One-Time Pad Cipher
Encrypt or decrypt with a one-time pad — provably unbreakable, but only if the key is truly random, used once, and kept secret.
About this tool
Letters are shifted using the matching letter of the key (like Vigenère, but the key must be as long as the message, truly random, and used only once — the classical requirements for a genuinely unbreakable cipher). Use "Generate random key" to get a fresh key exactly as long as your text, using your browser's cryptographic random source. Reusing a one-time pad key breaks its security completely — generate a new one for every message.