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Unformatted text preview: HASH FUNCTIONS 1 / 62 What is a hash function? By a hash function we usually mean a map h : D → { , 1 } n that is compressing, meaning  D  > 2 n . E.g. D = { , 1 } ≤ 2 64 is the set of all strings of length at most 2 64 . h n MD4 128 MD5 128 SHA1 160 RIPEMD 128 RIPEMD160 160 SHA256 256 Skein 256, 512, 1024 2 / 62 Collision resistance (CR) Definition: A collision for h : D → { , 1 } n is a pair x 1 , x 2 ∈ D of points such that h ( x 1 ) = h ( x 2 ) but x 1 negationslash = x 2 . If  D  > 2 n then the pigeonhole principle tells us that there must exist a collision for h . 3 / 62 Collision resistance (CR) Definition: A collision for h : D → { , 1 } n is a pair x 1 , x 2 ∈ D of points such that h ( x 1 ) = h ( x 2 ) but x 1 negationslash = x 2 . If  D  > 2 n then the pigeonhole principle tells us that there must exist a collision for h . 3 / 62 Collision resistance (CR) Definition: A collision for h : D → { , 1 } n is a pair x 1 , x 2 ∈ D of points such that h ( x 1 ) = h ( x 2 ) but x 1 negationslash = x 2 . If  D  > 2 n then the pigeonhole principle tells us that there must exist a collision for h . Function h is collisionresistant if it is computationally infeasible to find a collision. 3 / 62 Function families We consider a family H : { , 1 } k × D → { , 1 } n of functions, meaning for each K we have a map h = H K : D → { , 1 } n defined by h ( x ) = H ( K , x ) Usage: K $ ← { , 1 } k is made public, defining hash function h = H K . Note the key K is not secret. Both users and adversaries get it. 4 / 62 CR of function families Let H : { , 1 } k × D → { , 1 } n be a family of functions. A cradversary A for H • Takes input a key K ∈ { , 1 } k • Outputs a pair x 1 , x 2 ∈ D of points in the domain of H K −→ A −→ x 1 , x 2 A wins if x 1 , x 2 are a collision for H K , meaning • x 1 negationslash = x 2 , and • H K ( x 1 ) = H K ( x 2 ) Denote by Adv cr H ( A ) the probability that A wins. 5 / 62 CR of function families Let H : { , 1 } k × D → { , 1 } n be a family of functions and A a cradversary for H . Game CR H procedure Initialize K $ ← { , 1 } k Return K procedure Finalize( x 1 , x 2 ) Return ( x 1 negationslash = x 2 ∧ H K ( x 1 ) = H K ( x 2 )) Let Adv cr H ( A ) = Pr bracketleftBig CR A H ⇒ true bracketrightBig . 6 / 62 The measure of success Let H : { , 1 } k × D → { , 1 } n be a family of functions and A a cr adversary. Then Adv cr H ( A ) = Pr bracketleftBig CR A H ⇒ true bracketrightBig . is a number between 0 and 1. A “large” (close to 1) advantage means • A is doing well • H is not secure A “small” (close to 0) advantage means • A is doing poorly • H resists the attack A is mounting 7 / 62 CR security Adversary advantage depends on its • strategy • resources: Running time t Security: H is CR if Adv cr H ( A ) is “small” for ALL A that use “practical” amounts of resources....
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 Winter '08
 daniele
 HK, Cryptographic hash function, MD5

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