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\author{Sebastian Götte {\texttt<secureusb@jaseg.net>} @Mori lab, Waseda University}
\title{Securing the USB interface}
\date{December 12 2018}
\begin{document}
\maketitle

\section{Introduction}
\subsection{Problem definition}
A computer's USB interface is hard to secure. Though overall security is quite good today, the USB interface has not
received enough attention. In particular HIDs are a problem, as they are naturally very highly privileged.
Off-the-shelf USB HID attack tools exist. In particular from a security point of view extremely bad ideas such as
WebUSB\cite{misc01} are set to increase this already large attack surface even further.

\subsection{Contributions}
This work includes three key contributions. First, it demonstrates a practical implementation of a complete,
backwards-compatible secure USB system using QubesOS and a single new piece of security hardware.  Second, it shows a
novel interactive user-friendly cryptographic handshaking scheme based on out-of-band communication. Third, it shows and
proposes some techniques for the design of general secure protocols that are not limited to USB alone.

\section{The state of the art in mitigation}
Several ways to secure the USB interface have been proposed that can be broadly categorized as follows.
\begin{itemize}
    \item USB firewalls are software or hardware that protects the host from requests deemed invalid similar to a network firewall\cite{tian01,angel01,kang01,bates01,loe01}.
    \item USB device authentication uses some sort of user feedback or public key infrastructure to authenticate the device when it connects\cite{usb01,griscioli01,wang01,he01}.
    \item USB bus encryption encrypts the raw USB payloads to ward off eavesdroppers\cite{neugschwandtner01,weinstein01}.
    \item For wireless protocols, every conceivable pairing model has been tried. However, not many have been applied to
        USB\cite{arun01,uzun01,kobsa01,saxena01}.
    \item Compartmentalized systems such as QubesOS separate vulnerable components with large attack surface such as the USB device drivers into VMs to not inhibit exploitation but mitigate its consequences.
\end{itemize}

\begin{table}
    \setlength{\extrarowheight}{5pt}
    \begin{tabular}{l|P{10mm}|P{15mm}|P{15mm}|P{15mm}|P{17mm}|P{25mm}|}
                                &\multicolumn{3}{c|}{\bfseries Attacks}                                             & \multicolumn{2}{c|}{\bfseries Eavesdropping} & \multirow{2}{25mm}{\centering\bfseries Backwards\newline compatible} \\\cline{2-6}
                                & \bfseries HID     &\bfseries Host\newline exploit &\bfseries Device\newline exploit&\bfseries Bus-level &\bfseries Physical layer    & \\\hline
        Firewalls               & $\bigcirc$        & $\triangle$                   & $\times$                      & $\triangle$   & $\times$          & $\bigcirc$ \\
        Device authentication   & $\bigcirc$        & $\times$                      & $\times$                      & $\triangle$   & $\times$          & $\times$ \\
        Bus encryption          & $\triangle$       & $\times$                      & $\times$                      & $\bigcirc$    & $\bigcirc$        & $\times$ \\
        Plain QubesOS setup\footnotemark 
                                & $\triangle$       & $\triangle$                   & $\triangle$                   & $\triangle$   & $\times$          & $\bigcirc$ \\
        Our work                & $\bigcirc$        & $\bigcirc$                    & $\bigcirc$                    & $\bigcirc$    & $\bigcirc$        & $\bigcirc$
    \end{tabular}
    \caption{Comparison of approaches to USB security}
    \label{approach_comparison}
\end{table}
\footnotetext{Requires separate USB host controller for HIDs}

We compare these approaches w.r.t. several attacks in \ref{approach_comparison}. Overall we found that QubesOS is the
only advance towards securing this interface that is both \emph{practical} and \emph{effective}. Other approaches have
not been successful so far likely due to market inertia and backwards-incompatibility.
QubesOS approaches the problem by running a separate VM with the USB host controllers mapped through via IOMMU. This VM
runs a linux kernel with a small set of white-listed USB device drivers (HID and mass storage device) and a USB-over-IP
backend. A set of Qubes services pass through any HID input arriving inside this VM into dom0, and coordinate exporting
USB mass storage devices as Xen block devices. Any other USB devices can be passed-through to other VMs through
USB-over-IP-over-QubesRPC, a Xen vChan-based inter-VM communication system.
QubesOS is still lacking in that it's compartmentalization becomes essentially useless when it is used with a USB HID
keyboard that does not have its own dedicated PCIe USB host controller, as any normal desktop and most recent laptop
computers. The issue here is that USB HID is neither authenticated nor encrypted, and the untrusted USB VM sits in the
middle of this data stream, which thus allows it trivial privilege escalation via keystroke injection.

\subsection{Usage scenarios}

Today USB's level security is still adequate for most everyday users. In general, attacks against USB either require
special malicious hardware or require re-flashing of existing peripherals with custom malicious firmware. Today's
low-level cybercrime targeting everyday users is still focused on much easier tasks such as stealing passwords through
phishing, installing cryptolocker malware by means of malicious email attachments and extracting sensitive user data
with malicious browser addons. Fortunately, we have not yet entered an age where average computer users need to worry
about the type of attack this work defends against. Still, it can be expected that with the general increase of overall
computer security, eventually attackers will have to graduate to more advanced means--and since at this time the
landscape of effective defenses against USB attacks is very sparse, your author considers it important to explore the
avenues to effective defence ealier rather than later in order to be prepared for evolving attacks.

