Homework #4 Revision 1

Due: March 9, 2022
Points: 100

Questions

1. (25 points) Suppose the composite machine catdog (see Section 9.4.1) emits the same value from the left and the right. Show that it has received an even number of inputs from the left.

2. (25 points) Revisit the example for x := y + z in Section 17.1.1. 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_t) and H(z_s) and showing that H(y_s | x_t) < H(y_s) and H(z_s | x_t) < H(z_s).

3. (20 points) Consider the rule of transitive confinement. Suppose a process needs to execute a subprocess in such a way that the child can access exactly two files, one only for reading and one only for writing.
   1. Could capabilities be used to implement this? If so, how? If not, why not?
   2. Could access control lists be used to implement this? If so, how? If not, why not?

4. (30 points) Section 18.3.2.3 derives a formula for I(A; X). Prove that this formula is a maximum with respect to p when p = (M^(1/m))/(1+mM^(1/m)) (this is different than what is in the text).

Matt Bishop Office: 2209 Watershed Sciences Phone: +1 (530) 752-8060 Email: mabishop@ucdavis.edu

ECS 235B, Foundations of Computer and Information Security Version of February 28, 2022 at 11:06AM

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