Side-channel attack

A side-channel attack recovers secrets from the physical behaviour of an implementation rather than from any weakness in the algorithm. The classic channels are timing (how long an operation takes leaks which key bits were processed), power analysis (current draw traces reveal key material), electromagnetic emanation, acoustic leakage, and shared cache behaviour on multi-tenant hardware. The mathematics of AES can be flawless while the smartcard running it gives the key away through its power pins.

Countermeasures target the leak, not the cipher: constant-time implementations that take the same path regardless of key bits, power conditioning and blinding, physical shielding against emanation (the TEMPEST programme), and cache partitioning. Longer keys do not help, because the keyspace is never searched; a TPM or smartcard that leaks power traces surrenders an AES key without the attacker ever trying a single candidate.

Exam relevance: if a scenario mentions measuring timing, power consumption, emanations, or cache access patterns to extract a key, the answer is side-channel attack; if the fix offered is constant-time code or shielding, same answer. Contrast brute force, which attacks the keyspace by trying candidates and is defeated by key length; a side channel bypasses the keyspace entirely, which is why it defeats otherwise strong cryptography.