Despite the banality of everyday cybersecurity described above, there already are some people and organizations who face
advanced attacks including USB attacks. Due to their exceedingly simple execution, USB HID attacks are a very attractive
way to perform targeted attacks. For this reason, specialized USB attack hardware is already available commercially at
low cost. For users facing targeted attacks like this, SecureHID might already provide practical benefits.
The users most at risk of targeted attacks are those either working with highly sensitive data or working with highly
privileged access. The former group would include people such as journalists working with their sources and politicians
working with confidential information. The latter group would include law enforcement officials, often being endowed
with wide-ranging electronic access to databases and other confidential information. Further, system administrators and
computer programmers are often provided highly privileged access to critical systems for software deployment using
systems such as Ansible or uploading software packages into software repositories such as PyPI.

In all of these scenarios there are many users with very poweful adversaries. In case of a software developer or systems
administrator that would be competing companies or foreign intelligence agencies trying to gain access to internal
networks to steal confidential information. In case of a journalist that would be whoever they are writing about and
here the most interesting articles might come with the most powerful enemies. Finally, a security researcher would by
nature of their work, out of academic interest specifically be looking for the most dangerous targets they could find.

Some users might be able to reduce their attack surface to USB attacks by reducing their use of untrusted USB devices,
but in many everyday scenarios such as those described above this is not an option. A security researcher needs to
connect to untrusted devices in order to analyze them, and using a second, isolated machine for this is very
inconvenient. A journalist or politician will frequently have to read USB flash drives with documents for their work,
and again simply solving the problem by air-gapping is an effective but impractical mitigation. In all of these cases,
SecureHID would be an effective mitigation.

\section{Approach}
\subsection{System overview}
The goal of SecureHID is to enable the first reasonably secure system using both HID and arbitrary untrusted devices on
the same USB host controller, based on QubesOS. SecureHID consists of a USB HID encryption box to be put between
keyboard and computer and a piece of software run inside QubesOS. After initial pairing with the host software, the
encryption box will encrypt and sign any USB HID input arriving from the keyboard and forward the encrypted data to the
host. The host software running outside the untrusted USB VM will receive the encrypted and signed data from the
untrusted USB VM, verify and decrypt it, and inject the received HID input events into Qubes's input event handling
system.

A schematic diagram of a system employing SecureHID is shown in figure \ref{diagram_with}. Two major points that can be
seen here are that first, SecureHID requires no specialized hardware on either end and transparently plugs into the
existing USB stack. Second, a SecureHID-protected setup has two well-defined security boundaries, one inside the
SecureHID device between host and device side, and one in the host operating system between USB driver VM and
hypervisor.  These security boundaries allow a clean separation of a SecureHID setup into untrusted and trusted domains
and greatly simpliefies reasoning about overall system security.  Communication across these security boundaries is
limited to the simple SecureHID protocol. We describe the design of the SecureHID protocol in section \ref{prot_desc}
and elaborate its security properties in section \ref{prot_ver}. The security of the protocol's core components has been
formally verified in the past and the protocol has been kept simple enough to allow exhaustive verification and testing.

\begin{figure}[p]
    \includegraphics[scale=0.8]{system_diagram_with_secureusb.eps}
    \caption{Diagram of a SecureHID-protected system}
    \label{diagram_with}
\end{figure}

\subsection{System security properties}
This system is sufficient to secure any USB setup, especially unmodified desktop PCs or laptops where a USB host
controller is shared between both HIDs and other devices. Attack surface is reduced such that a \emph{full compromise}
of the system becomes unlikely, since plain HID is no longer supported. The remaining attack surface consists only of a
\emph{compromise of the USB VM}. This attack surface is small enough that other sensitive devices such as USB audio
devices can safely be connected. A compromise of the USB driver VM no longer gives full system access, but at best
allows listening in on the microphone. Since a compromised USB VM in general does not have network access, such an
attack will be mostly harmless in most scenarios. Additionally, the most likely attacking devices would be custom
hardware or a compromised smartphone. Custom hardware can easily be outfitted with a microphone, essentially turning it
into a bug irrespective of USB functionality, and smartphones already have microphones by definition.

A practical mitigation for potential information leakage through microphones, webcams and other sensitive devices would
be to simply unplug them when not used. Microphones could also be connected to a PCIe-based sound card (such as the
integrated sound card of most laptops) and webcams could potentially be isolated to a separate USB host controller.

\subsection{USB physical-level and bus-level attacks}
Since sensitive HIDs are isolated from other USB devices effectively on a separate bus, bus-level attacks such as
\textcite{neugschwandtner01} are entirely prevented. The much scarier physical attacks on USB such as \textcite{su01}
can be prevented thanks to the clear security boundary inside the SecureHID device. Since there is only four wires
needed between the trusted and untrusted sides (Ground, VCC, serial RX and serial TX) and the serial link is running at
a comparatively low speed (115.2kBd easily suffice), analog filtering is a viable measure against sidechannels. On the
ground and VCC rails extensive filtering using series inductors and large capacitors can be used to decouple both sides.
Additionally, both sides' microcontrollers can optionally be fed from separate voltage regulators powered off the USB 5V
rail to reduce side-channels. The serial link can be filtered to limit its analog bandwidth to above serial speeds
(50kHz) but much below the trusted microcontroller's system clock (72MHz). Finally, on the untrusted microcontroller
choosing UART pins that are not multiplexed to its internal ADC elminates the risk of direct measurements by a
compromised microcontroller firmware and leaves only indirect measurements of power supplies or coupling into other
pins' signals. This means that with a few very inexpensive hardware countermeasures (an additional voltage regulator and
a handful of capacitors, inductors and resistors for filtering) any analog side-channels between trusted and untrusted
side can be ruled out.

