Ciphers
Beaufort cipher
A reciprocal repeating-Key cipher using Key minus plaintext.
Also found as standard Beaufort · reciprocal Beaufort
How it works
Beaufort uses the same values and repeating keyword as Vigenère. The difference is the subtraction: Key minus plaintext. The operation is reciprocal because K − (K − P) gives P modulo 26. This means Encrypt and Decrypt run the same letter calculation in Decipher.ACA BeaufortMilitary Cryptanalysis, Part II
Cᵢ = (Kᵢ − Pᵢ) mod 26Pᵢ = (Kᵢ − Cᵢ) mod 26Worked example
- Keyword
RECIPROCAL- Plaintext
CEQUALSKMINUSP- Ciphertext
PAMOPGWSODEKKT
What to look for
Look for the period in much the same way as Vigenère, then split the text by Key position. The arithmetic is different: ordinary Vigenère with the same keyword gives the wrong result. Test the formula as well as the period.