Quantum cryptography utilizes quantum mechanics to enable perfectly secure cryptosystems, safeguarding against potential threats from future quantum computers that could break classical encryption methods. Key concepts include quantum key distribution (QKD) techniques such as BB84, which encodes information in photon polarization and detects eavesdropping through measurement alterations. The technology is being developed for practical applications, including mobile phone security, despite challenges such as high costs and range limitations.

1. Quantum Cryptography
Harsh Dawar
Sumit Kadyan

2. •Introduction
• Quantum cryptography is the one of the
most successful application of Quantum
Computing/InformationTheory.
• For the first time in history, we can use
the forces of nature to implement
perfectly secure cryptosystems.
• Quantum cryptography has been proved
to work practically.

3. •State of the art
• ClassicalCryptography(RSA) relies heavily on
the complexity of factoring integers.
• Future Quantum Computers can use Shor’s
Algorithm to efficiently break today’s
cryptosystems.
• Quantum computers can solve factoring
problems in polynomial time.
What happens when that day arrives?

4. •Today’s Talk
• Basic Ideas in
Cryptography
• Ideas from the
QuantumWorld
• Quantum Key
Distribution (QKD)
• BB84 without
eavesdropping
• BB84 with
eavesdropping
• Working Prototypes
• Practical
Implementation of
QC
• Conclusion

5. •Basic Ideas of Cryptography
• Cryptography: “the coding and decoding of secret
messages.”
• The basic idea is to modify a message so as to make
it unintelligible to anyone but the intended
recipient.
• For message (plaintext) M,
e(M, K) encryption - ciphertext
d[e(M, K), K] = M decryption

6. •Keys and key distribution
 K is called the key.
 The key is known only to
sender and receiver: it is
secret.
 Anyone who knows the key
can decrypt the message.
 Key distribution is the
problem of exchanging the
key between sender and
receiver.

7. •Perfect Secrecy and OTP
 There exist perfect
cryptosystems.
 Example: One-Time
Pad (OTP)
 The problem of
distributing the keys in
the first place remains.
 Key length has to be
same as message
length!!

8. •Quantum Key Distribution
 Using the laws of quantum mechanics, we can
distribute keys in perfect secrecy!!
 The Result:The Perfect Cryptosystem,
QC = QKD + OTP

9. •Quantum Physics Basics
 Physical Qubits
 Any subatomic
particle can be used to
represent a qubit, e.g.
an electron.
 A photon is a
convenient choice.
 A photon is an
electromagnetic
wave.

10. •Polarization
• A photon has a property called
polarization, which is the plane in which the
electric field oscillates.
• We can use photons of different polarizations
to represent quantum states:


0 state 0
90 state 1

11. •Ideas from Quantum World
 Measurement
 Observing, or measuring, a
quantum system will alter its
state.
 Example: the Qubit
 When observed, the state of a
qubit will collapse to
unpredictable random state 1 or
0.

12. •Polarizers & Basics
• A device called a polarizer allows us to place a
photon in a particular polarization
• The polarization basis is frame of reference for
quantum measurement.

13. •Measuring Photons
• A calcite crystal can be used to recover the
bits encoded into a stream of photons.
13
CaCO3
Diagonal
axis (X)
1 0 1 0

14. •Uncertainty Principle
• What if the crystal has the wrong
orientation?
14
CaCO3
RECTILINEAR
axis(+)
???
50% chance
of getting
right answer.
Either | or ---

15. •No Cloning theorem
• It is impossible to make a device that perfectly copies an unknown
qubit.
• Suppose there is a quantum process that implements: |q,_> ->|q,q>

16. •Quantum Key Distribution
• Quantum Key Distribution exploits the
effects discussed in order to thwart
eavesdropping.
• If an eavesdropper uses the wrong
polarization basis to measure the channel, the
result of the measurement will be random.

17. •QKD Protocols
• A protocol is a set of rules governing the
exchange of messages over a channel.
• There are three main security protocols for
QKD: BB84, B92, and Entanglement-Based
QKD.
• Lets discuss the BB84 in detail.

18. •BB84
• BB84 was the first security protocol
implementingQuantum Key Distribution.
• It uses the idea of photon polarization.
• The key consists of bits that will be
transmitted as photons.
• Each bit is encoded with a random
polarization basis!

