gifshuffle(1)
NAME
gifshuffle - GIF colourmap steganography program
SYNOPSIS
gifshuffle [ -CQS1 ] [ -p passwd ] [ -f file | -m message ] [ infile.gif [ outfile.gif ]]
DESCRIPTION
gifshuffle is a program for concealing messages in GIF images by
shuffling the colourmap. A shuffled image is visibly indistinguishable
from the original. gifshuffle works with all GIF images, including
those with transparency and animation.
Consider a pack of 52 cards. There are 52 factorial ways to sort the
pack, which means that any particular ordering of the cards can
represent a number in the range [0, 52!-1]. In other words, given n
cards, you can store approximately log2(n!) bits of information based
on their ordering.
GIF images contain a colourmap with up to 256 entries, resulting in a
maximum storage capacity of 1675 bits. The image itself consists of a
compressed array of indices into this colourmap. To conceal a message
within a GIF image the following steps take place.
1. Start with the message you want to conceal, specified on the
command line or in a file. Optionally compress and/or encrypt
this message. You are then left with a sequence of 1's and 0's.
2. Prepend a 1 to this sequence, giving you a binary number m
(probably quite large).
3. Next take a look at the GIF image that you want to conceal the
message in. Count the number of unique colours in the image,
and call the value n. If m > n!-1 then the message is too
large, and the procedure will be aborted.
4. The colours in the colourmap are first sorted into their
"natural" order (except when encryption is being used - see
details below). Each RGB colour is assigned the value (red *
65536 + green * 256 + blue), and the colours are sorted
according to these values. Any duplicate colours are removed.
5. Iterate i through the values 1 .. n. Each colour i-1 is
allocated a target position (m mod i), then m is divided by i.
6. Each colour is then in turn inserted into a new colourmap at its
target position. Colours previously occupying the target
position and above are moved up one place.
7. If the size of the colourmap is greater than the number of
unique colours, then the colourmap will be padded with the last
colour from the original colourmap.
8. The image component of the GIF is then uncompressed, the colour
indices are re-mapped to the new colourmap, and the image is re-
compressed. For animated GIFs this is repeated for each image.
Extracting a hidden message follows a similar procedure, but in
reverse. The ordering of the colourmap is used to construct a binary
number, which is then optionally decrypted and uncompressed before
being output.
gifshuffle provides rudimentary compression, using Huffman tables
optimised for English text. However, if the data is not text, or if
there is a lot of data, the use of the built-in compression is not
recommended, since an external compression program such as compress or
gzip will do a much better job.
Encryption is also provided, using the ICE encryption algorithm in
1-bit cipher-feedback (CFB) mode to encrypt the data (after
compression). Because of ICE's arbitrary key size, passwords of any
length up to 1170 characters are supported (since only 7 bits of each
character are used, this means keys up to 1024-bytes are supported).
As of gifshuffle 2.0 encryption is also applied to the ordering of the
colours in the colourmap. Instead of using their "natural" ordering,
the ordering of their encrypted hash is used. This has the benefit of
making colourmaps look random even when there is only a small message
concealed. To disable this feature (which is incompatible with version
1 of gifshuffle) use the -1 option.
If a message string or message file are specified on the command-line,
gifshuffle will attempt to conceal the message in the file infile.gif
if specified, or standard input otherwise. The resulting file will be
written to outfile.gif if specified, or standard output if not.
If no message string is provided, gifshuffle attempts to extract a
message from the input file. The result is written to the output file
or standard output.
OPTIONS
-C Compress the data if concealing, or uncompress it if extracting.
-Q Quiet mode. If not set, the program reports statistics such as
compression percentages and amount of available storage space
used.
-S Report on the amount of space available for hidden message in
the GIF colourmap. This is calculated from the number of unique
colours in the image.
-1 Retain compatibility with version 1 of gifshuffle by ordering
colours using their "natural" ordering, rather than their
encrypted ordering. This is only relevant if a password is
specified.
-p password
If this is set, the data will be encrypted with this password
during concealment, or decrypted during extraction.
-f message-file
The contents of this file will be concealed in the input GIF
image.
-m message-string
The contents of this string will be concealed in the input GIF
image. Note that, unless a newline is somehow included in the
string, a newline will not be printed when the message is
extracted.
EXAMPLES
The following command will conceal the message "Meet me at 6" in the
file infile.gif, with compression, and encrypted with the password
"hello world". The resulting text will be stored in outfile.gif.
gifshuffle -C -m "Meet me at 6" -p "hello world" infile.gif
outfile.gif
To extract the message, the command would be
gifshuffle -C -p "hello world" outfile.gif
Note that the resulting message will not be terminated by a newline.
The storage capacity of a file can be determined with the -S option.
gifshuffle -S infile.gif
AUTHOR
This application was written by Matthew Kwan, who can be reached at mkwan@darkside.com.au
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