About Cryptography

1. Seminar on Elements of Cryptography
CSE-6504
Presented by
Anisur Rahman
M Shahedul Islam
Nazmul Hasan
Supervisor
Prof. Dr. Md. Mahbubur Rahman

2. Quantum Cryptography
Alice
Bob
Eve

3. Sequence
• Cryptology – General View
• Quantum Cryptography - How it came up
• Theoretical Background
• History of Quantum Cryptography (QC)
• Quantum Key Distribution Protocol - BB84
• Quantum Key Distribution – Example
• Attack and Vulnerabilities
• Main Contribution of QC
• Security of QC
• State of the QC Technology
• Pros and Cons

4. Cryptology – General View
 Cryptography is the art of devising codes and
ciphers.
 Crypto analysis is the art of breaking them.
 Cryptology is the combination of the two i. e
Cryptography and Crypto analysis

5. Cryptology – General View

6. • Need
– > Secure Communication
– > Secure Data Transmission
• Two techniques
– Symmetric - key encryption (shared key)
• Key - distribution problem
– Asymmetric - key encryption (pair of public &
secret keys)
• Success based on hardware limitations, absence of
good algorithms
Cryptology – General View

7. Symmetrical (secret-key) cryptosystems:
- only provably secure cryptosystem known today
- not handy, key as long as message
- key only valid for one transmission
- how to send the key in a secure manner?
M: 1 0 1 0 1 0 1 0
K: 1 0 0 0 1 1 1 0
S: 0 0 1 0 0 1 0 0
Distribute key over secure channel
MM S
S: 0 0 1 0 0 1 0 0
K: 1 0 0 0 1 1 1 0
M: 1 0 1 0 1 0 1 0
XOR XOR
Cryptology – General View

8. Asymmetrical (public-key) cryptosystems:
- First implementation of RSA in 1978
- Very convenient, Internet
- Idea is based on computational complexity
- rely on unproven assumptions
Private Public
MessageMessage Encrypted message
Cryptology – General View

9. Quantum Cryptography - How it came up
 Quantum Cryptography is an effort to allow two users
of a common communication channel to create a body
of shared and secret information. This information,
which generally takes the form of a random string of
bits, can then be used as a conventional secret key for
secure communication.
 Quantum cryptography is the science of exploiting
quantum mechanical properties to perform
cryptographic tasks.
 The best known example of quantum cryptography is
quantum key distribution which offers an information-
theoretically secure solution to the key exchange
problem.

10. Theoretical Background
 Quantum Key Distribution (QKD) uses quantum mechanics
to guarantee secure communication. It enables two parties to
produce a shared random secret key known only to them,
which can then be used to encrypt and decrypt messages. It
is often called quantum cryptography
 An important and unique property of quantum key distribution
is the ability of the two communicating users to detect the
presence of any third party trying to gain knowledge of the
key.
 Quantum - minimum amount of any physical entity
 Photon Polarization - Quantum Superposition
 2 orthogonal states:
1. Vertical-Horizontal
2. Diagonal +- 45 degrees

11. Filter to distinguish
polarized photons
Correct Filter
applied
Wrong Filter
applied
Theoretical Background

12. • PBS – Photon Beam Splitter
Theoretical Background

13. • The Heisenberg Uncertainty Principle states that we
do not know exactly what will happen to each individual
photon, for in the act of measuring its behavior, it alters
its properties. That means - “observation causes
perturbation”
• The no-cloning theorem states that it is impossible to
create an identical copy of an arbitrary unknown
quantum state.
• Quantum entanglement is a physical phenomenon that
occurs when pairs or groups of particles are generated
or interact in ways such that the quantum state of each
particle cannot be described independently — instead, a
quantum state may be given for the system as a whole.
Theoretical Background

14. The Heisenberg Uncertainty principle, no-
cloning theorem and quantum
entanglement can be exploited for secured
communication in quantum Cryptography.
Theoretical Background

16. History of Quantum Cryptography
• Stephen Wiesner – early 1970s wrote paper "Conjugate
Coding”
• Paper by Charles Bennett and Gilles Brassard in 1984 is
the basis for Quantum Key Distribution (QKD) protocol
BB84. Prototype developed in 1991
• Another QKD protocol was invented independently by
Artur Ekert in 1991

17. Quantum Key Distribution Protocol - BB84
• First quantum cryptography protocol
• Goal: describe a scheme of two users who want
to communicate and exchange data securely.
• Idea: distribute a key securely, based on the laws
of physics.
• Security proofs:
– If someone reads the state of photon -> state
changes○ Not possible to copy the photon in order to
encode it with
– all possible ways (basis)

18. Quantum Key Distribution - Example
Step-1 :
To begin creating a key, Alice sends a photon
through either ‘0’ or ‘1’ slot of the rectilinear or
diagonal polarizing filters, while making a
record of previous orientations

19. Step- 2 :
For each incoming bit, Bob chooses randomly which filter
slot he uses for detection and writes down both the
polarization and bit values.
Quantum Key Distribution - Example

