Power analysis
Measuring a device's power consumption during a cryptographic operation to recover secret key bits.
Chips draw different amounts of current depending on the data they process, and that dependence can correlate with key bits. Simple power analysis (SPA) reads a single trace directly and often needs only one or a few measurements. Differential power analysis (DPA) uses statistics across many traces to pull out the tiny difference the key makes.
Paul Kocher and colleagues demonstrated the technique against smart cards in the late 1990s, and it has been a standing headache ever since. The countermeasures, such as masking, shuffling and balanced logic, cost performance and rarely give absolute protection. They raise the price for the attacker, which is much of what security engineering comes down to.