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CSCI6268L05 - Foundations of Network and Computer Security...

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Foundations of Network and Foundations of Network and Computer Security Computer Security J J ohn Black Lecture #5 Aug 31 st 2009 CSCI 6268/TLEN 5550, Fall 2009
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How about Triple-DES! Triple DES uses a 168-bit key K=(K1, K2, K3) TDES K (P) = DES K3 (DES K2 (DES K1 (P))) No known attacks against TDES Provides 112-bits of security against key-search Widely used, standardized, etc More often used in “two-key triple-DES” mode with EDE format (K is 112 bits like DDES): TDES K (P) = DES K1 (DES -1 K2 (DES K1 (P))) Why is the middle operation a decipherment?
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AES – The Advanced Encryption Standard If TDES is secure, why do we need something else? DES was slow DES times 3 is three times slower 64-bit blocksize could be bigger without adding much cost DES had other annoying weakness which were inherited by TDES We know a lot more about blockcipher design, so time to make something really cool!
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AES Competition NIST sponsored a competition Individuals and groups submitted entries Goals: fast, portable, secure, constrained environments, elegant, hardware-friendly, patent- free, thoroughly analyzed, etc Five finalists selected (Aug 1999) Rijndael (Belgium), MARS (IBM), Serpent (Israel), TwoFish (Counterpane), RC6 (RSA, Inc) Rijndael selected (Dec 2001) Designed by two Belgians
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AES – Rijndael Not a Feistel construction! 128 bit blocksize 128, 192, 256-bit keysize SP network Series of invertible (non-linear) substitutions and permutations Much faster than DES About 300 cycles on a Pentium III A somewhat risky choice for NIST
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Security of the AES Four rounds are easily broken via the “Square Attack” Within the scope of this class, but we won’t do it Six rounds breakable, but quite a bit harder Initial proposal was therefore using 8 rounds Objections during assessment phase caused designers to go to 10 rounds
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Block Ciphers – Conclusion There are a bunch out there besides AES and DES Some are pretty good (IDEA, TwoFish, etc) Some are pretty lousy LOKI, FEAL, TEA, Magenta, Bass-O-Matic If you try and design your own, it will probably be really really bad Plenty of examples, yet it still keeps happening
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Aren’t We Done?
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