package Number::Encode;
require 5.005_62;
use strict;
use warnings;
use Digest::MD5 qw(md5);
require Exporter;
our @ISA = qw(Exporter);
our %EXPORT_TAGS = ( 'all' => [ qw(nonuniform uniform) ] );
our @EXPORT_OK = ( @{ $EXPORT_TAGS{'all'} } );
our $VERSION = '1.00';
sub uniform ($) {
my $data = shift;
my $char = 0;
my $carry = 0;
my $cbits = 0;
my $hsh = '';
for my $c (split(//, $data))
{
$carry <<= 8;
$carry += ord($c);
$cbits += 8;
while ($cbits >= 4) {
my $n;
if ($cbits >= 4) { # More than 4 bits available
$n = $carry & 0xF;
if ($n > 9) {
$n ^= vec(md5($hsh . $carry . $n), $n, 4);
$n &= 0x7 if $n > 9;
}
$carry >>= 4;
$cbits -= 4;
}
$hsh .= chr(ord('0') + $n);
}
}
if ($cbits) {
$hsh .= chr(ord('0') + $carry);
}
return $hsh;
}
sub nonuniform ($) {
my $data = shift;
my $char = 0;
my $carry = 0;
my $cbits = 0;
my $hsh = '';
for my $c (split(//, $data))
{
$carry <<= 256;
$carry += ord($c);
$cbits += 8;
while ($cbits >= 4) {
my $n;
if ($cbits >= 4) { # More than 4 bits available
$n = $carry & 0xF;
if ($n <= 9) {
$carry >>= 4;
$cbits -= 4;
}
}
if ($cbits == 3 or $n > 9) {
$n = $carry & 0x7;
$carry >>= 3;
$cbits -= 3;
}
$hsh .= chr(ord('0') + $n);
}
}
if ($cbits) {
$hsh .= chr(ord('0') + $carry);
}
return $hsh;
}
1;
__END__
=head1 NAME
Number::Encode - Encode bit strings into digit strings
=head1 SYNOPSIS
use Number::Encode qw(nonuniform uniform);
=head1 DESCRIPTION
Provides a mechanism to convert arbitrary bit-strings into numeric
digit strings. The transformation can be uniform or non-uniform
depending on the type of distribution of the numeric digits achieved.
The former approach is useful for security-related applications such
as calling cards and the such, which require a uniform digit
distribution. The algorythm used to generate uniform distributions,
while deterministic, is more constly than the non-uniform variant.
This module is distributed under the same terms and warranty as Perl
itself.
=head2 EXPORT
This module provides the following exports:
=over
=item C
Converts a bit-string represented in the example by the scalar
C<$data> to a numeric string representation returned at
C<$number>.
The probabilistic distribution of the digits in the resulting number
is not uniform. Some digits will have up to twice the chance of others
of appearing at a given position.
=item C
Performs a transformation from the bit-string provided in C<$data> to
a numeric string returned at C<$number>. This transformation is a bit
more costly but has the advantage that the digit distribution is
uniform. This function is adequate for applications that require a
uniform composition of the numeric strings, such as password or PIN
number generators.
=back
=head1 HISTORY
=over 8
=item 1.00
Original version; created by h2xs 1.20 with options
-ACOXfn
Number::Encode
-v
1.00
=back
=head1 AUTHOR
Luis E. Munoz
=head1 SEE ALSO
perl(1).
=cut