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\title{GNU Taler}
%\subtitle{}

\setbeamertemplate{navigation symbols}{ \includegraphics[width=1cm]{tud-logo.pdf} \includegraphics[width=0.4cm]{logo-esen.pdf} \includegraphics[width=1cm]{logo-GlsBank.pdf} \includegraphics[width=0.6cm]{logo-MagNetBank.pdf} \includegraphics[width=0.4cm]{logo-ps.pdf} \includegraphics[width=0.4cm]{logo-nlnet.pdf} \includegraphics[width=0.4cm]{logo-HomoDigitalis.pdf} \includegraphics[width=0.4cm]{logo-codeblau.pdf}  \includegraphics[width=1.4cm]{logo-tue.pdf}  \includegraphics[width=0.6cm]{logo-visualvest.pdf} \includegraphics[width=1cm]{inria.pdf} \includegraphics[width=0.4cm]{logo-bfh.pdf} \includegraphics[width=1.6cm]{fub.pdf} \includegraphics[width=0.4cm]{ashoka.png}  \includegraphics[width=0.4cm]{gnu.png} \includegraphics[width=1cm]{taler-logo-2021-inkscape.pdf} \hfill}
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\author[C. Grothoff]{J. Burdges, F. Dold, {\bf C. Grothoff}, M. Stanisci}
\date{\today}
\institute{The GNU Project}


\begin{document}

\justifying

\begin{frame}
  \begin{center}
    \LARGE {\bf GNU}

    \vfill
%    \includegraphics[width=0.66\textwidth]{logo-2017-fr.pdf}
    \includegraphics[width=0.66\textwidth]{taler-logo-2021-inkscape.pdf}
  \end{center}

  \begin{center}
      \includegraphics[width=0.15\textwidth]{logo-EU.pdf}
      \includegraphics[width=0.15\textwidth]{logo-SBFI.pdf}
  \end{center}

\begin{textblock*}{6cm}(.5cm,7.7cm) % {block width} (coords)
    {\Large {\bf \href{https://taler.net/}{taler.net}} \\
    \href{https://twitter.com/taler}{taler@twitter}}
\end{textblock*}

% Substitute based on who is giving the talk!
 \begin{textblock*}{6cm}(6.5cm,7.7cm) % {block width} (coords)
   {\hfill {\bf Christian Grothoff} \\
    \hfill grothoff@taler.net }
\end{textblock*}

\end{frame}

\setbeamertemplate{navigation symbols}{\hfill \includegraphics[width=1cm]{taler-logo-2021-inkscape.pdf}}



\begin{frame}{Agenda}
  \tableofcontents
\end{frame}

\section{Motivation \& Background}


\begin{frame}{Surveilance concerns}
\begin{itemize}
\item Everybody knows about Internet surveilance.
\item But is it {\bf that} bad?\pause
  \begin{itemize}
  \item You can choose when and where to use the Internet
  \item You can anonymously access the Web using Tor
  \item You can find open access points that do not require authentication
  \item IP packets do not include your precise location or name
  \item ISPs typically store this meta data for days, weeks or months
  \end{itemize}
\end{itemize}
\end{frame}


\begin{frame}{A Social Problem}
%  \vfill
  This was a question posed to RAND researchers in 1971:

\begin{quote}
  ``Suppose you were an advisor to the head of the KGB, the Soviet Secret Police. Suppose you are given the assignment of designing a system for the surveillance of all citizens and visitors within the boundaries of the USSR. The system is not to be too obtrusive or obvious. What would be your decision?''
\end{quote}
%The result: an electronic funds transfer system that looks
%strikingly similar today's debit card system.
\pause
  \begin{center}
  \includegraphics[height=1cm]{pics/nsa_spy.jpg}
  \end{center}
\vfill
  \begin{center}
``I think one of the big things that we need to do, is we need
to get away from true-name payments on the Internet. The credit
card payment system is one of the worst things that happened for the
user, in terms of being able to divorce their access from their
identity.'' \hfill --Edward Snowden, IETF 93 (2015)
\end{center}

\end{frame}


\begin{frame}{What is worse:}
  \begin{itemize}
  \item When you pay by CC, the information includes your name
  \item When you pay in person with CC, your location is also known
  \item You often have no alternative payment methods available
  \item You hardly ever can use someone else's CC
  \item Anonymous prepaid cards are difficult to get and expensive
  \item Payment information is typically stored for at least 6 years
  \end{itemize}
\end{frame}


\begin{frame}{Banks have Problems, too!}

  3D secure (``verified by visa'') is a nightmare:

  \begin{minipage}{5cm}
    \begin{itemize}
    \item Complicated process
    \item Shifts liability to consumer
    \item Significant latency
    \item Can refuse valid requests
    \item Legal vendors excluded
    \item No privacy for buyers
     \end{itemize}
  \end{minipage}
  \begin{minipage}{5cm}
      \includegraphics[width=\textwidth]{illustrations/cc3ds.pdf}
  \end{minipage}
  \vfill
    Online credit card payments will be replaced, but with what?
\end{frame}


\begin{frame}{The Bank's Problem}
\vfill
  \begin{textblock*}{12cm}(0.5cm,1cm) % {block width} (coords)
    \begin{itemize}
    \item Global tech companies push oligopolies
    \item Privacy and federated finance are at risk
%    \item 30\% fees are conceivable
    \item Economic sovereignty is in danger
    \end{itemize}
\end{textblock*}
\begin{textblock*}{4cm}(3.5cm,5.2cm) % {block width} (coords)
 {\includegraphics[width=\textwidth]{../investors/competitor-logos/amazon.png}}
\end{textblock*}
\begin{textblock*}{2cm}(7cm,3cm) % {block width} (coords)
 {\includegraphics[width=\textwidth]{../investors/competitor-logos/alipay.jpeg}}
\end{textblock*}
\begin{textblock*}{2cm}(3cm,3.5cm) % {block width} (coords)
 {\includegraphics[width=\textwidth]{../investors/competitor-logos/paypal.jpeg}}
\end{textblock*}
\begin{textblock*}{2cm}(9cm,5cm) % {block width} (coords)
 {\includegraphics[width=\textwidth]{../investors/competitor-logos/applepay.jpeg}}
\end{textblock*}
\begin{textblock*}{2cm}(7.5cm,5.9cm) % {block width} (coords)
 {\includegraphics[width=\textwidth]{../investors/competitor-logos/samsungpay.jpeg}}
\end{textblock*}
\begin{textblock*}{1cm}(9.5cm,6.3cm) % {block width} (coords)
 {\includegraphics[width=\textwidth]{../investors/competitor-logos/android_pay.png}}
\end{textblock*}
\vfill
\end{frame}


\begin{frame}{Predicting the Future}
  \begin{itemize}
  \item Google and Apple will be your bank and run your payment system
  \item They can target advertising based on your purchase history, location and
        your ability to pay
  \item They will provide more usable, faster and broadly available
        payment solutions; our federated banking system will be history
%        just like SMTP is now Gmail.
  \item After they dominate the payment sector, they will start to charge fees
        befitting their oligopoly size
  \item Competitors and vendors not aligning with their corporate ``values''
        will be excluded by policy and go bankrupt
  \item The imperium will have another major tool for its financial warfare
  \end{itemize}
\end{frame}


\begin{frame}{The Distraction: Bitcoin}

\begin{itemize}
\item Unregulated payment system and currency:
\item[] $\Rightarrow$ lack of regulation is a feature!
\item Implemented in free software
\item Decentralised peer-to-peer system   \pause
\item Decentralised banking requires solving Byzantine consensus
\item Creative solution: tie initial accumulation to solving consensus \pause
\item[] $\Rightarrow$ Proof-of-work advances ledger
\item[] $\Rightarrow$ Very expensive banking
\end{itemize}
\end{frame}


\begin{frame}
  \frametitle{\includegraphics[height=0.5cm]{pics/bitcoin.png}?}
  \framesubtitle{Background: \url{https://blockchain.com/charts/}}
  \centering
\noindent
\includegraphics[width=\textwidth]{pics/btc-transaction-cost.png}

