**Due**: May 24, 2013**Points**: 100

- (
*30 points*) Consider a scheme that allows a recipient to reply to a message from a chain of Cypherpunk remailers. Assume that encipherment is used throughout the chain.- Bob selects a chain of remailers for the return path. He creates a set of keys and enciphers them so that only the key for the current remailer is visible to that remailer. Design a technique by which he could accomplish this. Describe how he would include this data in his message.
- How should Alice’s mailer handle the processing of the return address information?
- When Bob receives the reply, what does it contain? How can he obtain the cleartext reply?

*text*, problem 14.3) - (
*30 points*) Revisit the example for`x := y + z`in Section 16.1.1. Assume that*x*does not exist in state*s*. Confirm that information flows from*y*and*z*to*x*by computing*H*(*y*|_{s}*x*),_{t}*H*(*y*),_{s}*H*(*z*|_{s}*x*), and_{t}*H*(*z*) and showing that_{s}*H*(*y*|_{s}*x*) <_{t}*H*(*y*) and_{s}*H*(*z*|_{s}*x*) <_{t}*H*(*z*)_{s}

(*text*, problem 16.1) - (
*20 points*) Let*L*= (*S*,_{L}*<*) be a lattice. Prove that the structure_{L}*IL*= (*S*,_{IL}*<*) is a lattice, given the following definitions:_{IL}-
*S*= { [_{IL}*a*,*b*] |*a*,*b*∈*S*∧*a*<_{L}*b*} - <
_{IL}= { ([*a*_{1},*b*_{1}], [*a*_{2},*b*_{2}]) |*a*_{1}<_{L}*a*_{2}∧*b*_{1}<_{L}*b*_{2}} -
*lub*([_{IL}*a*_{1},*b*_{1}], [*a*_{2},*b*_{2}]) = (*lub*(_{L}*a*_{1},*a*_{2}),*lub*(_{L}*b*_{1},*b*_{2})) -
*glb*([_{IL}*a*_{1},*b*_{1}], [*a*_{2},*b*_{2}]) = (*glb*(_{L}*a*_{1},*a*_{2}),*glb*(_{L}*b*_{1},*b*_{2}))

*text*, problem 16.2, modified) -
- (
*20 points*) Why can we omit the requirement*lub*(__i__,) <*b*[*i*]from the requirements for secure information flow in the example for iterative statements (see Section 16.3.2.4)?*a*[*i*]

(*text*, problem 16.5)

- (
*30 points*) Prove that a system that meets the definition of generalized noninterference security also meets the definition of deducible security.

(*text*, problem 8.6)

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