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Authors:
James F. Kurose, Keith W. Ross
Chapter:
Security In Computer Networks
Exercise:
Problems
Question:12 | ISBN:9780132856201 | Edition: 6

Question

Suppose Alice and Bob share two secret keys: an authentication key S 1 and a symmetric encryption key S 2 . Augment Figure 8.9 so that both integrity and confidentiality are provided.

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Answer

Refer the Figure 8.9 in the text book.

Assume Alice and Bob share two secret keys: an authentication key S 1 and a symmetric encryption key S 2 .

For integrity:

  • Alice creates a message m, concatenates S1 with m to get m+S1, and calculate the hash as h=H(m+S1).
  • The MAC(Message Authentication Code) knows the hash function h. Next, Alice will send (m,h) to Bob.
  • Then, Bob receives the message (m,h) as Bob knows the key S1. 
  • Bob check the messaage H(m+S1) and h are same, then he can conclude the message is not altered. So, the integrity is continued using authentication key S1.

For Confidentiality:

  • Alice will encrypt the message (m,h) using symmetric encryption key S2 and send this message to Bob.
  • Bob receives the encrypted message and he decrypts the received message using key S2 to get (m,h), then he calculates H(m+s1) to check the message.
  • Hence, the confidentiality as well as integrity is maintained between both Alice and Bob.

 

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