Current average transaction value: $\approx$ 1000 USD
\end{frame}


\begin{frame}
  \frametitle{\includegraphics[height=0.5cm]{pics/zerocoin.png}?}

Bitcoin cryptography is rather primitive:
\begin{center}
  {\bf All Bitcoin transactions are public and linkable!}
\end{center}

\begin{itemize}
\item[] $\Rightarrow$ no privacy guarantees
\item[] $\Rightarrow$ enhanced with ``laundering'' services
\end{itemize}
ZeroCoin, CryptoNote (Monero) and ZeroCash (ZCash) offer anonymity.
\end{frame}


\begin{frame}
  \vfill
\begin{center}
{\bf Do you want to have a libertarian economy?}
\end{center}
  \vfill
\begin{center}
{\bf Do you want to live under total surveillance?}
\end{center}
\vfill
\end{frame}


\begin{frame}{The Bank of International Settlements}
  \framesubtitle{Central Bank Digital Currency vs. Cash}
  \begin{center}
    \movie[%scale=0.6,
           autostart,
           poster]
           {
               \includegraphics[height=0.6\textwidth,width=0.8\textwidth]{white.png}
           }
          {bis-cbdc.mp4}
  \end{center}
\end{frame}


\begin{frame}{The Emergency Act of Canada\footnote{Speech by Premier Kenney, Alberta, February 2022}}
  \begin{center}
    \movie[%scale=0.6,
           autostart,
           poster]
           {
               \includegraphics[height=0.6\textwidth,width=0.8\textwidth]{ca.png}
           }
          {emergencyact.mp4}

  {\tiny \url{https://www.youtube.com/watch?v=NehMAj492SA} (2'2022)}
  \end{center}
\end{frame}


\section{GNU Taler: Introduction}

\begin{frame}
  \vfill
  \begin{center}
    {\bf GNU Taler: Introduction}
  \end{center}
  \vfill
\end{frame}


\begin{frame}{GNU Taler}
  \vfill
  \begin{center}
    {\huge {\bf Digital} cash, made \textbf{socially responsible}.}
  \end{center}
  \vfill
  \begin{center}
  \includegraphics[scale=0.3]{taler-logo-2021-inkscape.pdf}
  \end{center}
  \vfill
  \begin{center}
    Privacy-Preserving, Practical, Taxable, Free Software, Efficient
  \end{center}
 \vfill
 \vfill
\ %
\end{frame}


\begin{frame}{What is Taler?}
  \framesubtitle{\url{https://taler.net/en/features.html}}  \noindent
Taler is
  \vfill
  \begin{itemize}
    \item a Free/Libre software \emph{payment system} infrastructure project
    \item ... with a surrounding software ecosystem
    \item ... and a company (Taler Systems S.A.) and community that wants to deploy it
      as widely as possible.
  \end{itemize}
  \vfill
\noindent
 However, Taler is
  \begin{itemize}
    \item \emph{not} a currency or speculative asset
    \item \emph{not} a long-term store of value
    \item \emph{not} a network or instance of a system
    \item \emph{not} decentralized
    \item \emph{not} based on proof-of-work or proof-of-stake
  \end{itemize}
\end{frame}


\begin{frame}{Design principles}
  \framesubtitle{https://taler.net/en/principles.html}
GNU Taler must ...
\begin{enumerate}
  \item {... be implemented as {\bf free software}.}
  \item {... protect the {\bf privacy of buyers}.}
  \item {... enable the state to {\bf tax income} and crack down on
    illegal business activities.}
  \item {... prevent payment fraud.}
  \item {... only {\bf disclose the minimal amount of information
    necessary}.}
  \item {... be usable.}
  \item {... be efficient.}
  \item {... avoid single points of failure.}
  \item {... foster {\bf competition}.}
\end{enumerate}
\end{frame}


\begin{frame}
\frametitle{Taler Overview}
\begin{center}
\begin{tikzpicture}
 \tikzstyle{def} = [node distance= 5em and 6.5em, inner sep=1em, outer sep=.3em];
 \node (origin) at (0,0) {};
 \node (exchange) [def,above=of origin,draw]{Exchange};
 \node (customer) [def, draw, below left=of origin] {Customer};
 \node (merchant) [def, draw, below right=of origin] {Merchant};
 \node (auditor) [def, draw, above right=of origin]{Auditor};
% \node (regulator) [def, draw, above=of auditor]{CSSF};

 \tikzstyle{C} = [color=black, line width=1pt]

 \draw [<-, C] (customer) -- (exchange) node [midway, above, sloped] (TextNode) {withdraw coins};
 \draw [<-, C] (exchange) -- (merchant) node [midway, above, sloped] (TextNode) {deposit coins};
 \draw [<-, C] (merchant) -- (customer) node [midway, above, sloped] (TextNode) {spend coins};
 \draw [<-, C] (exchange) -- (auditor) node [midway, above, sloped] (TextNode) {verify};
% \draw [<-, C] (regulator) -- (auditor) node [midway, above, sloped] (TextNode) {report};

\end{tikzpicture}
\end{center}
\end{frame}



\begin{frame}
  % TODO: replace with simplified NEW architecture picture!
\frametitle{Architecture of Taler}
\begin{center}
  \includegraphics[width=1\textwidth]{operations.png}
\end{center}
\end{frame}


\begin{frame}{Consumer Impact of Taler}
\begin{itemize}
\item {\bf Convenient:} pay with one click instantly --– in Euro,
Dollar, Yen or Bitcoin
\item {\bf Friction-free security:} Payments do not require sign-up,
login or multi-factor authentication
\item {\bf Privacy-preserving:} payment requires/shares no personal information
\item {\bf Bank account:} not required
\end{itemize}
\end{frame}


\begin{frame}{Merchant Impact of Taler}
\begin{itemize}
\item {\bf Instant clearance:} one-click transactions and instant clearance at par
\item {\bf Easy \& compliant:} GDPR \& PCI-DSS compliance-free and without any effort
\item {\bf Major profit increase:} efficient protocol $+$ no fraud $=$ extremely low costs
\item {\bf 1-click checkout:} without Amazon and without false
positives in fraud detection
\end{itemize}
\end{frame}


\begin{frame}{Taler: Unique Regulatory Features for Central Banks}
  \framesubtitle{\url{https://www.snb.ch/en/mmr/papers/id/working_paper_2021_03}}
  \begin{itemize}
    \item Central bank issues digital coins equivalent to issuing cash \\
          $\Rightarrow$ monetary policy remains under CB control
    \item Architecture with consumer accounts at commercial banks \\
          $\Rightarrow$ no competition for commercial banking (S\&L) \\
          $\Rightarrow$ CB does not have to manage KYC, customer support
    \item Withdrawal limits and denomination expiration \\
          $\Rightarrow$ protects against bank runs and hoarding
    \item Income transparency and possibility to set fees \\
          $\Rightarrow$ additional insights into economy and new policy options
    \item Revocation protocols and loss limitations \\
          $\Rightarrow$ exit strategy and handles catastrophic security incidents
    \item Privacy by cryptographic design not organizational compliance \\
          $\Rightarrow$ CB cannot be forced to facilitate mass-surveillance
  \end{itemize}
\end{frame}


