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10 - Lecture#10 SS G513 Network Security Cryptography and...

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Lecture #10 SS G513 Network Security
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Cryptography and Network Security Third Edition by William Stallings Lecture slides by Lawrie Brown
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Chapter 11 – Message Authentication and Hash Functions At cats' green on the Sunday he took the message from the inside of the pillar and added Peter Moran's name to the two names already printed there in the "Brontosaur" code. The message now read: “Leviathan to Dragon: Martin Hillman, Trevor Allan, Peter Moran: observe and tail.” What was the good of it John hardly knew. He felt better, he felt that at last he had made an attack on Peter Moran instead of waiting passively and effecting no retaliation. Besides, what was the use of being in possession of the key to the codes if he never took advantage of it? Talking to Strange Men, Ruth Rendell
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Secure Hash Algorithm (SHA-1) SHA was designed by NIST & NSA in 1993, revised 1995 as SHA-1 US standard for use with DSA signature scheme standard is FIPS 180-1 1995, also Internet RFC3174 nb. the algorithm is SHA, the standard is SHS produces 160-bit hash values now the generally preferred hash algorithm based on design of MD4 with key differences
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SHA Overview 1. pad message so its length is 448 mod 512 2. append a 64-bit length value to message 3. initialise 5-word (160-bit) buffer (A,B,C,D,E) to (67452301,efcdab89,98badcfe,10325476,c3d2e1f0) 1. process message in 16-word (512-bit) chunks: expand 16 words into 80 words by mixing & shifting use 4 rounds of 20 bit operations on message block & buffer add output to input to form new buffer value 2. output hash value is the final buffer value
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SHA-1 Compression Function each round has 20 steps which replaces the 5 buffer words thus: (A,B,C,D,E) <-(E+f(t,B,C,D)+(A<<5)+W t +K t ),A, (B<<30),C,D) a,b,c,d refer to the 4 words of the buffer t is the step number f(t,B,C,D) is nonlinear function for round W t is derived from the message block K t is a constant value derived from sin
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SHA-1 Compression Function
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SHA-1 verses MD5 brute force attack is harder (160 vs 128 bits for MD5) not vulnerable to any known attacks (compared to MD4/5) a little slower than MD5 (80 vs 64 steps) both designed as simple and compact optimised for big endian CPU's (vs MD5 which is optimised for little endian CPU’s)
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Revised Secure Hash Standard NIST have issued a revision FIPS 180-2 adds 3 additional hash algorithms SHA-256, SHA-384, SHA-512 designed for compatibility with increased security provided by the AES cipher structure & detail is similar to SHA-1 hence analysis should be similar
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Keyed Hash Functions as MACs
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