fun build(): HeldCertificate {
      // Subject keys & identity.
      val subjectKeyPair = keyPair ?: generateKeyPair()
      val subjectPublicKeyInfo =
        CertificateAdapters.subjectPublicKeyInfo.fromDer(
          subjectKeyPair.public.encoded.toByteString(),
        )
      val subject: List<List<AttributeTypeAndValue>> = subject()

      // Issuer/signer keys & identity. May be the subject if it is self-signed.