What Is a ‘Side-Channel Attack’ in the Context of Cryptography?

A side-channel attack is a non-invasive attack that exploits information leaked from the physical implementation of a cryptographic system, rather than weaknesses in the algorithm itself. Examples include analyzing timing variations, power consumption, electromagnetic emissions, or even sound during key operations.

In ECDSA, timing attacks can sometimes be used to deduce parts of the private key if the implementation is not constant-time, which is a key reason for EdDSA's improved design.