Editing Tempest (codename) (section)

==History==
During World War II, The [[Bell System]] supplied the U.S. military with the [[131-B2]] mixer device that encrypted teleprinter signals by [[Exclusive or|XOR]]’ing them with key material from [[one-time tape]]s (the [[SIGTOT]] system) or, earlier, a rotor-based key generator called [[SIGCUM]]. It used electromechanical relays in its operation. Later, Bell informed the Signal Corps that they were able to detect electromagnetic spikes at a distance from the mixer and recover the plain text. Meeting skepticism over whether the phenomenon they discovered in the laboratory could really be dangerous, they demonstrated their ability to recover plain text from a Signal Corps’ crypto center on Varick Street in Lower Manhattan. Now alarmed, the Signal Corps asked Bell to investigate further. Bell identified three problem areas: radiated signals, signals conducted on wires extending from the facility, and magnetic fields. As possible solutions, they suggested shielding, filtering and masking.

[[File:SIGCUM.jpg|thumb|left|[[Rotor machine]]s, like this [[SIGCUM]], were an early source of compromising TEMPEST effects]] 
Bell developed a modified mixer, the 131-A1 with shielding and filtering, but it proved difficult to maintain and too expensive to deploy. Instead, relevant commanders were warned of the problem and advised to control a {{convert|100|ft|m|adj=on}}-diameter zone around their communications center to prevent covert interception, and things were left at that. Then in 1951, the CIA rediscovered the problem with the 131-B2 mixer and found they could recover plain text off the line carrying the encrypted signal from a quarter mile away. Filters for signal and power lines were developed, and the recommended control-perimeter radius was extended to {{convert|200|ft|m}}, based more on what commanders could be expected to accomplish than any technical criteria.

A long process of evaluating systems and developing possible solutions followed. Other compromising effects were discovered, such as fluctuations in the power line as rotors stepped. The question of exploiting the noise of electromechanical encryption systems had been raised in the late 1940s but was re-evaluated now as a possible threat. Acoustical emanations could reveal plain text, but only if the pick-up device was close to the source. Nevertheless, even mediocre microphones would do. Soundproofing the room made the problem worse by removing reflections and providing a cleaner signal to the recorder.

[[File:Friden Flexowriter NIK 3615.jpg|thumb|[[Relay logic]], such as in this [[Friden Flexowriter|Flexowriter]] was another major early source of TEMPEST radiation.]]
In 1956, the [[Naval Research Laboratory]] developed a better mixer that operated at much lower voltages and currents and therefore radiated far less. It was incorporated in newer NSA encryption systems. However, many users needed the higher signal levels to drive teleprinters at greater distances or where multiple teleprinters were connected, so the newer encryption devices included the option to switch the signal back up to the higher strength. The NSA began developing techniques and specifications for isolating sensitive-communications pathways through filtering, shielding, grounding, and physical separation: of those lines that carried sensitive plain text—from those intended to carry only non-sensitive data, the latter often extending outside of the secure environment. This separation effort became known as the [[Red/black concept|Red/Black Concept]]. A 1958 joint policy called NAG-1 set radiation standards for equipment and installations based on a {{convert|50|ft|m|abbr=on}} limit of control. It also specified the classification levels of various aspects of the TEMPEST problem. The policy was adopted by Canada and the UK the next year. Six organizations—the Navy, Army, Air Force, NSA, CIA, and the State Department—were to provide the bulk of the effort for its implementation.

Difficulties quickly emerged. Computerization was becoming important to processing intelligence data, and computers and their peripherals had to be evaluated, wherein many of them evidenced vulnerabilities. The [[Friden Flexowriter]], a popular I/O typewriter at the time, proved to be among the strongest emitters, readable at distances up to {{convert|3,200|ft|km|abbr=on}} in field tests. The U.S. [[Communications Security Board]] (USCSB) produced a Flexowriter Policy that banned its use overseas for classified information and limited its use within the U.S. to the [[Classified information#Confidential|Confidential]] level, and then only within a {{convert|400|ft|m|abbr=on}} security zone, but users found the policy onerous and impractical. Later, the NSA found similar problems with the introduction of cathode-ray-tube displays ([[Cathode-ray tube|CRT]]s), which were also powerful radiators.

There was a multiyear process of moving from policy recommendations to more strictly enforced TEMPEST rules. The resulting Directive 5200.19, coordinated with 22 separate agencies, was signed by Secretary of Defense [[Robert McNamara]] in December 1964, but still took months to fully implement. The NSA's formal implementation took effect in June 1966.

Meanwhile, the problem of acoustic emanations became more critical with the discovery of some 900 microphones in U.S. installations overseas, most behind the [[Iron Curtain]]. The response was to build room-within-a-room enclosures, some transparent, nicknamed "fish bowls". Other units{{clarify|date=January 2020}} were fully shielded{{clarify|date=January 2020}} to contain electronic emanations, but were unpopular with the personnel who were supposed to work inside; they called the enclosures "meat lockers", and sometimes just left their doors open. Nonetheless, they were installed in critical locations, such as the embassy in Moscow, where two were installed: one for State Department use and one for military attachés. A unit installed at the NSA for its key-generation equipment cost $134,000.

TEMPEST standards continued to evolve in the 1970s and later, with newer testing methods and more nuanced guidelines that took account of the risks in specific locations and situations.<ref name=boaklectures>[http://www.governmentattic.org/18docs/Hist_US_COMSEC_Boak_NSA_1973u.pdf A History of U.S. Communications Security; the David G. Boak Lectures], National Security Agency (NSA), Volumes I, 1973, Volumes II 1981, partially released 2008, additional portions declassified October 14, 2015</ref>{{rp|Vol I, Ch. 10}} During the 80s, security needs were often met with resistance. According to NSA's David G. Boak, "Some of what we still hear today in our own circles, when rigorous technical standards are whittled down in the interest of money and time, are frighteningly reminiscent of the arrogant Third Reich with their Enigma cryptomachine." {{rp|''ibid'' p. 19}}