陳君翰 bb84 0718
TRANSCRIPT
Quantum Cryptography:Public key distribution
and coin tossing
Charles H.Bennett Gilles BrassardInternational Conference on Computers, Systems & Signal Processing,1984Speaker:陳君翰Quantum Information Security LabProfessor: Tzonelih Hwang
Outline
• Background
• Introduction
• BB84 Protocol
• Conclusion
Background
• Quantum Cryptography: Public key Distribution
• Charles H. Bennett, Gilles Brassard
• International Conference on Computers, Systems & Signal Processing,1984
• First Quantum key distribution protocol
Introduction
• Quantum cryptosystem:Base on property of quantum:1.The uncertainty of measurement
2.No cloning theory
BB84 Protocol• Definition
0 1
Bit:0 Bit:1
R Base
D Base
0 ° 90 °
45 ° -45 °
Quantum channel: with the property of quantum.
Classical channel: authentic channel
BB84 Protocol concept
T= 0,1
B= ,R D
T’=B’=
0,1
,R D
QB(T)
B’
B’’=Correct bases of B’
*P(B’’(1)),V(B’’(1))+………….*P(B’’(N/4)),V(B’’(N/4))+
Randomly choose half of B’’ to
discuss with Alice
ok or eavesdroppers existVerify
Case 1:No eavesdroppers
D R D R D R D R
1,3,6,8
(6,1),(8,1)
ok
1 0 1 0 1 1 1 1D D D D R R R R1 2 3 4 5 6 7 8
1 0 1 0 1 1 1 1
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 1 0 1 0 1D R D R D R D R1 2 3 4 5 6 7 8
Position 1,3,6,8 are the same base
1 0 1 0 1 1 1 1D D D D R R R R1 2 3 4 5 6 7 8
Randomly choose numbers
to discuss with Alice
Case 2:Have eavesdroppers
D R D R D R D R
1,3,6,8
(6,1),(8,0)
Eavesdropper exist!
1 0 1 0 1 1 1 1D D D D R R R R1 2 3 4 5 6 7 8
1 0 1 0 1 1 1 1
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 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 D D D R R R R1 2 3 4 5 6 7 8
Assume use D base
1 0 1 0 1 1 1 1D D D D R R R 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
Analysis BB84 Protocol
• Probability of eavesdropper :
1 bit:
N bit: 3
4
N
When N ↑,it’s easier to detect eavesdroppers exist or not.
1 1 1 3*
2 2 2 4
Analysis BB84 Protocol Cont.
Correct bases of B’
Qubit efficient:
(R,R) (D,D) (R,D)(D,R)
N/2 bits
N/4 bitsto discuss
N/4 bitsas key
(R,R) (D,D) (R,D)(D,R)
B’
DiscussionSecurity/Not security
N bits
• Qubit efficiency:1 1 1
*2 2 4
Utilization is only 25%!
Conclusion
• Advantages:
– Without exchanging information initially.
– Eavesdroppers can be detected.
• Disadvantages:
– Low utilization
– Photons may be lost in transmitting
Thank you