code: public static String rot13(String _input) {
          if (_input == null) {
              return null;
          }
          StringBuilder sb = new StringBuilder();
          for (int i = 0; i < _input.length(); i++) {
              char c = _input.charAt(i);
              if (c >= 'a' && c <= 'm') {
                  c += 13;
              } else if  (c >= 'A' && c <= 'M') {
                  c += 13;
              } else if  (c >= 'n' && c <= 'z') {
                  c -= 13;
              } else if  (c >= 'N' && c <= 'Z') {
                  c -= 13;
              }
              sb.append(c);
          }
          return sb.toString();
       }
docs: Simple rot13 implementation.
  
  @param _input input to scramble
  @return scrambled input (null if input was null)