Cryptography, or cryptology is the practice and study of techniques for secure communication in the presence of third parties called adversaries.

computer science

Description

Introduction

Cryptography, or cryptology is the practice and study of techniques for secure communication in the presence of third parties called adversaries. Until modern times, cryptography referred almost exclusively to encryption, which is the process of converting ordinary information (called plaintext) into unintelligible form (called ciphertext). Decryption is the reverse, in other words, moving from the unintelligible ciphertext back to plaintext. A cipher (or cypher) is a pair of algorithms that carry out the encryption and the reversing decryption. For this assignment you will write a smiple encryption/decryption program called GMUCipher. A GMUCipher uses a document as the cipher key, and the cipher itself uses numbers that reference the words within the text. For example, we can use an edition of The Declaration of Independence as the cipher key. The cipher you will write will use a pair of numbers corresponding to each letter in the text. The first number denotes the position of a word in the key text (starting at 0), and the second number denotes the position of the letter in the word (also starting at 0). For instance, given the following key text (the numbers correspond to the index of the first word in the line):

[0] 'Twas brillig, and the slithy toves Did gyre and gimble in the wabe;

[13] All mimsy were the borogoves, And the mome raths outgrabe.

[23] "Beware the Jabberwock, my son! The jaws that bite, the claws that catch!

[36] Beware the Jubjub bird, and shun The frumious Bandersnatch!"

[45] He took his vorpal sword in hand: Long time the manxome foe he soughtâ€”

The word "computer" can be encoded with the following pairs of numbers:

35,0 catch

5,1 toves

43,3 frumious

48,3 vorpal

22,1 outgrabe.

34,3 that

23,6 "Beware

7,2 gyre

Placing these pairs into a cipher text, we get the following: 35,0,5,1,43,3,48,3,22,1,34,3,23,6,7,2

If you are encoding a phrase, rather than just a single word, spaces in the original english phrase will also appear in the ciphered text. So, the phrase "all done" (using the above Jabberwocky poem) might appear as: 0,3,1,4,13,1 6,0,46,2,44,2,3,2

Only spaces in the key text should be considered delimiters. All other punctuation in the key text are to be considered part of a key word. Thus the first word in the Jabberwocky poem, 'Twas, will have the following characters and positions for key word 0:

Position 0: '

Position 1: T

Position 2: w

Position 3: a

Position 4: s

Developing the Program

You should approach this assignment in several parts. The first part will be to write a menu driven program that prompts the user for the following actions:

1) Read in the name of a text file to use as a cipher key

2) Create a cipher using the input text file (and save the result to a file)

3) Decode an existing cipher (prompt user for a file to read containing the cipher text)

4) Exit the program

For each choice, create a stub function that will be completed in the remaining steps.

After testing your menu, continue to fill in the stub functions with the following specifications:

Choice #1

For this menu choice, you will prompt the user for the name of a cipher text file (such as the Declaration of Independence). You will read this text file line by line, and place each word, in order, in an array of char strings. As you copy each word into the array, you will keep a running count of the number of words in the text file and convert the letters of each word to lower case. You may assume that you will use no more than the first 5,000 words in the text, and no more than the first 15 characters in a word. However, there is no guarantee that the text will have 5000 words or less and any word in the text is 15 characters or less.

Choice #2

If no file has been chosen for Choice #1 (i.e. the user goes directly to Choice #2 without first choosing Choice #1), you will first prompt the user to enter a cipher text and read it into memory (presumably by calling the function that would be called by Choice #1). You will prompt the user to enter a secret message (in plain text - such as "Computer Science is the best major at GMU!") that is terminated by pressing the "Enter" key. You can assume that the length of this message will be less than 1500 characters (including the carriage return and NULL terminator). You will then parse this message, character by character, converting them to lower case as you go, and find corresponding characters in the words found in the key text word array. You can do this by going through each word in the word array, and then each character in each word, until you find a match with the current message character. There are more efficient ways to perform this operation, and you are encouraged to implement them instead of the given method. Once a character match is found, you will write the index of the word and the index of the character to a character string that you will later write out to a text file. Spaces are to be placed into the text as found in the message and will be used to delimit the separate words in the secret message. Once the message has been encoded, prompt the user for the name of a file to save the encoded message to, and save it to that file.

Choice #3

You will prompt the user for the name of a file containing an encoded text (i.e. a file containing number pairs). Your program will read the file and decode the the text using the indexes pairs given for each character in the word and the text file chosen for Choice #1. If no file has been chosen for Choice #1 (i.e. the users goes directly to Choice #3 without first choosing Choice #1), you will prompt the user to enter a cipher text and read it into memory (presumably by calling the function that would be called by Choice #1). Spaces found in the file are to be treated as spaces in the decoded text. You can assume that the number of characters in the encoded text file is 5000 or less, including any carriage returns or NULL terminator characters. Once the text is decoded, print the message to standard output.

Choice #4

Exit the program.

In order to introduce some "randomness" in the specific character encoding, you will generate a random number i from 0..9 inclusive (use the last four digits of your G Number as the seed), and use the ith instance of that character found in the text. (If fewer than i instances of the character is found in the text, loop back and continue the search from the beginning of the document.)

Example: Suppose the letter to encode is a 'c'. Using the sentences just above, we find that there are the following 'c' characters:

In order to introduCe some "randomness" in the spe CifiC Chara Cter enCoding, you will generate a random number i from 0..9 inClusive (use the last four digits of your G Number as the seed), and use the ith instanCe of that CharaCter found in the text.

If the random number returns 6, then you will use the 'c' from the word "inclusive." (Start counting from 0). If the random number returns 2, you would the second c found in the word "specific."

If a given character in the secret message is not found in any word of the text, replace that character with the '#' character in the encoded text (a single '#' character replaces a word/position pair).

Files and filenames will follow the standard CS262 conventions (username, lab section, etc. at top of file, and as part of filename).

Each menu choice should call a separate function to perform the operation.

You can assume that the message to encrypt or decrypt will be less than 1500 characters.

Your program will be compiled using a Makefile

Predefined C functions that may be useful for this project:

strtok()

strlen()

tolower()

atoi()

Use dynamic memory for the arrays

Makefile

You will create a Makefile to compile your program.

The name of the source file will be Project2_<username>_<labsection>.c