Autokey Cipher

How Autokey Cipher works

Autokey Cipher handles letters with a Vigenere-like stream that starts from a primer keyword and then continues with the plaintext itself instead of repeating the short key forever. Encoding adds primer and message letters modulo 26. Decoding subtracts each available key letter and appends every recovered plaintext letter to the future keystream. The Autokey Cipher exposes its message, primer keyword, operation controls so this particular convention stays inspectable instead of hiding behind an unexplained preset.

ATTACKATDAWN with primer QUEENLY produces QNXEPVYTWTWP, and decoding that ciphertext with the same primer reconstructs ATTACKATDAWN. Cipher students can observe how incorporating recovered message text changes the repeating-pattern weakness of ordinary Vigenere while retaining a hand-calculable method. The Autokey Cipher presents a deterministic plain-text result that can be copied and compared with the worked value before it enters another workflow.

Limits and private processing

Spaces and punctuation pass through without advancing the stream. This historical cipher offers no modern confidentiality and should be used only for learning or puzzles. The Autokey Cipher performs its complete operation locally in your browser; neither the source text nor its settings are uploaded, stored remotely, or sent through an external request.