The "cipher disc" (in Italian "disco cifrante") was a cryptographic device invented by the Italian engineer Leon Battista Alberti in XVI Century.

He realized this device for the Pope, in order to deliver secret messages along the Chuch State.

The most impressive feature of this solution is that only one letter (today's byte) is needed as decryption key, so it's very easy to send that key out-of-band.
Moreover the lenght of encrypted text may randomly differ from the length of plain text, and the only way to decrypt the message is having an exact copy of the encryption device.

CipherDisc is a small Java application which simulates the Leon Battista Alberti's device.
You can download it here.

You can read how it works in the instructions page.