Quantum mechanics offers dramatically novel ways to process information. It is known that a fully functioning quantum computer would break some encrypting algorithms, such as RSA, which is used nowadays. However, quantum mechanics offers an alternative to standard cryptography, which cannot be broken by a quantum computer – the quantum key distribution. For this reason, designing and realizing quantum key distribution protocols in practice is a highly important task and involves a huge effort of theoreticians and engineers nowadays. Depending on the level of professional paranoia, there are different levels of trust toward devices realizing quantum key distribution, ranging from device dependent through semi-device dependent to fully device-independent quantum cryptography. Due to novel quantum hacker attacks which exploit imperfections of implementation, the lower levels of security need the significant effort of engineers to be useful for encryption.