

 
Manual Reference Pages  DATA::ENTROPY::SOURCE (3)
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NAME
Data::Entropy::Source  encapsulated source of entropy
CONTENTS
SYNOPSIS
use Data::Entropy::Source;
$source = Data::Entropy::Source>new($handle, "sysread");
$c = $source>get_octet;
$str = $source>get_bits(17);
$i = $source>get_int(12345);
$i = $source>get_int(Math::BigInt>new("1000000000000"));
$j = $source>get_prob(1, 2);
DESCRIPTION
An object of this class encapsulates a source of entropy
(randomness). Methods allow entropy to be dispensed in any
quantity required, even fractional bits. An entropy source object
should not normally be used directly. Rather, it should be used to
support higherlevel entropyconsuming algorithms, such as those in
Data::Entropy::Algorithms.
This type of object is constructed as a layer over a raw entropy source
which does not supply methods to extract arbitrary amounts of entropy.
The raw entropy source is expected to dispense only entire octets at
a time. The <B>/dev/randomB> devices on some versions of Unix constitute
such a source, for example. The raw entropy source is accessed
via the IO::Handle interface. This interface may be supplied by
classes other than IO::Handle itself, as is done for example by
Data::Entropy::RawSource::CryptCounter.
If two entropy sources of this class are given exactly the same raw
entropy data, for example by reading from the same file, and exactly the
same sequence of get_ method calls is made to them, then they will
return exactly the same values from those calls. (Calls with numerical
arguments that have the same numerical value but are of different
types count as the same for this purpose.) This means that a run of an
entropyusing algorithm can be made completely deterministic if desired.
CONSTRUCTOR
Data::Entropy::Source>new(RAW_SOURCE, READ_STYLE)

Constructs and returns an entropy source object based on the given raw
source. RAW_SOURCE must be an I/O handle referring to a source of entropy
that can be read one octet at a time. Specifically, it must support
either the getc or sysread method described in IO::Handle.
READ_STYLE must be a string, either getc or sysread, indicating which
method should be used to read from the raw source. No methods other
than the one specified will ever be called on the raw source handle,
so a full implementation of IO::Handle is not required.
The sysread method should be used with <B>/dev/randomB> and its ilk,
because buffering would be very wasteful of entropy and might consequently
block other processes that require entropy. getc should be preferred
when reading entropy from a regular file, and it is the more convenient
interface to implement when a nonI/O object is being used for the handle.


METHODS
$source>get_octet

Returns an octet of entropy, as a string of length one. This provides
direct access to the raw entropy source.

$source>get_bits(NBITS)

Returns NBITS bits of entropy, as a string of octets. If NBITS is
not a multiple of eight then the last octet in the string has its most
significant bits set to zero.

$source>get_int(LIMIT)

LIMIT must be a positive integer. Returns a uniformlydistributed
random number between zero inclusive and LIMIT exclusive. LIMIT may be
either a native integer, a Math::BigInt object, or an integervalued
Math::BigRat object; the returned number is of the same type.
This method dispenses a noninteger number of bits of entropy.
For example, if LIMIT is 10 then the result contains approximately 3.32
bits of entropy. The minimum nonzero amount of entropy that can be
obtained is 1 bit, with LIMIT = 2.

$source>get_prob(PROB0, PROB1)

PROB0 and PROB1 must be nonnegative integers, not both zero.
They may each be either a native integer, a Math::BigInt object,
or an integervalued Math::BigRat objects; types may be mixed.
Returns either 0 or 1, with relative probabilities PROB0 and PROB1.
That is, the probability of returning 0 is PROB0/(PROB0+PROB1), and the
probability of returning 1 is PROB1/(PROB0+PROB1).
This method dispenses a fraction of a bit of entropy. The maximum
amount of entropy that can be obtained is 1 bit, with PROB0 = PROB1.
The more different the probabilities are the less entropy is obtained.
For example, if PROB0 = 1 and PROB1 = 2 then the result contains
approximately 0.918 bits of entropy.


SEE ALSO
Data::Entropy,
Data::Entropy::Algorithms,
Data::Entropy::RawSource::CryptCounter,
Data::Entropy::RawSource::Local,
Data::Entropy::RawSource::RandomOrg,
IO::Handle
AUTHOR
Andrew Main (Zefram) <zefram@fysh.org>
COPYRIGHT
Copyright (C) 2006, 2007, 2009, 2011
Andrew Main (Zefram) <zefram@fysh.org>
LICENSE
This module is free software; you can redistribute it and/or modify it
under the same terms as Perl itself.
perl v5.20.3  DATA::ENTROPY::SOURCE (3)  20160317 
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