1. 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
2. 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]
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(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]
3. 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.
4. 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.
5. Presentation and referencing
Submitted code should provide suitable comments where possible.
6. 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.
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7. 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.
8. 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]

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