Side-channel attack
Extract secrets by measuring something that isn't the algorithm itself — time, power, EM radiation, acoustics, cache state.
Classics: timing attacks on RSA (read out the key by measuring how long signing takes), DPA (differential power analysis) against smart cards, cache timing against AES, acoustic attacks on 3D printers or keyboards. Spectre/Meltdown (2018) were massive CPU-cache side channels. Mitigations: constant-time implementations, electromagnetic shielding, address-space randomisation, mask-based crypto implementations.