Chaocipher

Chaocipher is the classical polyalphabetic machine invented by John F. Byrne in 1918 and kept secret for decades until the full algorithm was published in 2010. Unlike a fixed Caesar or Vigenère table, it keeps two circular alphabets—the left disk for ciphertext and the right disk for plaintext—and after every single letter both disks are rearranged by a strict zenith-and-nadir permutation. You supply the starting left and right alphabets as full 26-letter permutations of A–Z, the text to protect or recover, and a mode of encrypt or decrypt. The solver finds each letter on one disk, reads the aligned letter on the other, then rotates and slides both alphabets so the next mapping is completely different.

How the two alphabets and zenith–nadir rules work

Chaocipher encryption treats the right alphabet as the plaintext disk and the left alphabet as the ciphertext disk. For each plaintext letter you locate that letter in the current right alphabet and read the letter at the same index from the left alphabet; that is the ciphertext symbol for this step. Immediately afterward both alphabets are permuted with the stationary-zenith procedure used in Byrne’s model and documented by Moshe Rubin.

On the left disk the just-used ciphertext letter is rotated to the zenith (first position), the letter at zenith plus one is extracted, the block from zenith plus two through the nadir (position fourteen, zero-based index thirteen) shifts one place toward the zenith, and the extracted letter is reinserted at the nadir.

On the right disk the plaintext letter is rotated to the zenith, the whole alphabet is then rotated one more step so the next letter sits at zenith, the letter at zenith plus two is extracted, the block from zenith plus three through the nadir shifts left, and the extracted letter lands at the nadir. Decryption uses the inverse lookup—find the ciphertext letter on the left disk and read the same-index letter on the right—then applies exactly the same two permutations so the machine state stays synchronized.

Because each step depends on the previous letter, a single-letter change early in the message cascades through both alphabets and destroys long runs of monoalphabetic structure, which is why classical frequency analysis fails on long Chaocipher traffic when the starting disks are unknown.

Practical tips, limits, and related searches

Use well-shuffled starting alphabets rather than near-sequential A–Z strings so the early permutations do not leave long alphabetical runs near the zenith. The classic exhibit pair left HXUCZVAMDSLKPEFJRIGTWOBNYQ and right PTLNBQDEOYSFAVZKGJRIHWXUMC with plaintext WELLDONEISBETTERTHANWELLSAID is the standard self-check: encryption must yield OAHQHCNYNXTSZJRRHJBYHQKSOUJY, and decryption of that stream with the same starting disks must recover the original phrase. Round-trip checks are the best sanity test when you invent new disks—encrypt, then decrypt with the original left and right strings, never with the final alphabets from a previous run.

Prefer this capability when you need transparent classical dynamic substitution for education or puzzles; if you need real secrecy, use a modern authenticated cipher instead.