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COMP 273 Assignment 3

 Caesar and Vigenere Ciphers `

COMP 273 Assignment 3 - Winter 2021, Prof. Kry
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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, cipher.asm, i.e., you will need to rename the provided file! 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.
1 Overview
In this assignment, you will use the Run I/O console to encrypt and decrypt text using a simple ciphers.
Specifically you will encrypt and decrypt text with a Vigenere cipher, which is a simple extension of the `
well know Caesar cipher, where letters are shifted by a given number. For instance a shift of 3 would
change an A to D. At the end of the alphabet, we wrap back to the beginning, so a shift of 3 also changes
a Z to C.
We will assume that all text is ASCII, and we will work only with capital letters. The key will also be
written using letters where A corresponds to a shift of 0, B to 1, C to 2, and so on. Decryption is done by
simply shifting instead by a negative amount.
In a Vigenere cipher, to use a key with multiple letters, `
we repeat the key over and over again. Where the plain
text contains spaces and punctuation, we will leave the text
unchanged, and skip to the next letter in the key and the
next letter in the text. For instance ”LET’S GO TO THE CAT
MUSEUM!” with key ”AB” will becomes ”LFT’S GP UO TIE
CBT MVSFUN!”, that is, letters in even positions starting at
position zero are not shifted, while letters in odd positions
are shifted by 1.
There are many interesting ways to try to break Vigenere `
ciphers, but among the most simple is to look at letter
frequencies. In this assignment, you will be required to
compute the frequencies of letters to try to identify the key,
but this will not be an entirely automatic process. For a given
guess, you will need to also specify an assumed length of the key and what letter you believe to be most
probable. While the letter E is most common in large blocks of English text, other letters are also very
common. See the Figure inset at right.
Provided code and files
You ar provided some code to help you get started, along with a small collection of test files of encrypted
text. Once you’ve completed the objectives of the assignment, you should be able to discover the keys for
these encrypted texts. There are no marks associated for discovering the keys, so feel free to discuss the
keys and contents of the decrypted text with your classmates!
The provided code 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, guess an encryption
key, 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, guess, quit):r
Enter file name:plain0.txt
THE QUICK BROWN FOX JUMPS OVER THE LAZY DOG.
Commands (read, print, encrypt, decrypt, write, guess, quit):e
Enter key (upper case letters only):B
UIF RVJDL CSPXO GPY KVNQT PWFS UIF MBAZ EPH.
Commands (read, print, encrypt, decrypt, write, guess, quit):w
Enter file name:cipher0.txt
Commands (read, print, encrypt, decrypt, write, guess, quit):g
Enter a nubmer (the key length) for guessing:1
Enter guess of most common letter:O
B
Commands (read, print, encrypt, decrypt, write, guess, quit):p
UIF RVJDL CSPXO GPY KVNQT PWFS UIF MBAZ EPH.
Commands (read, print, encrypt, decrypt, write, guess, quit):d
Enter key (upper case letters only):B
THE QUICK BROWN FOX JUMPS OVER THE LAZY DOG.
Commands (read, print, encrypt, decrypt, write, guess, quit):q
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.
2 EncryptBuffer (3 marks)
Implement the EncryptBuffer procedure. This procedure takes two arguments. The first is a null terminated
ASCII string to encrypt (i.e., the text buffer), while the second is a null terminated ASCII string for the
key. You can assume that the letters in the buffer are all uppercase, and likewise, that the key is specified
with all uppercase letters. You should leave any non-letters unchanged (e.g., punctuation and white
spaces).
3 DecryptBuffer (3 marks)
Implement the DecryptBuffer procedure. This procedure takes two arguments. The first is a null terminated
ASCII string to decrypt (i.e., the text buffer), while the second is a null terminated ASCII string for the
key. You can assume that the letters in the buffer are all uppercase, and likewise, that the key is specified
with all uppercase letters. You should leave any non-letters unchanged (e.g., punctuation and white
spaces).
4 GuessKey (4 marks)
Implement the GuessKey procedure, and note that you should break down the problem into smaller parts
(i.e., write multiple procedures). The procedure takes 4 arguments: the size of the key to guess, the null
terminated text buffer, a block of memory in which to write the guessed key (i.e., you are returning the
guessed key to the caller in this memory), and finally the last parameter is the letter which you can assume
to be most common in the cypher text.
In the example interactive session above, you can see that the guess command was called with a key
length of 1 and a common letter guess of O. To guess the key, you must compute letter frequencies. For
the string ”UIF RVJDL CSPXO GPY KVNQT PWFS UIF MBAZ EPH”, because the letter P is the most
common, this is assumed to map to the letter O, so we guess that the key is B so as to shift all the letters
by one position.
You are free to organize your solution as you please, but must use register calling conventions and the
stack as necessary. You may find it useful to allocated space in the .data segment for an array of integers
for counting frequencies. You may find it useful to write a procedure for counting the letter frequencies.
More specifically for counting frequencies of letters in the position i where i mod n = k, that is, for trying
to figure out the k
th letter of a key where the assumption is that the key is of length n. You may find
it useful to write a helper function for zeroing a block of memory (i.e., your count buffer), or likewise
identifying a letter within the key given a set of frequencies and a presumed common letter.
Breaking your solution into multiple procedures will help you create an easy to understand solution. Feel
free to write additional procedures as necessary or even add commands to the menu that might help (e.g.,
printing letter frequencies might be useful).
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