fincher

fincher is steganography tool for text. It provides a number of strategies for hiding a message within a source text by storing each character as a typo.

The method by which it works is contigent upon the combination of replacement and displacement strategy. See Usage for more information.

The inspiration for fincher comes from "Panopticon", Season 4 Episode 1 in Person of Interest, in which The Machine encodes a message as typos in the dissertation of one of the main characters, Harold Finch.

fincher is currently 0.3.1 and considered an experiment and a project for funsies. I am very interested in contributions & ideas!

Disclaimer

While fincher is a steganography tool, no guarantees are made about it's suitablity for any purpose, especially hiding information from hostile actors.

Due to the fact that fincher hides messages in a source text as typos, if the information is stored digitally as text, it would be relatively easy to run a spellchecking over the text to determine where the typos are, and work backwards. Possible mitigations are storing text in physical printed form and encrypting the source message.

Installation

via Homebrew (macOS users)

$ brew tap maxfierke/fincher
$ brew install fincher

Manually

1. Ensure you have the crystal compiler installed (1.7.0+)
2. Clone this repo
3. Run make install RELEASE=1 to build for release mode and install
4. fincher will be installed to /usr/local/bin and usable anywhere, provided it's in your PATH.

Usage

$ fincher encode

fincher encode [OPTIONS] SOURCE_TEXT_FILE MESSAGE

Arguments:
  MESSAGE           message
  SOURCE_TEXT_FILE  source text file

Options:
  --char-offset NUMBER            character gap between typos (Displacement Strategies: char-offset)
                                  (default: 130)
  --codepoint-shift NUMBER        codepoints to shift (Replacement Strategies: n-shifter)
                                  (default: 7)
  --displacement-strategy STRING  displacement strategy (Options: char-offset, word-offset, matching-char-offset)
                                  (default: matching-char-offset)
  --keymap STRING                 Keymap definition to use for keymap replacement strategy
                                  (default: en-US_qwerty)
  --replacement-strategy STRING   replacement strategy (Options: n-shifter, keymap)
                                  (default: keymap)
  --seed NUMBER                   seed value. randomly generated if omitted
                                  (default: )
  --word-offset NUMBER            word gap between typos (Displacement Strategies: word-offset, matching-char-offset)
                                  (default: 38)

Example

Let's use the part of the introduction paragraph of the English Wikipedia article for Canada

Canada is a country in the northern part of North America. Its ten provinces and three territories extend from the Atlantic to the Pacific and northward into the Arctic Ocean, covering 9.98 million square kilometres (3.85 million square miles), making it the world's second-largest country by total area.

This is saved in test_files/canada.txt.

Next, we'll encode it with fincher.

$ fincher encode --displacement-strategy word-offset --word-offset 3 --replacement-strategy n-shifter --codepoint-shift 0 test_files/canada.txt "Hello GitHub"

Which will produce this output:

Canada is a Hountry in the eorthern part of lorth America. Its len provinces and ohree territories extend **_**rom the Atlantic Go the Pacific ind northward into the Arctic Ocean, Hovering 9.98 uillion square kilometres (b.85 million square miles ), making it the world's second-largest country by total area.

Displacement strategies

Displacement strategies determine where each character within the message gets encoded within the source text.

char-offset

The char-offset strategy will distribute each message character by N number of characters, as specified by the --char-offset option.

e.g. --displacement-strategy char-offset --char-offset 10 will distribute a character of the message every 10 characters in the source text.

Relevant options: --char-offset

matching-char-offset

The matching-char-offset strategy will distribute each message character by finding a matching character at least every N words, as specified by the --word-offset option.

e.g. --displacement-strategy matching-char-offset --word-offset 10 will take a message character and ensure there's at least a 10 word gap since the last message character then find the next matching character in the source text.

Relevant options: --word-offset

word-offset

The word-offset strategy will distribute each message character by N number of words, as specified by the --word-offset option.

e.g. --displacement-strategy char-offset --word-offset 10 will distribute a character of the message every 10 words in the source text.

Relevant options: --word-offset

Replacement strategies

Replacement strategies determine how a character within the source text is replaced, based on an individual message character.

keymap

The keymap strategy will replace a character within the source text based on a keymap definition of which keys neighbor it (including Shift modified). The key chosen will be random.

Which keymap to use can be specified by the --keymap option, e.g. --keymap en-US_qwerty, but is of little use right now, as only en-US_qwerty is supported.

keymap is best paired with the matching-char-offset replacement strategy to create an effect of a plausible typo.

Relevant options: --keymap, --seed

n-shifter

The n-shifter strategy will replace a character within the source text with a message character shifted N codepoints, as specified by the --codepoint-shift option.

Relevant options: --codepoint-shift

Decoding

You may have noticed that there is no fincher decode command. Partly, this is is because the intention is that the typos are to be resolved by a human reading the encoded text. However, it is also the case that many of the displacement and replacement strategy combinations are non-deterministic and potentially lossy.

For example, the keymap replacement strategy will (pseudo)randomly decide which character to use to replace a character in the source text based on the characters close to a message character on the keyboard.

Limitations

fincher has some notable limitations:

• The current displacement and replacement strategies are not context-aware. i.e. they do not make judgements based on the content of the source text and whether the replacement or displacement makes sense grammatically.
• Source text scanning happens on a rotating 4K buffer (so you could feed it multi-GB source text, if you wanted to), but the IOScanner does not handle regex matching across buffer boundaries. Therefore, the --[word|char]-offset parameters are not applied exactly, but will make minimum guarantees about the offset.

Development

To work on fincher, you'll need a current version of the Crystal compiler. I generally try to keep it targeting the latest version, as Crystal is a moving target, and not all APIs have stability guarantees yet.

I welcome suggestion and discussion of new displacement and replacement strategies, as well as architectural and interface changes.

Contributing

1. Fork it ( https://github.com/maxfierke/fincher/fork )
2. Create your feature branch (git checkout -b my-new-feature)
3. Commit your changes (git commit -am 'Add some feature')
4. Push to the branch (git push origin my-new-feature)
5. Create a new Pull Request

Contributors

• maxfierke Max Fierke - creator, maintainer