Microsoft Word

Your answer to each question should:

Not exceed one (1) page in length
Use 12 pt Times Roman font
Use left justification
Use single line spacing with no more than 6pt line space

Your submitted paper should:

• Not include a title page
• Set margins: Left = 1”, right = 1”, top = 1” and bottom = 1”
• Include your name in the header and a page number in the footer
• Use an additional page for the bibliography, if needed
• Include citations at the sentence level

Use Microsoft Word format

Question 1

Suppose you changed the protocol in Kaufman & Perlman §20.8.2 Plausible Deniability Based on Public Key Technology so Alice first signs S, and then encrypts with Bob’s public key. So, instead of sending [{S}_Bob]_Alice to Bob, she sends {[S]_Alice}_Bob. Written in the syntax of the textbook:

message M sent to Bob = (M, MAC<M, S>, {[ S ]_Alice}_Bob)

Does this work? (i.e., can Bob be sure that the message came from Alice, but not be able to prove it to a third party?) Your response needs to include a justification with citations.

Question 2

Consider the following format for Alice sending signed, encrypted email to Bob:

Alice invents a secret key S and sends {S}_Bob, the message encrypted with S, and the message digest of the message signed by Alice. Is this secure?  Your response needs to include a justification with citations. (Hint: Alice only signs the message digest. You may want to breakdown the information the question provides into the syntax of the textbook and compare the results with known methods.)