\begin{frame}{Usability of Taler}
  \vfill
  \begin{center}
    \url{https://demo.taler.net/}
  \end{center}
  \begin{enumerate}
  \item Install browser extension.
  \item Visit the {\tt bank.demo.taler.net} to withdraw coins.
  \item Visit the {\tt shop.demo.taler.net} to spend coins.
  \end{enumerate}
  \vfill
\end{frame}


\begin{frame}{Social Impact of Taler}
  \begin{center}
    \includegraphics[height=0.9\textheight]{../social-impact.pdf}
  \end{center}
\end{frame}


\begin{frame}{Use Case: Journalism}
  Today:
  \begin{itemize}
    \item Corporate structure % ($\Rightarrow$ filter)
    \item Advertising primary revenue % ($\Rightarrow$ dependence)
    \item Tracking readers critical for business success
    \item Journalism and marketing hard to distinguish
  \end{itemize}\vfill\pause
  With GNU Taler:
  \begin{itemize}
    \item One-click micropayments per article
    \item Hosting requires no expertise % (no PCI DSS)
    \item Reader-funded reporting separated from marketing
    \item Readers can remain anonymous
  \end{itemize}
\end{frame}


\begin{frame}[c]{Example: The Taler Snack Machine\footnote{by M. Boss and D. Hofer}}
  \framesubtitle{Integration of a MDB/ICP to Taler gateway.\\Implementation of a NFC or QR-Code to Taler wallet interface.}
	\vfill
	\begin{figure}
  \centering
  \includegraphics[width=1.0\textwidth]{design}
  \end{figure}
\end{frame}


\begin{frame}[t]{Software architecture for the Taler Snack Machine}
  \framesubtitle{Code at \url{https://git.taler.net/taler-mdb}}
\begin{figure}
  				\centering
  				\includegraphics[width=.9\textwidth]{software_stack}
				\end{figure}
\end{frame}


\begin{frame}[c]{User story: Install App on Android}
\framesubtitle{\url{https://wallet.taler.net/}}
		\begin{figure}
			\includegraphics[width=0.9\textwidth]{download_wallet.png}
		\end{figure}
\end{frame}

\begin{frame}{User story: Withdraw e-cash}
		\begin{figure}
			\includegraphics[width=0.9\textwidth]{get_taler_coins.png}
		\end{figure}
\end{frame}

\begin{frame}{User story: Use machine!}
		\begin{figure}
			\includegraphics[width=0.9\textwidth]{get_snacks.png}
		\end{figure}
\end{frame}


\begin{frame}{Real-world use}
\vfill
\begin{center}
\includegraphics[width=1.0\textwidth]{taler-in-use.png}
\end{center}
\vfill
\end{frame}


\section{Component Zoo}

\begin{frame}
  \vfill
  \begin{center}
    {\bf Component Zoo}
  \end{center}
  \vfill
\end{frame}


\begin{frame}{The Taler Software Ecosystem: Overview}
  \framesubtitle{\url{https://taler.net/en/docs.html}}
  Taler is based on modular components that work together to provide a
  complete payment system:
  \vfill
  \begin{itemize}
    \item {\bf Exchange:} Service provider for digital cash
      \begin{itemize}
        \item Core exchange software (cryptography, database)
        \item Air-gapped key management, real-time {\bf auditing}
        \item {\bf libeufin}: Modular integration with banking systems
        \item {\bf challenger}: KYC service with OAuth 2.0 API
      \end{itemize}
    \item {\bf Merchant:} Integration service for existing businesses
      \begin{itemize}
        \item Core merchant backend software (cryptography, database)
        \item {\bf Back-office interface} for staff
        \item {\bf Frontend integration} (E-commerce, Point-of-sale)
      \end{itemize}
    \item {\bf Wallet:} Consumer-controlled applications for e-cash
      \begin{itemize}
        \item Multi-platform wallet software (for browsers \& mobile phones)
        \item Wallet backup storage providers ({\bf sync} \& {\bf Anastasis})
      \end{itemize}
  \end{itemize}
\end{frame}


\begin{frame}{Taler Exchange}
  The {\bf Exchange} is the core logic of the payment system.

  \begin{itemize}
    \item One exchange at minimum must be operated per currency
    \item Offers a REST API for merchants and customers
    \item Uses several helper processes for configuration and to
          interact with RTGS and cryptography
    \item KYC support via OAuth 2.0, KycAID or Persona APIs
    \item Implemented in C on top of GNU libmicrohttpd
  \end{itemize}
\end{frame}


\begin{frame}{Taler: Exchange Architecture}
\begin{center}
\begin{tikzpicture}
 \tikzstyle{def} = [node distance=2em and 2.5em, inner sep=1em, outer sep=.3em];
 \node (origin) at (0,0) {};
 \node (httpd) [def,above=of origin,draw]{httpd};
 \node (secmod-rsa) [def, draw, right=of httpd] {secmod-rsa};
 \node (secmod-eddsa) [def, draw, left=of httpd] {secmod-eddsa};
 \node (postgres) [def, draw, below=of httpd]{Postgres};
 \node (aggregator) [def, draw, right=of postgres]{aggregator};
 \node (transfer) [def, draw, below left=of postgres]{transfer};
 \node (wirewatch) [def, draw, below right=of postgres]{wirewatch};
 \node (nexus) [def, draw, below=of postgres]{Nexus};

 \tikzstyle{C} = [color=black, line width=1pt]

 \draw [<->, C] (httpd) -- (postgres) node [midway, above, sloped] (TextNode) {};
 \draw [<->, C] (httpd) -- (secmod-rsa) node [midway, above, sloped] (TextNode) {};
 \draw [<->, C] (httpd) -- (secmod-eddsa) node [midway, above, sloped] (TextNode) {};
 \draw [<->, C] (aggregator) -- (postgres) node [midway, above, sloped] (TextNode) {};
 \draw [<->, C] (wirewatch) -- (postgres) node [midway, above, sloped] (TextNode) {};
 \draw [<->, C] (transfer) -- (postgres) node [midway, above, sloped] (TextNode) {};
 \draw [->, C] (transfer) -- (nexus) node [midway, above, sloped] (TextNode) {};
 \draw [<-, C] (wirewatch) -- (nexus) node [midway, above, sloped] (TextNode) {};
\end{tikzpicture}
\end{center}
\end{frame}


\begin{frame}{Taler Merchant}
  The {\bf Merchant} is the software run by merchants to accept\\
  GNU Taler payments.

  \begin{minipage}{6cm}
  \begin{itemize}
    \item REST API for integration with e-commerce
    \item SPA provides Web interface for administration
    \item Features include:
      \begin{itemize}
      \item Multi-tenant support
      \item Refunds
      \item Tipping (Website pays visitor)
      \item Webhooks
      \item Inventory management (optional)
      \end{itemize}
    \item Implemented in C on top of GNU libmicrohttpd
  \end{itemize}
  \end{minipage}
  \begin{minipage}{5cm}
  \includegraphics[width=5cm]{screenshots/merchant-spa-settings}
  \end{minipage}
\end{frame}


\begin{frame}
\frametitle{Taler: Merchant Perspective}
\begin{center}
\begin{tikzpicture}
 \tikzstyle{def} = [node distance= 3.5em and 2em, inner sep=1em, outer sep=.3em];
 \node (origin) at (0,0) {};
 \node (backend) [def,above=of origin,draw]{{\tiny taler-merchant-httpd}};
 \node (frontend) [def,above left=of backend,draw]{{\tiny E-commerce Frontend}};
 \node (backoffice) [def,above right=of backend,draw]{Backoffice};
 \node (postgres) [def, draw, below left=of backend] {Postgres};
 \node (sqlite) [def, draw, below=of backend] {Sqlite};
 \node (alt) [def, draw, below right=of backend] {...};

