Enter the keyword to convert, or choose “Find the key” if you don't have it.
Like Vigenère, Beaufort writes a repeating keyword under the message. Instead of adding each key letter to the message letter, it subtracts the message letter from the key letter: C = K − P, counting A as 0. With key letter G (6) over message letter T (19), the cipher letter is 6 − 19 = −13, which wraps to 13: N.
That subtraction makes Beaufort its own inverse. Apply it again with the same key and you're back where you started. There's no separate decode step, which is why the tool has one convert mode rather than two.
It's named after Sir Francis Beaufort, the hydrographer behind the Beaufort wind scale. It was a favourite of 19th-century code cards because one table does both jobs. It's no stronger than Vigenère and falls to the same attack.
A Beaufort ciphertext looks just like a Vigenère one: flat letter frequencies and a low index of coincidence. “Find the key” uses the same method. It works out the key length from the statistics, solves each column against English letter frequencies, and ranks the candidate keys. If the Vigenère tool can't crack a message that looks polyalphabetic, try this one. The cipher identifier tries both automatically.
Vigenère adds the key letter to the message letter. Beaufort subtracts the message letter from the key letter: C = K − P. That one change makes it reciprocal, so encoding twice gets you back to the start.
Because Beaufort is its own inverse. Put ciphertext in with the key and you get plaintext; put plaintext in and you get ciphertext.
Yes, the same way as Vigenère: find the key length from the index of coincidence, then solve each column against English letter frequencies. Choose “Find the key”.