1.\" 2.\" Copyright (c) 2015 3.\" Mark R V Murray 4.\" Copyright (c) 2000 5.\" The Regents of the University of California. All rights reserved. 6.\" 7.\" Redistribution and use in source and binary forms, with or without 8.\" modification, are permitted provided that the following conditions 9.\" are met: 10.\" 1. Redistributions of source code must retain the above copyright 11.\" notice, this list of conditions and the following disclaimer. 12.\" 2. Redistributions in binary form must reproduce the above copyright 13.\" notice, this list of conditions and the following disclaimer in the 14.\" documentation and/or other materials provided with the distribution. 15.\" 16.\" THIS SOFTWARE IS PROVIDED BY THE DEVELOPERS ``AS IS'' AND ANY EXPRESS OR 17.\" IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES 18.\" OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED. 19.\" IN NO EVENT SHALL THE DEVELOPERS BE LIABLE FOR ANY DIRECT, INDIRECT, 20.\" INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT 21.\" NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, 22.\" DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY 23.\" THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT 24.\" (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF 25.\" THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE. 26.\" 27.\" $FreeBSD$ 28.\" " 29.Dd December 26, 2019 30.Dt RANDOM 9 31.Os 32.Sh NAME 33.Nm arc4rand , 34.Nm arc4random , 35.Nm arc4random_buf , 36.Nm is_random_seeded , 37.Nm random , 38.Nm read_random , 39.Nm read_random_uio 40.Nd supply pseudo-random numbers 41.Sh SYNOPSIS 42.In sys/libkern.h 43.Ft uint32_t 44.Fn arc4random "void" 45.Ft void 46.Fn arc4random_buf "void *ptr" "size_t len" 47.Ft void 48.Fn arc4rand "void *ptr" "u_int length" "int reseed" 49.Pp 50.In sys/random.h 51.Ft bool 52.Fn is_random_seeded "void" 53.Ft void 54.Fn read_random "void *buffer" "int count" 55.Ft int 56.Fn read_random_uio "struct uio *uio" "bool nonblock" 57.Ss LEGACY ROUTINES 58.In sys/libkern.h 59.Ft u_long 60.Fn random "void" 61.Sh DESCRIPTION 62The 63.Fn arc4random 64and 65.Fn arc4random_buf 66functions will return very good quality random numbers, suited for 67security-related purposes. 68Both are wrappers around the underlying 69.Fn arc4rand 70interface. 71.Fn arc4random 72returns a 32-bit random value, while 73.Fn arc4random_buf 74fills 75.Fa ptr 76with 77.Fa len 78bytes of random data. 79.Pp 80The 81.Fn arc4rand 82CSPRNG 83is seeded from the 84.Xr random 4 85kernel abstract entropy device. 86Automatic reseeding happens at unspecified time and bytes (of output) 87intervals. 88A reseed can be forced by passing a non-zero 89.Fa reseed 90value. 91.Pp 92The 93.Fn read_random 94function is used to read entropy directly from the kernel abstract entropy 95device. 96.Fn read_random 97blocks if and until the entropy device is seeded. 98The provided 99.Fa buffer 100is filled with no more than 101.Fa count 102bytes. 103It is strongly advised that 104.Fn read_random 105is not used directly; 106instead, use the 107.Fn arc4rand 108family of functions. 109.Pp 110The 111.Fn is_random_seeded 112function can be used to check in advance if 113.Fn read_random 114will block. 115(If random is seeded, it will not block.) 116.Pp 117The 118.Fn read_random_uio 119function behaves identically to 120.Xr read 2 121on 122.Pa /dev/random . 123The 124.Fa uio 125argument points to a buffer where random data should be stored. 126If 127.Fa nonblock 128is true and the random device is not seeded, this function does not return any 129data. 130Otherwise, this function may block interruptibly until the random device is seeded. 131If the function is interrupted before the random device is seeded, no data is 132returned. 133.Pp 134The deprecated 135.Xr random 9 136function will produce a sequence of pseudorandom numbers using a similar weak 137linear congruential generator as 138.Xr rand 3 139(the 1988 Park-Miller LCG). 140It is obsolete and scheduled to be removed in 141.Fx 13.0 . 142It is strongly advised that the 143.Xr random 9 144function not be used to generate random numbers. 145See 146.Sx SECURITY CONSIDERATIONS . 147.Sh RETURN VALUES 148The 149.Fn arc4rand 150function uses the Chacha20 algorithm to generate a pseudo-random sequence of 151bytes. 152The 153.Fn arc4random 154function uses 155.Fn arc4rand 156to generate pseudo-random numbers 157in the range from 0 to 158.if t 2\u\s732\s10\d\(mi1. 159.if n (2**32)\(mi1. 160.Pp 161The 162.Fn read_random 163function returns 164the number of bytes placed in 165.Fa buffer . 166.Pp 167.Fn read_random_uio 168returns zero when successful, 169otherwise an error code is returned. 170.Sh ERRORS 171.Fn read_random_uio 172may fail if: 173.Bl -tag -width Er 174.It Bq Er EFAULT 175.Fa uio 176points to an invalid memory region. 177.It Bq Er EWOULDBLOCK 178The random device is unseeded and 179.Fa nonblock 180is true. 181.El 182.Sh AUTHORS 183.An Dan Moschuk 184wrote 185.Fn arc4random . 186.An Mark R V Murray 187wrote 188.Fn read_random . 189.Sh SECURITY CONSIDERATIONS 190Do not use 191.Fn random 192in new code. 193.Pp 194It is important to remember that the 195.Fn random 196function is entirely predictable. 197It is easy for attackers to predict future output of 198.Fn random 199by recording some generated values. 200We cannot emphasize strongly enough that 201.Fn random 202must not be used to generate values that are intended to be unpredictable. 203