TEMPEST and Electromagnetic Emanations Guide: Governing EMSEC for Classified Systems

Electronic equipment unintentionally radiates and conducts signals that can carry the information it processes, and for classified government and defence systems that leakage is a recognised compromise path that ordinary cyber controls do not address. Emanations security (EMSEC), long known by the codename TEMPEST, is the discipline of keeping those compromising emanations inside space you control. This guide explains how the risk arises at concept level, how national schemes govern it, which controls matter (zoning, equipment selection, separation, cabling, fibre and inspection), and how a security leader builds EMSEC into the system lifecycle. It is for security leaders, accreditors, facility security officers and engineers in government, defence and the defence industrial base, and for organisations outside that space deciding whether they need it at all. It goes beyond the Physical Security Guide for Security Teams and does not describe interception techniques.

Key takeaways

• EMSEC is a national, classification-driven discipline. NIST SP 800-53 PE-19(1) requires protection "in accordance with national Emissions Security policies and procedures based on the security category or classification", and those national documents are usually not public.
• Start early or pay later. The Australian ISM requires an emanation security risk assessment as early as possible in the system lifecycle because changes after design are far more costly.
• Distance and control come first. The cheapest countermeasure is keeping emanations within inspectable, controlled space; shielded equipment and facilities are for when distance is not enough.
• Fibre and separation are the everyday controls. Fibre-optic cabling does not produce electromagnetic emanations, and classified cables need their own bundles, conduits and inspection regime.
• Most organisations do not need TEMPEST. Outside classified work, standard EMC compliance, physical security and sensible placement cover the realistic risk; decide this deliberately rather than buying "TEMPEST" products by default.

What compromising emanations are

Any equipment that switches current, such as processors, displays, keyboards, printers, cables and crypto devices, produces unintended signals. Most are harmless noise. A compromising emanation is one that is correlated with the information being processed, so that someone who receives and analyses it could recover some of that information. NIST SP 800-53 describes this as information leakage from "electromagnetic signals emanations". Paths include:

• Radiated: signals emitted into free space by equipment and cables acting as unintended antennas.
• Conducted: signals coupled onto power lines, signal lines, earthing or other conductors that leave the controlled area.
• Acoustic and mechanical: sound from keyboards, printers or machines, which NSA's history classes alongside electrical emanations.
• Coupling between systems: classified (RED) signals leaking onto unclassified (BLACK) cables, equipment or networks that run nearby.

The problem is old and well documented. NSA's declassified history "TEMPEST: A Signal Problem" records that in 1943 Bell Labs noticed oscilloscope spikes each time a 131-B2 cipher mixer stepped, and that engineers recording from about 80 feet away recovered about 75% of the plaintext. Bell Labs proposed three suppression approaches (shielding, filtering and masking), and the Signal Corps advised controlling a zone of about 100 feet around communications centres. In 1951 the CIA reported reading plaintext about a quarter of a mile down a signal line, which led to power-line and signal-line filters. Those three ideas, together with controlled distance, still frame EMSEC today.

Who needs EMSEC

The Australian Information Security Manual (ISM, version 2026.09.4 as published in ASD's OSCAL catalogue) illustrates the gradient: ISM-0250 (equipment meets EMI and EMC standards) applies at every classification level including non-classified, ISM-1884 (comply with ASD emanation security doctrine) applies from OFFICIAL: Sensitive upwards, and the risk assessment controls ISM-1137, ISM-0249, ISM-0246 and ISM-1885 apply to SECRET and TOP SECRET.

For commercial organisations that are not bound by a national scheme, the question is whether an adversary capable of close-range technical collection is realistically in scope, for example for a board room during merger negotiations or a facility holding high-value cryptographic keys. If it is, the proportionate answer is usually placement, controlled space, fibre, a clear policy on wireless devices and periodic technical surveillance countermeasures sweeps, not rated equipment. Record the decision either way, so it can be revisited if the threat changes.

How national schemes govern TEMPEST

Each nation runs its own scheme, normally through a technical authority that sets standards, accredits test laboratories and professionals, and advises system owners. The detailed technical standards, including emission limits and test procedures, are generally classified or restricted. What is public is the governance model:

NATO and many member states use equipment levels and facility zoning schemes under NATO documents that are not publicly released; obtain them through your national security authority rather than vendor summaries. In the US, national TEMPEST policy is issued through the Committee on National Security Systems (CNSS) and implemented by departments; the FAA order, for example, implements NSTISSI 7000.

The countermeasure hierarchy

The FAA order states the principle clearly: select the most cost-effective countermeasures that keep compromising emanations within the inspectable space, as determined by the technical authority. Work down this hierarchy and stop when the assessed risk is met:

1. Distance and control: place classified processing far enough inside space you control and can inspect that emanations attenuate before reaching uncontrolled areas. This is often free if considered at site selection.
2. Placement within the building: keep classified equipment away from external walls, windows, shared risers and adjacent tenants.
3. Separation (RED/BLACK): keep classified equipment, cables and power physically separate from unclassified equipment, cables and anything that leaves the controlled space.
4. Fibre-optic cabling: ISM-1111 calls for fibre instead of copper because fibre does not produce electromagnetic emanations and is not influenced by them.
5. Filtering: filter power and signal lines that leave the controlled area so conducted emanations are suppressed.
6. Approved low-emanation equipment: where distance is insufficient, use equipment tested to the required national or alliance level.
7. Shielded enclosures and rooms: shielded racks or rooms where nothing else will meet the requirement; these are the most expensive and need ongoing maintenance and testing.

