What is the Beaufort cipher?

The Beaufort cipher uses a keyword like Vigenère, but instead of adding the key letter to the message letter, it subtracts the message letter from the key letter. A neat side effect is that encoding and decoding are the exact same step, so there is no mode to choose.

It was used in the British Royal Navy and on the Hagelin cipher machines of World War II. If a Vigenère decode with the right key gives nonsense, try Beaufort with the same key.

Example

CKMPVCPVWPIWUJOGIUAPVWRIWUUK with the key FORTIFICATION decodes to DEFENDTHEEASTWALLOFTHECASTLE.