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Mohamed G. Gouda                                		CS 386S
Spring 2009                                     		Quiz 5
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Consider an e-voting system that consists of a voting center c and an array
of 100 voters v[0], .., v[99]. Periodically, c sends a "motion" to every
voter v[i] which replies by sending back its vote, 0 or 1, on the motion to 
c. The protocol between c and each v[i] can be specified as follows:

	c --> v[i] : R.c <motion>
	c <-- v[i] : B.c <motion, vote, i, P.i>

where 

R.c 	is the private key of the voting center c,
B.c 	is the public key of the voting center c,
motion 	is an integer in the range 0..(k-1) for some large k,
vote 	is either 0 or 1,
i 	is a value in the range 0..99, and
P.i 	is the password of voter v[i] which is known by both c and v[i].

Explain why is this protocol vulnerable when P.i is poorly chosen. Show how
to strengthen this protocol without strengthening P.i.

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Student EID:
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Solution:

The values of "B.c", "motion", and "i" in the second message in the protocol
are all known. Also, the numbers of all possible values of "vote" and "P.i" 
are relatively small (since vote is either 0 or 1 and P.i is poorly chosen). 
Thus, an adversary can guess possible values vote' and P'.i for vote and P.i, 
and if it turns out that
	B.c <motion, vote', i, P'.i> = B.c <motion, vote, i, P.i>
then the adversary concludes that vote' = vote and P'.i = P.i. Thus the vote
cast by v[i] is no longer confidential.

To strengthen the protocol, v[i] should add a random confounder cn to the
second message in the protocol. The strengthened protocol becomes:

	c --> v[i] : R.c <motion>
	c <-- v[i] : B.c <motion, vote, cn, i, P.i>

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