#!/usr/bin/perl -w
# Extract low order bits from an image and make those high order bits.
# Useful for steganographic image extraction, eg from "ppmstegan" output.
# See also pnmchdim
#
# 3 Jan 2005
use strict;
use Image::PBMlib;
use vars qw( $id $in $keepmask $shiftbits $ref @pixels $pixel );
$id = $0;
$id =~ s:.*/::;
$keepmask = 3; # 00000011
$shiftbits = 6; # number of zeros above
while(defined($ARGV[0]) and substr($ARGV[0], 0, 1) eq '-') {
if (($ARGV[0] eq '-b') or ($ARGV[0] eq '--bits')) {
shift;
$shiftbits = shift;
if(!defined($shiftbits) or ($shiftbits !~ /^[1-7]$/)) {
print STDERR "$id: -b (--bits) requires a number from 1 to 7\n";
exit 2;
}
$shiftbits = 8 - $shiftbits;
$keepmask = 255 >> $shiftbits;
print STDERR "keep: $keepmask bits: $shiftbits\n";
} elsif ($ARGV[0] eq '--help') {
print STDERR "$id: usage\n", <<USAGEinfo;
ppmlowbit [options] stegan.ppm > evil.ppm
Options:
-b --bits NUM extract image from NUM low bits of stegan.ppm
PGM and PPM images supported. The output picture will be created with the
dimensions of the input picture, reguardless of original size. The low bits
will be shifted to high bits to brighten the image. See pnmchdim.
USAGEinfo
exit 2;
} else {
print STDERR "$id: $ARGV[0] not a recognized option, use --help for help\n";
exit 2;
}
}
$in = shift;
if(!defined($in)) {
die "$id: Missing input file\n";
}
if(!open(PPM, "< $in")) {
die "$id: Can't open $in: $!\n";
}
$ref = readppmheader(\*PPM);
if(defined($$ref{error})) {
die "$id: PPM error: $$ref{error}\n";
}
if($$ref{bgp} eq 'b') {
die "$id: PPM error: bitmaps do not have enough bits per pixel\n";
}
# Just dump the same header
print $$ref{fullheader};
# Read everything
@pixels = readpixels_raw(\*PPM, $$ref{type},
($$ref{width} * $$ref{height}) );
for $pixel (@pixels) {
if($$ref{bgp} eq 'g') {
print chr((ord($pixel) & $keepmask) << $shiftbits);
} else {
print chr((ord($$pixel[0]) & $keepmask) << $shiftbits);
print chr((ord($$pixel[1]) & $keepmask) << $shiftbits);
print chr((ord($$pixel[2]) & $keepmask) << $shiftbits);
}
}
exit;
__END__
=pod
=head1 NAME
ppmlowbit - extract an image from the lowbits of an image
=head1 DESCRIPTION
Very simple steganographic image extraction.
Most basic usage:
ppmlowbit steganography.ppm > evil.ppm
PGM and PPM input images supported. The output picture will be created with
the dimensions of the input picture, reguardless of original size. The low
bits will be shifted to high bits to brighten the image.
The output can be redimensioned with C<pnmchdim>.
=head1 USAGE
Usage:
ppmlowbit [options] stegan.ppm > evil.ppm
Options:
=over 4
=item *
-b --bits NUM
Extract evil.pnm from the lower NUM bits of stegan.ppm, defaults to 2.
=item *
--help
Prints a usage message and exits.
=back
The C<pnmchdim> tool can be usaged to massage one file type into another,
if the two require the same number of bits to represent.
=head1 SEE ALSO
C<ppmstegan> - merge two images stegangraphically
C<pnmchdim> - change dimensions or type of a PBM/PGM/PPM file
=head1 COPYRIGHT
Copyright 2005 by Eli the Bearded / Benjamin Elijah Griffin.
Released under the same license(s) as Perl.
=head1 AUTHOR
Eli the Bearded wrote the C<ppmstegan>, C<ppmlowbit>, and C<pnmchdim>
set of tools to examine a demo steganographic image.
=head1 CPAN INFO
=head1 SCRIPT CATEGORIES
Image
=head1 README
ppmlowbit - extract an image from the lowbits of an image
=head1 PREREQUISITES
This uses the C<strict>, C<vars>, and C<Image::PPMlib> modules.
=head1 COREQUISITES
The C<ppmstegan> and C<pnmchdim> scripts are recommended.
=head1 OSNAMES
Should be OS independent.
=cut
