SE547: Homework 1

Deadline: 5.30pm, Thursday 22 January 2004.


Consider the following protocol:

(1) A -> S: A, B, { Kab }Kas
(2) S -> A: { A, Kab }Kbs
(3) A -> B: { A, Kab }Kbs, { M }Kab
(4) B -> A: ack

Some goals for this protocol are:

  1. M is kept secret from the attacker.
  2. After receiving message (3), B knows that A sent M.
  3. After receiving message (4), A knows that B received M.

1. Show that none of these goals are met by this protocol, by showing one or more runs of the protocol which vioate each of the goals.

2. Edit the protocol to ensure that the protocol meets its goals, in the presence of a Dolev-Yao attacker. For each goal, give a one- or two-sentence justification for why your protocol meets that goal.

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