NOVEL QUANTUM ENCRYPTION SCHEMES FOR SECURE DATA TRANSMISSION AND STORAGE

Abstract

Quantum encryption is to protect the privacy of messages with unconditional security. During the last fifteen years, the research in this area is on the design and analysis of quantum key distribution protocols. In this thesis, two results on quantum encryption are presented. One result is the design of quantum encryption scheme that is different from all the previous quantum key distribution protocols. Another result is on improving the efficiency of the quantum key distribution. A quantum encryption scheme that is different from all the previous quantum key distribution protocols and classical ciphers is proposed in this thesis. This new scheme is able to protect the privacy of both the transmitted and stored data while all the previous quantum key distribution protocols can only be applied to protect the transmitted data. In this new scheme, an n- bit one-time secret key provides unconditional secu1ity protection to a message of size up to 2n bits on the most critical assumption that unlimited computing power is available to an adversary. On the contrary, an n-bit secret key in a classical encryption scheme can only provide unconditional security protection to a message of size about O(n) bits. The new quantum encryption scheme thus far more better than the quantum key distribution protocols and the classical ciphers. It is shown that in this new encryption scheme, info1mation leakage for each bit of the message is about 2-n bit, and to compromise the secret key under known-plaintext attack, it requires a message of size 2n bits being encrypted under the same key. To have noise tolerance, error correction coding is incorporated into the quantum encryption scheme. In particular, it is shown that when the simple (n ?, 1, n) coding is used, the effect on the security of the encryption scheme is negligible. To design a quantum encryption scheme that is efficient and has a large error tolerance, a very complicated scheme is required. The efficiency of the quantum key distribution has been improved for many years. In the first quantum key distribution scheme BB84, the sender and the receiver are using different polarization basis half of the times and the efficiency of BB84 is at most 50%. The efficiency was improved significantly by Ardehali, Chau and Lo recently (ACL98). In ACL98, the sender and the receiver use the same polarization basis almost all the times. As the number of transmitted signals increases the efficiency of ACL98 approaches 100% in the photon detection process. However, the error detection efficiency of ACL98 cannot reach the optimal value. In this thesis, a quantum key distribution scheme achieving higher efficiency than the ACL98 is designed. In this scheme, the sender and the receiver are using the same basis all the time. Each of them generates a pseudo-random sequence from the same generator with a secret key shared initially between them. The pseudo-random sequence is to determine the basis used for the photons. Thus the sender and the receiver are using the same basis all the time and the efficiency of the photon detection process reaches the optimal value. Also the error detection process in this new scheme can reach the optimal efficiency. The overall efficiency of this scheme is believed to be optimal.