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 09/20/2011

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 09/20/2011

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 09/20/2011

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 09/20/2011

Providing Msg Authentication by Asymmetric Encryption 09/20/2011

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 09/20/2011

Uses of MAC 09/20/2011

MAC Properties Cryptographic checksum Many-to-one function 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 09/20/2011

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 09/20/2011

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 09/20/2011