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Cryptography and Network Security
Message Authentication and Hash Functions
 At cats' green on the Sunday he took the message from the inside of the pillar and added Peter Moran's name to the two names already printed there in the "Brontosaur" code. The message now read: “Leviathan to Dragon: Martin Hillman, Trevor Allan, Peter Moran: observe and tail.” What was the good of it John hardly knew. He felt better, he felt that at last he had made an attack on Peter Moran instead of waiting passively and effecting no retaliation. Besides, what was the use of being in possession of the key to the codes if he never took advantage of it?
 —Talking to Strange Men, Ruth Rendell
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 requirements
 then three alternative functions used:
 message encryption
 message authentication code (MAC)
 hash function
Security Requirements
 disclosure
 traffic analysis
 masquerade
 content modification
 sequence modification
 timing modification
 source repudiation
 destination repudiation
Message Encryption
 message encryption by itself also provides a measure of authentication
 if symmetric encryption is used then:
 receiver know sender must have created it
 since only sender and receiver now key used
 know content cannot of been altered
 if message has suitable structure, redundancy or a checksum to detect any changes
 if public-key encryption is used:
 encryption provides no confidence of sender
 since anyone potentially knows public-key
 however if
 sender signs message using their private-key
 then encrypts with recipients public key
 have both secrecy 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 be reversible
 appended to message as a signature
 receiver performs same computation on message and checks it matches the MAC
 provides assurance that message is unaltered and comes from sender
MAC Properties
 a MAC is a cryptographic checksum
MAC = CK(M)
 condenses a variable-length message M
 using a secret key K
 to a fixed-sized authenticator
 is a many-to-one function
 potentially many messages have same MAC
 but finding these needs to be very difficult
Requirements for MACs
 taking into account the types of attacks
 need the MAC to satisfy the following:
1. knowing a message and MAC, is infeasible to find another message with same MAC
2. MACs should be uniformly distributed
3. MAC should depend equally on all bits of the message
Using Symmetric Ciphers for MACs
 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