======Project: SECRET AGENT======

A project for C/C++ Programming by Saad Malik for SPRING 2012.

This project was begun on 2/22/12 and is anticipated to take 2 days to complete. Project was completed on February 24, 2012.

=====Objectives=====
Create a program that will encipher a message (make it unreadable).

Create a second program that will take that unreadable message, and make it readable once again.
=====Prerequisites=====
In order to successfully accomplish/perform this project, the listed resources/experiences need to be consulted/achieved:

  * successful completion of project #0 and solid understanding of pertinent topics
  * familiarity with memory allocation via **malloc(3)**
  * familiarity with looking up C function parameters/information in the manual
  * familiarity with if statements, especially in use for error detection
  * familiarity with pointers and pointer arithmetic
  * familiarity with loops
  * file I/O

=====Background=====

The purpose of this project is to write a program that will encipher, and a program to decipher, a message.

This will basically involve the use of if blocks, looping, command line arguments, and sorting strings into arrays (and manipulating the data therein).
=====Scope=====
This project will test your familiarity with more involved algorithms, storage, and control structures.

Encoding is the process of converting a message into a coded form.

Decoding is the reverse- retrieving that original message from an encoded form.

In this project you will write a 2 programs: one that will take a plain text message and convert it into a coded form (largely indecipherable to the regular english expecting eye) using a given cipher key, and another that will reverse the process (or decipher), when given the appropriate key.

The key is a numeric used to rotate the alphabet a set amount. Where A would normally equal 1, and Z 26, using a cipher key of 1 to shift 1 position to the right; A would now equal 2 (or B), B is now 3 (or C), and Z would be 1 (A).

Your encoding program can operate as follows:

  * obtains its cipher key from a text file called "key.txt"
  * or, if you prefer, use command-line arguments to provide the key
  * obtains the input message from a file called "message.txt"
  * if "plain.txt" is blank, or does not exist, the program should prompt the user to enter the message via STDIN
  * outputs the ciphertext message to STDOUT **AND** saves it to a file called "cipher.txt"
  * implement error checking to avoid segfaults

The decoding operation is essentially the reverse (shifting to the left). I'd suspect you could reuse much of the same logic.

Your decoding program:

  * obtains its cipher key from a text file called "key.txt"
  * or, if you prefer, use command-line arguments to provide the key
  * obtains the input cipher from a file called "cipher.txt"
  * if "cipher.txt" is blank, or does not exist, the program should prompt the user to enter the message via STDIN
  * outputs the plaintext message to STDOUT **AND** saves it to a file called "plain.txt"
  * implement error checking to avoid segfaults

If you want, you may implement both functionalities into one program so long as you provide a mechanism for the user to access both operations (but note this is not required for successful completion of this project).

=====Code=====

==The encipher code:==

<code c>
#include <stdio.h>
#include <stdlib.h>

//THIS PROGRAM WILL ENCIPHER A MESSAGE GIVEN A COMMAND LINE ARGUMENT
//THAT WILL BE USED AS THE ENCIPHERING KEY

int main(int argc, char **argv)
{
        FILE *in, *out;
        int cipher;

        long long int character;

        in = fopen("plain.txt", "r");
        out = fopen("cipher.txt", "w");

//This if block will check to make sure that the user entered a cipher key
//as an argument when they ran the program

        if(*(argv+1) == NULL)
        {
                printf("\nERROR\n");
                printf("You did not provide an argument for the cipher key\n");
                printf("Please run the program again, with 1 integer argument that will be the cipher key\n");
                exit(1);
        }

//This if block will ensure user does not enter an ungodly cipherkey

        if(atoi(*(argv+1)) > 10000)
        {
                printf("\nPlease for the love of god enter a reasonable cipher key\n");
        exit(1);
        }


//This if block will allow the user to enter a message if the input file
//does not exist, else input the message from plain.txt.
//The message will also be ciphered and outputted to the cipher.txt and STDOUT.

