Outguess is a universal steganographic tool that allows the insertion of hidden information into the redundant bits of data sources. The nature of the data source is irrelevant to the core of outguess. The program relies on data specific handlers that will extract redundant bits and write them back after modification. Currently only the PPM (Portable Pixel Map), PNM (Portable Any Map), and JPEG image formats are supported, although outguess could use any kind of data, as long as a handler were provided.

Steganography is the art and science of hiding that communication is happening. Classical steganography systems depend on keeping the encoding system secret, but modern steganography are detectable only if secret information is known, e.g. a secret key. Because of their invasive nature steganography systems leave detectable traces within a medium's characteristics. This allows an eavesdropper to detect media that has been modified, revealing that secret communication is taking place. Although the secrecy of the information is not degraded, its hidden nature is revealed, defeating the main purpose of Steganography.

For JPEG images, OutGuess preserves statistics based on frequency counts. As a result, no known statistical test is able to detect the presence of steganographic content. Before embedding data into an image, the OutGuess system can determine the maximum message size that can be hidden while still being able to maintain statistics based on frequency counts.

OutGuess uses a generic iterator object to select which bits in the data should be modified. A seed can be used to modify the behavior of the iterator. It is embedded in the data along with the rest of the message. By altering the seed, OutGuess tries to find a sequence of bits that minimizes the number of changes in the data that have to be made.

A sample output from OutGuess is as follows:

Reading dscf0001.jpg....
JPEG compression quality set to 75
Extracting usable bits: 40059 bits
Correctable message size: 21194 bits, 52.91%
Encoded 'snark.bz2': 14712 bits, 1839 bytes
Finding best embedding...
    0:  7467(50.6%)[50.8%], bias  8137(1.09), saved:   -13, total: 18.64%
    1:  7311(49.6%)[49.7%], bias  8079(1.11), saved:     5, total: 18.25%
    4:  7250(49.2%)[49.3%], bias  7906(1.09), saved:    13, total: 18.10%
   59:  7225(49.0%)[49.1%], bias  7889(1.09), saved:    16, total: 18.04%
59, 7225: Embedding data: 14712 in 40059
Bits embedded: 14744, changed: 7225(49.0%)[49.1%], bias: 7889, tot: 40032, skip: 25288
Foiling statistics: corrections: 2590, failed: 1, offset: 122.585494 +- 239.664983
Total bits changed: 15114 (change 7225 + bias 7889)
Storing bitmap into data...
Writing foil/dscf0001.jpg....

The simple example script seek_script uses OutGuess to select an image that fits the data we want to hide the best, yielding the lowest number of changed bits. Because we do not care about the actual content of the cover data we send, this is a very viable approach.

Additionally, OutGuess allows to hide multiple messages in the data. Thus, it also provides plausible deniability. It keeps track of the bits that have been modified previously and locks them. A (23,12,7) Golay code is used for error correction to tolerate collisions on locked bits. Artificial errors are introduced to avoid modifying bits that have a high bias.

Currently OutGuess can insert only two different messages. This is an experimental feature.

OutGuess needs your help. If you are a programmer and want to help a nice project, this is your opportunity.

The original OutGuess went unmaintained; the source of the last version, 0.2, was imported from Debian or other repositories of the Internet. After, patches from Debian and elsewhere were applied to create the 0.2.1 release. The details of each release are registered in the ChangeLog file. Now, OutGuess is maintained by volunteers under Resurrecting Open Source Projects.

If you are interested in helping OutGuess, read the CONTRIBUTING.md file.

To do so, you need to choose (and potentially edit) an appropriate jconfig.h file. To get an idea which one you might want, have a look at their header comments.

You might do so like this (POSIX only):

head -n 1 src/jpeg-6b-steg/jconfig.*

The default one is jconfig.cfg. You may use it like this:

cd jpeg-6b-steg
ln -s jconfig.cfg jconfig.h
cd ..

However, in OutGuess 0.3 or newer, there is the option --with-generic-jconfig that will use jconfig.cfg automatically. See the Build and install OutGuess section below.

OutGuess has only been tested on OpenBSD, Linux, Solaris and AIX.

If you manually edited jconfig.h, you must use the following command sequence:

./autogen.sh
./configure
make
make install

Otherwise, if you prefer to use jconfig.cfg content as default for jconfig.h, without a manual action, you can use the following sequence:

./autogen.sh
./configure --with-generic-jconfig
make
make install

OutGuess needs a modified version of the JPEG library. Currently, the original lib (without changes) is available at https://www.ijg.org/files/. The tarball name for version 6b is jpegsrc.v6b.tar.gz (or jpegsr6b.zip).

There is a complete document about the JPEG in src/jpeg-<version>-steg/install.doc in OutGuess source code (this is plain text, not a traditional .doc).

The .diff file used to modify the original JPEG library is available at /doc in OutGuess source code.

OutGuess uses code from the following projects. Attributions can also be found in the sources.

• Markus Kuhn's Stirmark software, see doc/STIRMARK-README
• the Independent JPEG Group's JPEG software, see src/jpeg-6b-steg/README
• the Arc4 random number generator for OpenBSD, (C) 1996 by David Mazieres dm@lcs.mit.edu
• free MD5 code by Colin Plumb

For determining the redundant bits out of a JPEG image, the jpeg-jsteg-v4 patches by Derek Upham upham@cs.ubc.ca were helpful.

Special thanks to:

• Robert Morelos-Zaragoza morelos_zaragoza@yahoo.com, author of golay.c, that gently agreed to change its licensing to BSD.
• Dug Song dugsong@monkey.org for helping with the original configure file,
• Andrew Reiter andrewr@rot26.net for testing on Solaris.

OutGuess was originally developed by Niels Provos provos@citi.umich.edu, under the BSD software license. It is completely free for any use including commercial.

Currently, source code is maintained by volunteers. Newer versions are available at https://github.com/resurrecting-open-source-projects/outguess