\chapter{Testing}
\label{cha:testing}
With the setup described in the prevoius chapter, we created a system which is able to capture and process biometric data.
The system encapsulates 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 each part of the setup work.
Furthermore we analyze the footprint in memory and on the disk as well as timing if applicable.

\section{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 seamless with the kernel, even when using the manually generated unified kernel without GRUB.
Only when performing an update for GRUB, it will modify the entries in the EFI boot manager.
Consequently, we recommend a check of the boot entries after a system upgrade.

Having a backup boot option with GRUB is useful for maintenance tasks.
Especially for modifying the IMA configuration via kernel command line, it may be necessary to boot with a backup kernel.
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.

GRUB already supports trusted boot and activation requires a line in the corresponding config file.
Unlike GRUB, the unified kernel does not perform any measurements.
Only when asking the TPM for the disk encryption key, the initramfs must have the TPM stack available.
These files use about 62\,KB of space in initramfs which is negligible to the complete image at \texttt{/boot/initrd.img-5.4.0-84-generic} using about 80\,MB.
Furthermore, only in this case exists a measurable difference in the boot performace since asking the TPM takes around 1\,s.
This is the time when the boot process posts \texttt{unlocking via TPM} and stays there until the disk is unlocked.
The used amount of memory was not tested here, but it should be comparable to the results when generating a TPM key for the member.
According to these results, we assume that less than 10\,KB are used to hold the TPM stack in memory.

\section{IMA}
IMA appears to have an easy setup but a complex set of consequences.
When setting IMA in fixing or enforcing mode, the system's performance drop is significant.
We show in \autoref{tab:boottimes} the boot performance of the used test systems given a setup for a biometric sensor described in \autoref{cha:implementation} with TPM backed disk encryption enabled.
%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 reproducible file intensive process.
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 the \texttt{reboot} command is entered in the shell.
Again, the CLI login mask ends the rebooting process.
Further timing tests comparing the different IMA states are shown in the next section.

When enabling IMA, the file hashes will be stored as extended file attributes.
According to the xattr man page\cite{XattrMan2021}, the ext4 file system creates therefore a single block per file where all attributes must find place.
The block size of the used systems is 4\,KB.
The number of files is determined with the following command
\begin{lstlisting}[numbers=none]
  find / -fstype ext4 -type f -uid 0 | wc -l
\end{lstlisting}
System 1 holds after setup as Biometric Sensor and several system updates 156947 files.
This results in additional disk space of 613\,MB when adding a block for each file.
Additional attributes will, however, add no more disk space on ext4.

IMA holds all runtime information as virtual files in memory, including the IMA log.
In context of memory usage, it is clear that the size of the virtual files without the redundant information (like PCR number and log format) is a rough estimation for the memory usage within the kernel.
Both memory and disk usage do not change between IMA's fixing and enforcing mode since enforcing only adds file access control.

Saving the file to disk, takes about 2\,MB of size per 10000 lines in the log.
This indicates a linear dependency to the number of accessed files for the log and the kernel's memory usage.
The log can easily get over 100000 entries when the system is running long enough.
System 1, for example, had a log file with 214561 lines after about 15 days of uptime, resulting in about 40\,MB of size.
Saving that file on disk took several seconds which indicates that extracting the log from the kernel is very inefficient.
The log file showed that a major impact on the size was the performed system update.
Generating the iniramfs blob and compiling programs from source generates combined several 10000 lines in the log.

When IMA is set to enforcing, some unexpected problems 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.

Another relevant part for attestation 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 verification procedures regarding the sha256 bank of PCR 10 are currently not implemented.

\section{Processing and Sending Biometric Data}
Capturing and processing biometric data from the user is quite seamless.
Similarly, the cryptographic part of ECDAA is reliable enough for this prototype.
During the following tests, all software and hardware parts worked as expected.

Analyzing disk and memory usage is only meaningful for the member.
The implemented protoype of the issuer does only negotiate the membership key.
Revocation lists and group management is not implemented yet.
Similarly, the DAA verifier only checks the signature.
In productive state it must hold the revocation list and perform further checks with respect to received attestation message.



When IMA is in fixing or enforcing mode, the corresponding log will be filled according to \autoref{tab:imalogsize}.
\begin{table}
  \renewcommand{\arraystretch}{1.2}
  \centering
  \caption{Nuber of entries and IMA log size} \label{tab:imalogsize}
  %\rowcolors{2}{lightgray}{white}
  \begin{tabular}{rll}
    \toprule
    &\textit{Log entries}&\textit{Log size} \\
    \midrule
    \textbf{Boot}	&\textasciitilde\,27\,s &\\
    \textbf{DAA TPM enrollment} &\textasciitilde\,44\,s & \\
    \textbf{Image capturing}   &	&    \\
    \textbf{Image processing}	&\textasciitilde\,28\,s	&\\
    \textbf{Collecting DAA message} &\textasciitilde\,47\,s   &  \\
    \textbf{Sending DAA message}   &  &   \\
    \textbf{Sending 2nd message}   &  &   \\
    \bottomrule
  \end{tabular}
\end{table}
This

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.

Since the IMA log file is also essential for remote attestation, the information of this file must be transmitted to the DAA verifier.

\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.

