NAME
outguess - universal steganographic tool
SYNOPSIS
outguess [ -emt ] [ -r ] [ -k key ] [ -F [+-] ] [ -d datafile ] [ -s
seed ] [ -i limit ] [ -x maxkeys ] [ -p param ] [ inputfile [
outputfile ]]
DESCRIPTION
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,
PNM, and JPEG image formats are supported, although outguess could use
any kind of data, as long as a handler were provided.
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 bias is introduced that favors the modification of bits that were
extracted from a high value, and tries to avoid the modification of
bits that were extracted from a low value.
Additionally, Outguess allows for the hiding of two distinct messages
in the data, thus providing plausible deniablity. 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. Artifical errors are introduced to avoid
modifying bits that have a high bias.
OPTIONS
The following command line options, when specified as capital letters,
indicate options for the second message.
-F [+-]
Specifies that OutGuess should preserve statistics based on
frequency counts. As a result, no statistical test that is
based on frequency counts will be able to detect steganographic
content. This option is on by default.
-kK key
Specify the secret key used to encrypt and hide the message in
the provided data.
-dD datafile
Specify the filename containing a message to be hidden in the
data.
-sS seed
Specify the initial seed the iterator object uses for selecting
bits in the redundant data. If no upper limit is specified, the
iterator will use this seed without searching for a more optimal
embedding.
-iI limit
Specify the upper limit for finding an optimal iterator seed.
The maximum value for the limit is 65535.
-eE Use error correction for data encoding and decoding.
Other options that apply to the general execution of outguess:
-r Retrieve a message from a data object. If this option is not
specified, outguess will embed messages.
-x maxkeys
If the second key does not create an iterator object that is
successful in embedding the data, the program will derive up to
specified number of new keys.
-p param
Passes a string as parameter to the destination data handler.
For the JPEG image format, this is the compression quality, it
can take values between 75 and 100. The higher the quality the
more bits to hide a message in the data are available.
-m Mark pixels that have been modified.
-t Collect statistics about redundant bit usage. Repeated use
increases output level.
For embedding messages, you need to specify a source and a destination
filename. Outguess determines the data format by the filename
extension. If no filenames are specified outguess operates as a filter
and assumes the PPM data format.
EXAMPLES
To embed the message hidden.txt into the monkey.jpg image:
outguess -k "my secret pass phrase" -d hidden.txt monkey.jpg
out.jpg
And in the other direction:
outguess -k "my secret pass phrase" -r out.jpg message.txt
will retrieve the hidden message from the image.
If you want to embed a second message, use:
outguess -k "secret1" -d hide1.txt -E -K "secret2" -D hide2.txt
monkey.jpg out.jpg
Outguess will first embed hide1.txt and then hide2.txt on top of it,
using error correcting codes. The second message hide2.txt can be
retrieved with
outguess -k "secret2" -e -r out.jpg message.txt
SEE ALSO
cjpeg(1), djpeg(1), pnm(5), stirmark(1)
AUTHOR
Niels Provos <provos@citi.umich.edu>
1 May 2000