 \tikzstyle{C} = [color=black, line width=1pt]

 \draw [->, C] (frontend) -- (backend) node [midway, above, sloped] (TextNode) {REST API};
 \draw [->, C] (backoffice) -- (backend) node [midway, above, sloped] (TextNode) {REST API};
 \draw [<->, C] (backend) -- (postgres) node [midway, above, sloped] (TextNode) {SQL};
 \draw [<->, C] (backend) -- (sqlite) node [midway, above, sloped] (TextNode) {SQL};
 \draw [<->, C] (backend) -- (alt) node [midway, above, sloped] (TextNode) {SQL};
\end{tikzpicture}
\end{center}
\end{frame}



\begin{frame}{Taler Wallet}
  The {\bf Wallet} is the software run by consumers to store
  their digital cash and authorize transactions.

  \begin{minipage}{8cm}
  \begin{itemize}
    \item {\bf wallet-core} is the logic shared by all interfaces
    \item Applications exist for Android, F-Droid,
          WebExtension (Chrome, Chromium, Firefox, etc.), iOS ({\bf WiP})
    \item Features include:
      \begin{itemize}
      \item Multi-currency support
      \item Wallet-to-wallet payments (NFC or QR code)
      \item CRDT-like data model
      \end{itemize}
    \item {\bf wallet-core} implemented in TypeScript
  \end{itemize}
  Can be integrated into other Apps if desired.
  \end{minipage}
  \begin{minipage}{3cm}
  \includegraphics[width=3cm]{screenshots/Screenshot_20230225-103520.png}
  \end{minipage}
\end{frame}


\begin{frame}
\frametitle{Taler: Wallet Architecture}
  \framesubtitle{Background: \url{https://anastasis.lu/}}
\begin{center}
\begin{tikzpicture}
 \tikzstyle{def} = [node distance= 5em and 4.5em, inner sep=1em, outer sep=.3em];
 \node (origin) at (0,0) {};
 \node (gui) [def,above=of origin,draw]{wallet-gui};
 \node (core) [def,below=of gui,draw]{wallet-core};
 \node (sync) [def, draw, below left=of core] {Sync};
 \node (taler) [def, draw, below right=of core] {Taler};
 \node (anastasis) [def, draw, below=of core] {Anastasis};

 \tikzstyle{C} = [color=black, line width=1pt]
 \draw [<->, C] (gui) -- (core) node [midway, above, sloped] (TextNode) {};
 \draw [<->, C] (core) -- (sync) node [midway, above, sloped] (TextNode) {Backup};
 \draw [<->, C] (core) -- (taler) node [midway, above, sloped] (TextNode) {Payment};
 \draw [<->, C] (core) -- (anastasis) node [midway, above, sloped] (TextNode) {Key Escrow};
\end{tikzpicture}
\end{center}
\end{frame}


\begin{frame}[fragile]{RFC 8905: \texttt{payto:} Uniform Identifiers for Payments and Accounts}
  \vfill
  Like \texttt{mailto:}, but for bank accounts instead of email accounts!
  \vfill
  \begin{verbatim}
    payto://<PAYMENT-METHOD>/<ACCOUNT-NR>
      ?subject=InvoiceNr42
      &amount=EUR:12.50
  \end{verbatim}
  \vfill
  Default action:  Open app to review and confirm payment.
  \vfill
\includegraphics[width=0.25\textwidth]{einzahlschein-ch.jpeg}
\hfill
\includegraphics[width=0.2\textwidth]{de-ueberweisungsformular.png}
  \vfill
\end{frame}


\begin{frame}[fragile]{Benefits of {\tt payto://}}
  \begin{itemize}
    \item Standardized way to represent financial resources (bank account, bitcoin wallet)
      and payments to them
    \item Useful on the client-side on the Web and for FinTech backend applications
    \item Payment methods (such as IBAN, ACH, Bitcoin) are registered with
          IANA and allow extra options
  \end{itemize}
  \begin{center}
  {\bf Taler wallet can generate payto://-URI for withdraw!}
  \end{center}
\end{frame}


\begin{frame}{Taler Auditor}
  The {\bf Auditor} is the software run by an independent auditor
  to validate the operation of an Exchange.

  \begin{itemize}
    \item REST API for additional report inputs by merchants (optional)
    \item Secure database replication logic
    \item Implemented in C on top of GNU libmicrohttpd
  \end{itemize}
\end{frame}


\begin{frame}
\frametitle{Taler: Auditor Perspective}
\begin{center}
\begin{tikzpicture}
 \tikzstyle{def} = [node distance=2em and 2.5em, inner sep=1em, outer sep=.3em];
 \node (origin) at (0,0) {};
 \node (httpd) [def,above left=of origin,draw]{auditor-httpd};
 \node (report) [def,above right=of origin,draw]{auditor-report};
 \node (postgres-A) [def, draw, below=of origin] {Postgres (Auditor)};
 \node (postgres-E) [def, draw, below=of postgres-A] {Postgres (Bank)};

 \tikzstyle{C} = [color=black, line width=1pt]

 \draw [->, C] (postgres-E) -- (postgres-A) node [midway, above, sloped] (TextNode) {sync};
 \draw [<->, C] (httpd) -- (postgres-A) node [midway, above, sloped] (TextNode) {};
 \draw [<->, C] (report) -- (postgres-A) node [midway, above, sloped] (TextNode) {};
\end{tikzpicture}
\end{center}
\end{frame}


\begin{frame}{libeufin-nexus}
  libeufin-nexus allows Taler components to interact with a core banking system. It:

  \begin{itemize}
    \item provides an implementation of the Wire Gateway for the exchange
    \item supports EBICS 2.5
    \item other APIs such as FinTS or PSD2-style XS2A APIs can be added
      without requiring changes to the Exchange
    \item was tested with GLS Bank (DE) and Postfinance (CH) accounts and real EUR/CHF
  \end{itemize}
\end{frame}


\begin{frame}{libeufin-bank}
  libeufin-bank implements a standalone bank with a Web interface. It:

  \begin{itemize}
    \item provides the Taler Core Bank API for RESTful online banking
       using a Web interface (with multi-factor authentication)
    \item includes a Taler Wire Gateway for the exchange
    \item offers the Taler Bank Integration API to allow wallets
       to easily withdraw digital cash
    \item optionally provides the Taler Conversion Info API for currency
       conversion between fiat and regional currencies
    \item optionally integrates with libeufin-nexus to interact with
       a core banking system
  \end{itemize}
\end{frame}


\begin{frame}[fragile]{Taler: Bank Perspective}
\begin{adjustbox}{max totalsize={.9\textwidth}{.7\textheight},center}
\begin{tikzpicture}
 \tikzstyle{def} = [node distance= 5em and 6.5em, inner sep=1em, outer sep=.3em];
 \node (origin) at (0,0) {};
 \node (exchange) [def,above=of origin,draw]{Exchange};
 \node (nexus) [def, draw, below right=of exchange] {Nexus};
 \node (corebanking) [def, draw, below left=of nexus] {Core Banking};
 \node (nginx) [def, draw, above=of exchange]{Nginx};
 \node (postgres) [def, draw, below left=of exchange]{Postgres};
 \node (postgres-nexus) [def, draw, below right=of nexus]{Postgres};

 \tikzstyle{C} = [color=black, line width=1pt]

 \draw [<-, C] (exchange) -- (nginx) node [midway, above, sloped] (TextNode) {REST API};
 \draw [<-, C] (postgres) -- (exchange) node [midway, above, sloped] (TextNode) {SQL};
 \draw [<-, C] (postgres-nexus) -- (nexus) node [midway, above, sloped] (TextNode) {SQL};
 \draw [<-, C] (nexus) -- (exchange) node [midway, above, sloped] (TextNode) {Internal REST API};
 \draw [<-, C] (corebanking) -- (nexus) node [midway, above, sloped] (TextNode) {EBICS/FinTS};

