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

The purpose of this project is to encode and decode a message using a program.

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

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.

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).

The encipher code:

#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);
}

Decipher 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);
}

An example run of the enciphering process:

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$

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

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$ 

upon completion of project 1 i have learned a easy way to encipher and decipher and message.

In performing this project, the following resources were referenced:

• C ++ (the C anci)