Cypher.c

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

#define MAXSTR 50

char pt[MAXSTR], ct[MAXSTR], dt[MAXSTR] ;
int i = 0;

int trans(void){ /*Transposition Cypher
                  *Swaps adjacent pairs of letters
                  */


  while (pt[i] != '\0' && pt[i+1] != '\0') /* while the 2 characters are not line terminators */
    {
      ct[i] = pt [i+1]; /*swap the 2 characters*/
      ct[i+1] = pt [i];
    i = i+2; /*move on to next 2 characters*/
    }

  if (pt[i] == '\0') /*even number of characters*/
    {
    ct[i] = pt[i]; /*copy over line terminator*/
    }

  else 
    {
    ct[i] = pt[i]; /*copy over last character*/
    ct[i+1] = pt[i+1]; /*copy over line terminator*/
    }
 
    printf("encyphered text:\n%s\n", ct);
    return 0; 
}

int rot13(void){

  while (pt[i] != '\0')
    {
      switch (pt[i]){
      
      case 'A' : ct[i] = 'N'; break;
      case 'B' : ct[i] = 'O'; break;
      case 'C' : ct[i] = 'P'; break;
      case 'D' : ct[i] = 'Q'; break;
      case 'E' : ct[i] = 'R'; break;
      case 'F' : ct[i] = 'S'; break;
      case 'G' : ct[i] = 'T'; break;
      case 'H' : ct[i] = 'U'; break;
      case 'I' : ct[i] = 'V'; break;
      case 'J' : ct[i] = 'W'; break;
      case 'K' : ct[i] = 'X'; break;
      case 'L' : ct[i] = 'Y'; break;
      case 'M' : ct[i] = 'Z'; break;
      case 'N' : ct[i] = 'A'; break;
      case 'O' : ct[i] = 'B'; break;
      case 'P' : ct[i] = 'C'; break;
      case 'Q' : ct[i] = 'D'; break;
      case 'R' : ct[i] = 'E'; break;
      case 'S' : ct[i] = 'F'; break;
      case 'T' : ct[i] = 'G'; break;
      case 'U' : ct[i] = 'H'; break;
      case 'V' : ct[i] = 'I'; break;
      case 'W' : ct[i] = 'J'; break;
      case 'X' : ct[i] = 'K'; break;
      case 'Y' : ct[i] = 'L'; break;
      case 'Z' : ct[i] = 'M'; break;
      case 'a' : ct[i] = 'n'; break;
      case 'b' : ct[i] = 'o'; break;
      case 'c' : ct[i] = 'p'; break;
      case 'd' : ct[i] = 'q'; break;
      case 'e' : ct[i] = 'r'; break;
      case 'f' : ct[i] = 's'; break;
      case 'g' : ct[i] = 't'; break;
      case 'h' : ct[i] = 'u'; break;
      case 'i' : ct[i] = 'v'; break;
      case 'j' : ct[i] = 'w'; break;
      case 'k' : ct[i] = 'x'; break;
      case 'l' : ct[i] = 'y'; break;
      case 'm' : ct[i] = 'z'; break;
      case 'n' : ct[i] = 'a'; break;
      case 'o' : ct[i] = 'b'; break;
      case 'p' : ct[i] = 'c'; break;
      case 'q' : ct[i] = 'd'; break;
      case 'r' : ct[i] = 'e'; break;
      case 's' : ct[i] = 'f'; break;
      case 't' : ct[i] = 'g'; break;
      case 'u' : ct[i] = 'h'; break;
      case 'v' : ct[i] = 'i'; break;
      case 'w' : ct[i] = 'j'; break;
      case 'x' : ct[i] = 'k'; break;
      case 'y' : ct[i] = 'l'; break;
      case 'z' : ct[i] = 'm'; break;
      default : ct[i] = pt[i];
      }
      
      i = i + 1;

    }

    ct[i] = pt[i]; /*copy over the line terminator*/

    printf("encyphered text:\n%s\n", ct);
    return 0; 
}

int main(void){

  int X;
  
  printf("enter a string less than 50 characters>");
  scanf("%s", &pt);

  printf("0: Exit\n1: Transposition cypher\n2: ROT13\nEnter number > ");
  scanf("%d", &X);

  while (X != 0)
  {

    switch (X){
  
    case 1 : trans(); break;
    case 2 : rot13(); break;
    }

    printf("0: Exit\n1: Transposition cypher\n2: ROT13\nEnter number > ");
    scanf("%d", &X);
    strncpy (pt, ct, MAXSTR);
    printf("%s\n%s\n", pt, ct);
    
  }
  return 0;
}