CSCI6268L08 - 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 #8 Sep 14 th 2009 CSCI 6268/TLEN 5550, Fall 2009
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The Ubiquitous HMAC The most widely-used MAC (IPSec, SSL, many VPNs) Doesn’t use a blockcipher or any universal hash family Instead uses something called a “collision resistant hash function” H Sometimes called “cryptographic hash functions” Keyless object – more in a moment • HMAC K (M) = H(K opad || H(K ipad || M)) opad is 0x36 repeated as needed ipad is 0x5C repeated as needed
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Notes on HMAC Fast Faster than CBC MAC or XCBC Because these crypto hash functions are fast Slow Slower than UMAC and other universal-hash-family MACs Proven security But these crypto hash functions have recently been attacked and may show further weaknesses soon
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What are cryptographic hash functions? Output Message e.g., MD5,SHA-1 Hash Function A cryptographic hash function takes a message from {0,1} * and produces a fixed size output Output is called “hash” or “digest” or “fingerprint” There is no key The most well-known are MD5 and SHA-1 but there are other options MD5 outputs 128 bits SHA-1 outputs 160 bits % md5 Hello There ^D A82fadb196cba39eb884736dcca303a6 %
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T A << 5 + g t (B, C, D) + E + K t + W t SHA-1 ... M 1 M 2 M m for i = 1 to m do W t = { t-th word of M i 0 t 15 ( W t-3 W t-8 W t-14 W t-16 ) << 1 16 t 79 A H 0 i-1 ; B H 1 i-1 ; C H 2 i-1 ; D H 3 i-1 ; E H 4 i-1 for t = 1 to 80 do E D; D C; C B >> 2; B A; A T H 0 i A + H 0 i-1 ; H 1 i B + H 1 i-1 ; H 2 i C+ H 2 i-1 ; H 3 i D + H 3 i-1 ; H 4 i E + H 4 i-1 end end return H 0 m H 1 m H
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CSCI6268L08 - Foundations of Network and Computer Security...

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