COMP1212 Computer Processors

School of Computing: assessment brief
Module title Computer Processors
Module code COMP1212
Assignment title Assignment 2: Encryption using a Feistel Cipher
Assignment type
and description
In-course assessment. Requires design, implementation
and testing of code written in assembly language
Rationale
Provides an opportunity to write assembly code including
understanding the implementation of branching and
functions and learn how a Feistel Cipher works for encryption.
Word
limit and
guidance
This coursework should take less than 15 hours to complete.
Weighting
60%
Submission deadline
10am
9/5/24
Submission
method
Gradescope
Feedback provision Feedback will be provided through Gradescope
Learning outcomes
assessed
Explain how high level programming constructs, such
as ’if’ statements and ’for’ loops, are implemented at a
machine level
Module lead Andy Bulpitt
Other Staff contact Noleen K¨ohler

11. Assignment guidance
    The Feistel cipher is a symmetric block cipher encryption framework which is the basis
    of many modern day encryption algorithms. In this coursework you will implement
    a Feistel cipher system as a software implementation in Hack Assembly.
    In a Feistel cipher the plaintext, P, to be encrypted is split into two equal size parts
    L0 and R0 such that P = L0R0. A function F is applied to one half of the plaintext,
    combined with a key, and the result is XOR’d with the other half of the plaintext.
    Feistel ciphers often employ multiple rounds of this scheme. In general the scheme
    works as follows, for all i = 0, . . . , n,
    Li+1 = Ri
    Ri+1 = Li ⊕ F(Ri
    , Ki)
    To decrypt an encrypted message using this cipher we can apply the same procedure
    in reverse. For i = n, n − 1, . . . , 0,
    Ri = Li+1
    Li = Ri+1 ⊕ F(Li+1, Ki)
    For this coursework we are interested in the 16-bit Feistel cipher which uses 4 rounds.
    The function F(A, B) = A ⊕ ¬B.
    The keys are derived from a single 8-bit key K0 such that,
    K0 = b7b6b5b4b3b2b1b0
    K1 = b6b5b4b3b2b1b0b7
    K2 = b5b4b3b2b1b0b7b6
    K3 = b4b3b2b1b0b7b6b5
12. Assessment tasks
    (a) Write a program (XOR.asm) in HACK assembly that implements a bit-wise
    XOR function between two 16-bit values stored in RAM[3] and RAM[4] and
    stores the result in RAM[5].
    [4 marks]
    2(b) Write a program (Rotate.asm) in HACK assembly that implements an algorithm
    to rotate the bits of a 16-bit number left (Least Significant bit (LSb) to Most
    Significant bit (MSb)). The original number should be stored in RAM[3], the
    number of times to rotate the bits should be in RAM[4] and the result stored in
    RAM[5], i.e. 1010111100000000 rotated left 3 times would be 0111100000000101
    where the MSb is used to replace the LSb on each rotation.
    [8 marks]
    (c) Write a program (FeistelEncryption.asm) in HACK assembly, that implements
    the described Feistel encryption system. The initial key, K0, will be stored in
    RAM[1], and the plaintext to be encrypted will be represented by a 16-bit value
    stored in RAM[2]. The result of the encryption should be stored in RAM[0].
    [10 marks]
    [Total 22 marks]
13. General guidance and study support
    Tools required to simulate the hardware and CPU are provided on Minerva under
    Learning resources: Software. You may find it easier to implement cipher in a high
    level language first. This will also allow you to test the results of your HACK program.
    Support will be available during lab classes. Please ensure the files you upload work
    with the test files provided and use the filenames provided in this sheet. Do not
    alter the format of the lines of these test files in any way. The spacing in
    each line needs to be preserved You are of course welcome to build your own
    test files in the same format or add to these files.
14. Assessment criteria and marking process
    This coursework will be automatically marked using Gradescope. Feedback will be
    provided through Gradescope.
    Marks are awarded for passing the automated tests on the submitted programs.
    These will not necessarily be the same tests that are provided to help you develop
    the solution. You should therefore test your solution thoroughly using other values
    for the plaintext and keys before your final submission.
15. Presentation and referencing
    Submitted code should provide suitable comments where possible.
16. Submission requirements
    Links to submit your work can be found on Minerva under Assessment and feedback/Submit
    my work. The HACK assembly (asm) files for each part must be uploaded
    individually. Ensure you use only the filenames provided in this specification
    sheet.
17. Academic misconduct and plagiarism
    Academic integrity means engaging in good academic practice. This involves essential
    academic skills, such as keeping track of where you find ideas and information and
    referencing these accurately in your work.
    By submitting this assignment you are confirming that the work is a true expression
    of your own work and ideas and that you have given credit to others where their
    work has contributed to yours.
18. Assessment/marking criteria
    No marks will be awarded for tests which fail
    • Part a) is graded using 4 tests, each worth 1 mark. [max 4 marks]
    • Part b) is graded using 4 tests, each worth 2 marks. [max 8 marks]
    • Part c) is graded using 4 tests, each worth 2 marks and a further 2 marks for
    optimised solutions that require a lower number of operations to complete the
    encryption [max 10 marks]
    [Total 22 marks]
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