xref: /freebsd/lib/libc/gen/arc4random.3 (revision 190cef3d52236565eb22e18b33e9e865ec634aa3)
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 July 19, 2014
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() % ((unsigned)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