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.

To extract the message, the command would be

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.

AUTHOR¶

This application was written by Matthew Kwan, who can be reached at mkwan@darkside.com.au