code: function write_sub_packet(type, data) {
    const arr = [];
    arr.push(packet.writeSimpleLength(data.length + 1));
    arr.push(new Uint8Array([type]));
    arr.push(data);
    return util.concat(arr);
  }
docs: Creates a string representation of a sub signature packet
  @see {@link https://tools.ietf.org/html/rfc4880#section-5.2.3.1|RFC4880 5.2.3.1}
  @see {@link https://tools.ietf.org/html/rfc4880#section-5.2.3.2|RFC4880 5.2.3.2}
  @param {Integer} type subpacket signature type.
  @param {String} data data to be included
  @returns {String} a string-representation of a sub signature packet
  @private