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Chapter 12: Hash and MAC Algorithms

Chapter 12: Hash and MAC Algorithms Fourth Edition by William Stallings Lecture slides by Lawrie Brown (modified by Prof. M. Singhal, U of Kentucky) Hash and MAC Algorithms Hash Functions condense arbitrary size message to fixed size by processing message in blocks

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Chapter 12: Hash and MAC Algorithms

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  1. Chapter 12: Hash and MAC Algorithms Fourth Edition by William Stallings Lecture slides by Lawrie Brown (modified by Prof. M. Singhal, U of Kentucky)

  2. 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

  3. Hash Algorithm Structure

  4. Secure Hash Algorithm (SHA) • SHA originally designed by NIST & NSA in 1993 • was revised in 1995 as SHA-1 • US standard • based on design of MD4 • produces 160-bit hash values • recent 2005 results on security of SHA-1 have raised concerns on its use in future applications

  5. Revised Secure Hash Standard • NIST issued revision FIPS 180-2 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 is similar • but security levels are rather higher

  6. SHA-512 Overview

  7. 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 Kt that represents the first 64 bits of the fractional parts of the cube root of first 80 prime numbers

  8. SHA-512 Round Function

  9. SHA-512 Round Function

  10. Whirlpool • Whirlpool is a hash function • endorsed by European NESSIE project • 512 bit hash code • uses modified AES internals as compression function • Addresses the concerns on use of block ciphers seen previously • with performance comparable to dedicated algorithms like SHA

  11. Whirlpool Overview

  12. Whirlpool Block Cipher W • designed specifically for hash function use • with security and efficiency of AES • 512-bit block size • similar structure & functions as AES but • Matrix orientation: input is mapped row-wise (rather than column-wise) • has 10 rounds (not 10, 12, or 14) • a different primitive polynomial for GF(2^8) • uses different S-box design & values

  13. Whirlpool Block Cipher W

  14. Whirlpool Block Cipher • Substitute Bytes: a simple table lookup that provides a non-linear mapping. • Shift Columns: causes a circular downwards shift of each column. (ensures that 8 bytes of a row are spread into eight different rows.) • Mix rows: Each byte in a row is mapped into a new value that is a function of all 8 bytes in that row. (achieves diffusion).

  15. Whirlpool Performance & Security • Whirlpool is a very new proposal • hence little experience with use • but many AES findings should apply • does seem to need more h/w than SHA, but with better resulting performance (in terms of throughput).

  16. 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

  17. HMAC • specified as Internet standard RFC2104 • uses hash function on the message: HMACK = 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 -ipad =00110110 (36 in hex) repeated b/8 times -apad=01011100 (5C in hex) repeated b/8 times (b is number of bits in a block) • any hash function can be used • E.g., MD5, SHA-1, RIPEMD-160, Whirlpool

  18. HMAC Overview

  19. HMAC Security • The security of HMAC relates to that of the underlying hash algorithm • attacking HMAC requires either: • brute force attack on key used • choose hash function used based on speed versus security constraints

  20. CMAC • previously saw the DAA (CBC-MAC) • widely used in govt & industry • but has message size limitation • can overcome using 3 keys & padding • thus forming the Cipher-based Message Authentication Code (CMAC) • adopted by NIST SP800-38B

  21. CMAC Overview

  22. Summary • have considered: • some current hash algorithms • SHA-512 & Whirlpool • HMAC authentication using hash function • CMAC authentication using a block cipher

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