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Mohamed G. Gouda						Midterm 3
CS 395T: Network Protocol security				Fall 2006
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Time: One Hour Fifteen Minutes					Open Book
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1. (5 points)
Let r.i be a rule in a firewall F and let the following predicate, named rp.i, 
denote the resolving predicate of r.i.

	F.1 in S.1 ^  ...  ^ F.d in S.d  

Also let mp.j denote the matching predicate of the following rule r.j

	F.1 in T.1 ^  ...  ^ F.d in T.d  -->  accept

that occurs after rule r.i in firewall F. Assume that each S.k and each T.k 
is a finite and non-empty set of non-negative integers. Describe a simple 
procedure to check whether or not "rp.i implies mp.j". Also describe a simple
procedure to check whether or not "no packet satisfies both rp.i and mp.j".
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2. (5 points)
The voting system in Homework 3 has the following protocol between a voter,
the mix server and the voting server:

	voter --> mix    :	B.mix<B.voting<nonce n, myvote>>

	mix   --> voting :	B.voting<nonce n, myvote>

A second version of this protocol is as follows:

	voter --> mix    :	B.mix<rand, B.voting<nonce n, myvote>>

	mix   --> voting :	B.voting<nonce n, myvote>

where rand is a fixed length random number selected by the voter. Explain 
why does the second version of the protocol have better anonymity properties 
than the first version.
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3. (5 points)
Modify the second version of the protocol in problem 2 such that the voter
receives a proof, signed by the voting server, that the voting server has 
received their "myvote".
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