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Chapter 11 Message intergrity and authentication.ppt
- 1. 11.1 Copyright © TheMcGraw-Hill Companies, Inc. Permission required for reproduction or display. Chapter 11 Message Integrity and Message Authentication
- 2. Message Authentication messageauthentication 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
- 3. Message Encryption messageencryption 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
- 5. Digital Signatures havelooked at message authentication but does not address issues of lack of trust digital signatures provide the ability to: verify author, date & time of signature authenticate message contents be verified by third parties to resolve disputes hence include authentication function with additional capabilities
- 6. Digital Signature Properties must depend on the message signed must use information unique to sender to prevent both forgery and denial must be relatively easy to produce must be relatively easy to recognize & verify be computationally infeasible to forge with new message for existing digital signature with fraudulent digital signature for given message be practical save digital signature in storage
- 7. Message Encryption ifpublic-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
- 11. 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
- 12. Message Authentication Codes asshown the MAC provides confidentiality 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 than the encryption (eg. archival use) note that a MAC is not a digital signature
- 16. MAC Properties aMAC 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
- 17. 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
- 18. Using Symmetric Ciphersfor 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
- 20. Hash Functions condensesarbitrary message to fixed size usually assume that the hash function is public 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
- 21. Hash Function Properties a Hash Function produces a fingerprint of some file/message/data h = H(M) condenses a variable-length message M to a fixed-sized fingerprint assumed to be public
- 28. Requirements for HashFunctions 1. can be applied to any sized message M 2. produces fixed-length output h 3. is easy to compute h=H(M) for any message M 4. given h is infeasible to find x s.t. H(x)=h • one-way property 5. given x is infeasible to find y s.t. H(y)=H(x) • weak collision resistance 6. is infeasible to find any x,y s.t. H(y)=H(x) • strong collision resistance
Editor's Notes
- #2 Up till now, have been concerned with protecting message content (ie secrecy) by encrypting the message. Will now consider how to protect message integrity (ie protection from modification), as well as confirming the identity of the sender. Generically this is the problem of message authentication, and in eCommerce applications is arguably more important than secrecy.
- #3 The first two requirements belong in the realm of message confidentiality, and are handled using the encryption techniques already discussed. The remaining requirements belong in the realm of message authentication. At its core this addresses the issue of ensuring that a message comes from the alleged source and has not been altered. It may also address sequencing and timeliness. The use of a digital signature can also address issues of repudiation.
- #19 Can also use block cipher chaining modes to create a separate authenticator, by just sending the last block. However this suffers from being a bit too small for acceptable use today.
- #29 These are the specifications for good hash functions. Essentially it must be extremely difficult to find 2 messages with the same hash, and the hash should not be related to the message in any obvious way (ie it should be a complex non-linear function of the message). There are quite a few similarities in the evolution of hash functions & block ciphers, and in the evolution of the design requirements on both.
- #31 The Birthday Attack exploits the birthday paradox – the chance that in a group of people two will share the same birthday – only 23 people are needed for a Pr>0.5 of this. Can generalize the problem to one wanting a matching pair from any two sets, and show need 2m/2 in each to get a matching m-bit hash. Note that creating many message variants is relatively easy, either by rewording or just varying the amount of white-space in the message.