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Mohamed G. Gouda                                		CS 386S
Spring 2009                                     		Quiz 7
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There are two objectives of the anonymity protocol of David Chaum:

	1. A user process p can send a secret fore message to another 
	   user process q and later receive a secret back message 
	   from q without q knowing the identity of p.

	2. An adversary, that can observe all the messages that are 
	   exchanged between any user process and process mix, cannot
	   identify any pair of user processes that are communicating.

Explain how this protocol can be simplified if the adversary can observe only
the (fore or back) messages from any user process to process mix but cannot
observe the (fore or back) messages from process mix to any user process.

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The simplified protocol is as follows.

p   -->	 mix		: fore(B<q, B.q<msg,pad>>)
	 mix		: does not have to reorder received fore msgs
	 mix  -->   q	: fore(B.q<msg,pad>)
	 msg		: has a pair (B<p, pad'>, sk)
	 mix  <--   q	: back(B<p, pad'>, sk<reply>)
	 mix		: does not have to reorder received back msgs
p   <--	 mix		: back(sk<reply>)

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