Security in quantum cryptography

Quantum cryptography exploits principles of quantum physics for the secure processing of information. A prominent example is secure communication, i.e., the task of transmitting confidential messages from one location to another. The cryptographic requirement here is that the transmitted messages remain inaccessible to anyone other than the designated recipients, even if the communication channel is untrustworthy. In classical cryptography, this can usually be guaranteed only under computational hardness assumptions, such as when factoring large integers is infeasible. In contrast, the security of quantum cryptography relies entirely on the laws of quantum mechanics. Here this physical notion of security is reviewed, with a focus on quantum key distribution and secure communication.