* in the format of [System.currentTimeMillis]. Specify -1L for both values to use the default
     * interval, 24 hours starting when the certificate is created.
     */
    fun validityInterval(
      notBefore: Long,
      notAfter: Long,
    ) = apply {
      require(notBefore <= notAfter && notBefore == -1L == (notAfter == -1L)) {
        "invalid interval: $notBefore..$notAfter"
      }