Use the Secret Decoder to encrypt all your secret messages. Passing notes in class? The teacher won't be able to tell what you wrote. Your secret note will look something like this: G41 x851af xe1 zb95a3. Secret Decoders come in sets of two, so you can give one to your best friend. The secret decoder disk uses a Caesar Cipher to encrypt and decrypt your secret messages. This is the same technique Julias Caesar used to send important messages to his armies.