What are the left and right wheels?

The Chaocipher uses two alphabets imagined as wheels. The left wheel holds the cipher alphabet and the right wheel holds the plain alphabet. Their starting arrangements together form the secret key.

Why does the same letter encrypt differently each time?

After every letter both wheels are permuted, so the substitution alphabet changes for the next character. This dynamic re-keying is what made the Chaocipher remarkably strong for a hand cipher.

How does one letter get enciphered?

To encrypt, the plaintext letter is located on the right wheel and the letter at that same position on the left wheel becomes the ciphertext. To decrypt, the ciphertext letter is found on the left wheel and read off the right wheel.

What permutation happens after each letter?

Each wheel is rotated so the matched letter sits at a fixed point, then one letter is extracted from a set position and reinserted lower down the wheel. The left and right wheels use slightly different extraction points, scrambling both alphabets.

Does the simulator match the real algorithm?

Yes. It reproduces the steps Byrne's son published in 2010 and produces the canonical ciphertext OAHQHCNYNXTSZJRRHJBYHQKSOUJY for the famous WELLDONEISBETTERTHANWELLSAID example, then decrypts it back exactly.

Chaocipher Simulator

The Chaocipher was invented by John Francis Byrne in 1918. He spent decades trying to interest cryptographers in it, claiming it was unbreakable, but never revealed how it worked. The mechanism stayed secret until 2010, when his estate donated his papers and his son’s exposition to the National Cryptologic Museum. This simulator runs that published algorithm faithfully.

How it works

The cipher keeps two 26-letter alphabets, pictured as wheels. The left wheel is the cipher alphabet and the right wheel is the plain alphabet. To encipher a letter:

Find the plaintext letter on the right wheel. The letter at that same index on the left wheel is the ciphertext (decryption swaps the roles of the two wheels).
Permute both wheels so the substitution changes for the next letter.

The permutation is the clever part. Using the index where the match was found:

Left wheel: rotate it so the matched letter is at the top, then remove the letter now sitting just below the top and reinsert it further down the wheel (at the so-called nadir position).
Right wheel: rotate so the letter after the matched letter leads, then remove a letter from a fixed position and reinsert it further down.

Because both wheels move after every character, the same plaintext letter rarely maps to the same ciphertext letter twice — a property called dynamic substitution.

Example

Starting wheels HXUCZVAMDSLKPEFJRIGTWOBNYQ (left) and PTLNBQDEOYSFAVZKGJRIHWXUMC (right), the famous test message:

WELLDONEISBETTERTHANWELLSAID

encrypts to:

OAHQHCNYNXTSZJRRHJBYHQKSOUJY

Switching this tool to decrypt with the same two starting wheels recovers the original message.

Notes

Only letters A–Z are processed; spaces, digits and punctuation are dropped, so format your message in letter blocks if you need spacing.
The full secret key is the pair of starting wheel arrangements — there are an astronomical number of possible pairings.
Any letters you leave out of a wheel are appended in normal alphabetical order, so a partial entry still produces a valid 26-letter wheel.