Cadenus Cipher

The Cadenus Cipher is a polyalphabetic substitution cipher that uses a repeating keyword to alter the shift of each plaintext character. Named in reference to classical ciphering techniques, it is similar in principle to the Vigenère Cipher, but often employs a direct letter-to-letter mapping that adapts dynamically according to the keyword.

In the Cadenus Cipher, each letter of the plaintext is combined with a corresponding letter of the keyword, repeating the keyword as necessary to match the length of the message. The resulting shift or substitution produces a ciphertext that varies throughout the message, providing resistance to simple frequency analysis compared to monoalphabetic ciphers.

Cadenus Cipher: Encoding

To encrypt using the Cadenus Cipher, align the keyword with the plaintext and shift each plaintext letter according to the alphabetic position of the corresponding keyword letter:

Plaintext: HELLOCADENUS
Keyword:   KEYKEYKEYKEYK

Ciphertext:
RIJVSAKHCXYQ

Each letter of the plaintext is shifted by the alphabetic index of the corresponding keyword letter (A=0, B=1, … Z=25). For example, 'H' shifted by 'K' (10) → 'R', 'E' shifted by 'E' (4) → 'I', and so on.

Cadenus Cipher: Decoding

Decoding reverses the process by subtracting the keyword shifts from each ciphertext letter:

Ciphertext: RIJVSAKHCXYQ
Keyword:    KEYKEYKEYKEYK

Plaintext:
HELLOCADENUS

Correct decryption requires the exact keyword and the repeating alignment used during encryption. Any misalignment produces garbled text.

Cadenus Cipher: Notes

Key characteristics and usage notes for the Cadenus Cipher:

• Type: Polyalphabetic substitution cipher
• Key: alphabetic keyword (e.g., "KEY")
• Operation: shift each letter according to corresponding keyword letter
• Alphabet: standard A–Z
• Strengths: resists simple frequency analysis, simple to implement
• Weaknesses: vulnerable to repeated-key attacks if the keyword is short or known; not secure against modern cryptanalysis
• Practical use: educational purposes, simple text obfuscation, and demonstrations of polyalphabetic ciphers

The Cadenus Cipher illustrates the power of repeating-key polyalphabetic techniques and forms a bridge between classical substitution ciphers and more complex modern encryption methods.