1. (20 points) This question concerns the ability of attackers to crack UNIX passwords on a system where the password file is world-readable and contains the users' password hashes. Two approaches for reducing the probability that a password will be guessed are:
    1. increase the size of the salt from 12 bits to 24 bits in the obvious way (i.e., flipping all 24 sets of bits in the E table); or
    2. increase the length of the password to 16 characters by hashing the first 8 characters using the current hash function, the second set of 8 characters using the current hash function and the same salt, and concatenating the two.
    Assume an attacker is attempting to guess a particular user's password. Which method increases the estimated time of guessing the password the most? Why?
  2. (20 points) Three different protection mechanisms that we have discussed are capabilities, access control lists, and the UNIX rwx bits. For each of the following protection problems, tell which of these mechanisms can be used, and how. For the UNIX system, assume the groups faculty, administrator, student, and secretary have appropriate membership.
    1. Rick, a faculty member, wants his files readable by everyone except Jennifer, who is the only administrator with access to the computer.
    2. Helen and Anna, both students, want to share some secret files.
    3. Thomas, a secretary, wants some of his files to be readable by everyone.
  3. (20 points) Consider the following protection mechanism. Each object and each process is assigned a number. A process can only access an object if the object has a higher number than the process. Which of the mechanisms discussed in class (ACL, capabilities, lock-and-key, the MULTICS ring protection scheme) does this most closely resemble, and why? In what essential way does it differ from the scheme discussed in class?
  4. (20 points) What is the definition of "perfect secrecy"? What characteristics are necessary for a cipher to achieve this goal (ie., to provide perfect secrecy)?
  5. (20 points) Represent a security compartment label using the notation
    <security level; set of categories>.
    Can a user cleared for secret; { dog, cat, pig } have read or write access (or both) to documents classified in each of the following ways under the military security model?

    1. <top secret; { dog }>
    2. <secret; { dog }>
    3. <secret; { dog, cow }>
    4. <secret; { moose }>
    5. <confidential; { dog, pig, cat }>

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Department of Computer Science
University of California at Davis
Davis, CA 95616-8562

Page last modified on 2/13/98