Tempest ( codename ) Tempest ( often spelled TEMPEST ) is a codename referring to investigations and studies of compromising emanations ( CE ) . Compromising emanations are defined as unintentional intelligence - bearing signals which , if intercepted and analyzed , may disclose the information transmitted , received , handled , or otherwise processed by any information - processing equipment . Compromising emanations consist of electrical , mechanical , or acoustical energy intentionally or by mishap unintentionally emitted by any number of sources within equipment / systems which process national security information . This energy may relate to the original pre - or non - encrypted message , or information being processed , in such a way that it can lead to recovery of the plaintext . Laboratory and field tests have established that such CE can be propagated through space and along nearby conductors . The interception / propagation ranges and analysis of such emanations are affected by a variety of factors , e . g . , the functional design of the information processing equipment ; system / equipment installation ; and , environmental conditions related to physical security and ambient noise . The term " compromising emanations " rather than " radiation " is used because the compromising signals can , and do , exist in several forms such as magnetic - and / or electric field radiation , line conduction , or acoustic emissions . The term " Tempest " is often used broadly for the entire field of emission security or emanations security ( EMSEC ) . The term " Tempest " was coined in the late 1960s and early 1970s as a codename for the National Security Agency operation to secure electronic communications equipment from potential eavesdropper s and vice versa the ability to intercept and interpret those signals from other sources . The U . S . government has stated that the term " TEMPEST " is not an acronym and does not have any particular meaning . However , various backronyms have been suggested , including " Transmitted Electro - Magnetic Pulse / Energy Standards & Testing " , " Telecommunications ElectroMagnetic Protection , Equipment , Standards & Techniques " , " Transient ElectroMagnetic Pulse Emanation STandard " , and " Telecommunications Electronics Material Protected from Emanating Spurious Transmissions " . As a joke - but just as factually possible as other attempts - the phrase " Tiny ElectroMagnetic Particles Emitting Secret Things " has been suggested . Measurement standards . The US and NATO Tempest standards define three levels of protection requirements : Additional standards include : All these documents remain classified and no published information is available about the actual emission limits and detailed measurement procedures that they define . However , some very basic Tempest information has not been classified information in the United States since 1995 . Short excerpts from the main U . S . TEMPEST test standard , NSTISSAM TEMPEST / 1 - 92 , are now publicly available , but all the actual emanation limits and test procedures have been redacted from the published version . A redacted version of the introductory Tempest handbook NACSIM 5000 was publicly released in December 2000 . Equally , the NATO standard SDIP - 27 ( before 2006 known as AMSG 720B , AMSG 788A , and AMSG 784 ) is still classified . The United States Air Force has published a few mandatory standards regarding Emission Security : Certification . The information - security agencies of several NATO countries publish lists of accredited testing labs and of equipment that has passed these tests : The United States Army also has a Tempest testing facility , as part of the U . S . Army Information Systems Engineering Command , at Fort Huachuca , Arizona . Similar lists and facilities exist in other NATO countries . Tempest certification must apply to entire systems , not just to individual components , since connecting a single unshielded component ( such as a cable ) to an otherwise secure system could dramatically alter the system RF characteristics . RED / BLACK separation . TEMPEST standards require " RED / BLACK separation " , i . e . maintaining distance or installing shielding between circuits and equipment used to handle plaintext classified or sensitive information that is not encrypted ( RED ) and secured circuits and equipment ( BLACK ) , the latter including those carrying encrypted signals . Manufacture of TEMPEST - approved equipment must be done under careful quality control to ensure that additional units are built exactly the same as the units that were tested . Changing even a single wire can invalidate the tests . Correlated emanations . One aspect of Tempest testing that distinguishes it from limits on spurious emission s ( e . g . FCC Part 15 ) is a requirement of absolute minimal correlation between radiated energy or detectable emissions and any plaintext data that are being processed . Public research . In 1985 , Wim van Eck published the first unclassified technical analysis of the security risks of emanations from computer monitor s . This paper caused some consternation in the security community , which had previously believed that such monitoring was a highly sophisticated attack available only to governments ; van Eck successfully eavesdropped on a real system , at a range of hundreds of metres , using just $ 15 worth of equipment plus a television set . In consequence of this research such emanations are sometimes called " van Eck radiation , " and the eavesdropping technique van Eck phreaking , although government researchers were already aware of the danger , as Bell Labs noted this vulnerability to secure teleprinter communications during World War II and was able to produce 75 % of the plaintext being processed in a secure facility from a distance of 80 feet . Additionally the NSA published " Tempest Fundamentals , NSA - 82 - 89 , NACSIM 5000 , National Security Agency " ( Classified ) on February 1 , 1982 . In addition , the van Eck technique was successfully demonstrated to non - TEMPEST personnel in Korea during the Korean War in the 1950s . Markus Kuhn has discovered several low - cost techniques for reducing the chances that emanations from computer displays can be monitored remotely . With CRT displays and analogue video cables , filtering out high - frequency components from fonts before rendering them on a computer screen will attenuate the energy at which text characters are broadcast . With modern flat panel display s , the high - speed digital serial interface ( DVI ) cables from the graphics controller are a main source of compromising emanations . Adding random noise to the less significant bits of pixel values may render the emanations from flat - panel displays unintelligible to eavesdroppers but is not a secure method . Since DVI uses a certain bit code scheme for trying to transport an evenly balanced signal of 0 and 1 bits there may not be much difference between two pixel colours that differ very much in their colour or intensity . It may also be that the generated emanations may differ totally even if only the last bit of a pixel ' s colour is changed . The signal received by the eavesdropper does also depend on the frequency where the emanations are detected . The signal can be received on many frequencies at once and each frequency ' s signal differs in contrast and brightness related to a certain colour on the screen . Usually , the technique of smothering the RED signal with noise is not effective unless the power of the noise is sufficient to drive the eavesdropper ' s receiver into saturation and thus overwhelming the receiver input . LED indicators on computer equipment can be a source of compromising optical emanations . One such technique involves the monitoring of the lights on a network switch . Almost all network switches flash to show activity , and it is normal for the flashes to be directly taken from the data line . As such , a fast optical system can easily see the changes in the flickers from the data being transmitted down the wire . Further , recent research has shown it is possible to detect the radiation corresponding to a keypress event from not only wireless ( radio ) keyboards , but also from traditional wired keyboards , and even from laptop keyboards .