This document discusses quantum cryptography and potential vulnerabilities in practical quantum key distribution (QKD) systems. It describes how QKD works to securely distribute a key using photons and how systems aim to detect eavesdropping by measuring error rates. However, the document outlines several hacks that have been demonstrated, including a tailored illumination attack that uses bright light pulses to operate avalanche photodiodes in the linear mode and bypass the intended detection of single photons. While the security of QKD is provably secure in theory, vulnerabilities exist in practical systems that do not meet all theoretical assumptions.