code: public static boolean checkClientId(boolean isLegacyAttributeList, String farEndClientID) {
  		if (ZRTP.CLIENT_ID_LEGACY.equals(farEndClientID)) 
  			return true;
  		
  		if (ZRTP.CLIENT_ID_RFC.equals(farEndClientID)) 
  			return false;
  		
  		boolean isPrintable = true;
  		
  		for (int i = 0; i < farEndClientID.length(); i++) {
  			int singleChar = farEndClientID.charAt(i);
  			if (singleChar < 32 || singleChar >= 127) {
  				isPrintable = false;
  			}
  		}
  		return !isPrintable && isLegacyAttributeList;
  	}
docs: Legacy Client send ZRTP.CLIENT_ID_LEGACY or a non-printable ClientId