\section{Cryptographic design}

\subsection{Protocol description}
\label{prot_desc}
The basic protocol consists of two stages: \textsc{pairing} and \textsc{data}. When the device powers up, it enters
\textsc{pairing} state. When the host enumerates a new device, it enters \textsc{pairing} state. If any fatal
communication errors occur, both host and device re-enter \textsc{pairing} state. To make the implementation robust
against host software crashing, devices being unplugged etc. without opening it up to attacks, the host can request the
device to re-enter \textsc{pairing} state a limited number of times after powerup.

\textsc{pairing} state consists of a number of substates as set by \textcite{perrin01}. The device runs noise's
\textsc{XX} scheme, i.e. both host and device each contribute both one ephemeral key $e$ and one static key $s$ to the
handshake, and the public halves of the static keys are transmitted during handshake encrypted by the emphemeral keys.

The cryptographic primitives instantiated in the prototype are X25519 for the ECDH primitive, BLAKE2s as a hash and
ChaCha20-Poly1305 as AEAD for the data phase. ECDH instead of traditional DH was chosen for its small key size and fast
computation. Since no variant of RSA is used, key generation is fast. An ad-hoc prototype device-side random number
generator has been implemented based on BLAKE2s and the STM32's internal hardware RNG.

\begin{figure}[h]
    \centering
    \begin{sequencediagram}
        \newinst{kbd}{Keyboard}
        \newinst[3]{dev}{SecureHID}
        \newinst[5]{host}{Host}

        \mess{host}{}{dev}
        \path (mess from) -- (mess to) node[midway, above] {\emph{COBS sync (null byte)}};
        \mess{host}{}{dev}
        \draw[->,>=angle 60] (mess from) -- (mess to) node[midway, above] {\textsc{Initiate Handshake}};

        \begin{sdblock}{Noise XX handshake}{}
            \mess{host}{}{dev}
            \path (mess from) -- (mess to) node[midway, above] {$\textsc{Handshake}, e$};
            \mess{dev}{}{host}
            \path (mess from) -- (mess to) node[midway, above] {$\textsc{Handshake}, e,ee,s,es$};
            \mess{host}{}{dev}
            \path (mess from) -- (mess to) node[midway, above] {$\textsc{Handshake}, s,se$};
        \end{sdblock}

        \begin{sdblock}{Pairing}{Triggered by user interaction after unsuccessful handshake}
            \mess{dev}{}{host}
            \draw[->,>=angle 60] (mess from) -- (mess to) node[midway, above] {\textsc{Pairing Start}};
            \stepcounter{seqlevel}

            \mess{kbd}{keystroke}{dev}
            \addtocounter{seqlevel}{-1}
            \mess{dev}{}{host}
            \path (mess from) -- (mess to) node[midway, above] {$\textsc{Pairing Input},E(\text{keystroke})$};
            \stepcounter{seqlevel}
            \mess{kbd}{}{dev}
            \addtocounter{seqlevel}{-1}
            \path (mess from) -- (mess to) node[midway, above] {keystroke};
            \path (mess from) -- (mess to) node[midway, above, yshift=5mm] {$\vdots$};
            \mess{dev}{}{host}
            \path (mess from) -- (mess to) node[midway, above] {$\textsc{Pairing Input},E(\text{keystroke})$};
            \path (mess from) -- (mess to) node[midway, above, yshift=5mm] {$\vdots$};
            \stepcounter{seqlevel}

            \mess{kbd}{}{dev}
            \addtocounter{seqlevel}{-1}
            \path (mess from) -- (mess to) node[midway, above] {\emph{enter}};
            \mess{dev}{}{host}
            \draw[->,>=angle 60] (mess from) -- (mess to) node[midway, above] {\textsc{Pairing Success}};
        \end{sdblock}

        \begin{sdblock}{Input passthrough}{Started after successful handshake or pairing}
            \mess{kbd}{keystroke}{dev}
            \path (mess from) -- (mess to) node[midway, below, yshift=-2mm] {$\vdots$};

            \addtocounter{seqlevel}{-1}
            \mess{dev}{}{host}
            \path (mess from) -- (mess to) node[midway, above] {$\textsc{Data},E(\text{keystroke})$};
            \path (mess from) -- (mess to) node[midway, below, yshift=-2mm] {$\vdots$};
            \stepcounter{seqlevel}
        \end{sdblock}
    \end{sequencediagram}
    \caption{A successful prototype protocol pairing}
    \label{protocol_diagram}
\end{figure}

