08-10-2012, 05:00 PM
Quantum Cryptography
Quantum.ppt (Size: 112 KB / Downloads: 79)
One Time Pad Encryption
Conventional cryptosystem:
Alice and Bob share N random bits b1…bN
Alice encrypt her message m1…mN b1m1,…,bNmN
Alice send the encrypted string to Bob
Bob decrypts the message: (mjbj)bj = mj
As long as b is unknown, this is secure
Can be passively monitored or copied
No-Cloning Theorem
To determine the amplitudes of an unknown qubit, need an unlimited copies
It is impossible to make a device that perfectly copies an unknown qubit.
Suppose there is a quantum process that implements:
Contradicts the unitary/linearity restriction of quantum physics
Quantum Cryptography
In 1984 Bennett and Brassard describe how the quantum money idea with its basis {0,1} vs. {+,-} can be used in quantum key distribution protocol
Measuring a quantum system in general disturbs it and yields incomplete information about its state before the measurement
Security of BB84
Without knowing the proper basis, Eve not possible to
Copy the qubits
Measure the qubits without disturbing
Any serious attempt by Eve will be detected when Alice and Bob perform “equality check”
Arguments Against QKD
QKD is not public key cryptography
Eve can sabotage the quantum channel to force Alice and Bob use classical channel
Expensive for long keys: Ω(N) qubits of communication for a key of size N