Chaocipher

What is Chaocipher?

The Chaocipher is a cipher system invented by John F. Byrne in 1918. It uses two dynamically permuting alphabets, creating a cipher that remained unbroken for nearly 100 years until it was declassified by the NSA in 2010.

How It Works

The Basic Mechanism

1. Two circular disks each contain a scrambled alphabet
2. To encrypt: find plaintext letter in right disk, output corresponding left disk letter
3. After each letter, both disks are permuted in a specific way
4. This creates a polyalphabetic cipher with continuously changing substitution

The Permutation Rules

After encrypting each letter:

1. Left disk: Rotate until ciphertext letter is at top, extract position 1, insert at position 13
2. Right disk: Rotate until plaintext letter is at top +1, extract position 2, insert at position 13

Historical Significance

John F. Byrne spent over 40 years trying to get official recognition for his cipher. He corresponded with government agencies and cryptographers, but the cipher's true strength wasn't publicly recognized until after his death.

Chaocipher in Puzzles

This cipher appears in sophisticated puzzles because:

• High complexity: Very difficult to crack without the key
• Historical interest: The mystery of its unbroken status
• Mechanical appeal: Can be visualized as rotating disks
• Full key needed: Requires both initial alphabets

Breaking Chaocipher

Without the key, Chaocipher is very difficult to break:

• No repeating patterns like Vigenère
• Each letter position has different substitution
• Requires known-plaintext attacks or the initial alphabets