World War 2 Submarines
During World War II, submarines began used radio signals communicate. A variety of means where used to mask or protect communications, such as encrypting the message. The most famous example of this is the German Enigma machine.
"An Enigma machine (above) is any of a family of related electro-mechanical rotor machines used for the encryption and decryption of secret messages. The first Enigma was invented by German engineer Arthur Scherbius at the end of World War I. This model and its variants were used commercially from the early 1920s, and adopted by military and government services of several countries — most notably by Nazi Germany before and during World War II. Several different Enigma models were produced, but the German military models, the Wehrmacht Enigmas, are the ones most commonly discussed(Source)."