Which public function that maps a message of any length into a fixed length hash value which serves as its authenticator?
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CRYPTOGRAPHIC HASHFUNCTIONSB A S E D O N S TA L L I N G S & C A R B U N A RR I C K B L A Z E K , P H D . Show
MESSAGE AUTHENTICATIONAS A COUNTERMEASURE•Disclosure•Traffic analysis–Discovery of the pattern oftraffic between parties•Masquerade•Content modification•Sequence modification•Timing modification•Source repudiation•Destination repudiation2UseEncryptionUseAuthenticationDigital Signature HASH FUNCTIONUSES3•MessageIntegrity/Authentication Check (MAC)•Message OriginCheck (Signature)•Combined withencryption OTHER HASH FUNCTION USESCommonly used tocreate a one-waypassword fileWhen a user enters apassword, the hashof that password iscompared to thestored hash value forverificationThis approach topassword protectionis used by mostoperating systemsCan be used toconstruct apseudorandomfunction (PRF) or apseudorandomnumber generator(PRNG)A commonapplication for ahash-based PRF isfor the generation ofsymmetric keys4H(salt, pwd) HASH FUNCTIONSA hash function Haccepts a variable-length block of dataMas input and producesa fixed-size hash value–h =H(M)–Principal object isdata integrity6How is the sizereduced ? HASH FUNCTION•Hash function H is a lossy compression function•Repeated Compression using Block Chaining•Avoid Collision: H(x)=H(x’) for some inputs x≠x’•H(x) should look “random”•Cryptographic hash function–An algorithm for which it is computationally infeasible to findeither:•(a) a data object that maps to a pre-specified hash result (theone-way property)•(b) two data objects that map to the same hash result (thecollision-free property)7 Figure11.2Attack Against Hash Function(b) Man-in-the-middleattackAliceDarthBobBobAliceCOMPAREdata8Does notprotect againstMITM EKMH| |DKMH(M)HCompare(a)E(K, [M|| H(M)])Destination BSource A9M + HEncrypted wSymmetric KeyE(K, H(M)) MESSAGEAUTHENTICATIONCODE (MAC)•Also known as a keyed hash function•Typically used between two parties that share a secretkey to authenticate information exchanged between11Takes as input a secret key and a data block andproduces a hash value (MAC) which is associatedwith the protected message•If the integrity of the message needs to bechecked, the MAC function can be applied to themessage and the result compared with theassociated MAC value•An attacker who alters the message will beunable to alter the associated MAC value withoutknowledge of the secret key EXAMPLE MD5 &SHA112md5_digest("The quick brown fox jumps over the lazydog") =9e107d9d372bb6826bd81d3542a419d6md5_digest("The quick brown fox jumps over the lazycog") =1055d3e698d289f2af8663725127bd4bsha1_digest("The quick brown fox jumps over the lazydog”) =2fd4e1c67a2d28fced849ee1bb76e7391b93eb12sha1_digest("The quick brown fox jumps over the lazycog”) =de9f2c7fd25e1b3afad3e85a0bd17d9b100db4b3 MESSAGEAUTHENTICATIONFUNCTIONS•Two levels offunctionality:–Hash function– Upload your study docs or become a Course Hero member to access this document Upload your study docs or become a Course Hero member to access this document End of preview. Want to read all 36 pages? Upload your study docs or become a Course Hero member to access this document Tags Cryptographic hash function Which function is used to map data of any size to a fixed length?A hash function is any function that can be used to map data of arbitrary size to fixed-size values.
Is a public function of the message and a secret key that produces a fixed length value serves as the authenticator?Message Authentication Code (MAC): A public function of the message and a secret key that produces a fixed length value that serves as the authenticator.
Which technique is used in hash algorithm to produce fixed length string from variable length?A one-way hash function, also known as a message digest, is a mathematical function that takes a variable-length input string and converts it into a fixed-length binary sequence that is computationally difficult to invert—that is, generate the original string from the hash.
What are the authentication functions?Authentication functions:
Assures the data received are exactly as sent (i.e. contain no modification, insertion ,deletion or replay) Assures that identity of the sender is valid. When a hash function is used to provide message authentication, the hash function value is often referred to as a message digest.
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