Barack periodically comes up with brilliant ideas to stop the financial crisis, provide health care to every citizen, and save the polar bears. He wants to share these ideas with all the cabinet members but also get credit for the ideas. Extending the above approach, he shares a secret key k with all the cabinet members. Next, he broadcasts each idea z followed by value h(k||z). Does this

approach work or can Tim claim that he came up with the ideas instead of Barack? Justify your answer.

What will be an ideal response?

Barack is using a message authentication code (MAC). However, since all cab
inet members know the secret key, k, they infer that the idea z came from either Barack
or one of them. However, it is impossible to determine exactly the individual in this group
who had idea z. Indeed, any of the people who know key k can compute the MAC value of
an idea and claim ownership of the idea. Thus, Tim can claim he is the one who came up
with the ideas.