A successful pairing looks like this:
\begin{enumerate}
\item \textbf{Handshake.} \textsc{device} is connected to \textsc{host}
\item \textsc{host} initiates pairing by sending \textsc{initiate handshake} to device
\item \textsc{device} and \textsc{host} follow noise state machine for the \textsc{XX} handshake. See figure
	\ref{crypto_diagram} for a complete flowchart of cryptographic operations during this handshake. The handshake and
	subsequent Noise protocol communication are specified in \textcite{perrin01} and their security properties are
	formally verified in \textcite{kobeissi01}. Section \ref{sec_prop} analyzes the implications of these security
	properties for this research.
\item After the handshake completes, both \textsc{device} and \textsc{host} have received each other's static public key
    $rs$ and established a shared secret connection key. At this point, the possibility of an MITM attacker having
	actively intercepted the handshake remains. At this point \textsc{device} and \textsc{host} will both notice they do
	not yet know each other's static keys. \textsc{host} will respond to this by showing the pairing GUI dialog.
	\textsc{deivce} will sound an alarm to indicate an untrusted connection to the user.
\item \textbf{Channel binding.} Both \textsc{device} and \textsc{host} calculate the \emph{handshake hash} as per noise
	spec\cite{perrin01}. This hash uniquely identifies this session and depends on both local and remote ephemeral and
	static keys $le, re, ls, rs$.  Both parties encode a 64-bit part of this hash into a sequence of english words by
	dictionary lookup. This sequence of words is called the \emph{fingerprint} of the connection.
\item \textsc{host} prompts the user to enter the \emph{fingerprint} into a keyboard connected to \textsc{device}. The
	user presses the physical pairing button on \textsc{device} to stop the alarm and start pairing. This step prevents
	an attacker from being able to cause the device to send unencrypted input without user interaction by starting
	pairing.
\item As the user enters the \emph{fingerprint}, \textsc{device} relays any input over the yet-unauthenticated encrypted
    noise channel to \textsc{host}. \textsc{host} displays the received user input in plain text in a regular input
    field in the pairing GUI. This display is only for user convenience and not relevant to the cryptographic handshake.
    A consequence of this is that a MITM could observe the \emph{fingerprint}\footnote{
        A MITM could also modify the fingerprint information sent from \textsc{device} to \textsc{host}. This would be
        very obvious to the user, since the fingerprint appearing on the \textsc{host} screen would differ from what she
        types.
    }. We show in section \ref{prot_ver} that this does not reduce the protocol's security.
\item When the user has completed entering the fingerprint, the device checks the calculated fingerprint against the
    entered data. If both match, the host is signalled \textsc{success} and \textsc{data} phase is entered. If they do
    not match, the host is signalled \textsc{failure}\footnote{
        Note that this means a MITM could intercept the \textsc{failure} message and forge a \textsc{success} message.
        This means both are just for user convenience \emph{absent} an attacker. If an attacker is present, she will be
        caught in the next pairing step.
    } and \textsc{pairing} state is re-entered unless the maximum number of tries since powerup has been exceeded.
    Failure is indicated to the user by \textsc{device} through a very annoying beep accompanied by angrily flashing
    LEDs.
\item \textbf{Data phase.} \textsc{host} asks the user for confirmation of pairing \emph{in case the device did not
	sound an alarm} by pressing a button on the GUI. When the user does this, the host enters \textsc{data} state and
	starts input passthrough.
\end{enumerate}

Roughly speaking, this protocol is secure given that the only way to MITM a (EC)DH key exchange is to perform two (EC)DH
key exchanges with both parties, then relay messages. Since both parties have different static keys, the resulting two
(EC)DH sessions will have different handshake hashes under the noise framework. The channel binding step reliably
detects this condition through an out-of-band transmission of the \textsc{host} handshake hash to \textsc{device}.
The only specialty here is that this OOB transmission is relayed back from \textsc{device} to \textsc{host} allowing the
MITM to intercept it. This is only done for user convenience absent a MITM and the result is discarded by \textsc{host}.
Since the handshake hash does as a hash does not leak any sensitive information about the keys used during the
handshake, it being exposed does not impact protocol security.

\subsection{Protocol verifictation}
\label{prot_ver}
\subsubsection{Noise security properties}
\label{sec_prop}
According to \textcite{perrin01} and proven by \textcite{kobeissi01} Noise's XX pattern provides strong forward-secrecy,
sender and receiver authentication and key compromise impersonation resistance. Strong forward secrecy means an attacker
can only decrypt messages by compromising the receivers private key and performing an active impersonation.  Strong
forward secrecy rules out both physical and protocol-level eavesdropping attacks by malicious USB devices and implies
that an attacker can never decrypt past protocol sessions. An implication of the static key checks done on both sides of
the connection is that an attacker would need to compromise both host and device in order to remain undetected for e.g.
keylogging.  Compromising only one party the worst that can be done is impersonating the SecureHID device to perform a
classical HID attack. In this case, the attacker cannot read user input and the user would notice this by SecureHID
indicating a not connected status and thus the keyboard not working.
% FIXME possibly detect this by having session counter etc.?

To verify that these security properties extend to the overall SecureHID protocol it suffices to show the following
three properties.
\begin{enumerate}
\item The SecureHID implementation of Noise XX adheres to the Noise specification, i.e. the handshake is performed
	correctly. \label{adh0}
\item Both sides' static keys are verified. \label{adh1}
\item All sensitive data is encapsulated in Noise messages after the handshake has ended, and none is sent before.
	\label{adh2}
\end{enumerate}

\ref{adh0} has been validated by manual code review and cross-validation of our implementation against other Noise
implementations. % FIXME write more, do more here
\ref{adh1} has been validated by manual code review. %FIXME do simulation here
Since all sensitive data in our application is handled on the device in a single place (the USB HID request handling
routine), \ref{adh2} is easily validated by code review. USB HID reports are only transmitted either encrypted after the
handshake has been completed or in plain during pairing. Since the host will only inject reports into the input
subsystem that have been properly authenticated and encrypted (and not the unauthenticated reports sent during pairing),
the protocol is secure in this regard. Since pairing keyboard input is only passed through after the host's pairing
request has been acknowledged by the user with SecureUSB's physical pairing button the user would certainly notice an
attack trying to exfiltrate data this way. Were pairing input passed through automatically without explicit user
acknowledgement, an attacker could start pairing mode just as the user starts typing in a password prompt such as the
one of \texttt{sudo} or a password field and might not notice the attack until they have typed out their entire password
to the attacker.

