This document discusses quantum cryptography and its advantages over classical cryptography. It introduces the key distribution problem in classical cryptography. Quantum cryptography uses principles of quantum mechanics like quantum bits that cannot be copied and photon polarization to securely distribute keys. The document describes the BB84 protocol for quantum key distribution where Alice and Bob use different polarization bases to generate a random key and detect eavesdropping. While promising, challenges remain in scaling the technology to longer distances and developing affordable devices.

1. Quantum Cryptography
Presented By
Tabrej A.Khan
13223006

2. • Classical Cryptography
• Introduction to Quantum cryptography-
• Classical Cryptography and Key Distribution Problem.
• Quantum Communication .
• Elements of Quantum Theory
• Heisenberg Uncertainty Principle
• Quantum Key Distribution .
• Detecting Eavesdropper
• Technical Challenges of QKD

3. .
Classical Cryptography

4. Symmetric Algorithm
 Usually use same key for encryption and decryption.
 Require sender and receiver to agree on a key before
they communicate securely.
 Encryption key can be calculated from decryption
key and vice versa
 Security lies with the key.
 Also called secret key algorithms, singlekey
algorithms, or one-key algorithms
 Example: DES (1977), Triple DES (1998),AES

6. Asymmetric Algorithm
 Use different keys for encryption and decryption.
 Decryption key cannot be calculated from the
encryption key
 Anyone can use the key to encrypt data and send it to
the host; only the host can decrypt the data
 Also known as public key algorithms
 Example: Diffie-Hellman (1976) RSA (1977)

8. Vulnerabilities/Weakness to the
modern/classical cryptography
 There are three main problems with encryption
schemes
-first is key distribution
-the second is key management
-Thirdly as computing power increases, and new
classical computational techniques are developed,
the length of time that a message can be considered
secure will decrease, and numerical keys will no
longer be able to provide acceptable levels of secure
communications

9. Key Distribution Problem
 How to communicate the key securely between two
pair of users.
 it is not possible to check whether this medium was
intercepted – and its content copied – or not.
 Public key cryptography came as a solution to this,
but these too are slow and cannot be used to encrypt
large amount of data

10. Elements of Quantum Theory
 Light waves are made up of millions of discrete
quanta called Photons
 They are massless and have energy, momentum
and angular momentum called spin.
 Spin carries the polarization.

11. Quantum Communication
The Classical World
- Bits either 0 or 1.
- Bits can be copied.
- Bits can be observed without changing them. (So, eavesdropping cannot
be detected in classical cryptosystems.)
Quantum Bits
- A quantum bit (qubit) can be 0 or 1 at the same time.
- It can not be copied (no cloning theorem).
- Its state will collapse if it is observed (measured).
 If a qubit can be 0 or 1 at the same time, how many values can n qubits
have at the same time ?

12. Quantum Communication
 Quantum cryptography solves the key distribution problem by
allowing the exchange of a cryptographic key between two
remote parties with absolute security, guaranteed by the laws
of physics.
 Quantum Communication is based on two features of
Quantum mechanisms and photons.
-State indeterminancy based on Heisenberg principle .
-Entangled based protocols that means two entities can be
defined such that their properties are entangled altering one
effects the value of other.

13. Heisenberg Uncertainty Principle
 For any two observable properties linked together
like mass and momentum
• According to the principle two interrelated properties
cannot be measured individually without affecting the
other.
• Measuring the state of photon will affect it value

14. Quantum Key Distribution – BB84
Protocol

15. Quantum Mechanics for Cryptography –
Measurement Basis
 Basis – frame of reference for quantum
measurement
 Example – polarization
vertical/horizontal vs. diagonal
 Horizontal filter, light gets through = 0
 Vertical filter, light gets through = 1
 45 deg. filter, light = 0
 135 deg. filter, light = 1

16. Quantum Key Distribution – BB84
Protocol

17. Example

18. Detecting Eavesdroppers
 To check for the presence of eavesdropping Alice and
Bob now compare a certain subset of their remaining
bit strings.
 If any interceptor has gained any information about
the photons polarization, this will have introduced
errors in Bobs' measurements
 If more than p bits differ they abort the key and try
again, possibly with a different quantum channel, as
the security of the key cannot be guaranteed.

19. 19
Alice's Bit Sequence
0 1 0 - 0 1 1 1 1 - 1 0
- 1 - - 0 1 - - 1 - 1 0
Bob's Bases
Bob's Results
Key
Alice
Bob
Polarizers
Horizontal - Vertical
Diagonal (-45, +45)
H/V Basis
45 Basis
BB84 protocol: Eve  25% errors

20. Intercept and Resend Attack

21. Implementing Quantum
Cryptography(Real Case)
 BBN, Harvard, and Boston University built the DARPA quantum network,
the world’s first network that delivers end-to-end network security via
high-speed quantum key distribution, and tested that network against
sophisticated eavesdropping attacks.
 For the Bank of Austria, the novel technology was demonstrated by the
group of Professor Anton Zeilinger, Vienna University in collaboration with
the group Quantum Technologies of Seibersdorf research.
 The bank transfer was initiated by Vienna’s Mayor Dr. Michael Haupl, and
executed by the director of the Bank Austria Creditanstalt, Dr. Erich.
 The information was sent via a glass fiber cable from the Vienna City
 Hall to the Bank Austria Creditanstalt branch office “Schottengasse”.

22. TECHNICAL CHALLENGES OF QKD AND
FUTURE DIRECTION
 One of the challenges for the researchers, is distance
limitation.Currently, quantum key distribution
distances are limited to tens of kilometers because of
optical amplification destroys the qubit state.
 Also to develop optical device capable of generating,
detecting and guiding single photons; devices that
are affordable within a commercial environment .
 Also users need reassurance not only that QKD is
theoretically sound, but also that it has been securely
implemented by the vendors.

23. Summary
 Realization of practical quantum information
technologies can not be accomplished without
involvement of the network research community.
 The advances in computer processing power and the
threat of limitation for today’s cryptography systems
will remain a driving force in the continued research
and development of quantum cryptography.
 The technology has the potential to make a valuable
contribution to the network security among
government, businesses, and academic environment.

24. Future Prospects
 Ground-to-satellite, satellite-to-satellite links
 General improvement with evolving qubit-handling
techniques, new detector technologies

25. Thanks