TEMPEST

TEMPEST

[tem-pist]
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 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 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 60's and early 70's as a codename for the NSA operation to secure electronic communications equipment from potential eavesdroppers 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, laconically, including "Transmitted Electro-Magnetic Pulse / Energy Standards & Testing" "Telecommunications ElectroMagnetic Protection, Equipments, Standards & Techniques", "Transient ElectroMagnetic Pulse Emanation STandard and "Telecommunications Electronics Material Protected from Emanating Spurious Transmissions or, jokingly, Tiny ElectroMagnetic Particles Emitting Secret Things.

TEMPEST measurement standards

The US and NATO TEMPEST standards define three levels of protection requirements:

  • NATO SDIP-27 Level A (formerly AMSG 720B) and USA NSTISSAM Level I

"Compromising Emanations Laboratory Test Standard"
This is the strictest standard for devices that will be operated in NATO Zone 0 environments, where it is assumed that an attacker has almost immediate access (e.g. neighbour room, 1 m distance).

  • NATO SDIP-27 Level B (formerly AMSG 788A) and USA NSTISSAM Level II

"Laboratory Test Standard for Protected Facility Equipment"
This is a slightly relaxed standard for devices that are operated in NATO Zone 1 environments, where it is assumed that an attacker cannot get closer than about 20 m (or where building materials ensure an attenuation equivalent to the free-space attenuation of this distance).

  • NATO SDIP-27 Level C (formerly AMSG 784) and USA NSTISSAM Level III

"Laboratory Test Standard for Tactical Mobile Equipment/Systems"
An even more relaxed standard for devices operated in NATO Zone 2 environments, where attackers have to deal with about 100 m worth of free-space attenuation (or equivalent attenuation through building materials).

Additional standards include:

  • NATO SDIP-29 (formerly AMSG 719G)

"Installation of Electrical Equipment for the Processing of Classified Information"
This standard defines installation requirements, for example in respect to grounding and cable distances.

  • AMSG 799B

"NATO Zoning Procedures"
Defines an attenuation measurement procedure, according to which individual rooms within a security perimeter can be classified into Zone 0, Zone 1, Zone 2, or Zone 3, which then determines what shielding test standard is required for equipment that processes secret data in these rooms.

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.

TEMPEST 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 easily make it radiate dramatically more RF signal. This means that users who must specify TEMPEST certification could pay much higher prices, for obsolete hardware, and be severely limited in the flexibility of configuration choices available to them. A less-costly approach is to place the equipment in a fully shielded room.

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 (RED) and normal unsecured 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 emissions (e.g. FCC Part 15) is a requirement of absolute minimal correlation between radiated energy or detectable emissions and any plain text data that are being processed. It would stand to reason that this requirement holds in some form for other types of data as well.

Public research

In 1985, Wim van Eck published the first unclassified technical analysis of the security risks of emanations from computer monitors. 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 it is believed that government researchers were already aware of the danger, as the NSA published Tempest Fundamentals, NSA-82-89, NACSIM 5000, National Security Agency (Classified) on February 1, 1982. This technique is used as a plot point in Neal Stephenson's novel Cryptonomicon and in the Numb3rs "Sacrifice." In addition, the van Eck technique was successfully demonstrated to non-TEMPEST personnel in Korea during the Korean police action in the early 1950s.

Markus Kuhn discovered several low-cost software 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 displays, 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 he detects the emanations. 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. Otherwise, the covering noise must be on the order of 200 decibels higher than the RED signal.

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