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 L$0$ and R$0$ such that P $=$ L$0$R$0.$ 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 inreverse. Fori$=$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 K$0$ such that,

K$0=$ b$7$b$6$b$5$b$4$b$3$b$2$b$1$b$0$ K$1=$ b$6$b$5$b$4$b$3$b$2$b$1$b$0$b$7$ K$2=$ b$5$b$4$b$3$b$2$b$1$b$0$b$7$b$6$ K$3=$ b$4$b$3$b$2$b$1$b$0$b$7$b$6$b$5$

DO NOT USE $<<,>>$ OR $^$ AS THEY ARE NOT VALID IN HACK ASSEMBLY.

$1.$ 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$[5].$

$2.$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.

USE XOR.ASM AND ROTATE.ASM IN PART$3$:

$3.$ Write a program $($FeistelEncryption$.$asm$)$ in HACK assembly, that implements the described Feistel encryption system. The initial key, K$0,$ 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$[0].$