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Cryptography Section 8.4

Explore the history of cryptography, including private key and public key systems. Discover the future of cryptography with quantum systems.

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Cryptography Section 8.4

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  1. CryptographySection 8.4 Stacey Levine

  2. What I will cover • History of cryptography • Why cryptography? • Private Key Systems • Public Key Systems • Comparisons and PEM (not) • The future - Quantum Cryptography

  3. History of Crytography • Earliest recorded us around 1900BC in Egypt • Around 100BC Julius Caesar used substitution cipher • 1623 – Sir Francis Bacon described bilateral cipher • A type of steganography (hiding) • Lots of other uses/advances – most notable Enigma machine in WWII • 1970’s - Dr. Horst Feistal invented DES • 1977 magazine The Scientific American – RSA announced • 2007 Quantum Cryptography successfully used to transmit 50 miles [8]

  4. Security concerns • Message passing between authenticated principals • Authenticate message has digital signature

  5. Private Key Systems • 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)

  6. Private Key Systems cont. • Confidentiality kept by keeping algorithms secret. • Not practical over distributed systems – too many algorithms. • Solution is to decompose algorithm • Function - public • Key - private

  7. Private Key Systems cont. • Encryption algorithm with secret key Ke • Decryption key Kd • M=Dkd(Eke(M)) • Requirements of function (algorithm) • Different messages with same key  distinct result • Same message different key  distinct results • Key impossible to infer from plaintext/ciphertext

  8. Private Key Systems cont. • The keys Ke and Kd are different, but it is convenient to choose a key K that can be applied to both. • The longer the key (the more bits) the more secure it is

  9. Private Key Systems cont. • DES – developed by IBM • 56 bit key – sufficient because 256=7.2 * 1016 • According to the book this too large to enumerate with modern computers but our book is from 1998 • The plaintext is broken down into 64 bit blocks • Each block is encrypted using the key • Drawback is that if blocks are repetitive in plaintext, so will the ciphertext be giving a clue to the interlopers. • This can be addressed with chaining – each block is XOR’d with previous encrypted block BEFORE encryption.

  10. Private Key Systems cont. • Private key systems require [n*(n-1)]/2 keys • Keys must be agreed on before secure communication can start. • The keys can be distributed in a key distribution system which will be covered next week.

  11. Public Key Systems • Introduced by Diffie and Hellman • 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 persons Ke

  12. Public Key Systems Cont. • Basically a two key system • It is possible to make E and D public if Ke and Kd are kept private and impossible to infer • RSA uses this approach • E and D are public. And are inverse of each other. • Relies on computational complexity in factoring large numbers upon which keys are placed.

  13. Public Key Systems Cont. • Message is limited to k size 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

  14. Comparisons • 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 • Privacy Enhanced Email (PEM) initiative does this (NOTE: this is gone now..) – basically used certificates • PGP took over

  15. Quantum Cryptography • Based on Quantum theory • The act of observing affects what is being observed • Schrodinger’s Cat • quantum indeterminacy or the observer's paradox

  16. Sending Quantum Message[8] Al Sends Message Interloper Bob Gets Message

  17. References • Chow, Randy; Johnson, Theodore; Distributed Operating Systems & Algorithms, 1998 • http://en.wikipedia.org/wiki/Quantum_cryptography [April 2007] • What is Quantum Physics, http://library.thinkquest.org/3487/qp.html • Elliott, C., Pearson, D., and Troxel, G. 2003. Quantum cryptography in practice. In Proceedings of the 2003 Conference on Applications, Technologies, Architectures, and Protocols For Computer Communications (Karlsruhe, Germany, August 25 - 29, 2003). SIGCOMM '03. ACM Press, New York, NY, 227-238. DOI= http://doi.acm.org/10.1145/863955.863982 • Papanikolaou, N. 2005. An introduction to quantum cryptography. Crossroads 11, 3 (May. 2005), 3-3. DOI= http://doi.acm.org/10.1145/1144396.1144399 • Diffie, W. 2001. Ultimate cryptography. Commun. ACM 44, 3 (Mar. 2001), 84. DOI= http://doi.acm.org/10.1145/365181.365214 • Components for quantum cryptographyZbinden, H.; Ribordy, G.; Stucki, D. Optical Fiber Communication Conference, 2006 and the 2006 National Fiber Optic Engineers Conference. OFC 2006, Vol., Iss., 5-10 March 2006Pages: 3 pp.- • E.S.;”Hack-Proof Internet”, Popular Science Magazine, February 2007, pg 48-49 • http://www.cybercrimes.net/Cryptography/Articles/Hebert.html (April 2007)

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