Cryptography

1. Cryptography Instructor: Dr. Yanqing Zhang Presented by: Rajapaksage Jayampthi S

2. Outline • Section I (Theory) • Introduction • Symmetric Key Cryptography • Examples • Key Issues • Public Key Encryption • Algorithms • Comparison of Cryptographic systems • Hybrid Secret-Public Key Cryptography • Section II (Recent Work) • Quantum Cryptography: A New Generation of Information Technology Security System [Mehrdad S. Sharbaf, 2009] • Section III (Future Work)

3. Section I Theory

4. Introduction • Intruders can get the encrypted data, but can not do anything with it. • Encryption techniques are published, standardized and available to every one. • Must be some bit of secret information (key) that prevent an intruder from decrypting .

5. Alice’s encryption key Bob’s decryption key encryption algorithm decryption algorithm ciphertext plaintext plaintext K K A B Introduction (contd.) • Symmetric key cryptography: • encryption and decryption keys are identical. • the key must be kept secret. • The encryption and decryption functions used can be the same or different. • Public key cryptography: • different keys for encryption and decryption (one public, the other private).

6. Symmetric Key Cryptography • Cryptographic algorithms involve substituting one thing for another, in many possible ways. • Caesar cipher: • Substitution with an offset of β for all letters • Eg if β = 4 then a-> d b-> e • there are only 25 possible keys available. • Easy to break. • monoalphabetic cipher: substitute one letter for another; now there are 26! possibilities. • polyalphabetic cipher: plaintext: abcdefghijklmnopqrstuvwxyz ciphertext: mnbvcxzasdfghjklpoiuytrewq Plaintext: bob. i love you. alice ciphertext: nkn. s gktc wky. mgsbc

7. Symmetric Key Cryptography: Examples • Examples: • ROT13: Very simple rotation algorithm • Caesar cipher: Another (better) rotation algorithm • crypt: Original Unix encryption program • DES: Data Encryption Standard [NIST 1993] • AES: Advanced Encryption Standard • Skipjack: U.S. National Security Agency developed algorithm (classified) • DES:Data Encryption Standard • In 1997 DES was cracked in only 140 days by a team • In 1999 DES was cracked in little over 22 hours by a network of volunteers and special purpose computer.

8. Symmetric Key Cryptography (contd.) How to break simple encryption scheme • Brute force: attempt all possibilities • Simple with the Caesar cipher, but gets quite difficult with monoalphabetic or polyalphabetic ciphers. • Ciphertext-only attack: use statistics and other information to decrypt intercepted ciphertext • Known-plaintext attack: if some of the plaintext is known, one could uncover some of the plaintext-ciphertext mappings, making decryption easier. • Chosen-plaintext attack: the intruder can choose the plaintext message and receive the ciphertext form. • Can break the encryption scheme.

9. K K A-B A-B encryption algorithm decryption algorithm ciphertext plaintext plaintext message, m K (m) A-B K (m) m = K ( ) A-B A-B Symmetric Key Cryptography: Key Issues • How do sender and receiver agree on key value? • How is the agreed upon key distributed to both sender and receiver in a secure fashion?

10. Public Key Encryption • Diffie-Hellman 1976: the first public key approach proposed. • Sender and receiver do notshare secret key • Public key is available to every one • Private key is known by only receiver

11. + Bob’s public key K B - Bob’s private key K B + K (m) B encryption algorithm decryption algorithm plaintext message plaintext message, m ciphertext - + m = K (K (m)) B B Public Key Encryption (contd.)

12. - Alice’s private key K A + Alice’s public key K A - K (m) A encryption algorithm decryption algorithm plaintext message plaintext message, m ciphertext + - m = K (K (m)) A A Public Key Encryption (contd.)

13. K (K (m)) = m - B B + K (K (m)) - + = A A Public Key Encryption (contd.) • Result is the same • if one key can decrypt a message, it must have been encrypted by the other. • It must be extremely difficult, if not impossible, to deduce the private key when given a public key.

14. Public Key Encryption Algorithms • Diffie-Hellman: the first public key approach proposed. • RSA: the best known public key system, developed by Rivest, Shamir, and Adleman (hence RSA). • DSA: Digital Signature Algorithm, developed by the U.S. National Security Agency (NSA).

15. Comparison of Cryptographic systems • With suitable keys and algorithms, both methods can be secure enough for most purposes. • To use symmetriccryptography, both parties must know the secret key, which can be quite inconvenient. • To use public key cryptography, one only needs to find the public key to communicate with someone else, which can be a lot more convenient. • Encrypting and decrypting a lot of information with public key cryptography can be painfully slow in comparison to symmetric cryptography.

16. Hybrid Secret-Public Key Cryptography • combine the strengths of symmetric and public key cryptography, and avoid their weaknesses. • When two parties want to communicate securely, public key cryptography is used to exchange a random symmetric session key. • Since the session key is encrypted, we can ensure secrecy and mutual authentication. • Since secret key cryptography is used, this can be done relatively efficiently. • When done, both parties destroy the session key. If communication is required in the future, this process is repeated from the beginning to obtain a completely new session key.

17. Section II

18. Introduction • Apply the phenomena of quantum physics • Relies on • The Heisenberg Uncertainty principle • The principle of photon polarization • classical cryptography • communicating parties need to share the keys • protocols based on mathematical algorithms introduce security holes • rarely on refresh their cryptography keys • unproven computational assumptions • Not efficient • Can break

19. Quantum Cryptography • What are qubits? • both in state 0 and state 1 can exists • In classical register composed of three bits can store in a given moment of time only one out of eight different numbers • register composed of three qubits can store in a given moment of time all eight numbers in a quantum superposition

20. Quantum Cryptography (contd.) • Why Quantum Cryptography is secure? • when measuring the polarization of a photon, the choice of what direction to measure affects all subsequences measurements. • photons can be easily polarized (by photon polarization principle) • intruder can not copy unknown qubits (no-cloning theorem). • presence of the intruder can be determined • Harvard, and Boston University built the DARPA quantum network, the world’s first network that delivers end-to-end network security via highspeed quantum key distribution, and tested that network against sophisticated eavesdropping attacks.

21. Section III Future Work

22. Future Direction of Quantum Cryptography • Distance limitation • quantum key distribution distances are limited to tens of kilometers because of optical amplificationdestroys the qubit state. • Develop optical devices capable of generating, detecting and guiding single photons. • Lack of a security certification process or standard for the equipment. • Reassurance QKD is theoretically sound. (By experiments)

23. Referances • [1].http://en.wikipedia.org/wiki/Quantum_Cryptography • [2]. Mehrdad S. Sharbaf,” Quantum Cryptography: A New Generation of Information Technology Sec urity System”, 2009 IEEE • [3]. Computer Networking A Top-Down Approach Featuring the Internet James F. Kurose and Keith W. Ross • [4].http://www.quantiki.org/wiki/index.php/What_is_Quantum_Computation%3F • [5].http://www.quantiki.org/wiki/index.php/Shor%27s_factoring_algorithm