Variant Beaufort Cipher

The Variant Beaufort Cipher is a classical polyalphabetic substitution cipher closely related to the Vigenère Cipher and the original Beaufort Cipher. It uses a repeating keyword to transform plaintext letters, but differs in the way the substitution is calculated. Instead of adding or subtracting shifts directly, the Variant Beaufort Cipher uses a reversed tabula recta relationship, producing a distinct encryption pattern.

Like other polyalphabetic ciphers, the Variant Beaufort Cipher reduces the effectiveness of simple frequency analysis by changing the substitution alphabet for each letter based on the keyword. The same keyword is repeated to match the length of the plaintext, and each letter is encrypted using a position-dependent transformation derived from the key. This creates a more complex mapping than monoalphabetic systems while remaining simple enough for manual use.

Variant Beaufort Cipher: Encoding

To encrypt using the Variant Beaufort Cipher, the keyword is repeated to align with the plaintext. Each plaintext letter is then transformed using the Beaufort-style rule, where the key letter determines the substitution. For example:

Plaintext: HELLOBEAUFORT
Keyword:   KEYWORDKEYWORD

Ciphertext:
DANLAQZKKTIXY

The transformation is typically defined as: C = (K − P) mod 26, where P is the plaintext letter and K is the key letter. This differs from the Vigenère method (C = P + K), giving the Variant Beaufort Cipher its unique behavior.

Variant Beaufort Cipher: Decoding

Decoding uses the same process as encoding, making the Variant Beaufort Cipher reciprocal. Applying the same keyword and transformation restores the original plaintext:

Ciphertext: DANLAQZKKTIXY
Keyword:    KEYWORDKEYWORD

Plaintext:
HELLOBEAUFORT

This symmetry is a defining feature of Beaufort-based systems—encryption and decryption are identical operations when the same key is used.

Variant Beaufort Cipher: Notes

The Variant Beaufort Cipher offers a slightly different approach to polyalphabetic substitution compared to its relatives. Key characteristics include:

• Type: Polyalphabetic substitution cipher
• Key: repeating keyword aligned with plaintext
• Operation: C = (K − P) mod 26
• Alphabet: standard A–Z
• Symmetry: same process for encoding and decoding
• Strengths: resists simple frequency analysis better than monoalphabetic ciphers
• Weaknesses: vulnerable to Kasiski examination and modern cryptanalysis if key is short

The Variant Beaufort Cipher is a useful study in how small mathematical changes can significantly alter cipher behavior. It serves as a conceptual bridge between the Vigenère Cipher and the original Beaufort system, highlighting different approaches to key-based substitution.