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

A project for C++ by Michael during the Spring 2012.

This project was begun on feb 14 and is anticipated to take 2 weeks to complete. Project was completed on Feb 28th, 2012

=====Objectives=====
We are making an encoder and decoder in C.
=====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=====
Writting a program that will encode a text file by chagning the characters.\\
A different file will decode a text file by reversing the change in characters. \\
It asks for a cypher key, and you will need to know that key when you decode it.
=====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>
/*encodes*/
#include<stdio.h>
#include<stdlib.h>

int main()
{
	FILE *original, *encoded;
	int key = 0; //number of asci spaces moved
	int holder = 0; //used as a temp space holder
	char letter;
	original = fopen("file2.txt","r");
	encoded = fopen("encoded.txt","w");

	printf("Please pick your encyphering number: ");
	scanf("%d",&key);
	letter = fgetc(original);
	while(letter !=  EOF)
	{
		if ((letter >='A') && (letter <= 'Z'))
		{
			holder = letter - 65;
			holder = holder + key;
			holder = holder % 26;
			letter = holder + 65;
		}
		if ((letter >= 'a') && (letter <= 'z'))
		{
			holder = letter - 97;
			holder = holder + key;
			holder = holder % 26;
			letter = holder + 97;
		}
		fprintf(encoded, "%c", letter);
		letter = fgetc(original);
	}
	printf("Encoding complete\n");
	fclose(original);
	fclose(encoded);
	return(0);
}
</code>


Deciphering code.

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

int main()
{
	FILE *encoded, *decoded;
	char typed;
	int key = 0;
	char letter;
	unsigned char holder = 0;
	encoded = fopen("encoded.txt", "r");
	decoded = fopen("decoded.txt", "w");

	printf("Please enter the original key number: ");
	scanf("%d", &key);
	letter = fgetc(encoded);
	while(letter != EOF)
	{
		if ((letter >= 'A') && (letter <= 'Z'))
		{
			holder = letter - 65;
			holder = holder - key - 230;
			holder = holder % 26;
			letter = holder + 65;
		}
		if ((letter >= 'a') && (letter <= 'z'))
		{
			holder = letter - 97;
			holder = holder - key - 230;
			holder = holder % 26;
			letter = holder + 97;
		}
		fprintf(decoded, "%c", letter);
		letter = fgetc(encoded);
	}
	printf("Decoding complete, check decoded.txt \n");
	fclose(encoded);
	fclose(decoded);
	return(0);
}

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

An example run of the enciphering process:

<cli>

Please pick your encyphering number: 4
Encoding complete
</cli>

Example run of the decoding process:

<cli>

Please enter the original key number: 4
Decoding complete, check decoded.txt
</cli>
=====Reflection=====
I learned that this project was a pain in the butt.  I also know, I don't want to mess with stuff like this again.  It gave me a headache trying to figure it out, and I don't think I could easly rewrite this program.
=====References=====
In this project, I had a lot of help from Matt, our teacher.  I had so many issues that I couldn't figure out why they were happening.