\end{tikzpicture}
\end{adjustbox}
\end{frame}


\begin{frame}{Challenger}
  Challenger allows clients to obtain validated address (KYC) data about
  users:

  \begin{itemize}
    \item Customizable Web-based process for address validation
    \item Can validate phone numbers, e-mail addresses or physical mailing addresses
    \item Provides an exchange-compatible OAuth 2.0 API
  \end{itemize}
\end{frame}


\begin{frame}{Depolymerization}
  Depolymerization is a bridge between GNU Taler and blockchains,
  making Taler a layer 2 system for crypto-currencies (like Lightning).

  \begin{itemize}
    \item provides an implementation of the Wire Gateway for the exchange
    \item Works on top of Bitcoin and Ethereum
          crypto-currencies, with the DLTs as the ``RTGS''
    \item Provides same API to Exchange as libeufin-nexus
    \item Implemented in Rust
  \end{itemize}
  \begin{center}
      \url{https://bitcoin.ice.bfh.ch/}
  \end{center}
\end{frame}


\begin{frame}{Pretix Taler payment plugin}
\begin{center}
\includegraphics[width=0.5\textwidth]{screenshots/pretix.png}
\end{center}

  Pretix is a ticket sales system.

  \begin{itemize}
    \item Pretix payment plugin enables payments via GNU Taler
    \item Developed by Pretix.eu for \EUR{3,000} on behalf of Taler Systems SA
  \end{itemize}
\end{frame}


\begin{frame}{WooCommerce Taler payment plugin}
\begin{minipage}{6cm}
  \begin{itemize}
    \item WooCommerce is an e-commerce plugin for WordPress.
    \item WooCommerce payment plugin enables payments via GNU Taler
    \item Features include:
      \begin{itemize}
      \item Trivial configuration
      \item Support for refunds
      \item Full internationalization
      \end{itemize}
    \item WooCommerce and its plugins are implemented in PHP
  \end{itemize}
\end{minipage}
\begin{minipage}{5cm}
  \includegraphics[width=4cm]{screenshots/woocommerce-cart.png}
  \includegraphics[width=4cm]{screenshots/woocommerce-settings.png}
  \end{minipage}
\end{frame}


\begin{frame}{Joomla! Taler payment plugin}
\begin{minipage}{6cm}
  \begin{itemize}
    \item Joomla! is an e-commerce platform
    \item Joomla! payment plugin enables payments via GNU Taler
    \item Features include:
      \begin{itemize}
      \item Trivial configuration
      \item Support for refunds
      \item Full internationalization
      \end{itemize}
    \item Joomla! and its plugins are implemented in PHP
  \end{itemize}
\end{minipage}
% FIXME: add screenshots
%\begin{minipage}{5cm}
%  \includegraphics[width=4cm]{screenshots/woocommerce-cart.png}
%  \includegraphics[width=4cm]{screenshots/woocommerce-settings.png}
%  \end{minipage}
\end{frame}




\begin{frame}{Point-of-Sale App for Android}

\begin{minipage}{7cm}
  \begin{itemize}
    \item Allows merchant to generate orders against Taler backend
          and display QR code to enable customer to pay in person
    \item Patterned after ViewTouch restaurant UI
    \item Features include:
      \begin{itemize}
      \item Internet-based configuration
      \item Products sorted by categories
      \item Easy undo of every operation
      \item Manages multiple concurrent orders
      \end{itemize}
    \item The Point-of-Sale App is implemented in Kotlin
  \end{itemize}
\end{minipage}
\begin{minipage}{4cm}
  \includegraphics[width=4cm]{screenshots/Screenshot_20230224-194112.jpg}
  \includegraphics[width=4cm]{screenshots/Screenshot_20230224-194119.jpg}
  \includegraphics[width=4cm]{screenshots/Screenshot_20230224-195348.jpg}
\end{minipage}
\end{frame}


\begin{frame}{Cashier App for Android}
\begin{minipage}{4cm}
  \begin{itemize}
    \item Enables BFH staff to convert cash to e-cash
    \item Staff has special bank accounts with limited funds
    \item Students can pay staff in cash to receive e-cash
    \item The Cashier App is implemented in Kotlin
  \end{itemize}
  \end{minipage}
  \begin{minipage}{3cm}
  \includegraphics[width=3cm]{screenshots/Screenshot_20230225-103315.png}
  \end{minipage}
  \begin{minipage}{3cm}
  \includegraphics[width=3cm]{screenshots/Screenshot_20230225-103325.png}
  \end{minipage}
\end{frame}


\begin{frame}{TalDir (WiP)}
  TalDir is an extension to the existing
  peer-to-peer payment functionality.

  \begin{itemize}
    \item Registry to associate wallets with network addresses
    \item Extensible to different types of network services:
      \begin{itemize}
    \item E-mail
    \item SMS
    \item Twitter
    \item ...
     \end{itemize}
    \item Send payments or invoices to wallets associated with network address
    \item Will {\bf not} require sending wallet to use same network service
  \end{itemize}
\end{frame}




\section{Protocol Basics}

\begin{frame}
  \vfill
  \begin{center}
    {\bf Protocol Basics}
  \end{center}
  \vfill
\end{frame}


\begin{frame}{A Bachelor's Thesis Video}
  \begin{center}
    \movie[%scale=0.6,
           autostart,
           poster]
           {
               \includegraphics[height=0.6\textwidth,width=0.8\textwidth]{white.png}
           }
          {cs-movie.mp4}
  \end{center}
\end{frame}


\begin{frame}{How does it work?}
We use a few ancient constructions:
  \begin{itemize}
  \item Cryptographic hash function (1989)
  \item Blind signature (1983)
  \item Schnorr signature (1989)
  \item Diffie-Hellman key exchange (1976)
  \item Cut-and-choose zero-knowledge proof (1985)
  \end{itemize}
But of course we use modern instantiations.
\end{frame}


\begin{frame}{Definition: Taxability}
  We say Taler is taxable because:
  \begin{itemize}
  \item Merchant's income is visible from deposits.
  \item Hash of contract is part of deposit data.
  \item State can trace income and enforce taxation.
  \end{itemize}\pause
  Limitations:
  \begin{itemize}
  \item withdraw loophole
  \item {\em sharing} coins among family and friends
  \end{itemize}
\end{frame}


\begin{frame}{Exchange setup: Create a denomination key (RSA)}
   \begin{minipage}{6cm}
    \begin{enumerate}
    \item Pick random primes $p,q$.
    \item Compute $n := pq$, $\phi(n) = (p-1)(q-1)$
    \item Pick small $e < \phi(n)$ such that
          $d := e^{-1} \mod \phi(n)$ exists.
    \item Publish public key $(e,n)$.
    \end{enumerate}
  \end{minipage}
  \begin{minipage}{6cm}
  \begin{tikzpicture}
 \tikzstyle{def} = [node distance=1em and 1em, inner sep=0em, outer sep=.3em];
    \node (origin) at (0,0) {\includegraphics[width=0.2\textwidth]{dice.pdf}};
    \node (primes) [draw=none, below = of origin] at (0,0) {$(p, q)$};
    \node (seal) [def, draw=none, below left=of primes]{\includegraphics[width=0.15\textwidth]{seal.pdf}};
    \node (hammer) [def, draw=none, below right=of primes]{\includegraphics[width=0.15\textwidth]{hammer.pdf}};