20. Step- 3 :
If Eve, the eavesdropper, tries to spy on the train of photons,
quantum mechanics prohibits her from using both filters to
detect the orientation of a photon. If she chooses the wrong
filter, she may create errors by modifying their polarization.
If Eve has intruded the
communication, she will
DEFINITELY left some traces due to
Heisenberg Uncertainty Principle
(HUP) and non cloning theorem
Quantum Key Distribution - Example

21. Step- 4 :
After all the photons have reached Bob, he tells Alice over
public channel (telephone, email) the sequence of filters he
used for the incoming photon but not the bit values of
photons.
Quantum Key Distribution - Example

22. Step- 5 :
Alice tells Bob during the same conversation which filter she
chose correctly. Those instances constitute the bits that
Alice and Bob will use to form the key that they will use to
encrypt message.
Key : 0 0 1 1 1
Quantum Key Distribution - Example

24. ATTACKS
• In Quantum Cryptography, traditional
man-in-the-middle attacks are impossible due to
the Observer Effect
• If Alice and Bob are using an entangled photon
system, then it is virtually impossible to hijack
these, because creating entangled photons
would be easily detected

25. Vulnerabilities - Photon Number attack
Cause
• If more than one photon for each bit is sent
– Eve can steal extra photons to extract the stolen
photons information
Measure
• Ensure photon splitter only sends exactly ONE
photon at each time
• Single photon ensures quantum mechanic laws are
satisfied

26. Vulnerabilities - Spectral attack
Cause
• If photons are created by DIFFERENT laser photo
diodes, they have different spectral characteristics.
– Eve performs spectral attack by measuring COLOR,
and not polarization
Measure
Use single laser photo diode

27. Vulnerabilities – Random Numbers
Cause
• Are our random numbers really "Random"?
• Bob side, randomness is determined by Beam Splitter
• Alice side, randomness if a bit stream cannot be proven
mathematically
– "random" sequences by following specific patterns,
Algorithms generate NOT that random!
– Eve can use same algorithm to extract information
Measure
Entangled Photon
Pairs comes to the
rescue
(discussed in theoretical
background)

28. Vulnerabilities – Fake State Attack
Measure
• One possible solution - apply classical cryptography to ensure the
message’s authenticity.
• Another solution - uses trusted certificates created by quantum
mechanics
FSG: Fake State Generator

29. Noise
• Noise might introduce errors
• A detector might detect a photon even though there are
no photons
• Solution:
– send the photons according to a time schedule.
– then Bob knows when to expect a photon and can
discard those that doesn't fit into the scheme's time
window

30. Privacy Amplification
• Eve might have partial knowledge of the key
• Suppose there are n bits in the key and Eve has
knowledge of m bits.
• Alice randomly chose a hash function where
h(x): {0,1}n  {0,1} n-m-s
• Reduces Eve's knowledge of the key to 2–s / ln2 bits

31. Main Contribution of Quantum
Cryptography
• It solved the key distribution problem
• Once key is securely received it can be
used to encrypt/decrypt messages
transmitted by conventional channels

32. Security of Quantum Key
Distribution
• Quantum cryptography obtains its fundamental
security from the fact that
– each qubit is carried by a single photon, and
– each photon will be altered as soon as it is read
• This makes impossible to intercept message
without being detected

33. Security of Quantum Cryptography
• Eavesdropping on quantum signals can be detected
• Key generated from high-fidelity entangled states is private
• Using quantum error correction, high-fidelity entanglement
can be distilled from noisy entanglement
• “Prepare and measure” quantum key distribution,
augmented by error correction and privacy amplification is
secure (against any attack) if the bit error rate is low
• Quantum technologies are available today
• Other areas in quantum cryptography: digital signatures,
coin flipping, data hiding, etc

34. • Experimental implementations going on since
1990
• In 2004, QC is performed over distances of
30-40 km using optical fiber
• In general we need two capabilities:
(1)Single photon gun
(2) Being able to measure single photons
State of the Quantum
Cryptography Technology

35. Working Prototypes
• Quantum cryptography has been tried
experimentally over
– fibre-optic cables and,
– more recently, open air (23km)
RIGHT: The first prototype
implementation of quantum
cryptography
(IBM, 1989)

36. Pros & Cons
• Nearly Impossible to
steal
• Detect if someone is
listening
• “Secure”
• Distance Limitations
• Availability
– vulnerable to DOS
– keys can’t keep up with
plaintext

37. Future Scope (Prospects)
• The experiments suggests transmission to satellites
is possible, due to the lower atmospheric density at
higher altitudes
• The current commercial systems are aimed mainly at
governments and corporations with high security
requirements
• Factors preventing wide adoption of quantum
cryptography outside high security areas include the
cost of equipment, and the lack of a demonstrated
threat to existing key exchange protocols

38. CONCLUSION
 QKD systems are unconditionally secure, based on
the fundamental laws of physics
 However, physical realisations of those systems
violate some of the assumptions of the security
proof
 Eavesdroppers may thus intercept some of the sent
messages

39. CONCLUSION
Quantum cryptography is a major achievement
in security engineering
As it gets implemented, it will allow perfectly
secure bank transactions, secret discussions
for government officials, and well-guarded
trade secrets for industry!