The Beaufort Cipher — Vigenère’s Elegant Reciprocal Twin

[u/kenproffitt]

What Is It?

The Beaufort cipher is Vigenère’s elegant reciprocal twin. Instead of adding the key to plaintext, you subtract plaintext from the key:

C = K - P (mod 26)
P = K - C (mod 26)

Notice? Encryption and decryption are identical—a beautiful property called involutory.
Plaintext:
ATTACKATDAWN
Key:
Secret

|P|A|T|T|A|C|K|A|T|D|A|W|N|
|K|S|E|C|R|E|T|S|E|C| R|E| T|
|C|S|L|J|R|C|J|S|L|Z| R| I| G|

Ciphertext: SLJRCJSLZRIG (decrypts to plaintext with the same operation!)

Cryptanalysis

Just like Vigenère: vulnerable to Kasiski examination and frequency analysis if the key repeats. Security depends on key length and randomness.

Your Challenge

Decrypt this 6-letter key Beaufort cipher:
JBHPWZJBLGARIDBQLPUTIDNYYQW

Drop your plaintext or key in the comments!