\subsubsection{Handshake hash non-secrecy}

\begin{figure}[p]
	\centering
	\tikzset{%
		h/.style = {->, very thick},
		ck/.style = {->, very thick},
		key/.style = {->},
		ki/.style = {key, color=red},
		kr/.style = {key, color=red},
		ke/.style = {dashed},
		ks/.style = {solid},
		phaselbl/.style = {font=\bfseries, anchor=west}
	}
    \begin{tikzpicture}[scale=0.8]
        \node(protoname) at (0, 0){\texttt{"Noise\_XX\_25519\_ChaChaPoly\_BLAKE2s"}};
        \node[draw,thick,below=1em of protoname] (inithash) {$H$};
		\node[below=2em of inithash, xshift= 6.5em] (ck){$ck$};
		\node[below=2em of inithash, xshift=-6.5em] (h){$h$};
		\coordinate (labelbase) at (-5,0);

        \draw[->] (protoname) -- (inithash);
		\draw[->] (inithash) -- ++(0,-2em) coordinate(im0) -| (ck);
		\draw[->] (im0) -| (h);
		
		\node[right=7em of ck] (ei){$\mathbf{e_i}$};
		\node[right=8.5em of ck] (er){$\mathbf{e_r}$};
		\node[right=10em of ck] (si){$\mathbf{s_i}$};
		\node[right=11.5em of ck] (sr){$\mathbf{s_r}$};

		\node[draw,thick,below=3em of h] (mix0){MixHash};
		\node[right=1em of mix0] (str0){\texttt{""}};
		\node[left] at (str0 -| labelbase) (lbl0){\parbox{10em}{\raggedleft No preamble,\\use empty string}};

		\draw[h] (h) -- (mix0);
		\draw[->] (str0) -- (mix0);
		
%---
		\node[draw,thick,below=2em of mix0] (mix1){MixHash};
		\node[left] at (mix1 -| labelbase) (tok0){$e\rightarrow $};
		\draw[h] (mix0) -- (mix1);
		\draw[ki,ke] (ei) |- (mix1);

		\node[draw,thick,below=2em of mix1] (mix2){MixHash};
		\node[right=1em of mix2] (str2){\texttt{""}};
		\node[left] at (str2 -| labelbase) (lbl2){\parbox{10em}{\raggedleft EncryptAndHash\\No payload and\\$k$ unset}};
		\draw[->] (str2) -- (mix2);
		\draw[h] (mix1) -- (mix2);

		\coordinate (a) at ($ (mix0) !0.5! (mix1) $);
		\coordinate (b0) at (a -| lbl0.south west);
		\draw[dashed] (b0) -- ($ (b0 -| sr) + (1em,0) $);
%---
		\node[draw,thick,below=4em of mix2] (mix3){MixHash};
		\node[left] at (mix3 -| labelbase) (tok3){$e\leftarrow $};
		\draw[h] (mix2) -- (mix3);
		\draw[kr,ke] (er) |- (mix3);

		\coordinate (sync4) at (mix3 -| ck);
		\node[draw,thick,below=2em of sync4] (kmix1){MixKey};
		\node[left] at (kmix1 -| labelbase) (tok4){$ee\leftarrow $};
		\node[draw,thick,right=1em of kmix1] (dh0){ECDH};
		\draw[ck] (ck) -- (kmix1);
		\draw[ki,ke] (ei) |- ($ (dh0.east) + (0,0.2em) $);
		\draw[kr,ke] (er) |- ($ (dh0.east) - (0,0.2em) $);
		\draw[->] (dh0) -- (kmix1);

		\coordinate (sync5) at (kmix1 -| h);
		\node[draw,thick,below=2em of sync5] (mix5){MixHash};
		\node[draw,thick,right=2em of mix5] (enc5){$E$};
		\node[left] at (mix5 -| labelbase) (lbl5){$s\leftarrow $};
		\draw[h] (mix3) -- (mix5);
		\draw[->] (enc5) -- (mix5);
		\draw[->] ($ (kmix1.west) - (2em,0) $) -| (enc5);
		\draw[kr,ks] (sr) |- (enc5);

		\coordinate (sync6) at (mix5 -| ck);
		\node[draw,thick,below=2em of sync6] (kmix6){MixKey};
		\node[left] at (kmix6 -| labelbase) (tok6){$es\leftarrow $};
		\node[draw,thick,right=1em of kmix6] (dh6){ECDH};
		\draw[ck] (kmix1) -- (kmix6);
		\draw[ki,ke] (ei) |- ($ (dh6.east) + (0,0.2em) $);
		\draw[kr,ks] (sr) |- ($ (dh6.east) - (0,0.2em) $);
		\draw[->] (dh6) -- (kmix6);

		\coordinate (sync7) at (kmix6 -| h);
		\node[draw,thick,below=2em of sync7] (mix7){MixHash};
		\node[draw,thick,right=2em of mix7] (enc7){$E$};
		\node[right=1em of enc7] (str7){\texttt{""}};
		\node[left] at (mix7 -| labelbase) (lbl7){\parbox{10em}{\raggedleft DecryptAndHash\\ No payload}};
		\draw[h] (mix5) -- (mix7);
		\draw[->] (enc7) -- (mix7);
		\draw[->] (kmix1.west) -- ++(-2em,0) |- ($ (enc5.south) !0.25! (enc7.north) $) -- (enc7);
		\draw[->] (str7) -- (enc7);

		\coordinate (a) at ($ (mix2) !0.5! (mix3) $);
		\coordinate (b1) at (a -| lbl2.south west);
		\draw[dashed] (b1) -- ($ (b1 -| sr) + (1em,0) $);
% ---
		\node[draw,thick,below=4em of mix7] (mix8){MixHash};
		\node[draw,thick,right=2em of mix8] (enc8){$E$};
		\node[left] at (mix8 -| labelbase) (lbl8){$s\rightarrow $};
		\draw[h] (mix7) -- (mix8);
		\draw[->] (enc8) -- (mix8);
		\draw[->] (kmix6.west) -- ++(-2em,0) |- ($ (enc7.south) !0.25! (enc8.north) $) -- (enc8);
		\draw[ki,ks] (si) |- (enc8);

