Captain Crunch Decoder Ring

Since strings are stored as arrays, we can access the elements of the array and change them directly. For example, we can now write an implementation of the Captain Crunch secret decoder ring easily.

• A Captain Crunch decoder ring system is a trivial cryptographic system, a child's game.
• The Captain Crunch decoder ring works by mapping the characters a-z to a scrambled sequence: tfhxqjemupidckvbaolrzwgnsy

The Program

/*********************************************
** File: crunch.c
** Author: R. Chang
** Date: ?
** Modified by: Sue Evans
** Modification date: 9/26/05
** Section: 101
** EMail: bogar@cs.umbc.edu
*
** Description:
**   An implementation of the Captain Crunch
**   secret decoder ring.
**********************************************/
#include <stdio.h>
#include <ctype.h>

#define SIZE  25

int main() 
{
   char c, str[SIZE] = "this is a secret message";
   int index, i;

   /* Initialize our code */
   char code[26] = 
        {'t','f','h','x','q','j','e','m','u','p',
         'i','d','c','k','v','b','a','o','l','r',
         'z','w','g','n','s','y'} ;

   /* Print the original phrase */
   printf ("Original phrase: %s\n", str);

   /* Encrypt */
   for (i = 0; str[i] != '\0'; i++) 
   {
      if(isalpha(str[i])) 
      {
	 c = tolower(str[i]) ;
	 index =  c - 'a' ;
	 str[i] = code[index] ;  
      } 
   }
   printf("      Encrypted: %s\n", str ) ;

   /* Decrypt */
   for(i = 0; str[i] != '\0'; i++) 
   {
      if(isalpha(str[i])) 
      {
	 c = tolower(str[i]) ;

	 /* find matching character */
	 for(index = 0; index < 26; index++) 
         {
	    if(code[index] == c) 
	    {
	       str[i] =  'a' + index ;
	    }  
	 } 
      }
   } 
   printf("      Decrypted: %s\n", str ) ;

   return 0;
}

The Sample Run

Original phrase: this is a secret message
      Encrypted: rmul ul t lqhoqr cqllteq
      Decrypted: this is a secret message