FIT2093 Lecture Notes - Lecture 9: Public-Key Cryptography, Message Authentication Code, Hash Function
31 May 2018
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Week 10 Notes!
1. Briefly describe two main use of digital signature!
-Validate the authenticity and integrity of a message, software or digital documents!
-Non-repudiation!
-Verify source or issuer!
-Detect forgery or tampering!
2. Describe the stages of generating and verifying digital signature for long documents!
Generating digital signature: use one-way hash algorithm, hash the document first and sign the
hashed data which is normally short!
3. Discuss features of a good one-way hash function!
-For any size of data: can be applied to a block of data of any size!
-Output size: produces a fixed-length output!
-Easy to evaluate: should be fast!
-Hard to reverse: no feasible algorithm to ‘reverse’ a hashed value!
-Hard to find collisions: no feasible algorithm to find two or more input documents which are
shared into the same condensed output!
Can only be called one-way if it has 3rd and 4th properties!
4. Discuss digital signature requirements!
-Must depend on the message signed!
-Must use information unique to the originator: to prevent both forgery and denial!
-Must be relatively easy to produce!
-Must be relatively easy to recognise & verify!
-Be computationally infeasible to forge!
-New message for existing digital signature!
-Fraudulent digital signature for given message!
- Be practical to save digital signature in storage!
5. What is Message authentication?!
-protecting the integrity of a message and validating the identity of originator!
-Non-repudiation of the origin!
6. What are three requirements for MAC?!
Message Authentication Code - Integrity!
Encryption - Confidentiality!
-Knowing a message and MAC, is infeasible to find another message with the same MAC: deals
with message replacement attacks!
-MACs should be uniformly distributed across the messages: deals with the need to thwart a
brute-force attack based on chosen plaintext!
-MAC should depend equally on all bits of the message: dictates that the authentication
algorithm should not be weaker with respect to certain parts or bits of the message than others!
7. Sign_sk (M) = M^d mod n!
8. For!
9. List two disputes that can arise in the context of message authentication!
-non-repudiation dispute: they generate the same tag thus there can be argument of who is the
origin!
10. Theoretically there will be collision but you won’t be able to detect it, only in computational
that collision will not occur/impossible!
11. What is the difference between a message authentication code and a one-way hash?!
-Hash only takes the message and not the key as input to generate the message digest!
-MAC uses secret information whereas Hash does not!
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Document Summary
Week 10 notes: brie y describe two main use of digital signature. Validate the authenticity and integrity of a message, software or digital documents. Detect forgery or tampering: describe the stages of generating and verifying digital signature for long documents. Generating digital signature: use one-way hash algorithm, hash the document rst and sign the hashed data which is normally short: discuss features of a good one-way hash function. For any size of data: can be applied to a block of data of any size. Hard to reverse: no feasible algorithm to reverse" a hashed value. Hard to nd collisions: no feasible algorithm to nd two or more input documents which are shared into the same condensed output. Can only be called one-way if it has 3rd and 4th properties: discuss digital signature requirements. Must use information unique to the originator: to prevent both forgery and denial. Must be relatively easy to recognise & verify.
