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QUANTUM CRYPTOGRAPHY
Security through Uncertainty
CS 265
Spring 2003
Narayana D Kashyap
Classic Cryptography
 Keys by Transposition and Substitution
 Strength mainly on Long Keys
 KDC
 Mathematical Concepts
 Allows the eavesdropper in principle to measure
physical properties without disturbing them.
Quantum vs. Classic
 Physics instead of Math
 Exchange of information – very secure in a
strong sense.
 Laws of physics guarantee (probabilistically)
that the secret key exchange will be secure.
 The ACT OF MEASUREMENT is an integral part
of quantum mechanics, not just a passive,
external process as in Classic Crypto.
Fundamental Concepts
 Quantum Channel along with Quantum theory
 Heisenberg Uncertainty Principle –
certain pairs of physical properties are
complementary.
 Any effort to monitor the channel necessarily
disturbs the signal in some detectable way.
 The uncertainty principle is used to build
secure channel based on Quantum properties
of light.
 Photons & Polarization
How it works?
 Bob, informed about polarization being either 0
or 90 (rectilinear), can find out by his
photomultiplier as to how it was sent.
 Such an apparatus is useless for distinguishing
45 or 135 (diagonal) photons - unless the
apparatus is turned 45 degrees.
 Rectilinear and Diagonal polarizations are
complementary properties.
BB84
 Bennett and Brassard proposed in 1984
Quantum channel – Polarized Photons
Public channel – Normal Messages
 Alice = Sender & Bob = receiver
 Agreed before hand that
90 and 45 are 1’s
0 and 135 are 0’s
Quantum Key Distribution
Alice
Bob
Alice





Bob
1
1
0
0
1
Verification of QC
 Alice and Bob to compare the "parity"evenness or oddness of a publicly agreed on
random subset containing about half the bits in
their data.
 Alice could tell Bob, "I looked at the 1st, 3rd,
4th, 9th,...996th and 999th of my 1,000 bits of
data, and they include an even number of l's."
 It suffices to repeat the test 20 times, with 20
different random subsets, to reduce the chance
of an undetected error to less than one in a
million.
References
 Quantum Cryptography
http://www.cyberbeach.net/~jdwyer/quantum_cr
ypto/quantum1.htm
 Quantum Cryptography Tutorial
http://www.cs.dartmouth.edu/~jford/crypto.html
 Oxford Quantum Computation Group
http://www.qubit.org/