Alice and Bob want to establish a secure communication channel between them. The

Question

Alice and Bob want to establish a secure communication channel between them. They do not care about the confidentiality of the messages being transmitted, but they do want to ensure the integrity and authenticity of the messages. Assume A and B share a common key K. Answering the following questions.

a. (3 points) How can they achieve their goal only with secret key cryptography?

b. (3 points) How can they achieve their goal with hash function h and the key?

c. (4 points) Can they get non-repudiation? If yes, how? If no, why?

Explanation / Answer

Answer-(a):

To ensure integrity and authentication of the message sent by Alice to Bob through an insecure channel by using only a secret key, MAC (Message Authentication Code) can be used.

In this procedure, Alice first creates MAC. The message and the key are fed into a hash function and the MAC is created. Now, Alice sends the message along with the MAC to Bob.

Bob receives the message and then he creates MAC using the same secret key that is available to him and the message that is sent by Alice. Now, he compares the MAC with the MAC that is sent by Alice. If they are same then the message is not changed during transmission through the insecure channel.

Answer-(b):

Using hash function, the integrity and authentication of the message can be maintained. In this method, Alice creates a hash value of the message using a hash function and sends the hash value along with the message to Bob.

When Bob receives the message, he creates hash value of the message that he receives from Alice using the same hash function. If they are same, then the message is not changed during transmission through the insecure channel.

If anyone intercepts the message and changes the content of the message and sends it to Bob, Bob would not find the hash value created from the message that he receives and the hash value sent with the message to be same.

Answer- (c):

Here the answer is “No”. Secret key is not enough to ensure that someone who receives the message and decrypts it using the secret key and refuses after seeing the content of the message or repudiates.

Consider that Alice asks her bank to send $100 to Bob. When Bob receives the money, Alice can deny that she made such request. There is no proof that only Alice can make such request because the copy of the key can be available to anyone who intercepts the channel during the transmission of the request and Alice can give this excuse to prove that she never made such request to bank.

Related Questions

Navigate

• Browse (All)
• Browse A
• Subjects

Integrity-first tutoring: explanations and feedback only — we do not complete graded work. Learn more.