dev/rndvar.h 44.Ft void 45.Fn enqueue_randomness "int" 46.Sh DESCRIPTION 47The 48.Fn add_random 49functions below mix input into the system entropy pool, which is then used 50to create a key for the ChaCha stream cipher used in the 51.Fn arc4random 52series of functions. 53.Pp 54.Fn enqueue_randomness 55causes the supplied data argument to be added to the entropy pool, 56increasing the entropy counter by 32 bits. 57.Pp 58.Fn arc4random 59and 60.Fn arc4random_buf 61provide random numbers and are intended to be called in any 62circumstance where random numbers are required. 63.Pp 64.Fn arc4random_uniform 65will return a uniformly distributed random number less than 66.Fa upper_bound , 67avoiding "modulo bias" when the upper bound is not a power of two. 68In the worst case, this function may consume multiple iterations 69to ensure uniformity; see the source code to understand the problem 70and solution. 71.Pp 72.Fn arc4random , 73.Fn arc4random_buf , 74and 75.Fn arc4random_uniform 76may be called in nearly any kernel context. 77.Sh SEE ALSO 78.Xr arc4random 3 , 79.Xr pchb 4 , 80.Xr random 4 81