6.hash-MAC

6.hash-MAC - Cryptographic Hash Functions Message...

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Unformatted text preview: Cryptographic Hash Functions Message Authentication Digital Signatures Abstract We will discuss • Cryptographic hash functions • Message authentication codes – HMAC and CBC-MAC • Digital signatures 2 Encryption/Decryption • Provides message confidentiality . • Does it provide message authentication ? 3 Bob receives a message from Alice, he wants to know (Data origin authentication) whether the message was really sent by Alic e; (Data integrity) whether t h Message Authentication m • g g message authentication code e message has been modified. Solutions: Alice attaches a (MAC) to the message. Or she attach digital signatur es a to the message. e • g g 4 A hash function maps from a domain to a smaller range, typically many-to-one. Properties required of a hash function depend on its applications. Application s: Fast l o o Hash function • • • g kup (hash tables) Error detection/correction cryptographic ha Cryp sh f tography unctions : Others g g g 5 * * : , | | | |. For example, :{0,1} {0,1} :{0,1} :{0,1} {0,1} , . If Hash functi is ons: Cryptographic hash function n n k l h X Y X Y h h Z h k l X • → • → → → • ( 29 finite, is also called a compression function. A classical application: users/clients passwords are stored in a file not as username, password , but as username, h h • ( 29 using some cryptographic hash fu (password nctio ) n . h 6 Pre-image: if ( ) , is a pre-image of . Each hash value typically has multiple pre-images. Collision: a pair of ( , ), , s.t. ( ) ( ). A hash function is said t Security requirements h m y m y m m m m h m h m • = • ′ ′ ′ • ≠ = o be: if it is computationally infeasible to find a pre-image of a hash value. if it is computationally in Pre-image resistant C feasible to find a col ollision re lision sistant . A hash fu n • • • cryptographic hash functio ction is a if it is collision resista n nt. 7 8 • Collision-resistant hash functions can be built from collision-resistant compression functions using Merkle-Damgard construction. * * hash compression Construct a cryptographic function :{0,1} {0,1} from a function :{0,1} {0,1} . 1. For {0,1} , add to so that | '| is padding Merkle-Damgard construction n n b n h f m m m + • → → ∈ 1 2 1 a multiple of ....
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This note was uploaded on 01/22/2011 for the course CSE 651 taught by Professor Tenh.lai during the Spring '10 term at Ohio State.

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6.hash-MAC - Cryptographic Hash Functions Message...

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