What is the secret message?
COMP 273 Assignment 2 – Fall 2021
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Submission instructions
All work must be your own, and must be submitted by MyCourses. Include your name and student
number in a comment at the top of your source file. Also use comments at the top of your code
to provide any other information you would otherwise include in a README file. Submit only one file,
a2.asm. Do not use a zip archive. Check your submission by downloading your submission from the
server and checking that it was correctly submitted. You will not receive marks for work that is incorrectly
submitted.
Overview
In this assignment, you will encrypt and decrypt messages where letters are shifted by a given number,
which is called the key. For instance, a key of -3 change “D” to “A”. At the end of the alphabet, we wrap
back to the beginning, so a shift of -3 also changes “A” to “X”.
We will assume that the text is in ASCII, and we will work only with capital letters. The key is an integer
between -10 and 10. Where the plain text contains spaces and punctuations, we will leave the spaces and
punctuations unchanged, and skip to the next letter in the text. For instance ”LET’S GO TO THE CAT
MUSEUM!” with key ”3” will becomes ”OHW’V JR WR WKH FDW PXVHXP!”
Decryption is done by simply shifting by a negative amount of the key. For example, key=3 means shift
the letter by -3 when decrypting a message.
Provided code and files
You are provided some code to help you get started, along with a small collection of test files of encrypted
text (messages0.txt – messages4.txt). Once you’ve completed the objectives of the assignment, you
should be able to discover the original message for these encrypted texts. There are no marks associated
for discovering the original messages, but feel free to discuss your discovery with your classmates!
The provided code a2.asm implements a simple command menu to let you read a text file into a buffer,
print the text in the buffer, encrypt the buffer, decrypt the buffer, write the buffer to a file, and quit. The
code handles reading from and writing to files and a few other basic procedures to let you focus on the
job of writing procedures for encrypting text, decrypting text, and guessing keys. Here follows an example
session.
Commands (read, print, encrypt, decrypt, write, quit):r
Enter file name:plain0.txt
LET’S GO TO THE CAT MUSEUM!
Commands (read, print, encrypt, decrypt, write, quit):e
Enter key (upper case letters only):3
OHW’V JR WR WKH FDW PXVHXP!
Commands (read, print, encrypt, decrypt, write, quit):w
Enter file name:out.txt
Commands (read, print, encrypt, decrypt, write, quit):p
OHW’V JR WR WKH FDW PXVHXP!
Commands (read, print, encrypt, decrypt, write, quit):d
Enter key (upper case letters only):3
LET’S GO TO THE CAT MUSEUM!
Commands (read, print, encrypt, decrypt, write, guess, quit):q
Test your program
We provide a few files for you to test your program.
• plain0.txt contains a simple sentense. You can try to encode it and then decode it. You should
get exactly the same sentense back
• messages0.txt – messages4.txt are messages encoded with key = -3.
Implementations
Note that MARS will open files by default in the directory in which it is started. You can place your files
in that location for easy reading and writing, or specify the full path, or likewise specify the directory in
which to start the MARS jar.
Extra MIPS instructions
Here are two extra MIPS instructions that will help you with this assignment:
lbu $DistReg offset($AddrReg) # load a byte from offset($AddrReg) to $DistReg
sb $DataReg offset($AddrReg) # store a byte from $DataReg to offset($AddrReg)
The above two instructions are similar to lw and sw. Insteading of loading/storing a word, lbu and sb
loads/stores a byte (to/from the least significant byte of the register).
EncryptMessage
Write the MIPS assembly code to encrypt the messages, and place your code right after the label EncryptMessage.
You can assume that $a0 contains the address to the message stored in the memory, $s1 contains the
number of characters in the messages, and $a1 contains the key. You only need to encrypt the upper case
letters. The maximum size of the messages is 10000 characters.
DecryptMessage
Write the MIPS assembly code to decrypt the messages, and place your code right after the label DecryptMessage.
You can assume that $a0 contains the address to the buffer, $s1 contains the number of characters in the
messages, and $a1 contains the key. You only need to decrypt the upper case letters. The maximum size
of the messages is 10000 characters.