/**
     * Configure the certificate to generate a 2048-bit RSA key, which provides about 112 bits of
     * security. RSA keys are interoperable with very old clients that don't support ECDSA.
     */
    fun rsa2048() =
      apply {
        keyAlgorithm = "RSA"
        keySize = 2048
      }

    fun build(): HeldCertificate {
      // Subject keys & identity.
      val subjectKeyPair = keyPair ?: generateKeyPair()