Physical-layer security is the protection of systems against attacks that exploit hardware behavior, proximity, and the environment around a device. Instead of focusing only on software, it looks at what a device emits or reacts to in the real world: electromagnetic radiation, power draw, sound, vibration, heat, and wireless interference. These signals can become side channels that reveal sensitive information.
This matters because a secure application can still leak data through its hardware. Attackers may passively capture emissions or actively inject radio-frequency or other signals to make leakage easier to measure. Defenders respond with shielding, filtering, better device placement, hardened hardware design, and testing in realistic RF environments. In practice, physical-layer security is essential wherever privacy depends on the behavior of a peripheral, sensor, or communications device, not just on access control and encryption.