\chapter{Conclusion and Outlook}
\label{cha:conclusion}
With the setup described in the prevoius chapter, we created a system which is able to read biometric data.
The system encapsultes this data into an Attestation message and sends it to the PIA which is the DAA verifier.
We show in the following section how well the different parts of the setup work together.
\section{Test Results and Limitations}
We describe in the following the test results and give an overview how usable the components of the setup are.

\subsection{Trusted Boot}
The first part of the setup is trusted boot which is well integrated in recent releases of kernel and GRUB bootloader.
Furthermore, unlocking the optional disk encryption with the TPM works fine with the kernel, even when using the manually generated unified kernel.
Only when updating the unified kernel, EFI might have problems loading or finding the correct EFI blob on the boot partition.
This is the case when the update process modifies the EFI bootloader entries, e.g. during a GRUB update.
Hence, a backup boot process is strongly recommended for test setups.
This requires a backup password for the disk encryption since which allows to bypass the TPM during booting.
Otherwise there are no updates possible with the current setup since the affected PCRs are used by the EFI bootloader and cannot be precomputed.

\subsection{IMA}
The next part is IMA which appears to have an easy setup but a complex set of consequences.
When setting IMA in fixing or enforcing mode, logging is enabled and the system slows down significantly.
\autoref{tab:boottimes} shows the performance of system 1 given a setup for a biometric sensor described in \autoref{cha:implementation} with TPM backed disk encryption enabled.
Booting starts with pressing the power button when the system is in off state and ends when the CLI login mask appears.
Rebooting starts when \texttt{reboot} is entered in the shell.
Again the login mask ends the rebooting process.
%TODO measure the remaining systems with the different IMA settings
\begin{table}
  \renewcommand{\arraystretch}{1.2}
  \centering
  \caption{Booting and rebooting times with IMA} \label{tab:boottimes}
  %\rowcolors{2}{lightgray}{white}
  \begin{tabular}{rlll}
    \toprule
    &\textit{System 1}&\textit{System 2}&\textit{System 3} \\
    \midrule
    \textbf{Boot with IMA off}	&\textasciitilde\,27\,s & &\\
    \textbf{Boot with IMA fix} &\textasciitilde\,44\,s	&\\
    \textbf{Boot with IMA enforce}   &	&   &   \\
    \textbf{Reboot with IMA off}	&\textasciitilde\,28\,s	&	&\\
    \textbf{Reboot with IMA fix} &\textasciitilde\,47\,s &   &  \\
    \textbf{Reboot with IMA enforce}   &	&  &   \\
    \bottomrule
  \end{tabular}
\end{table}
The boot procedure is an example for a file intensive process which is reproducible the system.
When IMA is enabled the IMA log shows 2030 entries after a fresh boot.
This means---given that the (very slow) hardware TPM had to extend PCR 10 for every line in the log---the slowdown is mainly caused by the interaction with the TPM itself.

When IMA is set to enforcing, some unexpected problem appeared during updating Ubuntu.
During \texttt{apt upgrade}, the package manager downloads the deb packages into its cache folder at  \texttt{/var/cache/apt/}.
These files, however, do not have the \texttt{security.ima} attribute when the download is finished.
The kernel prevents due to theses missing attributes any access and breaks the upgrade process.
It is not clear why the files are not hashed although apt is run by root and every file created by root must be hashed with the active IMA policy.
Creating a text file via text editor shows exactly this behaviour.

The missing attributes are also a problem when attemping to turn IMA off again.
This requires an updated command line and hence an updated unified kernel.
Generating the new kernel work fine but moving the blob into the EFI partition fails due to the missing support of extended file attributes.
The copy process seems to create the file, but it fails when trying to write the first chunks.
As a result, the only way to update the kernel on this system is to boot a backup kernel and moving the file without IMA set to enforcing.
Applying customized IMA rules might solve both problems.

Other challenges come up when handling the IMA log itself.
This log file is an essential part of the attestation  procedure.
Unfortunately the file becomes several dozens of megabytes when using a common linux setup.
For remote attestation the transmitted data must be as small as possible which makes sending the log directly to a verifier unusable.
This is the reason why we only transmitted the log file hash in the DAA message.

