4. Quantum Key
Distribution
Outline
Introduction
Classical
Cryptography
Quantum
Cryptography
Quantum Key
Distribution
Pros and Cons
Conclusion
What is Cryptography?
Cryptography: the coding and decoding of secret
messages.
Cryptography is the art of rendering a message
unintelligible to any unauthorized party
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(Korean)
It is part of the broader ļ¬eld of cryptology, which also
includes cryptoanalysis, the art of code breaking
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8. Quantum Key
Distribution
Outline
Introduction
Classical
Cryptography
Quantum
Cryptography
Quantum Key
Distribution
Pros and Cons
Conclusion
Types of Cryptosystems
Symmetric - use the same (secret) key to encrypt and
decrypt a message
Asymmetric - use one key (the public key) to encrypt a
message and a diļ¬erent key (the private key) to decrypt it.
Asymmetric cryptosystems are also called public key
cryptosystems.
Figure: Process of cryprography
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9. Quantum Key
Distribution
Outline
Introduction
Classical
Cryptography
Quantum
Cryptography
Quantum Key
Distribution
Pros and Cons
Conclusion
One-Time Pad
ļ¬rst proposed by Gilbert Vernam in 1926
This cryptosystem is thus provably secure in the sense of
information theory (Shannon 1949)
Actually, this is today the only provably secure
cryptosystem
Only problem is distributing the keys and it is Diļ¬cult to
implement
Figure: One time pad
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11. Quantum Key
Distribution
Outline
Introduction
Classical
Cryptography
Quantum
Cryptography
Quantum Key
Distribution
Pros and Cons
Conclusion
Need of Quantum Cryptography?
The problem of distributing the keys in the OTP is
overcomed in Quantum Key Distribution
Using quantum eļ¬ects, we can distribute keys in perfect
secrecy!
The Result is Perfect Cryptosystem, QC = QKD + OTP
Classical Cryptography relies heavily on the complexity of
factoring integers.
Quantum Computers can use Shors Algorithm to
eļ¬ciently break todays cryptosystems
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12. Quantum Key
Distribution
Outline
Introduction
Classical
Cryptography
Quantum
Cryptography
Quantum Key
Distribution
Pros and Cons
Conclusion
Quantum Elements
Quantum Mechanics depends on 2 major elements of quantum
mechanics: i.e heisenberg uncertainity principle and principle of
photon polarization.
Heisenberg uncertainty principle : states that certain pairs
of physical properties are related in such a way that
measuring one property prevents the observer from
simultaneously knowing the value for other.
Principle of photon polarization: tells that an
eavesdropper cannot copy unknown Qubits due to
non-cloning algorithm.
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13. Quantum Key
Distribution
Outline
Introduction
Classical
Cryptography
Quantum
Cryptography
Quantum Key
Distribution
Pros and Cons
Conclusion
Overview of Quantum Cryptography
Unique property of quantum cryptography is the ability of
two communicating users to detect the presence of third
party trying to gain knowledge of the key.
By using quantum superposition/entanglement and
transmitting information in quantum states , a
communication system can be implemented which can
detect eavesdropping.
The polarization basis is the mapping we decide to use for
a particular state
Entangled pairs of photon are used that aļ¬ects
randomness of measurements
Figure: Photon Polarization using BasesApril 10, 2015 Shahrukh Ayaz Khan Quantum Key Distributionl 13/26
15. Quantum Key
Distribution
Outline
Introduction
Classical
Cryptography
Quantum
Cryptography
Quantum Key
Distribution
Pros and Cons
Conclusion
Quantum Key Distribution
Quantum Key Distribution exploits the eļ¬ects discussed in
order to thwart eavesdropping.
It enables two parties to produce a shared random bit
string known only to them, which can be used as a key for
encryption and decryption.
If an eavesdropper uses the wrong polarization basis to
measure the channel, the result of the measurement will
be random.
There are three main security protocols for QKD: BB84,
B92, and Entanglement-Based QKD.
We will only discuss BB84 here.
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16. Quantum Key
Distribution
Outline
Introduction
Classical
Cryptography
Quantum
Cryptography
Quantum Key
Distribution
Pros and Cons
Conclusion
BB84
BB84 was the ļ¬rst security protocol implementing
Quantum Key Distribution.
The key consists of bits that will be transmitted as
photons.
Each bit is encoded with a random polarization basis!
Alice and Bob talk on the telephone:
Alice chooses a subset of the bits (the test bits) and
reveals which basis she used to encode them to Bob.
Bob tells Alice which basis he used to decode the same
bits.
Where the same basis was used, Alice tells Bob what bits
he ought to have got.
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19. Quantum Key
Distribution
Outline
Introduction
Classical
Cryptography
Quantum
Cryptography
Quantum Key
Distribution
Pros and Cons
Conclusion
BB84 with Eavesdropping
If an eavesdropper Eve tries to tap the channel, this will
automatically show up in Bobs measurements.
In those cases where Alice and Bob have used the same
basis, Bob is likely to obtain an incorrect measurement
As Eve intercepts Alices photons, she has to measure them
with a random basis and send new photons to Bob.
The photon states cannot be cloned (non-cloneability).
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22. Quantum Key
Distribution
Outline
Introduction
Classical
Cryptography
Quantum
Cryptography
Quantum Key
Distribution
Pros and Cons
Conclusion
Future Scope
Future applications are in the most secure communication lines
for bank-to-ATM transactions, ļ¬nancial information protection
over the internet, and military and government communications
including the use for ship-to-ship, ground-to-satellite, and
satellite-to-satellite communication.
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25. Quantum Key
Distribution
Outline
Introduction
Classical
Cryptography
Quantum
Cryptography
Quantum Key
Distribution
Pros and Cons
Conclusion
References
1 N. Namekata, S. Mori, and S. Inoue, Quantum key
distribution over an installed multimode optical ļ¬ber local
area network, Optical Express, 2015.
2 A. Beresford and F. Stajano, Location privacy in pervasive
computing, IEEE Pervasive Computing, 2011.
3 K. G. Paterson, F. Piper, and R. Schack, Why quantum
cryptography?, Quantum physics, quant-ph/0406147,
June 2004.
4 Hidema Tanaka, Security Analysis of Generalized
Conļ¬dential Modulation for Quantum Communication
International Journal of Computer Networks &
Communications (IJCNC) Vol.5, No.5, September 2013
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