Goal

You will write a program in C, in the Ubuntu Linux environment, to provide the end user with the ability to encrypt and decrypt messages, using a secret key. Your program will prompt the user to indicate whether they wish to encrypt a readable ("plaintext") message or decrypt an already encrypted sequence of numbers ("ciphertext") back to its readable form. You will implement the encryption and decryption algorithm described in the instructions below. Your program will output the result (either the ciphertext or the plaintext) to the screen. You will begin with the skeleton code attached.

Learning Objectives

• write a small program in C that is modular, correctly designed and documented
• manipulate values at the bit level

Instructions
1. get/set/clear bit functions Implement the getBit, setBit and clearBit functions, exactly as we saw in class.

2. "skip xor" function Implement the skipx function, which works for both encryption and decryption, i.e. the same algorithm is used for translation in both directions. This function takes one source byte (either plaintext or ciphertext) and a key, and it returns the corresponding encrypted or decrypted resulting byte.

The skipx function must have the following prototype: unsigned char skipx(unsigned char source, unsigned char key) The function must use the following algorithm:

• begin with your resulting byte assigned the value of the source byte

• loop over every odd-numbered bit of the source byte, starting at bit position 7

• perform an xor operation between the current odd-numbered source bit, for example at bit position i and the even-numbered bit of the key corresponding to bit position (i-1)

• set the corresponding odd-numbered bit of the resulting byte to the result (0 or 1) of the xor operation

• return the resulting byte

Note:

• you must use the getBit, setBit and clearBit functions everywhere applicable 3. "rotr" function Implement the rotr function that performs a circular right shift on a byte. Your function will take one byte, shift all the bits one position to the right, taking the least significant bit and placing it into the most significant bit position, and return the result. The rotr function must have the following prototype: unsigned char rotr(unsigned char source)

4. Encryption and decryption Fill in the missing code in the main function of the provided skeleton code:

• if the user chooses to encrypt a message, the program:

mpptr s the user to enter a plaintext message

prompts the user to enter a numeric key (remember the valid range for unsigned chars!)

loops over each character (i.e. each byte) of the plaintext and uses the skipx function to encrypt it o updates the key at every iteration by performing a circular right shift on it  prints the encrypted message as a sequence of numbers to the screen

• if the user chooses to decrypt a message, the program:

mpptr s the user to enter an encrypted message (ciphertext) as a sequence of numbers (read as unsigned chars) terminated by -1

prompts the user to enter a numeric key (remember the valid range for unsigned chars!) o loops over each number of the ciphertext and uses the skipx function to decrypt it updates the key at every iteration by performing a circular right shift on it o prints the decrypted message to the screen

Notes:

• remember! you must use the same key value to decrypt a message as was used to encrypt it

• your program must perform all basic error checking

Testing: You will know that your program works correctly if it can decrypt the following sequence using key 178

114 199 105 196 34 90 242 229 71 208 105 197 111 79 242 249 0 193 103 221 108 94 160 236 82 205 101 136 50 -1

Constraints

• you must use the skeleton program provided and use the function prototypes exactly as stated

• do not use any global variables

• you must use all the functions that you implement and reuse functions everywhere possible

• your program must be thoroughly commented

Submission

You will submit one tar file that includes all the following:

• all source code

• a readme file that includes:

 a preamble (purpose, list of source/header/data files) o the exact compilation command
aunlching and operating instructions