The relative costs above are indicative, not from a national scheme; the technical authority's assessment decides what is required for a given site.

Decision guidance: a new build in a large controlled compound may meet requirements with distance, separation and fibre alone. A city-centre office floor with shared walls, or a vehicle, usually needs approved equipment and possibly shielding. Mobile and deployable systems need their own assessment because distance cannot be guaranteed (ISM-0249).

Cabling, separation and facility controls

Cabling is where EMSEC meets day-to-day facilities work, and the ISM gives concrete, public examples:

These controls also serve tamper detection: inspectable cabling makes it possible to find illicit taps or added devices. Keep a cable register and floor plans current, label cables and outlets by classification, and treat any unexplained cable, antenna or device in a controlled area as a security incident. Wireless devices are a related risk in classified areas; personal phones, smart watches and wireless peripherals need a clear policy. See the Physical and Environmental Security Policy Template.

Building EMSEC into the system lifecycle

1. Concept and site selection: record the classification and volume of information, and ask the national technical authority for an emanation risk assessment as early as possible (ISM-0246).
2. Design: apply the assessment's mitigation advice to layout, cabling, power, equipment choice and any shielding. Design cable routes for inspectability.
3. Procurement: buy approved equipment where the advice requires it, keep evidence of approval, and avoid unapproved changes to that equipment, which can invalidate its rating.
4. Installation and accreditation: verify separation, filtering and cabling against the design; where required, arrange instrumented testing by an accredited service, and keep test records, since PE-19 lists them as assessment evidence.
5. Operation: control changes to equipment, layout and cabling through the security authority; re-assess after significant changes such as new equipment, room moves or new adjacent tenants.
6. Exceptions: under the ISM, any mitigation advice not implemented must be accepted by the system's authorising officer (supporting ISM-1885); record it in the risk register.
7. Disposal: follow national direction for EMSEC-modified equipment (ISM-0321).

Rollback and change: because a single unapproved change (a new copper patch lead, a wireless keyboard, a moved desk against an external wall) can undo the assessment, treat the accredited configuration as a baseline and revert unapproved changes rather than assessing them after the fact.

Where identity fits

EMSEC rarely touches identity directly, but two links are real. First, cryptographic devices and the keys they hold are among the assets emanations historically exposed, so key-handling equipment should be in the most protected positions and meet the required equipment standard. Second, access to TEMPEST-controlled rooms, cabinets and cable routes should be limited to cleared, named people with logged entry, as ISM-0217 requires for shared TOP SECRET cabinets. See the HSM Deployment and Operations Guide.

Detection and measurement

• Inspection: scheduled inspection of cable routes, risers and outlets for unexplained cables, splitters or devices.
• Configuration audits: compare installed equipment, cabling and layout with the accredited baseline.
• Technical surveillance countermeasures: periodic sweeps of sensitive areas by authorised teams where policy requires.
• Change monitoring: facilities work orders that affect controlled areas routed through security approval.

Metrics:

• Percentage of classified systems with a current emanation risk assessment.
• Open mitigation actions from assessments, and the age of each.
• Unapproved changes found in configuration audits per quarter.
• Percentage of classified cabling that is fibre.
• Percentage of cable routes inspected on schedule.

Framework mapping

Common failure patterns

• Late engagement: asking for an emanation assessment after the room is built, then facing expensive retrofit or a long list of accepted risks.
• Buying "TEMPEST" without a requirement: paying for rated equipment where the assessment would have accepted distance and fibre, or buying equipment rated to the wrong scheme or level.
• Breaking separation during maintenance: classified and unclassified cables pulled into the same tray or conduit, contrary to controls such as ISM-0187.
• Unapproved modifications: replacing parts, cables or peripherals on approved equipment, which can invalidate its rating.
• Uninspectable cable routes: cables sealed in walls or ceilings with no access, so taps cannot be found.
• Personal wireless devices: phones, watches and wireless peripherals in classified areas without a clear and enforced policy.

Practitioner checklist

• Determine from classification and national policy whether EMSEC applies to each system and site.
• Request the emanation risk assessment at concept stage and track its mitigation advice to closure.
• Apply countermeasures in order: distance and control, placement, separation, fibre, filtering, approved equipment, shielding.
• Use fibre for classified cabling and keep classified cables in dedicated bundles and conduits.
• Keep cable routes inspectable, registered and labelled, and inspect them on schedule.
• Route every change to equipment, layout or cabling in controlled areas through security approval.
• Record any mitigation not implemented as a risk accepted by the authorising officer.
• Follow national direction for disposal of EMSEC-modified equipment.

Standards and references

• NIST: SP 800-53 Rev. 5, PE-19 Information Leakage
• Australian Signals Directorate: Information Security Manual (OSCAL)
• Australian Signals Directorate: ISM OSCAL mirror repository
• National Security Agency: TEMPEST, A Signal Problem (declassified)
• Federal Aviation Administration: TEMPEST Countermeasures for Facilities

Related NHI Mgmt Group resources: Physical Security Guide for Security Teams · Physical and Environmental Security Policy Template · Side-Channel Attacks Guide · Central Government Cybersecurity Programme Guide