第12章 Hash和MAC算法.ppt
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1、第12章 消息认证码,Message Authentication,message authentication is concerned with protecting the integrity of a message validating identity of originator non-repudiation of origin (dispute resolution)will consider the security requirementsthen three alternative functions usedmessage encryptionhash function
2、message authentication code (MAC),Security Requirements,disclosuretraffic analysismasqueradecontent modificationsequence modificationtiming modificationsource repudiationdestination repudiation,Symmetric Message Encryption,encryption can also provides authenticationif symmetric encryption is used th
3、en:receiver know sender must have created it, since only sender and receiver know key usedknow content cannot have been altered, if message has suitable structure, redundancy or a checksum to detect any changes,Public-Key Message Encryption,if public-key encryption is used:encryption provides no con
4、fidence of sendersince anyone potentially knows public-keyhowever if sender signs message using their private-keythen encrypts with recipients public keyhave both secrecy and authenticationagain need to recognize corrupted messagesbut at cost of two public-key uses on message,Hash and MAC Algorithms
5、,Hash Functionscondense arbitrary size message to fixed sizeby processing message in blocksthrough some compression functioneither custom or block cipher basedMessage Authentication Code (MAC)fixed sized authenticator for some messageto provide authentication for messageby using block cipher mode or
6、 hash function,Message Authentication Code (MAC),generated by an algorithm that creates a small fixed-sized blockdepending on both message and some keylike encryption though need not be reversibleappended to message as a signaturereceiver performs same computation on message and checks it matches th
7、e MACprovides assurance that message is unaltered and comes from sender,Message Authentication Code,a small fixed-sized block of datagenerated from message + secret keyMAC = C(K, M)appended to message when sent,Message Authentication Codes,as shown the MAC provides authenticationcan also use encrypt
8、ion for secrecygenerally use separate keys for eachcan compute MAC either before or after encryptionis generally regarded as better done beforewhy use a MAC?sometimes only authentication is needed (e.g., SNMP)sometimes need authentication to persist longer than the encryption (eg. archival use)note
9、that a MAC is not a digital signature,MAC Properties,a MAC is a cryptographic checksumMAC = CK(M)condenses a variable-length message Musing a secret key Kto a fixed-sized authenticatoris a many-to-one functionpotentially many messages have same MACbut finding these needs to be very difficult,Require
10、ments for MACs,taking into account the types of attacksneed the MAC to satisfy the following:knowing a message and MAC, is infeasible to find another message with same MACMACs should be uniformly distributedMAC should depend equally on all bits of the message,Security of MACs,like block ciphers have
11、:brute-force attacks exploitingstrong collision resistance hash have cost 2m/2 128-bit hash looks vulnerable, 160-bits betterMACs with known message-MAC pairscan either attack keyspace (cf key search) or MACat least 128-bit MAC is needed for security,Security of MACs,cryptanalytic attacks exploit st
12、ructurelike block ciphers want brute-force attacks to be the best alternativemore variety of MACs so harder to generalize about cryptanalysis,Keyed Hash Functions as MACs,want a MAC based on a hash function because hash functions are generally fastercode for crypto hash functions widely availablehas
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