static public final boolean verifyPassword(String plaintextPass, String sha512CryptText)
  {
    if (sha512CryptText.startsWith("$6$"))
      {
        return sha512CryptText.equals(Sha512_crypt(plaintextPass, sha512CryptText, 0));
      }
    else
      {
        throw new RuntimeException("Bad sha512CryptText");
      }
  }