CSCI6268L04 - 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 #4 Aug 31 st 2009 CSCI 6268/TLEN 5550, Fall 2009
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Let’s build a Better Blockcipher DES – The Data Encryption Standard 64-bit blocksize, 56 bit key Formerly called “Lucifer” Developed by Horst Feistel at IBM in early 70’s Tweaked by the NSA No explanation given for tweaks Some people worried that NSA was adding backdoors/weaknesses to allow it to be cracked! NSA shortened key from 64 bits to 56 bits (definite added weakness) Adopted by NIST (then called NBS) as a Federal Information Processing Standard (FIPS 46-3) NIST is retiring it as a standard this year after nearly 30 years
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The DES Key Was 64 bits But NSA added 8 parity bits Key is effectively only 56 bits! k0 k1 k2 k3 k4 k5 k6 k7 k8 k9 k60 k61 k62 k63 k0 k1 k2 k3 k4 k5 k6 P0 k8 k9 k60 k61 k62 P7
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Exhaustive Key Search -- DES This meant that instead of 2 64 keys there were only 2 56 keys Expected number of keys to search before finding correct value is 2 55 Note that we need a handful of plaintext-ciphertext pairs to test candidate keys NSA surely could do this in a reasonable amount of time, even in the 70’s
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Exhaustive Key Search -- DES In 1994, Michael Wiener showed that you could build a DES-cracking machine for $1,000,000 that would find the key in an expected 3.5 hours In 1998 he revised this to 35 minutes for the same cost In 1997, Rocke Verser used 10,000+ PCs to solve DES Challenge I to win $10,000 (Loveland, CO!) distributed.net solved the DES Challenge II in 41 days with 50,000 processors covering 85% of the keyspace
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This note was uploaded on 03/11/2010 for the course CSCI 6268 taught by Professor Black during the Winter '09 term at University of Colombo.

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CSCI6268L04 - Foundations of Network and Computer Security...

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