Homework on BAN logic
Assigned by: Avi Rubin
Due: Tuesday, November 19


BAN logic was used to show that the Needham and Schroeder protocol
has a design flaw.

1. What is wrong with the protocol?

2. Provide a modified Needham and Schroeder protocol that fixes
   the protocol.

3. Show the idealized protocol.

4. List the initial assumptions of your idealized protocol

5. Define the BAN logic goals of your protocol

6. Show a step by step BAN logic analysis of your new protocol and
   show that the old problems do not exist with your modified protocol.

7. Explain how a GNY analysis of your protocol is different from the
   BAN logic anlysis.

8. What are the advantages of GNY over BAN?

9. What is the biggest disadvantage of GNY over BAN?

** 10. Do a complete BAN analysis of the following protocol. (Show all work)
    Answer the 4 questions which BAN was designed to answer about
    the protocol.  Does the analysis reveal any flaws in the protocol? If so,
    fix the protocol, and show that analysis of the fixed protocol
    reveals no flaws. If not, explain why not.

    A -> B: A, Na
    B -> S: B, {A,Na,Nb}_Kbs
    S -> A: {B,Kab,Na,Nb}_Kas , {A,Kab}_Kbs
    A -> B: {A,Kab}_Kbs , {Nb}_Kab

* 11. Provide a concrete protocol that achieves the same goals as the original
    protocol in problem 10, but that requires fewer assumptions.