\subsection{The update mechanism}

The update mechanism is split into four main phases: checking for updates, reprogramming the device, calculating and verifying the cryptographic signature of the updated firmware, and - assuming that the update was successful - reconfiguring the boot process to use the new firmware.

\subsubsection{Checking for updates}
In order to inform the IoT devices of the availability of a new firmware version, the update server provides a file for each device type containing meta-information about the latest available firmware version.
The meta-information file has a simple line oriented ASCII format, which is easy to generate and efficient to parse within the limited constraints of the embedded device.
It consists of the version identifier and the cryptographic signatures of both of the firmware binaries.
The version identifier can be an arbitrary string as the content is not interpreted semantically but only compared to the version identifier used during build time.
The other two lines in the meta-information file provide the hexadecimal representation of the cryptographic signatures, one line for each firmware binary file.
These meta-information files are provided by the update server using \textit{HTTP 1.1} \cite{HTTP_1.1} under the following path pattern: \texttt{\$\{DEVICE\}.version} (whereas \texttt{\$\{DEVICE\}} gets replaced by the device type name).

Each device queries the update server regularly (initially when the boot process is finished and periodically once an hour) for the currently available firmware version. 
It uses the \texttt{UPDATER\_URL} option to identify the update server.
After the meta-information file has been downloaded successfully, the version identifier is extracted and compared to the version identifier of the running firmware.
If the version identifiers differ, the update process is initialized.
In cases where the download fails, the update server or network connection is not available, or any other error occurres, another attempt will be made automatically at the next regular interval.
In addition to the interval, a special \textit{MQTT} topic shared by all devices is subscribed on device startup: \texttt{\$\{MQTT\_REALM\}/update}.
Every time a message is received on this topic, a fetch attempt for the meta-information file is triggered and the process restarts.
This allows faster roll-outs of updates and finer control for manual maintenance.

\subsubsection{Reprogramming the device}
The firmware files provided on the update server are the exact same ones as used to initially flash the chip for the according version.
Using the same files for flashing and updating allows better debugging by eliminating errors related to the update process itself and eases development and initial installation.
Listing~\ref{lst:choosing_rom} shows the algorithm used to determine the download address and reconfigure the bootloader.

\begin{lstlisting}[caption={Configuring the updater to download the right firmware image and update the booloader accordingly.},
                            label=lst:choosing_rom, basicstyle=\ttfamily\scriptsize]
#define URL_ROM(slot) (( URL "/" DEVICE ".rom" slot ))

// Select rom slot to flash
const auto& bootconf = rboot_get_config();

// Add items to flash
if (bootconf.current_rom == 0) {
  updater.addItem(bootconf.roms[1], URL_ROM("1"));
  updater.switchToRom(1);
} else {
  updater.addItem(bootconf.roms[0], URL_ROM("0"));
  updater.switchToRom(0);
}
\end{lstlisting}

The update server provides these files in the exact same way as it provides the meta-information files, but the path pattern differs: the suffixes \texttt{.rom\{0,1\}} are used to provide the firmware image files for the first and second slot respectively.
For installing a firmware update, the new firmware image file is downloaded using an \textit{HTTP 1.1} \texttt{GET} request.

\subsubsection{Verifying the cryptographic signature}
While the image is being downloaded, each chunk received in the download stream is used to update the \textit{SHA256} hash before it is written to the flash.
When the write has been finished, the next chunk is received and the process continues until all chunks have been processed.

After downloading the new firmware image has been finished successfully, the calculated hash is checked against the signature of the according firmware image.
Therefore the cryptographically signed hash, which was provided in the meta-information file triggering the update, is verified against the \textit{Curve25519} public key stored as a constant in the running firmware.
Only if the checksum matches the provided signature, the firmware is considered valid and the process is continued.

\subsubsection{Reconfiguring the boot process}
For the bootloader, \textit{rBoot}\cite{rBoot} has been choosen as it is integrated within the \textit{Sming} framework and allows to boot to multiple ROM slots.
For configuration, an \textit{rBoot} specific structure is placed in the flash at a well-known location directly after the space reserved for the bootloader code.
This structure contains, among other things, the target offsets for all known ROM slots and the number of the ROM slot to boot on next startup.

To switch to the updated ROM slot after successful installation, the number ROM slot to boot on startup is changed in the configuration section and the device is restarted.