		\coordinate (sync9) at (mix8 -| ck);
		\node[draw,thick,below=2em of sync9] (kmix9){MixKey};
		\node[left] at (kmix9 -| labelbase) (tok9){$se\rightarrow $};
		\node[draw,thick,right=1em of kmix9] (dh9){ECDH};
		\draw[ck] (kmix6) -- (kmix9);
		\draw[ki,ks] (si) |- ($ (dh9.east) - (0,0.2em) $);
		\draw[kr,ke] (er) |- ($ (dh9.east) + (0,0.2em) $);
		\draw[->] (dh9) -- (kmix9);

		\coordinate (sync10) at (kmix9 -| h);
		\node[draw,thick,below=2em of sync10] (mix10){MixHash};
		\node[draw,thick,right=2em of mix10] (enc10){$E$};
		\node[right=1em of enc10] (str10){\texttt{""}};
		\node[left] at (mix10 -| labelbase) (lbl10){\parbox{10em}{\raggedleft EncryptAndHash\\ No payload}};
		\draw[h] (mix8) -- (mix10);
		\draw[->] (enc10) -- (mix10);
		\draw[->] (kmix9.west) -| (enc10);
		\draw[->] (str10) -- (enc10);

		\coordinate (a) at ($ (mix7) !0.5! (mix8) $);
		\coordinate (b2) at (a -| lbl2.south west);
		\draw[dashed] (b2) -- ($ (b2 -| sr) + (1em,0) $);
% ---
		\coordinate (sync11) at (mix10 -| ck);
		\node[draw,thick,below=5em of sync11,xshift=-1em] (finkdf){HKDF};
		\node[below=2em of finkdf,xshift=-1em] (k1){$\mathbf{k_1}$};
		\node[below=2em of finkdf,xshift= 1em] (k2){$\mathbf{k_2}$};
		\node[left=1em of finkdf,yshift=2em] (str11){\texttt{""}};
		\draw[ck] (kmix9) -- (finkdf.north -| kmix9);
		\draw[ck] (k1 |- finkdf.south) -- (k1);
		\draw[ck] (k2 |- finkdf.south) -- (k2);
		\draw[->] (str11) -| ($ (finkdf.north) - (1em,0) $);

		\coordinate (sync11a) at (finkdf.south -| h);
		\node[below=2em of sync11a] (hout){$\mathbf{h}$};
		\draw[h] (mix10) -- (hout);

		\node[left] at (finkdf -| labelbase) (lbl13){\parbox{10em}{\raggedleft Split}};

		\coordinate (a) at ($ (mix10) !0.5! (finkdf) $);
		\coordinate (b3) at (a -| lbl10.south west);
		\draw[dashed] (b3) -- ($ (b3 -| sr) + (1em,0) $);
%---
		\node[phaselbl, rectangle] at ($ (b0) !0.5! (b0 |- protoname) $) (setup){setup};
		\node[phaselbl, circle] at ($ (b0) !0.5! (b1) $) (ph0){0};
		\node[phaselbl, circle] at ($ (b1) !0.5! (b2) $) (ph1){1};
		\node[phaselbl, circle] at ($ (b2) !0.5! (b3) $) (ph2){2};
		\node[phaselbl, rectangle] at ($ (b3) !0.5! (b3 |- hout) $) (fin){fin};

    \end{tikzpicture}
    \caption{Cryptographic flowchart of Noise XX handshake.}
    \label{crypto_diagram}
\end{figure}

To analyze the impact of disclosing the handshake hash to an adversary we must consider it's definition. The noise
protocol specification gives its definition, but does not % FIXME double-check this 
guarantee that it can be disclosed to an adversary without compromising security. Figure \ref{crypto_diagram} contains a
flowchart of the derivation of both initiator-transmit and initiator-receive symmetric encryption keys $k_{1,2}$ and the
handshake hash $h$ during the Noise handshake. The definitions of MixHash and MixKey according to the Noise protocol
specification are as follows.

\begin{align}
	\text{MixHash}(h,\text{input}) &= h' = H(h || \text{input})\\
	\text{MixKey}(ck, \text{input}) &= (ck', k_\text{temp}) = \text{HKDF}(ck, \text{input}, 2)\\
\end{align}

Noise's hash-based key derivation function (HKDF) is defined using the HMAC defined in RFC2104\cite{rfc2104}. The hash
function $H$ employed here depends on the cipher spec used, in this work it is BLAKE2s.
\begin{equation}
	\text{HMAC}(K, \text{input}) = H\left(\left(K \oplus opad\right)
		|| H\left(\left(K \oplus ipad\right) || \text{input} \right)\right)
\end{equation}

The HKDF is defined for two and three outputs as follows.
\begin{equation}
	\text{HKDF}(ck, \text{input}, n_\text{out}) =
		\left\{\begin{array}{ll}
			(q_0, q_1) &: n_\text{out} = 2\\
			(q_0, q_1, q_2) &: n_\text{out} = 3\\
		\end{array}\right.
\end{equation}

