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HMAC: a MAC from SHA-256

Online Cryptography Course Dan Boneh. Collision resistance. HMAC: a MAC from SHA-256. The Merkle-Damgard iterated construction. m[0]. m[1]. m[2]. m[3] ll PB. Thm : h collision resistant ⇒ H collision resistant

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HMAC: a MAC from SHA-256

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  1. Online Cryptography Course Dan Boneh Collision resistance HMAC: a MAC from SHA-256

  2. The Merkle-Damgard iterated construction m[0] m[1] m[2] m[3] ll PB Thm: h collision resistant ⇒ H collision resistant Can we use H(.) to directly build a MAC? H(m) h h h h IV (fixed)

  3. MAC from a Merkle-Damgard Hash Function Given H( k ll m) can compute H( w ll k llm ll PB) for any w. Given H( k ll m) can compute H( k llm ll w ) for any w. Given H( k ll m) can compute H( k ll m ll PB ll w ) for any w. Anyone can compute H( k llm ) for any m. H: X≤L ⟶ T a C.R. Merkle-Damgard Hash Function Attempt #1: S(k, m) = H( k ll m) This MAC is insecure because:

  4. Standardized method: HMAC (Hash-MAC) Most widely used MAC on the Internet. H: hash function. example: SHA-256 ; output is 256 bits Building a MAC out of a hash function: HMAC: S( k, m ) = H( kopad, H( kipadll m ) )

  5. HMAC in pictures k⨁ipad m[0] m[1] m[2] ll PB Similar to the NMAC PRF. main difference: the two keys k1, k2 are dependent > h h h h > > > k⨁opad IV (fixed) IV (fixed) tag > h h >

  6. HMAC properties HMAC is assumed to be a secure PRF • Can be proven under certain PRF assumptions about h(.,.) • Security bounds similar to NMAC • Need q2/|T| to be negligible ( q << |T|½ ) In TLS: must support HMAC-SHA1-96

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