Microwaves are deeply tangled with hardware security in both attack and defense roles.
TDR for tamper detection. A sealed cable's versus time has a characteristic signature: every connector, bend, and transition shows up at a specific TDR position. If someone splices a tap, the added length and impedance discontinuity change the signature visibly. High-security installations monitor TDR continuously on critical cabling; deviations trigger alarms. The same idea is applied to chassis bonds, antenna feedlines, and (via OTDR) fiber-optic cables.
High-power microwave (HPM) weapons. A magnetron or vircator pulsed at gigawatts into a parabolic dish creates a directed beam that damages electronics inside vehicles or buildings hundreds of meters away. The pulse couples through any aperture (vents, gaps in shielding) and induces voltages that exceed input-pin or memory-cell breakdown. Counter-electronics weapons exist in military and improvised forms. Hardening: Faraday shielding, surge protection on every external pin, optical isolation, careful aperture control.
EM injection attacks. Coin-sized pulsed EM probes held millimeters from a chip can inject transients that flip bits or skip instructions during cryptographic operations. The probe is a tight coil driven by a fast pulser; the magnetic pulse couples into on-chip wires via mutual inductance. EM injection is non-contact, leaves no marks, and can skip instructions in a smartcard's PIN-comparison loop or fault an AES round to leak the key. Defenses: shielded packaging, on-die EM sensors, redundant computation with comparison.
TEMPEST at GHz. Flat-panel monitors, USB cables, and SDRAM buses radiate at GHz harmonics of their pixel or bit-clock frequencies. A directional antenna plus low-noise spectrum analyzer at the right harmonic can recover screen contents from another room. Defenses: shielded cables, ferrite chokes, conductive enclosures, spread-spectrum clocking.
RF side channels. A chip's clock harmonics radiate slightly differently depending on data on the bus. With a sensitive antenna and signal processing, an attacker correlates harmonic amplitude with key bits and recovers keys non-contact. A famous demo recovered AES keys from a smartcard via its 1 GHz harmonic using a $5 SDR.
Through-wall imaging. UWB radars at 3–10 GHz penetrate drywall to detect motion (or even cardiac signatures). Used by SWAT and search-and-rescue. Defenses: metal mesh in walls or thick concrete.
GPS jamming and spoofing. GPS L1 power at ground is dBm. A small jammer overpowers it within meters; a sophisticated spoofer transmits fake signals and steers the receiver to a false position. Pure microwave engineering with the components in this chapter.
Faraday-cage rooms (SCIFs). Continuous metal mesh on walls, ceiling, floor with all penetrations filtered. Reduces external RF emissions by 60–100 dB depending on quality. Tested with VNAs and signal generators outside, very sensitive receivers inside.