gen/FreeBSD/arc4random.3

   1 .\" $OpenBSD: arc4random.3,v 1.34 2014/07/19 16:11:16 naddy Exp $
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   3 .\" Copyright 1997 Niels Provos <provos@physnet.uni-hamburg.de>
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  16 .\"      This product includes software developed by Niels Provos.
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  31 .\" Manual page, using -mandoc macros
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  33 .Dd July 30, 2015
  34 .Dt ARC4RANDOM 3
  35 .Os
  36 .Sh NAME
  37 .Nm arc4random ,
  38 .Nm arc4random_buf ,
  39 .Nm arc4random_uniform
  40 .Nd random number generator
  41 .Sh SYNOPSIS
  42 .In stdlib.h
  43 .Ft uint32_t
  44 .Fn arc4random "void"
  45 .Ft void
  46 .Fn arc4random_buf "void *buf" "size_t nbytes"
  47 .Ft uint32_t
  48 .Fn arc4random_uniform "uint32_t upper_bound"
  49 .Sh DESCRIPTION
  50 .Pp
  51 These functions use a cryptographic pseudo-random number generator to generate
  52 high quality random bytes very quickly.  One data pool is used for all
  53 consumers in a process, so that consumption under program flow can act as
  54 additional stirring.  The subsystem is re-seeded from the kernel random number
  55 subsystem on a regular basis, and also upon
  56 .Xr fork 2 .
  57 .Pp
  58 This family of functions provides higher quality random data than those
  59 described in
  60 .Xr rand 3 ,
  61 .Xr random 3 ,
  62 and
  63 .Xr rand48 3 .
  64 They can be called in almost all environments, including
  65 .Xr chroot 2 ,
  66 and their use is encouraged over all other standard library functions for
  67 random numbers.
  68 .Pp
  69 .Fn arc4random
  70 returns a single 32-bit value.
  71 .Pp
  72 .Fn arc4random_buf
  73 fills the region
  74 .Fa buf
  75 of length
  76 .Fa nbytes
  77 with random data.
  78 .Pp
  79 .Fn arc4random_uniform
  80 will return a single 32-bit value, uniformly distributed but less than
  81 .Fa upper_bound .
  82 This is recommended over constructions like
  83 .Dq Li arc4random() % upper_bound
  84 as it avoids "modulo bias" when the upper bound is not a power of two.
  85 In the worst case, this function may require multiple iterations
  86 to ensure uniformity.
  87 .Sh RETURN VALUES
  88 These functions are always successful, and no return value is
  89 reserved to indicate an error.
  90 .Sh SEE ALSO
  91 .Xr rand 3 ,
  92 .Xr rand48 3 ,
  93 .Xr random 3 ,
  94 .Xr random 4
  95 .Sh HISTORY
  96 The original version of this random number generator used the RC4 (also known
  97 as ARC4) algorithm.  In OS X 10.12 it was replaced with the NIST-approved AES
  98 cipher, and it may be replaced again in the future as cryptographic techniques
  99 advance.  A good mnemonic is
 100 .Dq A Replacement Call for Random .