Homework 1

Due: October 9, 2024
Points: 100

  1. (15 points) Argue for or against the following proposition. Ciphers that the government cannot cryptanalyze should be outlawed. How would your argument change if such ciphers could be used provided that the users registered the keys with the government?

  2. (20 points) A computer security expert has said that without integrity, no system can provide confidentiality.
    1. Assume the system provides no integrity controls. Do you agree with the noted computer security expert? Justify your answer.
    2. Now suppose the system has no confidentiality controls. Can this system provide integrity without confidentiality? Again, justify your answer.

  3. (15 points) A system has two processes, p1 and p2. The system has two files, f1 (owned by p1) and f2 (owned by p2). The rights on the system are r (read), w (write), x (execute), and o (own). The distinguished right o allows the owner to change the owned object’s column in the access matrix. Without an o right, a subject may not make those changes. Initially, each process can read and write the file that it owns.
    1. Please show the access control matrix for this system.
    2. p1 wants to give permission to p2 to write f2. Can p1 do this? If so, please explain why and draw the access control matrix that results from p1 doing so. If not, please explain why not.
    3. p1 wants to give p2 permission to execute f1. Can p1 do this? If so, please explain why and draw the access control matrix that results from p1 doing so. If not, please explain why not. (If the previous part made any changes to the access control matrix, ignore them; use the access control matrix in the first part of the problem as representing the current protection state of the system.)

  4. (30 points) A system has 2 subjects, s1, s2, and 4 objects o1, o2, o3, and o4. Assume that discretionary access controls allow anyone access.
    1. Using the minimum number of security levels, assign security levels to both subjects and objects according to the Bell-LaPadula model, such that the following conditions hold:
            s1 can write only into o3 and o4.
            s2 can write only into o3.
      Hint: Don’t forget to include o1 and o2 in your answer.
    2. Determine which objects can be read by which subjects under the assignment of part a.
    3. Modify the assignment of part a so that s1 cannot read o4. (That is, all the conditions of part a hold, and in addition s1 cannot read o4.)

  5. (20 points) Given the integrity levels HIGHEST, HIGH, MEDIUM, LOW, and LOWEST (ordered from highest to lowest) and the categories X, Y, and Z, specify what type of access (read, write, both, or neither) is allowed by the Biba model in each of the following situations. Assume that discretionary access controls allow anyone access.
    1. A process cleared for (HIGHEST, { X, Y }) wants to access a document classified (LOW, { Y }).
    2. A process cleared for (LOWEST, { X }) wants to access a document classified (LOWEST, { Y }).
    3. A process cleared for (MEDIUM, { Y, Z }) wants to access a document classified (MEDIUM, { X, Y, Z }).
    4. A process cleared for (LOW, { Z }) wants to access a document classified (LOW, { Z }).
    5. A process cleared for (HIGH, { X, Z }) wants to access a document classified (HIGHEST, { Y, Z }).
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Matt Bishop
Office: 2209 Watershed Sciences
Phone: +1 (530) 752-8060
Email: [email protected]
ECS 235A, Computer and Information Security
Version of September 24, 2024 at 11:56AM

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