Autokey cipher decode
Autokey cipher decode, classical cipher, a puzzle not a protection, computed on your device.
Autokey cipher decode rebuilds the same growing keystream one letter at a time, priming key first, then each recovered plaintext letter, subtracted from the matching ciphertext letter. The priming key, default key, has to match the one used to encode. Non-letters pass through untouched, and a wrong priming key produces fluent-looking nonsense rather than an error.
Questions
How does decoding rebuild the keystream?
One letter at a time: the priming key first, then each plaintext letter as it is recovered, since the keystream depends on output the decoder has produced.
What if my priming key does not match?
The output is fluent-looking nonsense rather than an error, since a wrong key still produces valid letters at every step.
What happens to non-letters?
They pass through unchanged and are skipped when advancing the keystream.
What if the key is empty?
The tool refuses with "key must contain at least one letter".
Can I use this from the command line?
Yes, it runs the same on the page, the CLI and MCP.