    \tikzstyle{C} = [color=black, line width=1pt]

    \draw [<-, C] (primes) -- (origin) node [midway, above, sloped] (TextNode) {};
    \draw [<-, C] (seal) -- (primes) node [midway, above, sloped] (TextNode) {};
    \draw [<-, C] (hammer) -- (primes) node [midway, above, sloped] (TextNode) {};
  \end{tikzpicture}
%  \includegraphics[width=0.4\textwidth]{seal.pdf}
  \end{minipage}
\end{frame}


\begin{frame}{Merchant: Create a signing key (EdDSA)}
  \begin{minipage}{6cm}
    \begin{itemize}
  \item pick random $m \mod o$ as private key
  \item $M = mG$ public key
  \end{itemize}
  \end{minipage}
  \begin{minipage}{6cm}
  \begin{tikzpicture}
   \tikzstyle{def} = [node distance= 1em and 1em, inner sep=0em, outer sep=.3em];
    \node (origin) at (0,0) {\includegraphics[width=0.2\textwidth]{dice.pdf}};
    \node (m) [draw=none, below = of origin] at (0,0) {$m$};
    \node (seal) [draw=none, below=of m]{M};
   \tikzstyle{C} = [color=black, line width=1pt]

    \draw [<-, C] (m) -- (origin) node [midway, above, sloped] (TextNode) {};
    \draw [<-, C] (seal) -- (primes) node [midway, above, sloped] (TextNode) {};
  \end{tikzpicture}
  \end{minipage}
  \parbox[t]{3cm}{{\bf Capability:} $m \Rightarrow$ }
  \raisebox{\dimexpr-\height+\baselineskip}{\includegraphics[width=0.1\textwidth]{merchant-sign.pdf}}
\end{frame}


\begin{frame}{Customer: Create a planchet (EdDSA)}
  \begin{minipage}{8cm}
  \begin{itemize}
  \item Pick random $c \mod o$ private key
  \item $C = cG$ public key
  \end{itemize}
  \end{minipage}
  \begin{minipage}{4cm}
  \begin{tikzpicture}
   \tikzstyle{def} = [node distance= 1em and 1em, inner sep=0em, outer sep=.3em];
    \node (origin) at (0,0) {\includegraphics[width=0.2\textwidth]{dice.pdf}};
    \node (c) [draw=none, below = of origin] at (0,0) {$c$};
    \node (planchet) [draw=none, below=of c]{\includegraphics[width=0.4\textwidth]{planchet.pdf}};
    \tikzstyle{C} = [color=black, line width=1pt]

    \draw [<-, C] (c) -- (origin) node [midway, above, sloped] (TextNode) {};
    \draw [<-, C] (planchet) -- (c) node [midway, above, sloped] (TextNode) {};
  \end{tikzpicture}
  \end{minipage}
  \parbox[t]{3cm}{{\bf Capability:} $c \Rightarrow$ }
  \raisebox{\dimexpr-\height+\baselineskip}{\includegraphics[width=0.1\textwidth]{planchet-sign.pdf}}
\end{frame}


\begin{frame}{Customer: Blind planchet (RSA)}
  \begin{minipage}{6cm}
    \begin{enumerate}
    \item Obtain public key $(e,n)$
    \item Compute $f := FDH(C)$, $f < n$.
    \item Pick blinding factor $b \in \mathbb Z_n$
    \item Transmit $f' := f b^e \mod n$
    \end{enumerate}
  \end{minipage}
  \begin{minipage}{6cm}
  \begin{tikzpicture}
   \tikzstyle{def} = [node distance= 2em and 0.5em, inner sep=0em, outer sep=.3em];
    \node (origin) at (0,0) {\includegraphics[width=0.2\textwidth]{dice.pdf}};
    \node (b) [def, draw=none, below = of origin] at (0,-0.2) {$b$};
    \node (blinded) [def, draw=none, below right=of b]{\includegraphics[width=0.2\textwidth]{blinded.pdf}};
    \node (planchet) [def, draw=none, above right=of blinded]{\includegraphics[width=0.15\textwidth]{planchet.pdf}};
    \node (exchange) [node distance=4em and 0.5em, draw, below =of blinded]{Exchange};
    \tikzstyle{C} = [color=black, line width=1pt]

    \draw [<-, C] (b) -- (origin) node [midway, above, sloped] (TextNode) {};
    \draw [<-, C] (blinded) -- (planchet) node [midway, above, sloped] (TextNode) {};
    \draw [<-, C] (blinded) -- (b) node [midway, above, sloped] (TextNode) {};
    \draw [<-, C] (exchange) -- (blinded) node [midway, above, sloped] (TextNode) {{\small transmit}};
  \end{tikzpicture}
  \end{minipage}
\end{frame}


\begin{frame}{Exchange: Blind sign (RSA)}
   \begin{minipage}{6cm}
    \begin{enumerate}
    \item Receive $f'$.
    \item Compute $s' := f'^d \mod n$.
    \item Send signature $s'$.
    \end{enumerate}
   \end{minipage}
  \begin{minipage}{6cm}
  \begin{tikzpicture}
   \tikzstyle{def} = [node distance= 2em and 0.5em, inner sep=0em, outer sep=.3em];
    \node (hammer) [def, draw=none] at (0,0) {\includegraphics[width=0.15\textwidth]{hammer.pdf}};
    \node (signed) [def, draw=none, below left=of hammer]{\includegraphics[width=0.2\textwidth]{sign.pdf}};
    \node (blinded) [def, draw=none, above left=of signed]{\includegraphics[width=0.15\textwidth]{blinded.pdf}};
    \node (customer) [node distance=4em and 0.5em, draw, below =of signed]{Customer};
    \tikzstyle{C} = [color=black, line width=1pt]

    \draw [<-, C] (signed) -- (hammer) node [midway, above, sloped] (TextNode) {};
    \draw [<-, C] (signed) -- (blinded) node [midway, above, sloped] (TextNode) {};
    \draw [<-, C] (customer) -- (signed) node [midway, above, sloped] (TextNode) {{\small transmit}};
  \end{tikzpicture}
  \end{minipage}
\end{frame}


\begin{frame}{Customer: Unblind coin (RSA)}
  \begin{minipage}{6cm}
   \begin{enumerate}
    \item Receive $s'$.
    \item Compute $s := s' b^{-1} \mod n$ % \\
    % ($(f')^d = (f b^e)^d = f^d b$).
    \end{enumerate}
   \end{minipage}
  \begin{minipage}{6cm}
  \begin{tikzpicture}
   \tikzstyle{def} = [node distance= 2em and 0.5em, inner sep=0em, outer sep=.3em];
    \node (b) [def, draw=none] at (0,0) {$b$};
    \node (coin) [def, draw=none, below left=of b]{\includegraphics[width=0.2\textwidth]{coin.pdf}};
    \node (signed) [def, draw=none, above left=of coin]{\includegraphics[width=0.15\textwidth]{sign.pdf}};
    \tikzstyle{C} = [color=black, line width=1pt]

