Poly-Alphabetic Substitution Cipher Tools

The Beaufort system is closely related to the Viginere system. To crack them you must first carry out frequency analysis on a repeating pattern across the cipher. If the keyword has 3 letters then you need to do frequency analysis on letters 1,4,7 then 2,5,8 and 3,6,9 etc. Use a product moment correlation coefficient (PMCC)to check the findings of your frequency analysis.
If you don't know the length of the keyword then use PMCC blind on gradually increasing keyword lengths or look for the periodicity of repeating groups within the message.
For a viginere the PMCC coefficient needs to be checked down the alphabet whilst for a Beaufort you need to check in the opposite direction.

The Beaufort system is a symmetrical cipher so the same program can be used for encryption and decryption. The table below gives the details operating modulo 26.

Viginere cipher = plain + key plain = cipher - key

Beaufort cipher = key - plain plain = key- cipher

alphabet = "ABCDEFGHIJKLMNOPQRSTUVWXYZ"

Plaintext

Beaufort Cipher System. Codeword