        char *message;
        message = (char*)malloc(sizeof(char) * 127);
        int count = 0;

        if(in == NULL)
        {
                printf("\nPlease enter the message you want to encrypt\n");
                fgets(message, 127, stdin);
                printf("\nThe message that you want to encrypt is: \n");

                while(*(message+count) != '\0')
                {
                        printf("%c", *(message+count));
                        count = count+1;
                }

                int cipher = atoi(*(argv+1));
                count = 0;

                printf("\nThe ciphered message is: \n");
                while(*(message+count) != '\0')
                {
                        character = 1;

                        if(*(message+count) >= 65 && *(message+count) <= 90)
                        {
                                character = *(message+count) + cipher;

                                while(character > 90)
                                {
                                        character = character - 26;
                                }
                        }

                        if(*(message+count) >= 97 && *(message+count) <= 122)
                        {
                                character = *(message+count) + cipher;

                                while(character > 122)
                                {
                                        character = character - 26;
                                }
                        }
                        if(character == 1)
                        {
                                character = 32;
                        }

                        printf("%c", character);
                        fprintf(out, "%c", character);
                        count = count+1;
                }

                printf("\nThe enciphered message is located in cipher.txt\n");
        }

        else
        {
                fgets(message, 127, in);

                printf("\nThe message that you want to encrypt is: \n");

                while(*(message+count) != '\0')
                {
                        printf("%c", *(message+count));
                        count = count+1;
                }

                int cipher = atoi(*(argv+1));
                count = 0;

                printf("\nThe ciphered message is: \n");
                while(*(message+count) != '\0')
                {
                        character = 1;

                        if(*(message+count) >= 65 && *(message+count) <= 90)
                        {
                                character = *(message+count) + cipher;
                                     while(character > 90)
                                {
                                        character = character - 26;
                                }
                        }

                        if(*(message+count) >= 97 && *(message+count) <= 122)
                        {
                                character = *(message+count) + cipher;

                                while(character > 122)
                                {
                                        character = character - 26;
                                }
                        }
                        if(character == 1)
                        {
                                character = 32;
                        }

                        printf("%c", character);
                        fprintf(out, "%c", character);
                        count = count+1;
                }

                printf("\nThe enciphered message is located in cipher.txt\n");
        }
        fclose(in);
        fclose(out);

        return (0);
}                                                                                                                 
</code>

==The decipher code:==

<code c>
#include <stdio.h>
#include <stdlib.h>

//THIS PROGRAM WILL TAKE A CIPHERED MESSAGE AND DECIPHER IT
//AS LONG AS YOU PROVIDE THE CORRECT COMMAND LINE ARGUMENT
//FOR A DECIPHER KEY

int main(int argc, char **argv)
{

        FILE *in, *out;
        int cipher;

        long long int character;

        in = fopen("cipher.txt", "r");
        out = fopen("plain.txt", "w");

//This if block will check to make sure that the user entered a cipher key
//as an argument when they ran the program

        if(*(argv+1) == NULL)
        {
                printf("\nERROR\n");
                printf("You did not provide an argument for the cipher key\n");
                printf("Please run the program again, with 1 integer argument that will be the cipher key\n");
                exit(1);
        }


//This if block will allow the user to enter a message if the input file
//does not exist, else input the message from plain.txt.
//The message will also be ciphered and outputted to the cipher.txt and STDOUT.

        char *message;
        message = (char*)malloc(sizeof(char) * 127);
        int count = 0;

        if(in == NULL)
        {
                printf("\nPlease enter the message you want to un-encrypt\n");
                fgets(message, 127, stdin);
                printf("\nThe message that you want to un-encrypt is: \n");

                while(*(message+count) != '\0')
                {
                        printf("%c", *(message+count));
                        count = count+1;
                }

                int cipher = atoi(*(argv+1));
                count = 0;

                printf("\nThe un-ciphered message is: \n");
                while(*(message+count) != '\0')
                {
                        character = 1;

