Every switching digital circuit (a pixel clock, a bus, a keyboard matrix) radiates a faint electromagnetic replica of the data it carries โ the effect exploited by TEMPEST-class signals intelligence and defended against by NSTISSAM TEMPEST/1-92-style emission security (EMSEC). Free-space propagation follows the inverse-square law, so received power falls off with distance:
P_r(dBm) = P_t โ 20ยทlog10(d / d0) โ S
P_t = transmitted emanation level (0 dBm reference)
d = distance to the eavesdropper (m), d0 = 1 m
S = shielding attenuation (dB) from the Faraday enclosure
โ bypassed entirely by any conductor (I/O cable, power
line, ground strap) that exits the enclosure unfiltered
Correlation ฯ โ how well the leaked emanation lets an eavesdropper reconstruct the original signal โ is modeled as a sigmoid of the signal-to-noise ratio against a โ90 dBm receiver noise floor, centered on the 10 dB detection threshold used in the formula below:
SNR = P_r โ (โ90 dBm)
ฯ = 1 / (1 + e^โ((SNR โ 10) / 5))
- Clock harmonic frequency โ higher-frequency digital logic radiates more efficiently and produces tighter, more decodable pulses; it does not change the inverse-square falloff itself, only the emanation's visual pulse rate here.
- Eavesdropper distance โ moving the receiver away costs 6 dB every time the distance doubles.
- Faraday enclosure + shielding effectiveness โ a certified enclosure adds S dB of attenuation to any wavefront that must cross its boundary radiatively.
- Unshielded I/O cable โ the single most common real-world TEMPEST failure: a cable that exits the shielded boundary without filtering carries the full unattenuated signal out with it, defeating the enclosure entirely regardless of its rated dB. Toggle it on to see the verdict flip even at high shielding.
This model is illustrative of the physics and doctrine (emission security, Red/Black separation of cabling, zoned shielding), not a working interception tool โ no real signal-decoding technique is implemented.