* [rfc_7468]: https://tools.ietf.org/html/rfc7468
   */
  fun privateKeyPkcs8Pem(): String {
    return buildString {
      append("-----BEGIN PRIVATE KEY-----\n")
      encodeBase64Lines(keyPair.private.encoded.toByteString())
      append("-----END PRIVATE KEY-----\n")
    }
  }

  /**
   * Returns the RSA private key encoded in [PKCS #1][rfc_8017] [PEM format][rfc_7468].
   *