19. •BB84 With no Eavesdropping
• Alice is going to send Bob a key.
• She begins with a random
sequence of bits.
• Bits are encoded with a random
basis, and then sent to Bob:
Bit 0 1 0 1 1
Basis + × × + ×
Photon

20. •BB84 With no Eavesdropping
• Bob receives the photons and must decode
them using a random basis.
• Some of his measurements are correct.
Photon
Basis? + + × + ×
Bit? 0 0 0 1 1

21. •BB84 With no eavesdropper
• Alice and Bob talk on the public encrypted
channel:
• 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.

22. •Comparing measurements
Alice’s Bit 0 1 0 1 1
Alice’s
Basis + × × + ×
Photon
Bob’s
Basis + + × + ×
Bob’s Bit 0 0 0 1 1
The test bits allow Alice and Bob
to test whether the channel is
secure.

23. •The Catch
• As long as no errors and/or eavesdropping
have occurred, the test bits should agree.
• Alice and Bob have now made sure that the
channel is secure.The test bits are removed.
• Alice tells Bob the basis she used for the
other bits, and they both have a common set
of bits: the final key!

24. •Deriving the final key
Alice’s Bit 0 1 0 1 1
Alice’s
Basis + × × + ×
Photon
Bob’s
Basis + + × + ×
Bob’s Bit 0 0 0 1 1
Final Key = 01
Test bits
discarded

25. •In the presence of eavesdropper
• If an eavesdropper Eve tries to tap the channel, this
will automatically show up in Bob’s measurements.
• In those cases where Alice and Bob have used the
same basis, Bob is likely to obtain an incorrect
measurement: Eve’s measurements are bound to
affect the states of the photons.

26. Eavesdropper Effect

27. •In the presence of Eavesdropper
• As Eve intercepts Alice’s photons, she has to
measure them with a random basis and send new
photons to Bob.
• The photon states cannot be cloned (non-
cloneability).
• Eve’s presence is always detected: measuring a
quantum system irreparably alters its state.

28. •QKD - Review

29. •BB84 – Key Distillation
 Sifting –Unmatched Bases;
“stray” or “lost” qubits
 Error Correction – Noise &
Eaves-dropping detected –
Uses “cascade” protocol –
Reveals information to Eve so
need to track this.
 Privacy Amplification – reduces
Eve’s knowledge obtained by
previous EC
 Authentication – avoid man-in-
middle attacks .Many protocols
in development!!

30. Researchers at Nokia and the University of Bristol in
UK have come up with quantum cryptography in
mobile phones.
Goal:
To perform "quantum key distribution" (QKD)
using simple and potentially inexpensive
"client" electronics that could be integrated
within a single chip.

31. •Making use of twisting and turning
• Makes use of the Reference
independent frame QKD[RfiQKD]
• Polarization or phase is measured in
context with a reference frame
• They select a specific threshold value
and drop below this value signals there
is an eavesdropper .

32. •Advantages
• It will help protect the password that are
transmitted using mobile phones
• Nokia has patented the technology and
looking forward to engineer into mobile
devices.

33. •Quantum City Project(Durban)
• 4 Node network connected in a star
configuration.
• Quantum Links as long as 26Kms
• Data Transmitted : data
records, telephones, and internet traffic.
• 99.3% of the time the system functioned
effectively and was stable.

34. •Limitations of QC
• Expensive :- Quantum Cryptography needs
expensive dedicated equipment to be set up
before keys can be exchanged.
Approx. 25 Millions $ for 2000 Km network!!
Survey of 500 prospective QC users show they
prefer QC for banking/military purpose.

35. •Limitations
• Range :- The current Range of QC systems
are a maximum of 250 kms. Lot of active
research is being done to increase range!!
• Since one cant use
repeaters or amplifiers
Trusted nodes are
needed to extend the
Range.

36. •Breaking Quantum Crypto
• Break underlying cryptography??
• Not possible!! Governed by law of physics
• Attack the implementation
• Hardware Attack
What if the laser generates more than
one photon

37. •Breaking Quantum Crypto
• Measure the color of the spectrum and
not the state of photon!!

38. •Future of Quantum Cryptography
• QKD with satellites:-
Research groups are actively working to
demonstrate key exchange with satellites.

39. •Thank you!!