hide4pgp.doc

把信息隐藏到图片中

Hide4PGP v2.0 - third public version Feb 10, 2000

copyrighted software by Heinz Repp, Germany
                 email: [e-mail]

Contents:

1 - Introduction

2 - Commandline Parameters

3 - Technical

4 - Sourcecode

5 - Other Sources

6 - History

7 - Legal Stuff



1 - Introduction

The idea of writing this program arose with the public discussion of
limiting or disallowing cryptography for the public, as in France,
upcoming also in the US and Germany. The tradeoff is to make
encrypted data invisible by hiding them in other files - this is
called steganography. As encrypted data themselves already resemble
pure (white) noise they should only add some extra noise to other
data. Henry Hastur was one of the first to add a little program
(Stealth) to the PGP distribution (Pretty Good Privacy by Phil
Zimmermann) to remove (and later add) the PGP header leaving behind
pure white noise. I use PGP since years, so I had PGP and Stealth in
mind when planning to write this program. Stealth acts like a UNIX
filter, so my program should receive or send the encryted data from/to
a pipe. It also works with I/O-redirection ('<' and '>' on the
commandline). Unlike other available steganography programs it has no
own encryption. I think that PGP itself is unbreakable, additional
encryption doesn't make it any stronger. In the contrary: adding an
own cipher adds a header that can be traced. Imagine extracting
every 1, 2, ... least significant bits and looking there for known
header structures or coded end of file marks. PGP files treated with
Stealth have no such weakness. Additionally Hide4PGP transforms the
hidden data in a very simple fashion I call 'scrambling' to make them
look more 'noisy' even when using plaintext or repeated character
patterns.

I also had some look at White Noise Storm (tm) by Ray Arachelian and
Steganos v1.4 by Fabian Hansmann. Steganos came closest to what I
wanted, and the choice of VOC, WAV and BMP files for steganography was
inspired by it. Both programs did a good job with real quantitative
data, that means true color bitmaps. Greyscale pictures sometimes
make problems, and 256 colors palette BMP usually show dramatic
changes. What is the reason?  Pixels in 8 bit bitmaps do not hold
a colors but indeces to a color table, the palette. Changing the
least significant bit with real quantitative data makes a difference
of 1 in a range of 255, less than 0.4%, 2 bits mean 3 out of 255
or 1.2% - still hard to recognize. Changing 1 bit with palettes
entries means a different (maybe opposite) color. Greyscale pictures
also have a palette. Often this palette contains grey 'colors' in
strictly ascending order of brightness making the palette indices
resemble true gray values, but sometimes that palette too is
arbitrarily mixed.

The first versions of Hide4PGP tried to cope with these circumstances
by rearranging the palette pairs of similar colors, and duplicating
frequent colors to unused palette positions. In mid 1999 Luke Natraj
presented me the program 'Spyder' he had written together with a
friend, Philip Tellis. It works only on palettized bitmaps, but this
it did markedly better than Hide4PGP. The idea was to assign the
information bits to the palette colors, and then coding each pixel bit
by leaving it unchanged or changing it to the most similar color with
opposite bit assignment. This is much more favourable than the pairing
I did before, because the partner colors need not be mutual nor
exclusive, and this allows choosing better suited partner colors.
With permission of Luke Natraj I integrated his algorithm into
Hide4PGP combining it with Hide4PGP's color matching using CIE L*a*b*
color space and different criteria for assinging bit values. The
result is further enhanced by slight changes to the palette colors
to undo partly the pixel color changes - here Hide4PGP uses also
different calculations than Spyder.

