Captain Crunch Decoder Ring

In the late 60's, John Draper (aka Captain Crunch) discovered that a toy whistle packaged in boxes of Cap'n Crunch cereal could be modified to emit a tone at precisely 2600 hertz -- the frequency used by AT&T to indicate that a trunk line was ready and available to route a new phone call.

Knowing that strings are really arrays allows us to 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 (disparagingly) refers to a trivial cryptographic system
• Our Captain Crunch decoder ring works by mapping the characters in a-z to a scrambled sequence: tfhxqjemupidckvbaolrzwgnsy

The Program

/*********************************************
** File: crunch.c
** Author: S. Bogar
** Date: 1/2/44
** 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 = (int) 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] = (char) '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