Project Assignment 1: Introduction to Computer Security

Written Assignment 1 &
Project Assignment 1
CS 458/558: Introduction to Computer Security

Assignments
• Written assignment
– Three problems
– Due on September 20 (after one week)
• Programming assignment
– Implement RC6
– Please don’t copy code from Internet
• easy to find out
– Due on September 27 (after two weeks)
– With a daily 10% penalty
– Grace period: two days (i.e., latest by September 29)
Problem 1
An affine cipher is a type of simple substitution where each letter
is encrypted according to the following rule c = (a p + b) mod
26. Here, p, c, a, and b are each numbers in the range of 0 to 25,
where p represents the plaintext letter, c the ciphertext letter, and
a and b are constants. For the plaintext and ciphertext, 0
corresponds to "a," 1 corresponds to "b," and so on. Consider the
ciphertext QJKES REOGH GXXRE OXEO, which was
generated using an affine cipher. Determine the constants a and b
and decipher the message. Hint: Plaintext "t" encrypts to
ciphertext "H" and plaintext "o" encrypts to ciphertext “E.”
Problem 2
Consider a Feistel cipher with four rounds. Then the
plaintext is denoted as P = (L0, R0) and the
corresponding ciphertext is C = (L4, R4). What is the
ciphertext C, in terms of L0, R0, and the subkey, for each
of the following round functions? (You should get the
most concise solution.)
• A. F(Ri-1, Ki
) = 0
• B. F(Ri-1, Ki
) = Ri-1
• C. F(Ri-1, Ki
) = Ki
• D. F(Ri-1, Ki
) = Ri-1 Å Ki
(Note that for each of cases A – D, the cipher uses four rounds.)
Problem 3
Suppose that we use a block cipher to encrypt according to the
rule :
C0=IVÅE(P0, K),
C1=C0ÅE(P1, K),
C2
=C1ÅE(P2, K),
...
• a. What is the corresponding decryption rule?
• b. Give two security disadvantages of this mode as
compared to CBC mode.
Programming Assignment
Read the paper (particularly Section 1, 2 and Appendix)
about Ron Rivest’s RC6 (version 1.1), which can be
downloaded from:
http://people.csail.mit.edu/rivest/pubs/RRSY98.pdf.
Implement the RC6-w/r/b, where w = 32, and r = 20. So you
can use the test vectors on page 20 to debug your program.
The input of your program should be the user key and either
plaintext (for encryption) or ciphertext (for decryption). The
output of your program should be the ciphertext (for
decryption) or plaintext (for encryption). Use a README
file to explain how the program should be executed. It’s OK
to discuss with your classmates about the details of the
paper, but you should finish programming by yourself.
Submission guidelines
Please hand in your source code and a Makefile electronically (please do not
submit .o or executable code). You must make sure that your code compiles
and runs correctly on a Linux machine.
Write a README file (text file, do not submit a .doc file) which contains
§ Your name and email address
§ Whether your code was tested on bingsuns.
§ Your program runs as follows: “./run ./input.txt ./output.txt”, where input.txt
is the input file, and output.txt is the output file. The format of input.txt and
output.txt is shown on the next slides.
§ Briefly describe your algorithm or anything special about your submission
that the TA should take note of.
Place all your files under one directory with a unique name (such as p1-[userid]
for assignment 1, e.g., p1-ghyan).
Tar the contents of this directory using the following command: tar –cvf
[directory_name].tar [directory_name] E.g., tar -cvf p1-ghyan.tar p1-ghyan/
Use the Blackboard to upload the tared file you created above.
Encryption
Decryption