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

A project for C++ by Robert Matsch during the Spring 2012.

This project was begun on febuary 23 and is anticipated to take 2 days to complete. Project was completed on 2/25/2012
=====Objectives=====
The purpose of this project is to encode and decode a message using a program.
=====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 the project is to take a message and encipher it with a key. The enciphered message should be sent to a file which you can read. you should also have program that thakes and encipher message and can decipher the message to come up with the original message.

=====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>

int main(){
        char letter1=0;
        char msg[250];
        FILE *in, *out, *key,*nf;
        char letter = 0;
        int keyvalue;

        nf = fopen("nofile.txt", "w");
        in = fopen("message.txt", "r");
        out = fopen("cipher.txt", "w");
        key = fopen("key.txt","r");

        if (in == NULL){
                fputs("please enter a message to cipher:\n", stdout);
                fgets(msg, sizeof msg, stdin);
                fprintf(nf, "%s", msg);
                fclose(nf);
                nf = fopen("nofile.txt", "r");
                letter1 = fscanf(nf,"%c", &letter);
        }else{
                 letter1 = fscanf(in, "%c", &letter);

        }if (key == NULL){
                printf("please enter a key to encipher with :\n");
                scanf("%d", &keyvalue);
        }else{
                fscanf(key,"%d",&keyvalue);
        }
        while (letter1 != EOF)
        {
                if((letter >= 'A') && (letter <= 'Z'))
                        letter = (((abs(letter - 65) + keyvalue)%26)+65);
                else if((letter >= 'a') && (letter <= 'z'))
                        letter = (((abs(letter - 97) + keyvalue)%26)+97);
 fprintf(out,"%c",letter);
                if (in == NULL){
                        letter1 = fscanf(nf, "%c", &letter);
                }else{
                        letter1 = fscanf(in, "%c", &letter);
                }

        }if (in == NULL){
                fclose(nf);
        }else{
                fclose(in);
        }
        if (key != NULL){
                fclose(key);
        }


        
        fclose(out);
        return(0);
}
</code>
Decipher code.
<code>
#include <stdio.h>
#include <stdlib.h>
#include <math.h>

int main(){
        char letter1=0;
        char msg[250];
        FILE *in, *out, *key,*nf, *cipher;
        char letter = 0;
        int keyvalue;

        cipher = fopen("cipher.txt", "r");
        out = fopen("plain.txt", "w");
        letter1 = fscanf(cipher, "%c",&letter);
        printf("the message to decipher is %c\n", letter);

        printf("please enter a key to decipher\n");
        fscanf(stdin, "%d", &keyvalue);

        while (letter1 != EOF)
        {
                if((letter >= 'A') && (letter <= 'Z'))
                        letter = (((abs(letter - 65) - keyvalue)%26)+65);
                else if((letter >= 'a') && (letter <= 'z'))
                {
                        letter = ( letter-97)-keyvalue;
                        if (letter<0){
                                letter =(26+letter);
                        }
                        letter = letter +97;
                }
                fprintf(stdout, "%c", letter);
                fprintf(out,"%c",letter);
                letter1 = fscanf(cipher, "%c",&letter);
        }



        fclose(cipher);
        fclose(out);
        return(0);
}


</code>
=====Execution=====

An example run of the enciphering process:

<cli>
lab46:~/src/cprog$ ./prog1x4
please enter a message to cipher:
hey
please enter a key to encipher with :
2
lab46:~/src/cprog$

lab46:~/src/cprog$ cat cipher.txt
jga
lab46:~/src/cprog$
</cli>

Now, we switch gears and decipher a different the message:

<cli>
lab46:~/src/cprog$ echo "12" > key.txt
lab46:~/src/cprog$ echo "jga" > cipher.txt
lab46:~/src/cprog$ ./prog1x4
Cipher key found in key.txt: 12

 Cipher is: hja
Message is: hey
lab46:~/src/cprog$ cat plain.txt
hey
lab46:~/src/cprog$ 
</cli>
=====Reflection=====
upon completion of project 1 i have learned a easy way to encipher and decipher and message.
=====References=====
In performing this project, the following resources were referenced:

  * C ++ (the C anci)
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