Thread: Encryption program using bitwise operators fails during the encryption stage.

Hey guys i am writing an encryption program that uses bitshifts to encrypt characters in a message and then receives a key from the user to perform an opposite shift to restore the original message.I divided the program into three functions: read_msg(), encrypt(), decrypt(). but it fails at the encryption part. i donot want to use another array there i just want to perform the bitshift on the same array and store the values.

Code:

/****************************************************************************************************************************
 * FileName: crypto.c                                                                                                       *
 * Developer: Nyah Check, INK Corp.                                                                                         *
 * Usage   : This program encrypts data following a key entered by the user. and prints the encrypted message.              *
 *                                                                                                                          *
 * Copyright: The code for this program is free to be distributed                                                           *
 *            and licenced freely following the opensource idea                                                             *
 *            no money should be demanded for its usage and the                                                             *
 *            user is responsible for what he/she does with it.                                                             *
 * INK Corporation, 2012..                                                                                                  *
 ****************************************************************************************************************************/

#include <stdio.h>
#include <stdlib.h>
#include <string.h>

#define MAX 140
#define getchar()     getc(stdin)

int read_line(unsigned char s[]);
int encrypt(unsigned char s[], int n);
int decrypt(unsigned char s[], int key);
int i, key;


/**************************************************************************************************************************************
 *Main: This  calls three functions read_line(), encrypt(), and decrypt() for the decryption sequence given by the key.               *
 *                                                                                                                                    *
 *                                                                                                                                    *
 **************************************************************************************************************************************/

int  main()
{
    unsigned char msg[MAX + 1];

    printf("\nThanks for using our encryption program that encrypts text messages.Takes a maximum of 140 characters. ");
    printf("\nDecryption is done using an input key. If you wish to quit the program, ENTER '&'(ampersand)...\n");
    printf("\nINK Corporation, 2012.\n\n");

        printf("\nEnter the plain text: ");
        read_line(msg);

        printf("\nEncrpted text: ");
        encrypt(msg, i);

        printf("Enter decryption code: ");
        key = getc(stdin);

        switch(key)
        {
            case ' ': case '\n': case '\0': printf("\nNo key entered. Try again!!!");
                                            break;

            default:                        decrypt (msg, key);
                                            break;
        }
    return 0;
}

/**************************************************************************************************************************************
 *Function Name: read_line():                                                                                                         *
 *Function:      This function reads input from the keyboard and then passes them to the  encrypting function which then encrypts the *
 *                message.
 *Precondition:  (plaintext).                                                                                                         *
 *Postcondition:    Encrypted message.                                                                                                *
 **************************************************************************************************************************************/

 int read_line( unsigned char s[])
{
    int ch;

    while ( (ch = getchar()) != '\n')
    {
        if ( ch == '&')
        {
            printf("\nThanks for trusting the services of INK Corporation");
            printf(". We would like to receive your comments and feedbacks to better our services");
            printf("\nINK Corporation, PALO ALTO CA. 2012.");
            exit(EXIT_FAILURE);
        }

        else if ( i < MAX)
            s[i++] = ch;

    }

     s[i] = '\0';

     if (i < 0)
     {
        printf("A matching failure occured while writing message.");
        exit(EXIT_FAILURE);
     }

     return i;
}

/******************************************************************************
 *encrypt(): This function encrypts a message depending on a predetermined key*
 *           then prints the encrypted message.                               *
 ******************************************************************************/
int encrypt(unsigned char s[], int i)
{
    int t = 0;
    unsigned char nstr[i];

    while ( s[t] != '\0')
    {

        if ( t < i)
        nstr[t++]  = 20 << s[t];
    }
   nstr[t] = '\0';
   if (nstr == NULL)
   {
        printf("\nAn matching failure occured while encrypting message.");
        exit(EXIT_FAILURE);
   }


    printf("%s\n",nstr);

    return t;
}

/*********************************************************************************
 *decrypt(): This function decrypts the message entered by the user depending on *
 *           the decryption key entered by the user.                             *
 *********************************************************************************/

 int decrypt( unsigned char s[], int key)
 {

     int res, t = 0;

     while ( s[t] != '\0')
     {
          if ( t < i)
            s[t++] = key >> s[t];
     }
     s[t] = '\0';

     if (s == NULL)
     {
        printf("\nAn error occured while decrypting message.");
        exit(EXIT_FAILURE);
    }

     printf("\nDecrypted message: %s", s);

     printf("\nWas the Decryption Succesful (y, n)? ");

     if ((res = getc(stdin)) == 'y' || res ==  'Y')
         printf("\nCongratulations on decrypting your messages.");

     else
        printf("\nTry better next time.. Courage!!!!");

     return t;
 }