ECE 428/CS 425/CSE 424 (Fall 2005)
Distributed Systems

Homework 5

Assigned 10/25/05
Due by start of class on 11/1/05

This homework relates to Security.

(a) Define a certificate, and explain when certificates are needed.

(b) Spam and phishing email take advantage of the lack of authenticity 
and integrity in email systems. Suggest methods by which email can 
provide authenticity and integrity. Non-repudiation may be an 
undesirable property in email. Can your scheme provide selective 
non-repudiation without compromising authenticity?

(c) Construct a stream cipher based on a generic block cipher.

(d) Suppose you are the systems designer for a national intelligence 
agency. The cryptographers in your agency believe RSA to provide strong 
digital signatures, but cannot come up with any second preimage collision 
resistant hash functions. You want your spies to send signed 
(non-repudiable) messages back from the field, and these messages might 
be longer than the public key of the spy. Design a protocol for signing 
such messages that does not rely on a hash function. If Alice is signing 
messages for Bob, we want the following:
	(i)   Alice cannot repudiate a signed message
	(ii)  Bob cannot permute signature blocks from the same message
	(iii) Bob cannot mix-and-match signature blocks from different messages
(These properties are listed in decreasing order of importance. If you 
cannot come up with a protocol that provides all of them, submit one 
that satisfies properties (i) and (ii)).