Another essential part is to use the IMA log file do recalculate the produces hashes.
This means on one hand recalculating the hash in the IMA log entry.
On the other hand the chain of hashes should result into the value held in PCR 10.
A small script shown in \autoref{code:verifyimash} tries to recalculate this value.
\begin{lstlisting}[numbers=none,float, caption={Attempt to recalculate the value of PCR 10}, label={code:verifyimash}]
  #!/usr/bin/bash
  set -e
  tpm2_pcrreset 16
  cut -d ' ' -f 2 /sys/kernel/security/ascii_runtime_measurements > ima.hashes
  while read i ;
  do tpm2_pcrextend 16:sha1=$i ;
  done < ima.hashes
  tpm2_pcrread sha1:10,16
\end{lstlisting}
It uses the debug register PCR 16 which is resetable without reboot and has the same initial value as the first 10 PCRs.
When IMA is off, the log holds only one entry of the boot aggregate.
Then the SHA1 value can be computed with that script.
However, comprehending the PCR 10 value with IMA enabled was not possible.
Our tests took into account that PCR and log file could be modified when loading programs to read these resources the first time.
Loading the log several times eventually ends up in a stable log and PCR value (it does not change anymore even when reading the log another time).
The value of PCR 10 was still not reproducible.

Furthermore the documentation of calculating these vaules did not mention how the sha256 hash in PCR 10 is calculated.
\texttt{tpm2\_pcrextend} requires a sha256 hash as input for the corresponding PCR bank, but the IMA log only provides sha1 hashes.
Any PCR 10 extensions regarding the sha256 bank are currently not verifiable.

\subsection{Processing and Sending Biometric Data}
\begin{itemize}
  \item Puts together all pieces
  \item payload without IMA log about 15KB
  \item No encryption for payload, but doable -- depends on the way how Sensor and PIA can communicate together
  \item IMA log much too large
  \item Test results how long the process of capturing takes -- with and without IMA
\end{itemize}

\section{Limitations}
\begin{itemize}
  \item Documentation available for TPM APIs, but no changelog for \texttt{tpm2-tools}.
  \item Trusted boot and IMA can just handle static resources like files, kernel modules and firmware of hardware components.
  Code transmitted over network or otherwse dynamically generated can not be recognized.
  This is an open door for non-persistent attacks.
  \item Documentation on IMA is mostly outdated and so are some tools.
  Further customization of rules may be useful to reduce log size.
  However major Linux distributions support IMA by default on recent releases.
  \item Complexity of verifying system state is too high and is connected to system complexity.
  Reducing number of dependencies and relevant file count is key for this problem.
  \item Implemented DAA does not support a full dynamic group scheme.
  This might be useful in the future, maybe with a custom implementation of a recent DAA version.
\end{itemize}

\section{Future Work}
\begin{itemize}
  \item Remove building tools on target device - just deliver binaries
  \item Remove complex runtime environments like Java, Python, etc. to reduce bloating the integrity logs
  \item Set file system read only, just use e.g. a ramdisk for working files
  \item Integrate USB sensors into the trusted/integrity environment, including device firmware.
\end{itemize}
\subsection{Closing the chain of trust between TPM manufacturer and DAA issuer}
Activate a credential with to certify that the Membership key is in the Endorsement hierarchy, which can be verified with the TPM certificate.
\begin{itemize}
  \item Theoretical concept in the \emph{Practical Guide to TPM 2.0, pp 109 ff}
  \item Practical approach: with EK, AK and AIK to show validity of EK:\\ \url{https://ericchiang.github.io/post/tpm-keys/?utm_campaign=Go%20Full-Stack&utm_medium=email&utm_source=Revue%20newsletter#credential-activation}
\end{itemize}

Further integration in the Digidow environment if DAA is useful for that.

\section{Outlook}
Hardening of the system beyond IMA useful.
Minimization also useful, because the logging gets shorter.

