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# 陳君翰 BB84 0718

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### 陳君翰 BB84 0718

1. 1. Quantum Cryptography:Public key distributionand coin tossing<br />Charles H.Bennett Gilles Brassard<br />International Conference on Computers, Systems & Signal Processing,1984<br />Speaker:陳君翰<br />Quantum Information Security Lab<br />Professor: Tzonelih Hwang<br />
2. 2. Outline<br />Background<br />Introduction<br />BB84 Protocol<br />Conclusion<br />
3. 3. Background<br />Quantum Cryptography: Public key Distribution<br />Charles H. Bennett, Gilles Brassard<br />International Conference on Computers, Systems & Signal Processing,1984<br />First Quantum key distribution protocol<br />
4. 4. Introduction<br />Quantum cryptosystem:Base on property of quantum:1.The uncertainty of measurement2.Nocloning theory<br />
5. 5. BB84 Protocol<br />Bit:0<br />Bit:1<br /><ul><li>Definition</li></ul>R Base<br />0 °<br />90 °<br />D Base<br />45 °<br />-45 °<br />Quantum channel: with the property of quantum.<br />Classical channel: authentic channel<br />
6. 6. BB84 Protocol concept<br />QB(T)<br />B’<br />Alice<br />T’=<br />T=<br />B’=<br />B=<br />B’’=Correct bases of B’<br />Randomly choose half of B’’ to discuss with Alice<br />Bob<br />[P(B’’(1)),V(B’’(1))]………….[P(B’’(N/4)),V(B’’(N/4))]<br />Verify<br />ok or eavesdroppers exist<br />
7. 7. Case 1:No eavesdroppers<br />1 0 1 0 1 1 1 1<br />D R D R D R D R<br />Alice<br />1 0 1 0 1 1 1 1D DDDR RRR1 2 3 4 5 6 7 8 <br />1 0 1 101 0 1D RD R D R D R1 2 3 4 5 6 7 8 <br />1 0 1 1 0 1 0 1D R D R D R D R1 2 3 4 5 6 7 8 <br />1 0 1 0 1 1 1 1D DDDR RRR1 2 3 4 5 6 7 8<br />1,3,6,8<br />Position 1,3,6,8 <br />are the same base<br />Randomly choose numbers to discuss with Alice<br />Bob<br />(6,1),(8,1)<br />ok<br />
8. 8. Case 2:Have eavesdroppers<br />Assume use D base<br />1 0 1 0 1 1 1 1<br />D R D R D R D R<br />Alice<br />1 0 1 0 1 1 1 1D DDDR RRR1 2 3 4 5 6 7 8 <br />1 0 1 111 10D RD R D R D R1 2 3 4 5 6 7 8 <br />1 0 1 1 1 1 1 0D R D R D R D R1 2 3 4 5 6 7 8<br />1 0 1 0 1 1 1 1D DDDR RRR1 2 3 4 5 6 7 8<br />1 0 1 0 1 1 1 1D DDDR RRR1 2 3 4 5 6 7 8<br />1 0 1 1 1 1 1 0D R D R D R D R1 2 3 4 5 6 7 8<br />1,3,6,8<br />Bob<br />Eve<br />(6,1),(8,0)<br />Eavesdropper exist!<br />
9. 9. Analysis BB84 Protocol<br />Probability of eavesdropper :<br />1 bit:<br />N bit:<br />When N ↑,it’s easier to detect eavesdroppers exist or not.<br />
10. 10. Analysis BB84 Protocol Cont.<br />Qubit efficient:<br />N bits<br />B’<br />Alice<br />Correct bases of B’<br />(R,R) <br />(D,D) <br />(R,D)<br />(D,R)<br />N/2 bits<br />(R,R) <br />(D,D) <br />(R,D)<br />(D,R)<br />Discussion<br />Security/Not security<br />Bob<br />N/4 bits<br />to discuss<br />N/4 bits<br />as key<br />
11. 11. Qubit efficiency:<br />Utilization is only 25%! <br />
12. 12. Conclusion<br />Advantages:<br />Without exchanging information initially.<br />Eavesdroppers can be detected.<br />Disadvantages:<br />Low utilization<br />Photons may be lost in transmitting<br />
13. 13. Thank you<br />