1. CSCE 715: Network Systems Security
Chin-Tser Huang
University of South Carolina

2. Next Topic in Cryptographic Tools
Symmetric key encryption
Asymmetric key encryption
Hash functions and message digest
Nonce
09/20/2011

3. 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

4. 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
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5. 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
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6. Providing Msg Authentication by Asymmetric Encryption
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7. 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
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8. Uses of MAC
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9. 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

10. 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
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11. 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