Computer Science 530 - Assignment #1 -- Fall 2022

Due: Wednesday, September 14, 2022, 11:00 p.m.

  1. Why do we use encryption modes of operation to convert block ciphers into stream ciphers? What is important about the initialization vector (IV) in a stream cipher, and what happens if the IV is known by the adversary? Explain your answer. What happens if we always use an IV of 0000 or 1111? Can a protocol be designed so it is just as safe to use the same IV to encrypt a message stream, as it is to use a different IV for the stream each time? Explain your answer.
  2. In RSA, an encryption key of e = 3 can be used so long as (p-1)(q-1) is not divisible by 3. For p = 11 and q = 23, let e = 3. Find d. Show how one enciphers the plaintext m = 5 with e = 3 into a value for c. Then show that deciphering c with d yields m again.
  3. When using XOR with a random key as a method of encryption why is it important that they key be used only once? What is the method of encryption called? This form of encryption is provably secure with respect to certain security goals, but it is absolutely vulnerable with respect to other goals. State the purposes for which it is secure and the purposes for which it is weak and explain the reasons for yor assessment.


The report must be submitted by 11:00 p.m. on September 14, 2022. The report should be approximately 3 pages, or roughly 1200 to 1500 words. To submit your report you will use the USC DEN D2L Dropbox submission mechanism. You will use this method regardless of whether you are an on-campus student or a DEN student. Please be sure to include your name in the body of the assignment (i.e. within the Word, PDF, or Text File). It is the individual student's responsibility to follow the submission instruction. Submissions that do not follow this instructions, e.g., submitted late, or only "Saved" and not submitted. may be penalized or may not be graded at all.

For the three reading reports in this course (of which this is one), students may receive an automatic extension of 48 hours total that may be applied across the three homework assignments. If you turn in one of your assignments 8 hours late, then you will only have 40 hours remaining in extensions to use on subsequent assignments. I suggest not using the whole 48 hours on the first assignment, because if you have an unforseen scheduling issue that arises later in the semester, it will be your problem. Late assignments (beyond any extension) will be assesed 1 full letter penalty per day they are late, and if the topic of an assignment is covered in the lecture following the due date, then the assignment will not be accepted beyond that lecture.


This is a lot to cover in so few words - so our advice is to write a first pass at your answer that is longer, and then edit out material that is redundant or not to the point. The use of tables can be very effective in conveying your ideas in a small area, but the tables must be integrated with your textual discussion, and not the only item in your submission.