Due in class on March 2nd.  Please turn in on paper, not
electronically.

1.  How would you improve this class?  Please submit your answer to
    this question anonymously on a separate sheet of paper.


2.  Attack Trees (taken from Wagner, Vazirani, Joseph, and Tygar)

An attack tree is a tree in which each node is labeled with either an
AND or an OR.  The root of the attack tree is the ultimate goal of an
attacker.  Each child of the root is a subgoal that can help the
adversary achieve his root goal.  If the root node is an AND node,
then the attacker must accomplish all the subgoals to achieve the root
goal.  If the root node is an OR node, then the attacker need only
accomplish one subgoal to succeed.  Each subgoal can then be used as
the root of another attack tree, breaking down the steps to achieve
the primary goal to a sufficient level of detail.  Here is an attack
tree for gaining access to my office:

              Get in office
                   OR
                 /    \
    Steal my key         Break lock
        AND                  OR
      /      \             /     \
  break      get my      acid    dynamite
  into       key and
  my house   get out


A.  Construct an attack tree for reading the /etc/shadow file on
a typical UNIX system.  The /etc/shadow file is by default installed
owned by root and only readable/writable by root.  You can consider
both attacks in the system and attacks that work outside the system, 
e.g. social engineering.  Obviously, there are many, many ways to 
achieve this goal, so just fill out your attack tree to 15-30 nodes.

B. Now label each _leaf_ in your attack tree with a cost in dollars 
(don't forget that time == money).  You can then compute the cost of 
all the other nodes in the attack tree by
  - AND node: sum cost of children
  - OR node: take min cost of children
How much would it cost to gain access to /etc/shadow?

C. How would you increase the cost of gaining access to this file?
Give two or three suggestions.  Update your attack tree to reflect
your suggested changes.  How much did the system's security improve?

D. What are the strengths and weaknesses of attack trees as security
assessment tools?


3. Suppose an attacker discovers the decryption exponent, d, for an
   RSA public key (N, e).  Show how she can factor N efficiently.


4. A traditional implementation of Bell-Lapadula access control uses
   labels on files to control access to those files.  The OS compares
   the label on a process with the labels on the files it attempts to
   access and allows the access only if the process' label dominates
   the file's.

   Consider an alternative implementation in which the files are
   encrypted and the access rights of a process are determined by the
   set of keys given to it at startup.  So, for example, a top-secret
   file would be encrypted by K_topsecret, and only top-secret
   processes would have access to K_topsecret.

   A. Describe how to implement compartments using this scheme.
   Recall that a process should only be able to access a file for
   which it has rights to ALL the compartments.

   B. What are the advantages and disadvantages of this approach?