And that is what has come of it: a program that handles OS/2 1.x,
Windows 3.x or OS/2 2.x uncompressed single image bitmaps with 256 or
16.7 mio colors, WAV with 8, 12 or 16 bit uncompressed mono or stereo
sound samples and 8 bit VOC files. I decided to support only bitmaps
because nearly every graphics program can read and write it, and it is
easy to convert the bitmaps into another graphics format after the
steganographic process. The same is true for audio programs and WAV
files. When converting to another format it is necessary that if
compression is used it must not be lossy, so JPEG or MP3 don't work,
but GIF, TIFF, ...  do. With the new algorithm for palettized bitmaps
the preservation of the palette order is no longer required. This
allows using most of the pictures e.g. in the WWW / Internet for
steganography. Use the GIF's of your Homepage ...


2 - Commandline Parameters

The syntax is simple: Hide4PGP needs always a multimedia file to work
on (BMP, WAV, or VOC), optionally a file name for the hidden data and
optionally some switches. It does not depend on the proper file
extension for the multimedia file: Hide4PGP recognizes the format by
the header, not by the name. If no file name for the secret data is
specified, they are read from the standard input, usually the
keyboard, resp. sent to the standard output, usually the screen. In
both cases you may redirect in- or output to a pipe ('|' symbol) or
a file (with the '<' or '>' symbol). An example with PGP and Stealth
in series may explain this:

pgp -ef user-ID < file-2-hide | stealth | Hide4PGP datafile

encrypts the file <file-2-hide> for the receiver <user-ID>, then
removes the header information leaving behind pure pseudo random data,
and hides those in the <datafile>. Alternatively you may specify

Hide4PGP datafile secret.dat

to hide secret.dat in datafile.

Hide4pgp -x datafile | stealth -a user-ID | pgp -f > decrypted-file

extracts the data previously stored in <datafile>, then adds a header
for <user-ID>, decrypts the file and stores the plaintext data in
<decrypted-file>. Hint: when using PGP for decryption in filter mode
you must set PGPPASS to your passphrase in advance (or use the -z
switch)!  Alternatively,

Hide4PGP -x datafile secret.dat

extracts the data to secrets.dat.

Options or switches may be anywhere on the commandline, starting with
'-' or '/'. Options may be in any order and case, and separate or
combined. Legal options are:

h or ?          Help; if Hide4PGP finds this option, it shows a help
                page with short description of all parameters and
                terminates.

i               Info; display some characteristics of the multimedia
                file and the maximum number of bytes that may be
                hidden within

v               Verbose: gives detailed information about the data
                file and the steganographic process.

q               Quiet: suppresses all output except reports of fatal
                error situations that lead to program abort (UNIX
                style: say nothing as long everything is o.k.).

x               eXtract; the default action is hiding.

n...            Not; n together with another character turns a feature
                off; see below

nf              do Not Fake File modication timestamp; by default
                Hide4PGP changes the multimedia timestamp to the value
                it had before it wrote to it; this option turns it off

nc              do Not Change Colors (256 colors BMP only); by default
                Hide4PGP optimizes the palette colors to make the
                picture appear similar to how it looked before; this
                option turns this behaviour off; note that there is
                ALWAYS a change in the least significant bit of one
                channel of some palette colors to store the bit
                assignments!

u/nu            use more/No UnUsed palette entries (256 colors BMP
                only); if a bitmap does not use all palette entries,
                Hide4PGP by default takes some of the unused entries
                and copies colors to them for which it could not find
                adequate partners otherwise. Usually this is only
                necessary in rare cases; the 'u' switch lets Hide4PGP
                use more of these otherwise unused entries (it raises
                the criterias for what it regards as 'adequate
                partner'); 'nu' suppresses the use of unused palette
                entries completely

The only file needed for the operation is the executable for your
operating system. You might wish to copy it to a directory in your
PATH.

The batch files for DOS, OS/2 and Windows NT were provided as a
convenience for users who feel uncomfortable with the pipe/streaming
syntax of Hide4PGP. The version 2.0 of Hide4PGP has the alternative
syntax already built in. For those used to these batch files they are
still included. You can think of these batch commands as a variant of
the COPY command. To hide <file 1> in <file 2> use

  Hide <file 1> <file 2> [options ...]