SIGINT (signals intelligence) is the monitoring, collection, and analysis of communications intelligence and covers the monitoring of radio and telephone message traffic and the decryption and analysis of coded messages. The related but separate activity known as electronic intelligence (ELINT) mainly covers the detection and analysis of radar, telemetry, and other non-communication signals to discover performance parameters and develop countermeasures.
During WW I, listening to and jamming enemy signals became routine. SIGINT enabled the German army to crush the Russians at Tannenberg and the Masurian Lakes, and also enabled the British Admiralty to intercept the German High Seas fleet at the Dogger Bank and Jutland. The opposing armies were so close to each other on the western front that with primitive crystal-set receivers they could pick up the traffic passing over telegraph and telephone wires through electrical induction. Personnel on both sides were responsible for locating enemy transmitters, monitoring transmissions, and intercepting and deciphering enemy traffic.
During WW II, ULTRA gave the Allies an inestimable advantage over the Axis, although the Abwehr also broke the Royal Navy codes. The US Army Signal Corps developed an automatic machine for enciphering and deciphering Morse messages called SIGABA which, unlike Enigma and the Japanese system, was never broken. Another US development was the radio telephone system called SIGSALY. A third security system developed during the war was the SIGTOT and the British TYPEX which permitted two-way teletype conferences—both were reserved for senior commanders.
The Achilles' heel of SIGINT arises both from the perishability of the product and the fact that if it is acted upon too openly, the source will likely dry up as the enemy finds less vulnerable means of communication. This was a major difficulty in the use of the information provided by ULTRA (see Crete, battle of).
During the latter part of the Cold War, thanks to the treachery of the Walker family and friends, the Soviets could read all US navy traffic, which might have been an advantage akin to ULTRA if it had ever become hot. All other nations must work on the assumption that major SIGINT and ELINT establishments such as those maintained by the USA, Russia, and the UK can read all their communications if they wish to. In general any machine encipherment system can be broken if sufficient computer time is devoted to it, hence modern military communications are protected only for the notional length of time (against computer attack) necessary to preserve operational secrecy. This can be quite brief.
Bibliography
- Bauer, F. L., Decrypted Secrets: Methods and Maxims of Cryptology (New York, 1997).
- Kahn, David, The Codebreakers: The Story of Secret Writing (rev. edn., New York, 1996)
— Danny M. Johnson/Hugh Bicheno
