Chaocipher Encryption and Decryption

What is Chaocipher?

Chaocipher is a substitution cipher invented by American cryptographer John F. Byrne in the 1930s. Unlike traditional substitution ciphers that use fixed replacement rules, Chaocipher employs a dynamic, changing alphabet to encrypt messages, making it much harder to crack.

The core of the algorithm involves two disks (a left disk and a right disk) that are used for encryption and decryption. The letters on both disks change according to a specific rule during the encryption process. Each letter's encryption is not fixed; instead, it changes as the encryption progresses, resulting in a highly complex encryption structure.

How Does Chaocipher Encrypt?

The encryption process for Chaocipher is as follows:

1. Prepare the Plaintext: First, arrange the plaintext in alphabetical order, removing any spaces and punctuation, ready for encryption.
2. Initialize the Letter Disks: Chaocipher uses two disks:
   • Left Disk: Used to generate the ciphertext. Initially, it contains all 26 letters of the alphabet.
   • Right Disk: Used to generate the plaintext. It also initially contains all 26 letters of the alphabet.
3. Encryption Process: Take each letter from the plaintext and find its position on the right disk. The letter on the right disk is then replaced by the letter in the same position on the left disk. After each replacement, both the left and right disks "rotate" according to a specific rule.
   • Each time a letter is "read" from the right disk, it moves to a new position on the left disk, changing the replacement rule for the following letters.
   • This rotating mechanism alters the arrangement of the letters on both disks, causing each letter to depend on the previous encryption result.
4. Repeat the Process: Continue the process until all letters are encrypted, forming the final ciphertext.

How Does Chaocipher Decrypt?

The decryption process is the reverse of the encryption process. Since Chaocipher is a symmetric encryption algorithm, decryption requires using the same two disks and rotating and replacing the letters in the same way. The decryption process is as follows:

1. Extract Each Letter from the Ciphertext: For each letter in the ciphertext, find its position on the left disk.
2. Restore the Letter: The letter on the left disk is restored based on its position on the right disk.
3. Rotate the Disks: After each decryption, both the left and right disks rotate according to the same rule.
4. Repeat the Process: Continue this process until all letters are decrypted, ultimately restoring the original plaintext.