The Enigma cipher machine was slow and cumbersome to use. Sending a message was a complicated procedure requiring the participation of several operators (see photograph 24). The process started with the German plain-language, known as the ‘clear’ or the ‘plaintext’. Encrypting this produced the ‘ciphertext’. Typically, the plaintext or clear consisted of ordinary German words mixed with military abbreviations and jargon (such as WEWA for Wetter Warte, meaning ‘weather station’, and BINE, literally ‘bee’, meaning ‘very very urgent’). A cipher clerk typed the plaintext at the keyboard of an Enigma machine (see the diagram on page 17). Each time the clerk pressed a key, a letter on the lampboard would light. For example, typing HITLER might produce the letters FLKPIM. As the letters of the ciphertext appeared one by one at the lampboard, they were painstakingly noted down by an assistant. Various items of information were then added to the ciphertext, including the intended recipient’s radio call-sign, and a radio operator transmitted the complete message in Morse code. At the receiving end, the process had to be carried out in reverse. The radio operator turned the dit-dit-dahs of the Morse transmission back into letters of ciphertext and handed the result to the cipher clerk. The clerk typed the ciphertext at the keyboard of an Enigma, which had been set up identically to the sender’s machine. The letters of the plaintext lit up at the lampboard one by one and were recorded by the assistant. The Tunny system was much more sophisticated. The process of sending and receiving a message was largely automated. Encryption and decryption were entirely automatic. The transmitted ciphertext was never even seen by the German operators. At the sending end, a single operator typed plaintext at the keyboard of a teleprinter. At the receiving end, the plaintext was printed out automatically by another teleprinter. (A teleprinter is called a teletypewriter in the US.) The sender could switch his teleprinter equipment from ‘hand mode’ to ‘auto mode’. In auto mode, a pre-punched paper tape was fed into the equipment. The plaintext punched on the tape was encrypted and transmitted at high speed.
Darkrooms, paper tape, punch cards and grease pencils: doing crystallography at Caltech in the early 1970s