Encrypt and decrypt with the autokey cipher where the plaintext extends the keyword stream. Autokey Vigenère Cipher is a free online tool from the ToolsRift Text Encoding Tools collection. It runs entirely in your browser — nothing to install, no signup, and your data never leaves your device.
Instead of repeating the keyword, the autokey cipher appends the plaintext to the keyword to build the key stream. This non-repeating key removes the periodicity that makes the standard Vigenère cipher easier to break.
The keyword (primer) is a short starting key. Encryption uses the keyword followed by the plaintext letters as the key stream; decryption recovers the plaintext one letter at a time and feeds it back into the key.
Yes. Encrypting 'ATTACKATDAWN' with keyword 'QUEENLY' gives 'QNXEPVYTWTWP', the classic textbook autokey result, and decoding with the same keyword restores the original.