This is an implementation of the paper “Efficient Cryptanalysis of Homophonic Substitution Ciphers” by Amrapali Dhavare, Richard M. Low and Mark Stamp. The code was originally written by Amrapali Dhavare in 2011 and later improved by Markus Amalthea Magnuson in 2016.

Included in the file texts/zodiac_340_ciphertext.txt is the ciphertext of the world’s probably most infamous homophonic substitution cipher, the unsolved 340-character cipher by the Zodiac killer, which is discussed in the latter part of the original paper. Zodiac sent many letters, some of which have been confirmed authentic and others not. The file texts/known_zodiac_letters_combined.txt contains a concatenation of all confirmed letters and can be used as digram frequency comparison instead of the standard English.

Run make to build or make clean to remove compiled files.

./HomophonicCipher <ciphertext_file> [<digram_statistics_file>] [<final_key_output_file>]

• The first argument is required and should be a text file containing the ciphertext.
• The second argument is optional and is a text file from which to calculate the digram frequencies that should be compared against when scoring potential solutions. If no such file is passed, the comparison will be against English digram frequencies.
• The third argument is optional and is the name of a text file to which the final resulting key should be written when done. The result will always be printed to screen as well.

Example usage:

# Compare against English digram frequencies:
./HomophonicCipher texts/zodiac_340_ciphertext.txt

# Compare against confirmed Zodiac letter frequencies:
./HomophonicCipher texts/zodiac_340_ciphertext.txt texts/known_zodiac_letters_combined.txt