  With the emergence of an information society, the idea of protecting
sensitive data is steadily gaining importance. Conventional encryption methods
may not be sufficient to guarantee data protection in the future. Quantum key
distribution (QKD) is an emerging technology that exploits fundamental physical
properties to guarantee perfect security in theory. However, it is not easy to
ensure in practice that the implementations of QKD systems are exactly in line
with the theoretical specifications. Such theory-practice deviations can open
loopholes and compromise the security. Several of such loopholes have been
discovered and investigated in the last decade. These activities have motivated
the proposal and implementation of appropriate countermeasures, thereby
preventing future attacks and enhancing the practical security of QKD. This
article introduces the so-called field of quantum hacking by summarizing a
variety of attacks and their prevention mechanisms.