The outputs $q_i$ are derived from chained HMAC invocations. First, a temporary key $t'$ is derived from the chaining key $ck$
and the input data using the $HMAC$, then depending on $n_\text{out}$ the HMAC is chained twice or thrice to produce
$q_{\{0,1,2\}}$.
\begin{gather}
	t' = \text{HMAC}(ck, \text{input})\\
	\underbrace{\text{HMAC} \Bigl(t',
	\underbrace{\text{HMAC} \bigl(t',
	\underbrace{\text{HMAC} (t',
		   1_{16})}_{q_0}
		|| 2_{16}\bigr)}_{q_1}
		|| 3_{16}\Bigr)}_{q_2}
\end{gather}

Relevant to this protocol implementation's security are the following two properties, both of which can be derived from
figure \ref{crypto_diagram}:
\begin{enumerate}
\item Initiator and responder ephemeral and static keys are all mixed into the handshake hash at least once.\label{fp1}
\item Knowledge of the handshake hash does not yield any information on the symmetric AEAD keys $k_1$ and $k_2$.\label{fp2}
\end{enumerate}

\ref{fp1} is evident since $e_i$ and $e_r$ are mixed in directly and $s_i$ and $s_r$ are mixed in after encryption with
temporary encryption keys derived from $ck$ at the $s\rightarrow$ and $s\leftarrow$ steps during the handshake.
We can see \ref{fp2} applies by following the derivation of $h$ backwards. If an attacker learned anything about $k1$ or
$k2$ during an attack by (also) observing $h$ that they did not learn before, we could construct an oracle allowing both
reversal of $H$ in the final invocation of $MixHash$ and breaking $E$ using this attacker. The attacker would have to
reverse $H$ at some point since $h = H(\hdots)$ in the final invocation of MixHash. The attacker would have to recover
the key of $E$ in at least one invocation since $s_i$ and $s_r$ are only mixed into $h$ after either being encrypted
using $E$ or being used after ECDH to generate a key for $E$. Since the result of ECDH on $e_i$ and $e_r$ is mixed into
$h$ in the $ee\leftarrow$ and following DecryptAndHash steps, $h$ is blinded to an attacker so that they cannot even
determine a given $k_1$ and $k_2$ match a given $h$ without compromising ECDH security.
This means that given the underlying primitives are secure, we do not leak any information on $k1$ or $k2$ by disclosing
$h$.

% FIXME find and consistently apply a nice name for this handshake method
\subsection{Alternative uses for interactive public channel binding}

The channel binding method described above can be used in any scenario where a secure channel between two systems must
be established where one party has a display of some sort and the other party has an input device of some sort. 

\paragraph{Adaption to mice} Instead of a keyboard, a mouse can be used for pairing without compromising protocol
security. The host would encode the fingerprint bit string into a permutation
$\sigma(i) : \{n\in\mathbb N, n\le m\} \rightarrow \{n\in\mathbb N, n\le m\}$ 
and then display the sequence $\sigma(i)$ in a grid of buttons similar to a minesweeper field with an emualted mouse
cursor driven by pairing input on top. The user would then click the buttons on the grid in numeric order. The device
would do the same mouse emulation invisible to the user and would be able to observe the permutation this way. The
fingerprint can finally be checked by decoding the permutation into a bit string and comparing. The security level for
this method in bits is $\eta = \log_2(m!)$ or better than 80bit for $m=25$ in case of a 5x5 grid.  See figure
\ref{mouse_mockup} for a mockup of what such a system might look like. 

\begin{figure}[h]
    \centering
    \includegraphics[width=7cm]{mouse_mockup.png}
    \caption{A mockup of what mouse-based interactive pairing might look like}
    \label{mouse_mockup}
\end{figure}

\paragraph{Adaption to button input}
Adaption to button input using few buttons is a little bit harder. The obvious but impractical solution here is to have
the user enter a very long numeric string. Entering an 80-bit number on a two-button binary keyboard is not
user-friendly. One other option would be to emulate an on-screen keyboard similar to the ones used in arcade and console
video games for joystick input. This would be more user-friendly and would likely be a natural interface to many users
familiar with such games. One possible attack here is that if the host were to ignore dropped input packets, an attacker
might selectively drop packets in order to cause a desynchronization of host and device fingerprint input states. The
user would likely chalk up this behavior to sticky or otherwise unreliable keys and while they might find it
inconvenient, they might not actually abort the procedure. Thus it is imperative that the host verify there are no
dropped packets during pairing. This same observation is also true for keyboard or mouse-based pairing as explained
above, but an attack would be much more noticeable there to users as mice and keyboards are generally regarded reliable
by most users.

\paragraph{Relation to screen-to-photodiode interfaces}
% FIXME citations
There have been many systems using a flashing graphic on a screen to transmit data to a receiver containing a photodiode
held against the screen. Such systems have been used to distribute software over broadcast television but have also been
used for cryptographic purposes. One widely-deployed example is the ``Flickertan'' system used for wire transfer
authorization in Germany where a smartcard reader with five photodiodes is held against a flickering image on the bank
website's wire transfer form\cite{schiermeier01,schiermeier02,braun01}. Systems such as this one do not benefit from the
interactive channel binding process described in this paper since they do not require any direct user interaction. They
could however be used as an alternative means for channel binding in any system also supporting interactive pairing as
described above. The handshake fingerprint would simply be encoded into the flicker signal and transmitted to the other
endpoint in that way.  Similarly, QR-codes or other barcodes could be used to a device containing a camera. The primary
advantage of photodiode-based systems is that they incur lower implementation complexity and don't require a potentially
expensive camera.

\section{Hardware implementation}
\subsection{Hardware overview}
To demonstrate the practicality of this approach and to evaluate its usability in an everyday scenario, a hardware
prototype has been built. Based on an initial prototype consisting of a microcontroller development board and a bundle
of wires a custom PCB fitting an off-the-shelf case has been produced that allows future % FIXME
usability testing in practical settings.

