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Module 4 Hash Functions

Module 4 Hash Functions. Highline Community College Seattle University University of Washington in conjunction with the National Science Foundation. Definition of a Hash Function. Originally created to verify that a transmission was successful

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Module 4 Hash Functions

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  1. Module 4Hash Functions Highline Community College Seattle University University of Washington in conjunction with the National Science Foundation

  2. Definition of a Hash Function • Originally created to verify that a transmission was successful • If not, then the transmission would be re-sent • Mainly error detection • Newer ones are for data integrity

  3. CRC – Cyclic Redundancy Check • Checksum • Detects errors after transmission or storage • Problem – if you move a word in the document, but don’t delete it, the checksum remains the same • Cannot be used reliably for data integrity

  4. CRC – (page 2) • Several varieties, 8 bit to 64 bit, a few 128 bit checksums • Polynomials • CRC-1x + 1 (Used in hardware, also known as parity bit) • CRC-5 x5 + x2 + 1 (used in USB token packets) • CRC-7 x7 + x3 + 1 (used in some telecom systems) • CRC-12 x12 + x11 + x3 + x2 + x + 1 (used in telecom systems)

  5. Crytographic Hash Function • Should not be able to predict the hash value of a message • Two messages should not have the same hash value • Any change should result in a different hash value

  6. Message Digest 2 • Created by Ronald Rivest in 1989 • 128 bit hash value • Is still used for public key encryption and digital signatures • Done at MIT

  7. MD4 • Developed by Rivest to address the problems with MD2 • Created in 1990 • 128 bit hash • 32 digit hexadecimal • First a message is padded to be 64 bits shy of a multiple of 512 bits

  8. How MD4 works • 64 bit data stream which contains the length of the original message is now appended to the padded message • A four word (32 bits each) buffer is used to generate the message digest • Process each 16 word block of the message • Output the MD • http://www.faqs.org/rfcs/rfc1320.html

  9. MD5 • Developed in 1991 to address weaknesses in MD4 • 128 bit cryptographic hash • Very similar to MD4, but with some changes • Still reliable for data integrity, but not for malicious attacks

  10. One MD5 operation — MD5 consists of 64 of these operations, grouped in four rounds of 16 operations. F is a nonlinear function; one function is used in each round. Mi denotes a 32-bit block of the message input, and Ki denotes a 32-bit constant, different for each operation. http://en.wikipedia.org/wiki/MD5

  11. Collisions with MD5 • August 2004 collisions for the full MD5 were announced by Xiaoyun Wang, Dengguo Feng, Xuejia Lai and Hongbo Yu. • Their analytical attack was reported to take only one hour on an IBM p690 cluster. (Up to 1TB of ECC Chipkill system memory)

  12. SHA Group • SHA-0 Developed in 1993 • Secure Hash Algorithm • Pushed by the NSA • Problems arose, replaced by SHA-1 • 160 bit digest • SHA-0 had near collisions with 142 of the 160 being equal

  13. One iteration within the SHA-1 compression function. A, B, C, D and E are 32-bit words of the state; F is a nonlinear function that varies; <<< denotes a left bit rotation by s places; s varies for each operation. The red square denotes addition modulo 232. Kt is a constant.

  14. Collisions with SHA-1 • In February 2005, an attack by Xiaoyun Wang, Yiqun Lisa Yin, and Hongbo Yu was announced. • The attacks can find collisions in the full version of SHA-1, requiring fewer than 2^69 operations http://en.wikipedia.org/wiki/SHA_hash_functions

  15. What’s Next? • Stronger hashes • SHA-256, SHA-384, SHA-512 • Does it affect you? • Maybe…

  16. As an Expert Witness • Feel free to tell what you use hashes for • Do not attempt to describe them, simply say I am not a mathematician • Do say, it is approved by my agency and by the NSA or NIST

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