code: function SecretKey(date=new Date()) {
    publicKey.call(this, date);
    /**
     * Packet type
     * @type {module:enums.packet}
     */
    this.tag = enums.packet.secretKey;
    /**
     * Encrypted secret-key data
     */
    this.encrypted = null;
    /**
     * Indicator if secret-key data is encrypted. `this.isEncrypted === false` means data is available in decrypted form.
     */
    this.isEncrypted = null;
  }
docs: A Secret-Key packet contains all the information that is found in a
  Public-Key packet, including the public-key material, but also
  includes the secret-key material after all the public-key fields.
  @memberof module:packet
  @constructor
  @extends module:packet.PublicKey