The hardware implementation consists of two ARM microcontrollers, one for the untrusted host side and one for the
trusted device side. Both are linked using a simple UART interface. Both microcontrollers have been chosen by their USB
functionality. For the integrated USB host controller, we had to chose a rather powerful microcontroller on the trusted
device side even though a much less powerful one would have sufficed even though we are doing serious cryptography on
this microcontroller. AES encryption is done on every data packet and must compelete in time for the overall system to
meet its latency requirement, % FIXME go into details on input latency vs. usability somewhere else
but is fast enough by a large margin. % FIXME benchmarks
Similarly, the hash and ECDH operations during the cryptographic handshake are fast enough by a large margin. %FIXME benchmark
Additionally, those operations are only invoked infrequently any time the device is disconnected or the host suspends.

\subsection{Hardware security measures}
% separate power supplies, possible future filtering of power/gnd and comms link signals

\subsection{Usability considerations}
% buzzer, leds etc.

\section{Evaluation}
% FIXME

\section{Future work}

\section{Conclusion}
% FIXME

%\section{PCB design renderings}
%\label{ch:renderings}

%\begin{figure}[H]
%    \centering
%    \begin{subfigure}[b]{0.8\textwidth}
%        \centering
%        \includegraphics[width=0.8\textwidth]{pcb_front.png}
%        \caption{PCB front}
%    \end{subfigure} %
%    \begin{subfigure}[b]{0.8\textwidth}
%        \centering
%        \includegraphics[width=0.8\textwidth]{pcb_back.png}
%        \caption{PCB back}
%    \end{subfigure}
%    \caption{PCB design 3D renderings}
%    \label{fig:pcb3d}
%\end{figure}
%
%\begin{figure}[H]
%    \centering
%    \includegraphics[width=0.6\textwidth]{takachi_case.png}
%    \caption{Off-the-shelf enclosure the PCB is made to fit}
%    \label{fig:case}
%\end{figure}

%\subsection{Attack surface in reasonably secure systems}
%\begin{figure}
%\tikzstyle{block} = [rectangle, draw, text centered, minimum height=4em]
%\begin{tikzpicture}[node distance=2cm, auto]
%   \node[block](matrix){Key matrix}
%   \node[block](hidctrl){Keyboard controller}
%   \node[block](hubs){USB hubs}
%   \node[block](roothub){USB host controller}
%   \node[block](pcie){PCIe bus}
%   \node[block](sys-usb-kernel){USB VM kernel}
%   \node[block](sys-usb-agent){USB VM userspace agent}
%   \node[block](dom0){dom0 agent}
%\end{tikzpicture}
%\label{qubes-hid-stack}
%\caption{The USB HID input stack in a QubesOS setup}
%\end{figure}

\nocite{*}
\printbibliography

\appendix
\section{Project state}
A working prototype has been completed.

\subsection{Completed}
\begin{itemize}
    \item Rough protocol design
    \item Protocol implementation based on \textcite{perrin01} using noise-c (microcontroller) and noiseprotocol (python/host)
    \item SRAM-based key storage with SRAM wear levelling
    \item host/device signature checking
    \item host/device key generation
    \item proper circuit design because I was bored last weekend (see appendix \ref{ch:renderings})
\end{itemize}

\subsection{Open issues}
\begin{itemize}
    \item Both noise-c and noiseprotocol have poor code and API quality. Since most noise functionality is not needed,
        just implement the protocol in bare C/python based on cryptographic primitives and scrap higher-level protocol
        implementations (though they've been useful so far during prototyping).
    \item Implement HID mouse host support
    \item Test USB hub support
    \item Replace the serial link with a custom USB link using an STM32F103 instead of the CH340G USB/serial converter
    \item Properly integrate prototype host client with qubes infrastructure
    \item Implement photodiode/monitor-based pairing side-channel
\end{itemize}

\section{Possible directions}
\begin{itemize}
%    \item Elaborate handshake security properties
%        \begin{itemize}
%            \item Possibly investigate other applications of this type of interactive handshake
            \item Possibly contrast to carmera/other backchannel systems
%            \item IMHO the pairing scheme is the most interesting part of this project from a scientific point of view
% 			\item Prove security
%        \end{itemize}
%    \item Elaborate overall security properties of QubesOS-based system
    \item Elaborate possible DisplayPort/HDMI-based display encryption $\rightarrow$ Bunnie's NeTV2 w/ HDMI/eDP converter
    \item Elaborate possible encrypted remote input (SSH) setups
        \begin{itemize}
            \item This might turn out to be really interesting
            \item For this to be usable the host needs to tell the device at least which keyslot to use which could turn
                out to be complex to implement securely
            \item Considering complexity, this might turn into its own research project
        \end{itemize}
    \item Showcase secure hardware interface design, contrast with wireguard protocol design
        \begin{itemize}
            \item Formally derive handshake security properties
            \item Formally derive host/device protocol security properties using noise spec
            \item Formally verify and thouroughly unit-test the host/device protocol implementation on all layers
            \item IMHO this is the most interesting part of this project from an engineering point of view
        \end{itemize}
% Waiting
    \item Create custom hardware prototype
    \item Benchmark cryptography routines (will likely turn out to be ``wayyy fast'' for HID, fast enough for full-speed
        USB. High-speed cannot be done with the current architecture as we can't get data out the chip at high-speed
        data rates. \textcite{srivaths01} raise the issue of running crypto on embedded systems, but in this case it
        turns out with somewhat modern hardware and cryptography there is no problem at all.
\end{itemize}
\end{document}
