How does variant Beaufort differ from standard Beaufort?

Standard Beaufort computes ciphertext as key minus plaintext, modulo 26. The variant Beaufort reverses the operands and computes plaintext minus key, modulo 26. This makes the variant equivalent to a Vigenère decryption step, so encryption and decryption are not the same operation as they are in standard Beaufort.

How do I decrypt a variant Beaufort message?

Add the key back. Encryption is C = (P minus K) mod 26, so decryption is P = (C plus K) mod 26. Use the same keyword you used to encrypt and select the decrypt mode.

Does it preserve case and punctuation?

Yes. Uppercase and lowercase letters keep their case after enciphering, and any non-letter character such as a space, digit, or punctuation mark passes through unchanged without consuming a key character.

What if my key contains non-letters?

Non-letter characters in the key are ignored. Only the alphabetic characters of the keyword form the repeating key stream, so spaces or symbols inside the key are skipped.

Is this cipher secure?

No. Like all classical polyalphabetic ciphers it is vulnerable to Kasiski examination and frequency analysis once the key length is found. Treat it as a puzzle or teaching tool, not real-world encryption.

Variant Beaufort Cipher

The variant Beaufort cipher is a polyalphabetic substitution cipher closely related to both Beaufort and Vigenère. Where standard Beaufort enciphers a letter as key minus plaintext, the variant flips the subtraction to plaintext minus key, which is exactly the operation a Vigenère cipher uses when decrypting.

How it works

Letters map to 0–25 (A=0 … Z=25). The keyword repeats over the message, advancing only on alphabetic characters:

P = letter index of plaintext (0..25)
K = letter index of key letter (0..25)
encrypt: C = (P - K + 26) mod 26
decrypt: P = (C + K) mod 26

Because encryption subtracts and decryption adds, the two directions are genuinely different — a point that trips people up because the closely named standard Beaufort is its own inverse.

Example and notes

With the key LEMON, the plaintext ATTACKATDAWN enciphers (plain minus key) to PPHOPQHPRPHA-style ciphertext, and decrypting that same string with LEMON recovers ATTACKATDAWN. Keep the keyword as long and unpredictable as possible: short or repetitive keys leak the period to a Kasiski attack and make the message easy to break.