Acoustic cryptanalysis
A side-channel attack that recovers keys from the sound a computer makes while it computes.
Components in a computer, especially coils in the power supply, emit sound that varies with the operations the processor is performing. A microphone placed near the machine can capture those variations and, with the right analysis, derive key bits from an RSA computation.
In 2014 Genkin, Shamir and Tromer showed that an RSA key could be extracted from the sound of a laptop. The attack required the victim to decrypt ciphertexts the attacker had chosen, for example through an encrypted email, plus a microphone within a few metres. It was slow and leaked little per trial. It still shows something unpleasant: a mathematically correct algorithm can leak through the physics of the hardware.
The defences are the usual ones from the side-channel world: constant-time implementations and blinding of the computation. Silencing the machine is possible but expensive, so in practice the protection goes into the implementation rather than relying on nobody listening.