144 lines
4.6 KiB
Groff
144 lines
4.6 KiB
Groff
.\" $OpenBSD: arc4random.3,v 1.35 2014/11/25 16:45:24 millert Exp $
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.\"
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.\" Copyright 1997 Niels Provos <provos@physnet.uni-hamburg.de>
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.\" All rights reserved.
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.\"
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.\" Redistribution and use in source and binary forms, with or without
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.\" modification, are permitted provided that the following conditions
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.\" are met:
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.\" 1. Redistributions of source code must retain the above copyright
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.\" notice, this list of conditions and the following disclaimer.
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.\" 2. Redistributions in binary form must reproduce the above copyright
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.\" notice, this list of conditions and the following disclaimer in the
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.\" documentation and/or other materials provided with the distribution.
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.\" 3. All advertising materials mentioning features or use of this software
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.\" must display the following acknowledgement:
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.\" This product includes software developed by Niels Provos.
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.\" 4. The name of the author may not be used to endorse or promote products
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.\" derived from this software without specific prior written permission.
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.\"
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.\" THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR
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.\" IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES
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.\" OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED.
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.\" IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT,
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.\" INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT
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.\" NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
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.\" DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
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.\" THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
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.\" (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF
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.\" THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
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.\"
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.\" Manual page, using -mandoc macros
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.\" $FreeBSD$
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.\"
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.Dd March 21, 2019
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.Dt ARC4RANDOM 3
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.Os
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.Sh NAME
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.Nm arc4random ,
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.Nm arc4random_buf ,
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.Nm arc4random_uniform
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.Nd random number generator
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.Sh LIBRARY
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.Lb libc
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.Sh SYNOPSIS
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.In stdlib.h
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.Ft uint32_t
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.Fn arc4random "void"
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.Ft void
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.Fn arc4random_buf "void *buf" "size_t nbytes"
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.Ft uint32_t
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.Fn arc4random_uniform "uint32_t upper_bound"
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.Sh DESCRIPTION
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This family of functions provides higher quality data than those
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described in
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.Xr rand 3 ,
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.Xr random 3 ,
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and
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.Xr rand48 3 .
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.Pp
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Use of these functions is encouraged for almost all random number
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consumption because the other interfaces are deficient in either
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quality, portability, standardization, or availability.
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These functions can be called in almost all coding environments,
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including
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.Xr pthreads 3
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and
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.Xr chroot 2 .
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.Pp
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High quality 32-bit pseudo-random numbers are generated very quickly.
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On each call, a cryptographic pseudo-random number generator is used
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to generate a new result.
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One data pool is used for all consumers in a process, so that consumption
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under program flow can act as additional stirring.
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The subsystem is re-seeded from the kernel random number subsystem using
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.Xr getentropy 2
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on a regular basis, and also upon
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.Xr fork 2 .
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.Pp
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The
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.Fn arc4random
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function returns a single 32-bit value.
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The
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.Fn arc4random
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function returns pseudo-random numbers in the range of 0 to
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.if t 2\u\s731\s10\d\(mi1,
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.if n (2**32)\(mi1,
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and therefore has twice the range of
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.Xr rand 3
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and
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.Xr random 3 .
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.Pp
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.Fn arc4random_buf
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fills the region
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.Fa buf
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of length
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.Fa nbytes
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with random data.
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.Pp
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.Fn arc4random_uniform
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will return a single 32-bit value, uniformly distributed but less than
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.Fa upper_bound .
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This is recommended over constructions like
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.Dq Li arc4random() % upper_bound
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as it avoids "modulo bias" when the upper bound is not a power of two.
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In the worst case, this function may consume multiple iterations
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to ensure uniformity; see the source code to understand the problem
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and solution.
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.Sh RETURN VALUES
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These functions are always successful, and no return value is
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reserved to indicate an error.
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.Sh EXAMPLES
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The following produces a drop-in replacement for the traditional
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.Fn rand
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and
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.Fn random
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functions using
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.Fn arc4random :
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.Pp
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.Dl "#define foo4random() (arc4random_uniform(RAND_MAX + 1))"
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.Sh SEE ALSO
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.Xr rand 3 ,
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.Xr rand48 3 ,
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.Xr random 3
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.Sh HISTORY
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These functions first appeared in
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.Ox 2.1 .
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.Pp
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The original version of this random number generator used the
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RC4 (also known as ARC4) algorithm.
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In
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.Ox 5.5
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it was replaced with the ChaCha20 cipher, and it may be replaced
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again in the future as cryptographic techniques advance.
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A good mnemonic is
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.Dq A Replacement Call for Random .
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.Pp
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The
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.Fn arc4random
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random number generator was first introduced in
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.Fx 2.2.6 .
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The ChaCha20 based implementation was introduced in
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.Fx 12.0 ,
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with obsolete stir and addrandom interfaces removed at the same time.
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