1. Quantum Cryptography
Arjun Vinod
S3 EC
Roll No:17

2. Introduction
Quantum cryptography is the single most successful application of Quantum Computing/Information Theory.
For the first time in history, we can use the forces of nature to implement perfectly secure cryptosystems.
It relies on 2 major elements of quantum mechanics:
i.e heisenberg uncertainity principle and principle of photon polarization.

3. Need of quantum cryptography
Classical Cryptography relies heavily on the complexity of factoring integers.
Quantum Computers can use Shor’s Algorithm to efficiently break today’s cryptosystems.
We need a new kind of cryptography!

4. Basic idea in 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
Cryptosystem (Cipher System) – method of disguising messages so that only certain people can read them
Cryptography – Art of creating and using Cryptosystems
Cryptanalysis – Art of breaking Cryptosystems
Cryptography – study of Cryptography and Cryptosystems

5. The Process Sender Recipient Key Plaintext Encryption Cryptotext
Secure
transmission
Cryptotext
Decryption
Key ready for use
Recipient
Plaintext
Message encryption
Secure key distribution
Hard Problem for conventional
encryption

6. The Classic Cryptography
Encryption algorithm and related key are kept secret.
Breaking the system is hard due to large numbers of possible keys.
For example: for a key 128 bits long
there are
keys to check using brute force.
The fundamental difficulty is key distribution to parties
who want to exchange messages.

7. Factoring A Product Of Two Large Primes
The best known conventional algorithm requires the solution time proportional to:
For p & q 65 digits long T(n) is approximately
one month using cluster of workstations.
For p&q 200 digits long T(n) is astronomical.

8. Quantum Computing Algorithm For Factoring.
In 1994 Peter Shor from the AT&T Bell Laboratory showed that in principle a quantum computer could factor a very long product of primes in seconds.
Shor’s algorithm time computational complexity is
Once a quantum computer is built the RSA method would not be safe.

9. Binary Information
A user can suggest a key by sending a stream of randomly polarized photons.
This sequence can be converted to a binary key.
If the key was intercepted it could be discarded and a new stream of randomly polarized photons sent.

10. Quantum Key Distribution
Quantum Key Distribution exploits the effects 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.

11. 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.

12. Noise The presence of noise can impact detecting attacks.
Eavesdropper and noise on the quantum channel are indistinguishable.
(1) Malicious eavesdropper can prevent communication.
(2) Detecting eavesdropper in the presence of noise is hard.

13. The Main contribution of Quantum Cryptography.
It solved the key distribution problem.
Unconditionally secure key distribution method proposed by: Charles Bennett and Gilles Brassard in 1984.
The method is called BB84.
Once key is securely received it can be used to encrypt messages transmitted by conventional channels.

14. State of the Quantum Cryptography Technology.
Experimental implementations have existed since 1990.
Current (2004) QC is performed over distances of kilometers using optical fiber.
In general we need two capabilities.
Single photon gun.
(2) Being able to measure single photons.

15. Conclusion
Quantum cryptography promises to revolutionize secure communication by providing security based on the fundamental laws of physics, instead of the current state of mathematical algorithms or computing technology.
The devices for implementing such methods exist and the performance of demonstration systems is being continuously improved.
Within the next few years, if not months, such systems could start encrypting some of the most valuable secrets of government and industry.

16. Thank You