                        if(*(message+count) >= 65 && *(message+count) <= 90)
                        {
                                character = *(message+count) - cipher;

                                while(character < 65)
                                {
                                        character = character + 26;
                                }
                        }

                        if(*(message+count) >= 97 && *(message+count) <= 122)
                        {
                                character = *(message+count) - cipher;

                                while(character < 97)
                                {
                                        character = character + 26;
                                }
                        }
                        if(character == 1)
                        {
                                character = 32;
                        }

                        printf("%c", character);
                        fprintf(out, "%c", character);
                        count = count+1;
                }

                printf("\n");
        }

        else
        {
                fgets(message, 127, in);

                printf("\nThe message that you want to un-encrypt is: \n");

                while(*(message+count) != '\0')
                {
                        printf("%c", *(message+count));
                        count = count+1;
                }

                int cipher = atoi(*(argv+1));
                count = 0;

                printf("\nThe un-ciphered message is: \n");
                while(*(message+count) != '\0')
                {
                        character = 1;

                        if(*(message+count) >= 65 && *(message+count) <= 90)
                        {
                                character = *(message+count) - cipher;

                                while(character < 65)
                                {
                                        character = character + 26;
                                }
                        }

                        if(*(message+count) >= 97 && *(message+count) <= 122)
                        {
                                character = *(message+count) - cipher;
                                while(character < 97)
                                {
                                        character = character + 26;
                                }
                        }
                        if(character == 1)
                        {
                                character = 32;
                        }

                        printf("%c", character);
                        fprintf(out, "%c", character);
                        count = count+1;
                }
                printf("\nThe un-enciphered message is located in plain.txt\n");
        }
        fclose(in);
        fclose(out);

        return (0);
}

</code>
=====Execution=====
This is the program running a cipher.  Without an argument, it gives you an error to rerun with an argument.

It inputs the message from a txt file, and then enciphers it.  If the file doesn't exist, then it will prompt the user to enter a message.

The enciphered message gets stored in cipher.txt.

<cli>
lab46:~/src/cprog/Projects$ ./Project1

ERROR
You did not provide an argument for the cipher key
Please run the program again, with 1 integer argument
lab46:~/src/cprog/Projects/Project1$ ./encipher 9183829183

Please for the love of god enter a reasonable cipher key
lab46:~/src/cprog/Projects/Project1$ ./encipher 284

The message that you want to encrypt is: 
Hello my name is Pastor John xyz XYZ  
The ciphered message is: 
Fcjjm kw lykc gq Nyqrmp Hmfl vwx VWX  
The enciphered message is located in cipher.txt
lab46:~/src/cprog/Projects/Project1$ 

</cli>

==DECIPHER==

<cli>
lab46:~/src/cprog/Projects/Project1$ ./decipher

ERROR
You did not provide an argument for the cipher key
Please run the program again, with 1 integer argument that will be the cipher key
lab46:~/src/cprog/Projects/Project1$ ./decipher 284

The message that you want to un-encrypt is: 
Fcjjm kw lykc gq Nyqrmp Hmfl vwx VWX  
The un-ciphered message is: 
Hello my name is Pastor John xyz XYZ  
The un-enciphered message is located in plain.txt
lab46:~/src/cprog/Projects/Project1$ 

</cli>
=====Reflection=====
The text files are created by the program, however if they do not exist the program does prompt the user to enter a message instead.

Also, the program has checks in place in case the user does not enter a command line argument.

This project really helped me fully understand how to manipulate strings of data, arrays, and command line arguments.  All the other concepts used I knew fairly well, but it strongly reinforced these three big ones and I can really appreciate that.
=====References=====
In performing this project, the following resources were referenced:

http://www.asciitable.com/                          -- Helped deal with the ASCII aspect of the project.

http://www.cprogramming.com/tutorial/lesson9.html   -- This webpage helped me make sense of how to handle strings in C.

Matthew Haas also introduced the nice fgets and atoi functions.