1. Quantum Cryptography:Public key distributionand coin tossing Charles H.Bennett Gilles Brassard International Conference on Computers, Systems & Signal Processing,1984 Speaker:陳君翰 Quantum Information Security Lab Professor: Tzonelih Hwang

2. Outline Background Introduction BB84 Protocol Conclusion

3. Background Quantum Cryptography: Public key Distribution Charles H. Bennett, Gilles Brassard International Conference on Computers, Systems & Signal Processing,1984 First Quantum key distribution protocol

4. Introduction Quantum cryptosystem:Base on property of quantum:1.The uncertainty of measurement2.Nocloning theory

6. BB84 Protocol concept QB(T) B’ Alice T’= T= B’= B= B’’=Correct bases of B’ Randomly choose half of B’’ to discuss with Alice Bob [P(B’’(1)),V(B’’(1))]………….[P(B’’(N/4)),V(B’’(N/4))] Verify ok or eavesdroppers exist

7. Case 1:No eavesdroppers 1 0 1 0 1 1 1 1 D R D R D R D R Alice 1 0 1 0 1 1 1 1D DDDR RRR1 2 3 4 5 6 7 8 1 0 1 101 0 1D RD R D R D R1 2 3 4 5 6 7 8 1 0 1 1 0 1 0 1D R D R D R D R1 2 3 4 5 6 7 8 1 0 1 0 1 1 1 1D DDDR RRR1 2 3 4 5 6 7 8 1,3,6,8 Position 1,3,6,8 are the same base Randomly choose numbers to discuss with Alice Bob (6,1),(8,1) ok

8. Case 2:Have eavesdroppers Assume use D base 1 0 1 0 1 1 1 1 D R D R D R D R Alice 1 0 1 0 1 1 1 1D DDDR RRR1 2 3 4 5 6 7 8 1 0 1 111 10D RD R D R D R1 2 3 4 5 6 7 8 1 0 1 1 1 1 1 0D R D R D R D R1 2 3 4 5 6 7 8 1 0 1 0 1 1 1 1D DDDR RRR1 2 3 4 5 6 7 8 1 0 1 0 1 1 1 1D DDDR RRR1 2 3 4 5 6 7 8 1 0 1 1 1 1 1 0D R D R D R D R1 2 3 4 5 6 7 8 1,3,6,8 Bob Eve (6,1),(8,0) Eavesdropper exist!

9. Analysis BB84 Protocol Probability of eavesdropper : 1 bit: N bit: When N ↑,it’s easier to detect eavesdroppers exist or not.

10. Analysis BB84 Protocol Cont. Qubit efficient: N bits B’ Alice Correct bases of B’ (R,R) (D,D) (R,D) (D,R) N/2 bits (R,R) (D,D) (R,D) (D,R) Discussion Security/Not security Bob N/4 bits to discuss N/4 bits as key

11. Qubit efficiency: Utilization is only 25%!

12. Conclusion Advantages: Without exchanging information initially. Eavesdroppers can be detected. Disadvantages: Low utilization Photons may be lost in transmitting

13. Thank you