How does the Beaufort cipher work?

For each letter it computes C = (K − P) mod 26, where K is the current keyword letter and P is the text letter, both as 0–25. Subtracting the plaintext from the key letter is what makes Beaufort different from Vigenère.

Why can I decrypt with the same key?

The formula is reciprocal. Because subtraction is symmetric here, applying C = (K − P) mod 26 to the ciphertext with the same keyword recovers the plaintext. There is no separate decrypt mode needed.

How is it different from the Vigenère cipher?

Vigenère adds the key letter (C = P + K) and needs the inverse subtraction to decrypt. Beaufort subtracts the plaintext from the key (C = K − P), which is its own inverse, so encryption and decryption are identical operations.

What happens to spaces and punctuation?

Non-letter characters such as spaces, digits and punctuation pass through unchanged and do not advance the keyword. Only letters consume a keyword position, and case is preserved.

Is the Beaufort cipher secure?

No. As a classical polyalphabetic cipher it is vulnerable to Kasiski examination and frequency analysis once the key length is found. Use it for puzzles, learning and historical interest, not for protecting real secrets.

Beaufort Cipher Encoder & Decoder

The Beaufort cipher is a polyalphabetic substitution cipher invented by Sir Francis Beaufort. It is closely related to the Vigenère cipher but is reciprocal: the same keyword and the same operation both encrypt and decrypt. That symmetry made it convenient for mechanical cipher devices, and it remains a neat illustration of self-inverse encryption.

How it works

Each letter is turned into a number from 0 (A) to 25 (Z). The keyword repeats across the message, supplying a key letter K for each text letter P. The output letter is computed as:

C = (K - P) mod 26

Subtracting the plaintext value from the key value is the crucial detail. In the Vigenère cipher the rule is C = (P + K) mod 26, which is not its own inverse, so decryption needs the opposite subtraction. In Beaufort, applying (K - P) mod 26 a second time to the ciphertext returns the original plaintext, because K - (K - P) = P. This is why there is only one operation: encrypting and decrypting are the same.

Only letters are enciphered, and they advance the keyword. Spaces, digits and punctuation are copied straight through and do not consume a keyword position, so the keyword stays aligned with the letters of the message.

Example

Encipher the letter A (P=0) with key letter K (K=10): C = (10 − 0) mod 26 = 10, which is K. Encipher G (P=6) with the same key letter: C = (10 − 6) mod 26 = 4, which is E. Feeding those ciphertext letters back through the same keyword recovers A and G.

Notes

Choose a long, non-obvious keyword: short or dictionary keywords are easy to attack.
Beaufort should not be confused with the “variant Beaufort”, which uses C = (P − K) mod 26 and is not reciprocal.
For a cipher whose key extends itself with the message, try the Autokey cipher.