Beaufort cipher
The Beaufort cipher subtracts each text letter from the corresponding letter of a repeating keyword. It is reciprocal: encrypting twice with the same key returns the original text, so a single operation handles both directions.
Break without the key
Paste Beaufort ciphertext with an unknown key and the solver recovers it by trying every key length and scoring candidate decryptions against English bigram statistics. It needs roughly four times as much text as the key is long.
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About the Beaufort cipher
The cipher is named after Sir Francis Beaufort, the Royal Navy officer better known for the Beaufort wind scale. Where Vigenère adds the key letter to the plaintext letter, Beaufort subtracts the plaintext letter from the key letter. That small change makes it reciprocal: encrypting a ciphertext with the same key gives back the plaintext, so one operation serves both directions.
The reciprocal property made it popular in mechanical crypto equipment, most famously the Hagelin M-209 cipher machine used by the US Army through the Second World War and the Korean War. Cryptographically it is exactly as strong, and as weak, as the Vigenère cipher: the same statistical attacks recover the key once enough ciphertext is available.