Type your text and a starting keyword, and the tool encrypts it with the autokey cipher, where your own plaintext extends the key stream after the keyword runs out.
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.