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Learn about symmetric key encryption, asymmetric key encryption, hash functions, and message authentication techniques. Understand how to protect the integrity of messages and validate the identity of the originator.
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CSCE 715:Network Systems Security Chin-Tser Huang huangct@cse.sc.edu University of South Carolina
Next Topic in Cryptographic Tools • Symmetric key encryption • Asymmetric key encryption • Hash functions and message digest • Nonce
Message Authentication • Message authentication is concerned with • protecting the integrity of a message • validating identity of originator • non-repudiation of origin (dispute resolution) • Three alternative functions to provide message authentication • message encryption • message authentication code (MAC) • hash function
Providing Msg Authentication by Symmetric Encryption • Receiver knows sender must have created it because only sender and receiver know secret key • Can verify integrity of content if message has suitable structure, redundancy or a checksum to detect any modification
Providing Msg Authentication by Asymmetric Encryption • Encryption provides no confidence of sender because anyone potentially knows public key • However if sender encrypts with receiver’s public key and then signs using its private key, we have both confidentiality and authentication • Again need to recognize corrupted messages • But at cost of two public-key uses on message
Message Authentication Code (MAC) • Generated by an algorithm that creates a small fixed-sized block • depending on both message and some key • like encryption though need not to be reversible • Appended to message as a signature • Receiver performs same computation on message and checks if it matches the MAC • Provide assurance that message is unaltered and comes from claimed sender
MAC Properties • Cryptographic checksum MAC = CK(M) • condenses a variable-length message M • using a secret key K • to a fixed-sized authenticator • Many-to-one function • potentially many messages have same MAC • make sure finding collisions is very difficult
Requirements for MACs • Should take into account the types of attacks • Need the MAC to satisfy the following: • knowing a message and MAC, it is infeasible to find another message with same MAC • MACs should be uniformly distributed • MAC should depend equally on all bits of the message
Using Symmetric Ciphers for MAC • Can use any block cipher chaining mode and use final block as a MAC • Data Authentication Algorithm (DAA) is a widely used MAC based on DES-CBC • using IV=0 and zero-pad of final block • encrypt message using DES in CBC mode • and send just the final block as the MAC • or the leftmost M bits (16≤M≤64) of final block • But final MAC is now too small for security