Why this tool is useful
Use plaintext Autokey for educational examples where a short primer is extended with the recovered message rather than repeated indefinitely.
Text & List Tools · runs locally
Plaintext Autokey Cipher Tool
Encode or decode the plaintext-autokey Vigenère variant with a validated primer and locally reconstructed key stream.
Ready. Your working data stays in this browser.
How it works
Encoding appends plaintext letters to the primer; decoding extends its stream only with letters already recovered, while nonletters remain unchanged.
Practical example
Primer QUEENLY encodes ATTACKATDAWN as QNXEPVYTWTWP, and local sequential decoding reconstructs ATTACKATDAWN.
Validation
The calculation is deterministic and explains validation errors instead of silently changing invalid input.
Privacy boundary
Input and output remain in this tab. Copy and download happen through browser APIs without a server upload.
Result passport
Every run reports its tool mode, timestamp and input/output size so transformed data can be audited.