Hash and MAC Algorithms Dr. Monther Aldwairi New York Institute of Technology- Amman Campus 12/3/2009 INCS 741: Cryptography 12/3/20091Dr. Monther Aldwairi.

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Presentation transcript:

Hash and MAC Algorithms Dr. Monther Aldwairi New York Institute of Technology- Amman Campus 12/3/2009 INCS 741: Cryptography 12/3/20091Dr. Monther Aldwairi

Hash and MAC Algorithms Hash Functions – condense arbitrary size message to fixed size – by processing message in blocks – through some compression function – either custom or block cipher based Message Authentication Code (MAC) – fixed sized authenticator for some message – to provide authentication for message – by using block cipher mode or hash function

Hash Algorithm Structure

Secure Hash Algorithm SHA originally designed by NIST & NSA in 1993 was revised in 1995 as SHA-1 US standard for use with DSA signature scheme – standard is FIPS , also Internet RFC3174 – nb. the algorithm is SHA, the standard is SHS based on design of MD4 with key differences produces 160-bit hash values recent 2005 results on security of SHA-1 have raised concerns on its use in future applications

Revised Secure Hash Standard NIST issued revision FIPS in 2002 adds 3 additional versions of SHA – 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 but security levels are rather higher

SHA-512 Step 1: Append padding bits Step 2: Append length Step 3: Initialize hash buffer Step 4: Process the message in 1024-bit (128- word) blocks, which forms the heart of the algorithm Step 5: Output the final state value as the resulting hash 11/8/2009Dr. Monther Aldwairi6

SHA-512 Overview

SHA-512 Compression Function heart of the algorithm processing message in 1024-bit blocks consists of 80 rounds – updating a 512-bit buffer – using a 64-bit value Wt derived from the current message block – and a round constant based on cube root of first 80 prime numbers

11/8/2009Dr. Monther Aldwairi9

SHA-512 Round Function

Function Elements Ch(e,f,g) = (e AND f) XOR (NOT e AND g) Maj(a,b,c) = (a AND b) XOR (a AND c) XOR (b AND c) ∑(a) = ROTR(a,28) XOR ROTR(a,34) XOR ROTR(a,39) ∑(e) = ROTR(e,14) XOR ROTR(e,18) XOR ROTR(e,41) + = addition modulo 2^64 – Kt = a 64-bit additive constant – Wt = a 64-bit word derived from the current 512-bit input block. 11/8/2009Dr. Monther Aldwairi11

SHA-512 Round Function

Keyed Hash Functions as MACs want a MAC based on a hash function – because hash functions are generally faster – code for crypto hash functions widely available hash includes a key along with message original proposal: KeyedHash = Hash(Key|Message) – some weaknesses were found with this eventually led to development of HMAC

HMAC specified as Internet standard RFC2104 uses hash function on the message: HMAC K = Hash[(K + XOR opad) || Hash[(K + XOR ipad)||M)]] where K + is the key padded out to size and opad, ipad are specified padding constants overhead is just 3 more hash calculations than the message needs alone any hash function can be used – eg. MD5, SHA-1, RIPEMD-160, Whirlpool

HMAC Overview