9 Quantum Cryptography

Information protection has been an important part of human life from ancient time. In computer society, information security becomes more and more important for humanity and new technologies are emerging in an endless stream. Cryptography or cryptology is a technology to convert the information from readable state into nonsense, or do the contrary transformation. Information transmission and storage can be effectively protected by cryptography. Modern cryptography has been rapidly developed since World War II, along with the fast progress of electronics and computer science. Symmetric-key cryptography and public-key cryptography are two major fields of modern cryptography, depending on if encryption and decryption keys are same or not. One of the symmetric encryption algorithms named one-time pad (OTP) has been proven to be impossible to crack no matter how powerful the computing power is (Shannon, 1949), however, to generate and distribute the true random key steam the same size as the plaintext is a rigorous requirement. Quantum cryptography can provide a secure approach to exchange keys between legitimate users and can be used with OTP to fulfill secure communication sessions. The concept of quantum cryptography was originally proposed by Wiesner in 1960s (Wiesner, 1983), though its real development should be recorded from the first quantum key distribution (QKD) protocol presented by Bennett and Brassard in 1984 (Bennett, & Brassard, 1984). The research fields of quantum cryptography are wider than QKD, including quantum secret sharing , quantum authentication, quantum signature and so on. QKD is a major aspect of quantum cryptography and will be the only topic discussed in this chapter. Unlike publickey cryptography, the security of QKD is guaranteed by quantum mechanics rather than computational complexity. The most important property of QKD is to detect the presence of the behavior to intercept the key information illegally. The single photon used in QKD cannot be divided into smaller parts, which means the eavesdropper cannot hold a part of the particle then measure it. The eavesdropper cannot precisely duplicate a particle with the same quantum state as the original one unknown to her due to the quantum no-cloning theorem. To measure an unknown quantum system will cause it to "collapse" from a range of possibilities into one of its eigenstates. This character, together with the uncertainty principle, ensure the eavesdropper cannot get total information without disturbing the original quantum system.