Beaufort Cipher

Use a repeating keyword with the reciprocal Beaufort rule C = K - P mod 26.

Key and method

Keyword

Only letters in the key are used. Spaces, numbers, and punctuation are ignored.

Beaufort uses a repeating key with the reciprocal rule C = K - P mod 26. Encrypt and decrypt use the same keyed transformation.

Type: Polyalphabetic substitution Direction: Reciprocal Formula: K - text Normalised key: FORTIFICATION Key length: 13

Keystream preview: -

First mapping: A with F becomes F

Options

Preserve case

Keep punctuation & numbers

Remove whitespace

Group letters

Beaufort is reciprocal: use the same key and operation again to recover the original text.

Output stats

Mode: Encrypt Key letters: 0 Input letters: 0 Input chars: 0 Output chars: 0 Blocks: 0

Example: with key FORTIFICATION, ATTACK AT DAWN becomes FVYTGV IJ XTMB.

Input

Output

Using This Tool: Guide & Notes Show guide

The Beaufort cipher is a reciprocal keyword cipher. It uses the keyword letter minus the plaintext letter, modulo 26.

How to use it

Enter a keyword, then paste the message into the input box.
Use Encrypt or Decrypt for the task you are doing. Beaufort is reciprocal, so the core operation is the same in both directions.
Check the output as you change the keyword or formatting settings.
Use Load example to compare against a known Beaufort result.

Options and settings

Keyword supplies the repeated key stream. A different keyword completely changes the output.
Preserve case keeps the readable case style of the original message.
Keep punctuation and numbers leaves unsupported characters in place.
Group letters creates fixed-size blocks for classical ciphertext formatting.

Notes

Beaufort is not the same formula as Vigenere, even though both use repeating keywords.
Wrong keyword alignment usually makes the whole decrypted text unreadable.

Beaufort Cipher: Vigenère in the Mirror

How a reversed keyword rule made one of classical cryptography's neatest self-undoing ciphers.

Read the article

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A note from the team

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