Chap5&6

Question 1

How does MAC assure the receiver that the message received is not changed?

Answer 1

If an attacker changes the message but not the MAC, the receiver will calculate a different MAC from
the MAC received

Question 2

Why can’t the attacker also change the MAC?

Answer 2

The symmetric key is known only to the sender and receiver
* The attacker does not have the key to calculate and replace the MAC after changing the message.

Question 3

How does MAC assure the receiver the message is from the correct sender?

Answer 3

The symmetric key is known only to the sender and receiver.
* Only the sender is able to use the key to calculate the MAC.

Question 4

How does the calculation of MAC differ from symmetric algorithm?

Answer 4

Symmetric algorithm: reversible, involves both encryption and decryption
* MAC: one-way (irreversible), involves only encryption
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Question 5

What is the key difference between a message digest (MD) and a message authentication code
(MAC)?

Answer 5

• MD provides data integrity but does not provide data origin authentication
• MAC provides both data integrity and data origin authentication
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Question 6

What are the disadvantages of using HMAC?

Answer 6

The biggest drawback is HMAC’s reliance on symmetric secret key
* Symmetric key exchange problem
* Multiple recipients: using multiple symmetric keys needs key management
* Multiple recipients: using single shared key has authentication problem

Question 7

Can HMAC be used as a digital signature? Why?

Answer 7

• No.
• Both sender and receiver has the same symmetric key