code: function SecretSubkey(date=new Date()) {
    secretKey.call(this, date);
    this.tag = enums.packet.secretSubkey;
  }
docs: A Secret-Subkey packet (tag 7) is the subkey analog of the Secret
  Key packet and has exactly the same format.
  @memberof module:packet
  @constructor
  @extends module:packet.SecretKey