COMP 2401B — Assignment #4




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COMP 2401B — Assignment #4

Collaboration: You may work in groups of no more than two (2) students
Timmy Tortoise and Harold the Hare are still being held captive by an evil wizard who wants to use their
technical skills to help him get revenge on his rivals. Our heroes were hatching an escape plan, but the
wizard found them out! He has retaliated by locking up our heroes in separate cells. Luckily, Timmy
managed to stash away some of the wizard’s communications equipment and has a plan to use it for
sending secret messages to Harold. Using the VM provided for the course and the socket TCP/IP code
we saw in class, you will assist Timmy by implementing an encrypted chat utility. Your program will help
Timmy and Harold send encrypted messages to each other that the evil wizard is unable to decrypt.
Learning Outcomes
You will practice problem solving and designing modular functions to implement a solution. You will
implement a peer-to-peer system that communicates over TCP/IP sockets. You will also be integrating
into your code a function that is provided as object code only.
1. From client-server to peer-to-peer
Your program will consist of a single executable that combines the client and server code that we saw
in class. When your executable starts up, it will check to see if there was a command line argument
entered after the executable name. If there is no command line argument, the program will wait for a
connection request to come in, as if it were a server waiting for a client to contact it. If there is a
command line argument, it will be the IP address that the program needs to connect to. In that case, it
will initiate a connection request to that IP address, as it it were a client connecting to a server.
When you run your program, you will launch the first instance of it in one window, using no command
line argument. That program will go into wait mode. In another window, you will launch a second
instance of the same program, using the home IP address as a command line argument (in
principle, this could be any IP address in the world, but we’ll keep it simple so that we don’t have deal
with firewalls). Once both programs are connected to each other, the encrypted chatting can begin.
2. Design your program
The first step is to break down your program into modular and reusable functions. You must determine
which functions you need to implement, given the TCP/IP code that we used in class. You need to blend
that client-server code into a single program, and it has to be made up of modular functions.
You are also required to divided up your code into separate source files and provide a header file. Do
not put all the code in the main() function, and do not put it all in the same source file. You will also
provide a Makefile to facilitate program building.
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3. Encrypted chat utility
Once two instances of the program have established a connection, as explained in Instruction #1, the
program will go into chat mode. Chat mode works as follows:
• the users will take turns; at each turn:
º the user whose turn it is will be prompted for a message; this message can be multiple words long
º the message will be encrypted using the algorithm described below
º the encrypted message will be sent over TCP/IP to the other user
º the receiving program instance will decrypt the message (using the same algorithm as encryption),
and it will display both the encrypted message and the decrypted equivalent to its user
º the receiving user will then take their turn
• the program instance that initiated the connection (the one behaving as client) will be first to prompt
its user to enter a message
• the chat ends when one of the users enters “quit” as their message; when this happens:
º the “quit” message is encrypted and sent to the other user
º both program instances end
NOTE: It’s recommended that you do not use the strlen() function to compute the length of an
encrypted message. The encrypted messages sometimes contain zeros within them, which the
strlen() function will treat as a null terminator. This can result in the wrong number of bytes being
decrypted or printed to the screen, and the encryption counters in the two program instances will get
desynchronized. Instead, you should use the return value of the recv() system call, which indicates the
number of bytes that were actually received.
A sample execution is shown in Figure 1.
Figure 1: Sample execution
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4. Encryption and decryption of messages
You will download the a4Posted.tar file from cuLearn and un-tar it in your own VM. This archive
contains a single file: a4­util.o which is an object file that you will link with your own code in the
Makefile that you provide. This file provides the encrypt() function, which has the following prototype:
unsigned char encrypt(unsigned char c, unsigned char k)
It encrypts the given character c with the given key k, and returns the encrypted character as the
return value.
You will use the following encryption/decryption algorithm, which is based on the Cipher Block Chaining
technique in Counter mode (CBC-CTR):
• you will declare two global variables as unsigned chars: one variable for the encryption/decryption
key, and one variable for the counter
◦ we’ll use Timmy’s favourite numbers for initialization: you will initialize the key to 101 and the
counter to 87
• every time the program sends a message, it will encrypt the plain text message into its ciphertext
equivalent before transmission:
◦ the program will loop over every character of the plain text message; for every character:
▪ the global counter is encrypted with the global key using the provided encrypt() function
▪ the encrypted counter is xor’d with the plain text character to produce the corresponding
ciphertext character (the xor operator in C is ^ , as in the expression a^b)
▪ the global counter is incremented by 1
◦ once the entire plain text message has been encrypted into its ciphertext equivalent, the cipher
text is sent to the receiving program
• every time the program receives a message, it will decrypt the received ciphertext into its plain text
equivalent using the exact same algorithm as for encryption. It does not perform the steps in
reverse order! The same steps in the same order will work, as long as the same key and counter
values are used. For example, if the string “hello” is encrypted with the counter beginning at value
87 and ending at 91, it must be decrypted using a counter with those same values.
• your program must be correctly designed and separated into modular, reusable functions
• your program must reuse functions everywhere possible
• your program must perform all basic error checking
• your program must be thoroughly documented, including each function and parameter
• compound data types must always be passed by reference
• all dynamically allocated memory must be explicitly deallocated
• do not use any global variables, except where explicitly permitted
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You will submit in cuLearn, before the due date and time, one tar or zip file that includes the following:
• all source code, including the code provided, if applicable
• a Makefile
• a readme file that includes:
º a preamble (program author, purpose, list of source/header/data files)
º the exact compilation command
º launching and operating instructions
If you are working with a partner:
• only one partner submits the assignment, and the other partner submits nothing
• the submitting partner must enter the names of both partners in the Online Text box of the cuLearn
submission link
• the readme file must contain the names of both partners
Grading (out of 100)
Marking components:
• 20 marks: correct connection sequence, based on command line argument
• 40 marks: correct implementation of the main chat loop
• 40 marks: correct implementation of encryption/decryption algorithm
• Packaging errors:
º 100 marks for an incorrect archive type that is not supported by the VM
º 50 marks for an incorrect archive type that is supported by the VM
º 10 marks for missing readme
º 20 marks for missing Makefile
º 10 marks for not separating the code into different source files
• Major programming and design errors:
º 50% of a marking component that uses global variables, unless otherwise permitted
º 50% of a marking component that is incorrectly designed
º 50% of a marking component that doesn’t pass compound data types by reference
º 100% of a marking component where the function prototype has been modified, where applicable
• Minor programming and design errors:
º 10 marks for consistently missing comments or other bad style
º 10 marks for consistently failing to perform basic error checking
º up to 10 marks for memory leaks
• Execution errors:
º 100% of a marking component that cannot be tested because it doesn’t compile or execute in VM
º 100% of a marking component that cannot be tested because it’s not used in the code
º 100% of a marking component that cannot be proven to run successfully due to missing output
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