Cryptography

1. Cryptography Yiwei Wu

2. Outline • Why have cryptography • History • Modern cryptography • Private key • Public key • Pretty Good Privacy • Conclusion • Reference

3. Why Have Cryptography [1] • In the most abstract sense, we can describe a distributed system as a collection of clients and servers communicating by exchange of messages. • Authentication of principals and messages is the major issue in secure distributed systems.

4. Why Have Cryptography [3,4] • Security Requirements • Confidentiality • Protection from disclosure to unauthorisedpersons • Integrity • Maintaining data consistency • Authentication • Assurance of identity of person or originator of data • Availability • Legitimate users have access when they need it • Access control • Unauthorisedusers are kept out

5. History [2, 3,4] • Earliest recorded us around 1900BC in Egypt • Atbash cipher (Old Testament, reversed Hebrew alphabet, • 600BC) • Around 100BC Julius Caesar used substitution cipher • letter = letter + 3 => ‘fish’ -> ‘ilvk’ • Cipher Machines • most notable Enigma machine in WWII • Block Ciphers • Originated with early 1970’s IBM effort to develop banking security systems • 1970’s - Dr. Horst Feistal invented DES • RSA 1977 • Rivest-Shamir-Adelman

6. Modern cryptography [4] • Private key cryptography • Problem of communicating a large message in secret is reduced to communicating a small key in secret

7. Private key cryptography [1] • Encryption algorithm E turns plain text message M into a cipher text C • C = E(M) • Decrypt C by using decryption algorithm D which is an inverse function of E • M = D(C)

8. Private key cryptography [1] • Confidentiality kept by keeping algorithms secret. • Not practical over distributed systems – too many algorithms. • Solution is to decompose algorithm • Function - public • Key – private

9. Private key cryptography [1] • Encryption algorithm with secret key Ke • Decryption key Kd • M=Dkd(Eke(M)) • The function must have the properties that different messages with the same key and a same message with different keys will result in distinct cipher text. • It is easy to compute the cipher text from the plaintext but difficult the other way.

10. Private key cryptography [4] • DES – developed by IBM, 1977 • Key size = 56 bits • Brute force = 255 attempts • The plaintext is broken down into 64 bit blocks • DES was designed for efficiency in early-70’s hardware. • Made it easy to build pipelined brute-force breakers in late-90’s hardware • EFF (US non-profit organization) broke DES in 2½ days • AES • Advanced Encryption Standard, replacement for DES

11. Private key cryptography [4] • Hash Functions • Creates a unique “fingerprint” for a message • Hash has to be protected in some way

12. Private key cryptography [4] • Message authentication codes (MACs) • secret key is used to authenticate the hash value

13. Public key cryptography [4] • A significant disadvantage of symmetric ciphers is the key management necessary to use them securely. • Uses matched public/private key pairs • Anyone can encrypt with the public key, only one person can decrypt with the private key

14. Public key cryptography [1, 2] • Each principal keeps a set of encryption keys (Ke & Kd) • Encryption algorithm E is public and so is the key Ke • Decryption algorithm D and decryption key Kd is kept private • Data sent to a principal is encrypted using that corresponding Ke • E and D can be made public if Ke and Kd are chosen such that it is impossible to infer Kd from Ke.

15. Public key cryptography [1, 2] • RSA (Rivest-Shamir-Adelman), 1977 • The algorithms E and D are inverses • Plain text messages are limited to a size is limited to k bits • Integer k is chosen such that 2k < N • N =p * q where p & q are LARGE prime numbers • Kp (public encyrption key) and Ks (private decryption key) are derived from p & q • Relies on computational complexity in factoring large numbers upon which keys are placed.

16. Public key cryptography [4] • public-key cryptography can be used to implement digital signature schemes

17. digital signature [4] • Signature checking:

18. Public key cryptography [4] • DH (Diffie-Hellman), 1976 • Key exchange algorithm • the discrete logarithm problem • Elgamal • DH variant, one algorithm for encryption, one for signatures • Attractive as a non-patented alternative to RSA (before the RSA patent expired)

19. Pretty Good Privacy (PGP) [7] • PGP encryption uses public-key cryptography and includes a system which binds the public keys to a user name and/or an e-mail address.

20. Pretty Good Privacy (PGP) [5,6,8] • Early history • Phil Zimmermann created the first version of PGP encryption in 1991. • PGP very rapidly acquired a considerable following around the world. • RSA complains to Phil that PGP violates their PK patents . • USG decides that they don't like PRZ because the NSA can't tap all those internet mail messages anymore. • Zimmermann was criminally investigated by the Customs Service and the FBI for several years. • Investigation was dropped in January 1996 with no charges laid.

21. PGP [8] • How PGP encryption works

22. PGP [8] • How PGP decryption works

23. Conclusion • Private Key DES is computationally efficient • Public Key RSA is computationally expensive • Possible best use is RSA for short/important data and DES for long or less critical • One or more security mechanisms are combined to provide a security service

24. Reference • Chow, Randy; Johnson, Theodore; “Distributed Operating Systems & Algorithms”, 1998 • Aiden A. Bruen and Mario A. Forcinito, “Cryptography, Information Theory, and Error-Correction a Handbook for the 21st Century”, John Wiley & Sons, Inc. (2005) ISBN 0-471-65317-9. • Coron, J.-S., “What is cryptography?” Security & Privacy, IEEEVolume 4,  Issue 1,  Jan.-Feb. 2006 Page(s):70 - 73 • http://www.cs.auckland.ac.nz/~pgut001/tutorial/, Oct. 2008 • http://en.wikipedia.org/wiki/Cryptography, Oct. 2008 • http://www.cypherspace.org/adam/timeline, Oct. 2008 • http://en.wikipedia.org/wiki/Pretty_Good_Privacy, Oct. 2008 • http://www.pgpi.org/doc/pgpintro/, Oct. 2008

25. Q & A