    \draw [<-, C] (coin) -- (b) node [midway, above, sloped] (TextNode) {};
    \draw [<-, C] (coin) -- (signed) node [midway, above, sloped] (TextNode) {};
  \end{tikzpicture}
  \end{minipage}
\end{frame}

\begin{frame}{Withdrawing coins on the Web}
  \begin{center}
    \includegraphics[height=0.9\textheight]{figs/taler-withdraw.pdf}
  \end{center}
\end{frame}


\begin{frame}{Customer: Build shopping cart}
  \begin{center}
  \begin{tikzpicture}
   \tikzstyle{def} = [node distance= 1em and 1em, inner sep=0em, outer sep=.3em];
    \node (origin) [draw=none] at (0,0) {\includegraphics[width=0.2\textwidth]{cart.pdf}};
    \node (merchant) [node distance=4em and 0.5em, draw, below =of origin]{\includegraphics[width=0.2\textwidth]{shop.pdf}};
    \tikzstyle{C} = [color=black, line width=1pt];
    \draw [<-, C] (merchant) -- (origin) node [midway, above, sloped] (TextNode) {{\small transmit}};
  \end{tikzpicture}
  \end{center}
\end{frame}


\begin{frame}{Merchant Integration: Contract}
  % \begin{figure*}[t!]
  {\tiny
 \lstset{language=JavaScript}
 \lstinputlisting{figs/taler-contract.json}
%   \caption{Minimal Taler contract over a digital article with a value of \EUR{0.10}. The merchant will pay transaction fees up to \EUR{0.01}.  The hash over the wire transfer information was truncated to make it fit to the page.}
%   \label{listing:json-contract}
 % \end{figure*}
 }
\end{frame}


\begin{frame}{Merchant Integration: Payment Request}
% \begin{figure}[p!]
 \lstset{language=HTML5}
 \lstinputlisting{figs/taler-402.html}
%  \caption{Sample HTTP response to prompt the wallet to show an offer.}
%   \label{listing:http-contract}
% \end{figure}

% \begin{figure*}[p!]
% \lstset{language=HTML5}
% \lstinputlisting{figs/taler-contract.html}
% \caption{Sample JavaScript code to prompt the wallet to show an offer.
%          Here, the contract is fetched on-demand from the server.
%          The {\tt taler\_pay()} function needs to be invoked
%          when the user triggers the checkout.}
% \label{listing:contract}
% \end{figure*}
\end{frame}



\begin{frame}{Merchant: Propose contract (EdDSA)}
   \begin{minipage}{6cm}
   \begin{enumerate}
    \item Complete proposal $D$.
    \item Send $D$, $EdDSA_m(D)$
    \end{enumerate}
   \end{minipage}
  \begin{minipage}{6cm}
  \begin{tikzpicture}
   \tikzstyle{def} = [node distance=2em and 0.5em, inner sep=0em, outer sep=.3em];
    \node (cart) [def, draw=none] at (0,0) {\includegraphics[width=0.15\textwidth]{cart.pdf}};
    \node (proposal) [def, draw=none, below right=of cart]{\includegraphics[width=0.5\textwidth]{merchant_propose.pdf}};
    \node (customer) [node distance=4em and 0.5em, draw, below =of proposal]{Customer};
    \tikzstyle{C} = [color=black, line width=1pt];
    \node (sign) [def, draw=none, above right=of proposal] {$m$};
    \tikzstyle{C} = [color=black, line width=1pt]

    \draw [<-, C] (proposal) -- (sign) node [midway, above, sloped] (TextNode) {};
    \draw [<-, C] (proposal) -- (cart) node [midway, above, sloped] (TextNode) {};
    \draw [<-, C] (customer) -- (proposal) node [midway, above, sloped] (TextNode) {{\small transmit}};
  \end{tikzpicture}
  \end{minipage}
\end{frame}


\begin{frame}{Customer: Spend coin (EdDSA)}
  \begin{minipage}{6cm}
   \begin{enumerate}
    \item Receive proposal $D$, $EdDSA_m(D)$.
    \item Send $s$, $C$, $EdDSA_c(D)$
    \end{enumerate}
   \end{minipage}
  \begin{minipage}{6cm}
  \begin{tikzpicture}
   \tikzstyle{def} = [node distance=2em and 0.4em, inner sep=0em, outer sep=.3em];
    \node (proposal) [def, draw=none] at (0,0) {\includegraphics[width=0.15\textwidth]{merchant_propose.pdf}};
    \node (contract) [def, draw=none, below right=of cart]{\includegraphics[width=0.3\textwidth]{contract.pdf}};
    \node (c) [def, draw=none, above=of contract] {$c$};
    \node (merchant) [node distance=4em and 0.5em, draw, below=of contract]{Merchant};
    \node (coin) [def, draw=none, right=of contract]{\includegraphics[width=0.2\textwidth]{coin.pdf}};
    \tikzstyle{C} = [color=black, line width=1pt]

    \draw [<-, C] (contract) -- (c) node [midway, above, sloped] (TextNode) {};
    \draw [<-, C] (contract) -- (proposal) node [midway, above, sloped] (TextNode) {};
    \draw [<-, C] (merchant) -- (contract) node [midway, above, sloped] (TextNode) {{\small transmit}};
    \draw [<-, C] (merchant) -- (coin) node [midway, below, sloped] (TextNode) {{\small transmit}};
  \end{tikzpicture}
  \end{minipage}
\end{frame}


\begin{frame}{Merchant and Exchange: Verify coin (RSA)}
   \begin{minipage}{6cm}
 \begin{equation*}
   s^e \stackrel{?}{\equiv} FDH(C) \mod n
   \end{equation*}
   \end{minipage}
  \begin{minipage}{6cm}
  \begin{minipage}{0.2\textwidth}
    \includegraphics[width=\textwidth]{coin.pdf}
  \end{minipage}
  $\stackrel{?}{\Leftrightarrow}$
  \begin{minipage}{0.2\textwidth}
    \includegraphics[width=\textwidth]{seal.pdf}
  \end{minipage}
  \end{minipage}
  \vfill
  The exchange does not only verify the signature, but also
  checks that the coin was not double-spent.
  \vfill
  \pause
  \begin{center}
  {\bf Taler is an online payment system.}
  \end{center}
  \vfill
\end{frame}


\begin{frame}{Payment processing with Taler}
  \begin{center}
    \includegraphics[height=0.9\textheight]{figs/taler-pay.pdf}
  \end{center}
\end{frame}


\begin{frame}{Giving change}
  It would be inefficient to pay EUR 100 with 1 cent coins!
  \begin{itemize}
  \item Denomination key represents value of a coin.
  \item Exchange may offer various denominations for coins.
  \item Wallet may not have exact change!
  \item Usability requires ability to pay given sufficient total funds.
  \end{itemize}\pause
  Key goals:
  \begin{itemize}
  \item maintain unlinkability
  \item maintain taxability of transactions
  \end{itemize}\pause
  Method:
  \begin{itemize}
    \item Contract can specify to only pay {\em partial value} of a coin.
    \item Exchange allows wallet to obtain {\em unlinkable change}
      for remaining coin value.
  \end{itemize}
\end{frame}


\begin{frame}{Diffie-Hellman (ECDH)}
  \begin{minipage}{8cm}
   \begin{enumerate}
    \item Create private keys $c,t \mod o$
    \item Define $C = cG$
    \item Define $T = tG$
    \item Compute DH \\ $cT = c(tG) = t(cG) = tC$
    \end{enumerate}
   \end{minipage}
  \begin{minipage}{6cm}
  \begin{tikzpicture}
   \tikzstyle{def} = [node distance= 2em and 0.5em, inner sep=0em, outer sep=.3em];
    \node (t) [def, draw=none] at (0,0) {$t$};
    \node (ct) [def, draw=none, below left=of b]{\includegraphics[width=0.2\textwidth]{dh.pdf}};
    \node (c) [def, draw=none, above left= of ct]  {$c$};
    \tikzstyle{C} = [color=black, line width=1pt]

