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Authors:
James F. Kurose, Keith W. Ross
Chapter:
Security In Computer Networks
Exercise:
Review
Question:11 | ISBN:9780132856201 | Edition: 6

Question

Consider a variation of the MAC algorithm (Figure 8.9) where the sender sends (m, H(m) + s), where H(m) + s is the concatenation of H(m) and s. Is this variation flawed? Why or why not?

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Answer

Still the intruder can sniff the sender’s message (m, H(m)+s) with variation in the MAC (Message Authentication Code) algorithm.

The intruder can extract ‘s’  from the message ‘m’ by performing hashing H(m). The extraction is performed as (H(m)+s-H(m)).

Also, there is a scope to Trudy send a modified message as he knows the secret key. The message is generated as (m’,H(m’)+s).

Therefore, the MAC algorithm with the given variation contains flaws.

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