code: public static String randomString(int length) {
  	StringBuilder sb = new StringBuilder();
  	for (int i = 0; i < length; i++) {
  	    sb.append(chars[AbstractCaptcha.rnd(0, chars.length - 1)]);
  	}
  	return sb.toString();
      }
docs: creates a random String in given length
  
  @param length length of the string to create
  @return