NAME

       Data::Entropy::Source - encapsulated source of entropy

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 higher-level entropy-consuming 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 /dev/random 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 entropy-using 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 /dev/random 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 non-I/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 uniformly-distributed random number between zero
           inclusive and LIMIT exclusive.  LIMIT may be either a native integer, a "Math::BigInt" object, or an
           integer-valued "Math::BigRat" object; the returned number is of the same type.

           This method dispenses a non-integer number of bits of entropy.  For example, if LIMIT is 10 then the
           result contains approximately 3.32 bits of entropy.  The minimum non-zero amount of entropy that can
           be obtained is 1 bit, with LIMIT = 2.

       $source->get_prob(PROB0, PROB1)
           PROB0 and PROB1 must be non-negative integers, not both zero.  They may each be either a native
           integer, a "Math::BigInt" object, or an integer-valued "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.