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Chapter 11 – Message
Authentication and Hash
Functions
At cats' green on the Sunday he took the message fromthe inside of the pillar and added Peter Moran's name tothe two names already printed there in the "Brontosaur"code. The message now read: “Leviathan to Dragon:Martin Hillman, Trevor Allan, Peter Moran: observe andtail.” What was the good of it John hardly knew. He feltbetter, he felt that at last he had made an attack on PeterMoran instead of waiting passively and effecting noretaliation. Besides, what was the use of being inpossession of the key to the codes if he never tookadvantage 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
Message Encryption
message encryption by itself also providesa 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, redundancyor a checksum to detect any changes
Message Encryption
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
Message Authentication
Codes
as shown the MAC provides authentication can also use encryption for secrecy generally use separate keys for each can compute MAC either before or after encryption is generally regarded as better done before why use a MAC? sometimes only authentication is needed sometimes need authentication to persist longer thanthe encryption (eg. archival use) note that a MAC is not a digital signature
Requirements for MACs
taking into account the types of attacks
need the MAC to satisfy the following:
knowing a message and MAC, is infeasibleto find another message with same MAC
MACs should be uniformly distributed
MAC should depend equally on all bits of themessage
Using Symmetric Ciphers for
MACs
can use any block cipher chaining modeand 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 ( ≤ M ≤
- of final block
but final MAC is now too small for security
Hash Functions
condenses arbitrary message to fixed size
h = H(M)
usually assume that the hash function ispublic and not keyed
cf. MAC which is keyed
hash used to detect changes to message
can use in various ways with message
most often to create a digital signature
Hash Functions & Digital
Signatures
Simple Hash Functions
are several proposals for simple functions
based on XOR of message blocks
not secure since can manipulate anymessage and either not change hash orchange hash also
need a stronger cryptographic function(next chapter)
Birthday Attacks
might think a 64-bit hash is secure but by Birthday Paradox is not birthday attack works thus: opponent generates 2 m /^2 variations of a valid message all with essentially the same meaning opponent also generates 2 m /^2 variations of a desired fraudulent message two sets of messages are compared to find pair withsame hash (probability > 0.5 by birthday paradox) have user sign the valid message, then substitute theforgery which will have a valid signature conclusion is that need to use larger MAC/hash
Hash Functions & MAC
Security
like block ciphers have:
brute-force
attacks exploiting
strong collision resistance hash have cost 2 m /^2
- have proposal for h/w MD5 cracker• 128-bit hash looks vulnerable, 160-bits better MACs with known message-MAC pairs - can either attack keyspace (cf key search) or MAC• at least 128-bit MAC is needed for security
Hash Functions & MAC
Security
cryptanalytic attacks exploit structure like block ciphers want brute-force attacks to be thebest alternative have a number of analytic attacks on iteratedhash functions CV i^ = f[CV i- , M ]; H(M)=CVi N typically focus on collisions in function f like block ciphers is often composed of rounds attacks exploit properties of round functions
