*   extensions      [3]  EXPLICIT Extensions OPTIONAL -- If present, version MUST be v3
   * }
   * ```
   */
  internal val tbsCertificate: BasicDerAdapter<TbsCertificate> =
    Adapters.sequence(
      "TBSCertificate",
      Adapters.INTEGER_AS_LONG
        .withExplicitBox(tag = 0L)
        // v1 == 0.
        .optional(defaultValue = 0),
      Adapters.INTEGER_AS_BIG_INTEGER,

        Signature.getInstance(tbsCertificate.signatureAlgorithmName).run {
          initSign(issuerKeyPair.private)
          update(CertificateAdapters.tbsCertificate.toDer(tbsCertificate).toByteArray())
          sign().toByteString()
        }

      // Complete signed certificate.
      val certificate =
        Certificate(
          tbsCertificate = tbsCertificate,