What is a Lorenz SZ42?

During World War II, the German High Command required a fast and secure way of transmitting messages between themselves and the army commanders out on the battle fronts. C. Lorenz AG in Berlin created a set of cipher attachment machines, the Lorenz SZ40, SZ42A and SZ42B, which could encode messages directly from a teleprinter and transmit them via radio to a duplcate machine at the receiving end where the original message would be decoded automatically.

This website has an online simulation of the Lorenz machines so you can actually encipher and decipher secret messages, exactly as they did during the war.