Autokey cipher encode
Autokey cipher encode, classical cipher, a puzzle not a protection, computed on your device.
Autokey cipher encode shifts each letter with a keystream starting as key, default key, then extended by the message's own letters once that key runs out. Since the keystream never repeats before the message ends, it resists attacks that break a repeating Vigenere key. Non-letters pass through untouched without advancing the keystream, and an empty key is refused.
Questions
What makes autokey different from Vigenere?
The keystream is the priming key extended by the plaintext's own letters once the key runs out, so it never repeats short of the whole message, unlike a cycled Vigenere key.
What is the default key?
The word key. Set your own key to change where the keystream starts; the message itself extends it after that.
What happens to non-letters?
They pass through untouched and do not advance the keystream.
What if my key is empty?
The tool refuses with "key must contain at least one letter".
How do I decode it?
Use Autokey cipher decode with the same priming key.
Can I use this from the command line?
Yes, the same code runs on the page, the CLI and MCP.