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

A project for C/C++ by Daniel McKinney during the fall 2012.

This project was begun on 2/24/12 and is anticipated to take 2 days to complete. Project was completed on 2/25/12.

=====Objectives=====
create 2 programs, one to enchipher and one to dechipher a statment in a .txt file
=====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=====
State the idea or purpose of the project. What are you attempting to pursue?

Upon approval, you'll want to fill this section out with more detailed background information. DO NOT JUST PROVIDE A LINK.

Providing any links to original source material, such as from a project page, is a good idea.

You'll want to give a general overview of what is going to be accomplished (for example, if your project is about installing a web server, do a little write-up on web servers. What is it, why do we need one, how does it work, etc.)

=====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 *message, *cipher, len=0, x, pos=0;
        int acipher;
        message=(char*) malloc(sizeof(char)*24);
        fprintf(stdout,"Enter a message to be encrypted:");
        fscanf(stdin, "%s",message);
        fprintf(stdout,"Enter a cipher shift integer:");
        fscanf(stdin, "%s",cipher);
        acipher=atoi(cipher);
        x=*(message+pos);
        while((x!='\0')&&(x!='\n'))
        {
                len++;
                pos++;
                x=*(message+pos);
        }
        for(pos=0;pos<len;pos++)
        {
                fprintf(stdout, "%c", *(message+pos)+acipher);
        }
        fprintf(stdout, "\n");
        return(0);
}


</code>

The Decipher code
<code c>
#include<stdio.h>
#include<stdlib.h>
int main()
{
        char *message, *cipher, len=0, x, pos=0;
        int acipher;
        message=(char*) malloc(sizeof(char)*24);
        fprintf(stdout,"Enter a message to be decipher:");
        fscanf(stdin, "%s",message);
        fprintf(stdout,"Enter a cipher shift integer:");
        fscanf(stdin, "%s",cipher);
        acipher=atoi(cipher);
        x=*(message+pos);
        while((x!='\0')&&(x!='\n'))
        {
                len++;
                pos++;
                x=*(message+pos);
        }
        for(pos=0;pos<len;pos++)
        {
                fprintf(stdout, "%c", *(message+pos)-acipher);
        }
        fprintf(stdout, "\n");
        return(0);
}
</code>


=====Execution=====

An example run of the enciphering process:

<cli>
lab46:~$ ./encipher
Enter a message to be encrypted:daniel
Enter a cipher shift integer:1
ebojfm
lab46:~$

</cli>

Now, we switch gears and decipher a different (previously enciphered) message:

<cli>
lab46:~$ ./decipher
Enter a message to be decipher:daniel
Enter a cipher shift integer:1
c`mhdk
lab46:~$

</cli>
=====Reflection=====
In this project i learned how to shift chars to the right and left, to "encipher" and "decipher" a word.

=====References=====
In performing this project, the following resources were referenced:

  * URL1
  * URL2
  * URL3 (provides useful information on topic)
  * URL4

Generally, state where you got informative and useful information to help you accomplish this project when you originally worked on it (from Google, other wiki documents on the Lab46 wiki, etc.)
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