1 .\" $OpenBSD: arc4random.3,v 1.37 2019/09/29 16:30:35 jmc Exp $
3 .\" Copyright 1997 Niels Provos <provos@physnet.uni-hamburg.de>
4 .\" All rights reserved.
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16 .\" This product includes software developed by Niels Provos.
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31 .\" Manual page, using -mandoc macros
40 .Nm arc4random_uniform
41 .Nd random number generator
49 .Fn arc4random_buf "void *buf" "size_t nbytes"
51 .Fn arc4random_uniform "uint32_t upper_bound"
53 This family of functions provides higher quality data than those
60 Use of these functions is encouraged for almost all random number
61 consumption because the other interfaces are deficient in either
62 quality, portability, standardization, or availability.
63 These functions can be called in almost all coding environments,
69 High quality 32-bit pseudo-random numbers are generated very quickly.
70 On each call, a cryptographic pseudo-random number generator is used
71 to generate a new result.
72 One data pool is used for all consumers in a process, so that consumption
73 under program flow can act as additional stirring.
74 The subsystem is re-seeded from the kernel
78 on a regular basis, and also upon
83 function returns a single 32-bit value.
86 function returns pseudo-random numbers in the range of 0 to
87 .if t 2\u\s731\s10\d\(mi1,
89 and therefore has twice the range of
101 .Fn arc4random_uniform
102 will return a single 32-bit value, uniformly distributed but less than
104 This is recommended over constructions like
105 .Dq Li arc4random() % upper_bound
106 as it avoids "modulo bias" when the upper bound is not a power of two.
107 In the worst case, this function may consume multiple iterations
108 to ensure uniformity; see the source code to understand the problem
111 These functions are always successful, and no return value is
112 reserved to indicate an error.
114 The following produces a drop-in replacement for the traditional
121 .Dl "#define foo4random() (arc4random_uniform(RAND_MAX + 1))"
127 .%A Daniel J. Bernstein
128 .%T ChaCha, a variant of Salsa20
130 .%O Document ID: 4027b5256e17b9796842e6d0f68b0b5e
131 .%U http://cr.yp.to/papers.html#chacha
134 These functions first appeared in
137 The original version of this random number generator used the
138 RC4 (also known as ARC4) algorithm.
141 it was replaced with the ChaCha20 cipher, and it may be replaced
142 again in the future as cryptographic techniques advance.
144 .Dq A Replacement Call for Random .
148 random number generator was first introduced in
150 The ChaCha20 based implementation was introduced in
152 with obsolete stir and addrandom interfaces removed at the same time.