Quantum Cryptography
Presented By
Tabrej A.Khan
13223006

• 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

.
Classical Cryptography

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

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)

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

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

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.

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 ?

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.

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

Quantum Key Distribution – BB84
Protocol

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

Quantum Key Distribution – BB84
Protocol

Example

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
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

Intercept and Resend Attack

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”.

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.

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.

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

Thanks