Chaocipher

The Chaocipher is a cipher system invented by John Francis Byrne in 1918. Byrne, an American author and cryptologist, created this cipher with the hope of presenting a complex and supposedly "unbreakable" encryption system that could be used in the military and for diplomatic purposes. However, unlike many other historical ciphers, the Chaocipher remained a mystery for a long time, as Byrne kept the details of its inner workings a secret, even after presenting it to various cryptographic experts of his time. He demonstrated the cipher's security by sending encrypted messages to prominent cryptographers, challenging them to decode it without revealing the method.

For decades, the Chaocipher intrigued cryptologists because Byrne never disclosed the mechanism, and he even included it as an unsolved challenge in his autobiography. The inner mechanics of the cipher were finally revealed only in 2010, almost a century later, when Byrne's family donated his notes to the National Cryptologic Museum.

The Chaocipher works using two rotating alphabets, which change positions after encrypting each character, making it a polyalphabetic substitution cipher. The two alphabets interact in a way that makes each letter dependent on all previous letters in the message, resulting in a complex, interwoven encryption. Unlike other polyalphabetic ciphers where a keyword guides encryption, the Chaocipher relies on systematic rotations of the alphabets, which vary with every letter in the plaintext, creating a different substitution for each character.

Here’s a simplified example to illustrate the general process of the Chaocipher's mechanism. Note that a complete example is difficult without knowing the exact rotation rules and positions, as they are unique to the Chaocipher’s design.

Suppose we want to encrypt the word HELLO with the Chaocipher. We start with two wheels of alphabets:

1. Left Wheel (Plaintext Alphabet)
2. Right Wheel (Ciphertext Alphabet)

Each letter is encrypted by matching it from the Left Wheel to the corresponding letter in the Right Wheel. After each letter, both wheels are rotated in a specific way that changes the subsequent mappings, making each letter encrypted differently based on the evolving positions of the alphabets.

For example:

Each step would result in new configurations for the wheels, which makes reconstructing the encryption or decryption without knowing the rotations virtually impossible. This complexity was the essence of the Chaocipher’s mystery and its reputed security.