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.\" $FreeBSD$
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.Dd September 25, 2000
.Os
.Dt RANDOM 9
.Sh NAME
.Nm arc4random ,
.Nm random ,
.Nm read_random ,
.Nm srandom
.Nd supply pseudo-random numbers
.Sh SYNOPSIS
.In sys/libkern.h
.Ft void
.Fn srandom "u_long seed"
.Ft u_long
.Fn random "void"
.Ft u_int32_t
.Fn arc4random "void"
.Pp
.In sys/random.h
.Ft int
.Fn read_random "void *buffer" "int count"
.Sh DESCRIPTION
The
.Fn random
function will by default produce a sequence of numbers that can be duplicated
by calling
.Fn srandom
with
.Ql 1
as the
.Ar seed .
The
.Fn srandom
function may be called with any arbitrary
.Ar seed
value to get slightly more unpredictable numbers.
It is important to remember that the
.Fn random
function is entirely predictable, and is therefore not of use where
knowledge of the sequence of numbers may be of benefit to an attacker.
.Pp
The
.Fn arc4random
function will return very good quality random numbers, slightly better
suited for security-related purposes.
The random numbers from
.Fn arc4random
are seeded from the entropy device if it is available.
.Pp
The
.Fn read_random
function is used to return entropy directly from the entropy device
if it has been loaded. If the entropy device is not loaded, then
the
.Ar buffer
is filled with output generated by
.Fn random .
The
.Ar buffer
is filled with no more than
.Ar count
bytes. It is advised that
.Fn read_random
is not used; instead use
.Fn arc4random .
.Pp
All the bits generated by
.Fn random ,
.Fn arc4random
and
.Fn read_random
are usable. For example,
.Sq Li random()&01
will produce a random binary value.
.Sh RETURN VALUES
The
.Fn random
function
uses a non-linear additive feedback random number generator employing a
default table of size 31 long integers to return successive pseudo-random
numbers in the range from 0 to
.if t 2\u\s731\s10\d\(mi1.
.if n (2**31)\(mi1.
The period of this random number generator is very large, approximately
.if t 16\(mu(2\u\s731\s10\d\(mi1).
.if n 16*((2**31)\(mi1).
.Pp
The
.Fn arc4random
function
uses the RC4 algorithm to generate successive pseudo-random
numbers in the range from 0 to
.if t 2\u\s732\s10\d\(mi1.
.if n (2**32)\(mi1.
.Pp
The
.Fn read_random
function returns the number of bytes placed in
.Ar buffer .
.Sh AUTHORS
.An Dan Moschuk
wrote
.Fn arc4random .
.An Mark R V Murray
wrote
.Fn read_random .