    \draw [<-, C] (ct) -- (c) node [midway, above, sloped] (TextNode) {};
    \draw [<-, C] (ct) -- (t) node [midway, above, sloped] (TextNode) {};
  \end{tikzpicture}
  \end{minipage}
\end{frame}


\begin{frame}{Strawman solution}
  \begin{minipage}{8cm}
    Given partially spent private coin key $c_{old}$:
   \begin{enumerate}
%    \item Let $C_{old} := c_{old}G$ (as before)
    \item Pick random $c_{new} \mod o$ private key
    \item $C_{new} = c_{new}G$ public key
    \item Pick random $b_{new}$
    \item Compute $f_{new} := FDH(C_{new})$, $m < n$.
    \item Transmit $f'_{new} := f_{new} b_{new}^e \mod n$
   \end{enumerate}
   ... and sign request for change with $c_{old}$.
   \end{minipage}
  \begin{minipage}{4cm}
  \begin{tikzpicture}
   \tikzstyle{def} = [node distance= 1.5em and 0.5em, inner sep=0em, outer sep=.3em];
    \node (blinded) [def, draw=none]{\includegraphics[width=0.15\textwidth]{blinded.pdf}};
    \node (planchet) [def, draw=none, above left= of blinded]  {\includegraphics[width=0.15\textwidth]{planchet.pdf}};
    \node (cnew) [def, draw=none, above= of planchet]  {$c_{new}$};
    \node (bnew) [def, draw=none, above right= of blinded]  {$b_{new}$};
    \node (dice1) [def, draw=none, above = of cnew]{\includegraphics[width=0.2\textwidth]{dice.pdf}};
    \node (dice2) [def, draw=none, above = of bnew]{\includegraphics[width=0.2\textwidth]{dice.pdf}};
    \node (exchange) [node distance=4em and 0.5em, draw, below =of blinded]{Exchange};

    \tikzstyle{C} = [color=black, line width=1pt]

    \draw [<-, C] (cnew) -- (dice1) node [midway, above, sloped] (TextNode) {};
    \draw [<-, C] (planchet) -- (cnew) node [midway, above, sloped] (TextNode) {};
    \draw [<-, C] (bnew) -- (dice2) node [midway, above, sloped] (TextNode) {};
    \draw [<-, C] (blinded) -- (planchet) node [midway, above, sloped] (TextNode) {};
    \draw [<-, C] (blinded) -- (bnew) node [midway, above, sloped] (TextNode) {};
    \draw [<-, C] (exchange) -- (blinded) node [midway, above, sloped] (TextNode) {{\small transmit}};
  \end{tikzpicture}
  \end{minipage}
  \pause
  \vfill
  {\bf Problem: Owner of $c_{new}$ may differ from owner of $c_{old}$!}
\end{frame}


\begin{frame}{Customer: Transfer key setup (ECDH)}
  \begin{minipage}{8cm}
    Given partially spent private coin key $c_{old}$:
   \begin{enumerate}
    \item Let $C_{old} := c_{old}G$ (as before)
    \item Create random private transfer key $t \mod o$
    \item Compute $T := tG$
    \item Compute $X := c_{old}(tG) = t(c_{old}G) = tC_{old}$
    \item Derive $c_{new}$ and $b_{new}$ from $X$
    \item Compute $C_{new} := c_{new}G$
    \item Compute $f_{new} := FDH(C_{new})$
    \item Transmit $f_{new}' := f_{new} b_{new}^e$
    \end{enumerate}
   \end{minipage}
  \begin{minipage}{4cm}
  \begin{tikzpicture}
   \tikzstyle{def} = [node distance= 1.5em and 0.5em, inner sep=0em, outer sep=.3em];
    \node (t) [def, draw=none] at (0,0) {$t$};
    \node (dice) [def, draw=none, above = of t]{\includegraphics[width=0.2\textwidth]{dice.pdf}};
    \node (dh) [def, draw=none, below left=of b]{\includegraphics[width=0.2\textwidth]{ct.pdf}};
    \node (d) [def, draw=none, above left= of dh]  {$c_{old}$};
    \node (cp) [def, draw=none, below left= of dh]  {$c_{new}$};
    \node (bp) [def, draw=none, below right= of dh]  {$b_{new}$};
    \node (blinded) [def, draw=none, below right=of cp]{\includegraphics[width=0.15\textwidth]{blinded.pdf}};
    \node (exchange) [node distance=4em and 0.5em, draw, below =of blinded]{Exchange};

    \tikzstyle{C} = [color=black, line width=1pt]

    \draw [<-, C] (dh) -- (d) node [midway, above, sloped] (TextNode) {};
    \draw [<-, C] (dh) -- (t) node [midway, above, sloped] (TextNode) {};
    \draw [<-, C] (t) -- (dice) node [midway, above, sloped] (TextNode) {};
    \draw [<-, C] (cp) -- (dh) node [midway, above, sloped] (TextNode) {};
    \draw [<-, C] (bp) -- (dh) node [midway, above, sloped] (TextNode) {};
    \draw [<-, C] (blinded) -- (cp) node [midway, above, sloped] (TextNode) {};
    \draw [<-, C] (blinded) -- (bp) node [midway, above, sloped] (TextNode) {};
    \draw [<-, C] (exchange) -- (blinded) node [midway, above, sloped] (TextNode) {{\small transmit}};
  \end{tikzpicture}
  \end{minipage}
\end{frame}


\begin{frame}{Cut-and-Choose}
  \begin{minipage}{4cm}
  \begin{tikzpicture}
   \tikzstyle{def} = [node distance= 1.5em and 0.5em, inner sep=0em, outer sep=.3em];
    \node (t) [def, draw=none] at (0,0) {$t_1$};
    \node (dice) [def, draw=none, above = of t]{\includegraphics[width=0.2\textwidth]{dice.pdf}};
    \node (dh) [def, draw=none, below left=of b]{\includegraphics[width=0.2\textwidth]{ct.pdf}};
    \node (d) [def, draw=none, above left= of dh]  {$c_{old}$};
    \node (cp) [def, draw=none, below left= of dh]  {$c_{new,1}$};
    \node (bp) [def, draw=none, below right= of dh]  {$b_{new,1}$};
    \node (blinded) [def, draw=none, below right=of cp]{\includegraphics[width=0.15\textwidth]{blinded.pdf}};
    \node (exchange) [node distance=4em and 0.5em, draw, below =of blinded]{Exchange};

    \tikzstyle{C} = [color=black, line width=1pt]

    \draw [<-, C] (t) -- (dice) node [midway, above, sloped] (TextNode) {};
    \draw [<-, C] (dh) -- (d) node [midway, above, sloped] (TextNode) {};
    \draw [<-, C] (dh) -- (t) node [midway, above, sloped] (TextNode) {};
    \draw [<-, C] (cp) -- (dh) node [midway, above, sloped] (TextNode) {};
    \draw [<-, C] (bp) -- (dh) node [midway, above, sloped] (TextNode) {};
    \draw [<-, C] (blinded) -- (cp) node [midway, above, sloped] (TextNode) {};
    \draw [<-, C] (blinded) -- (bp) node [midway, above, sloped] (TextNode) {};
    \draw [<-, C] (exchange) -- (blinded) node [midway, above, sloped] (TextNode) {{\small transmit}};
  \end{tikzpicture}
  \end{minipage}
  \begin{minipage}{4cm}
  \begin{tikzpicture}
   \tikzstyle{def} = [node distance= 1.5em and 0.5em, inner sep=0em, outer sep=.3em];
    \node (t) [def, draw=none] at (0,0) {$t_2$};
    \node (dice) [def, draw=none, above = of t]{\includegraphics[width=0.2\textwidth]{dice.pdf}};
    \node (dh) [def, draw=none, below left=of b]{\includegraphics[width=0.2\textwidth]{ct.pdf}};
    \node (d) [def, draw=none, above left= of dh]  {$c_{old}$};
    \node (cp) [def, draw=none, below left= of dh]  {$c_{new,2}$};
    \node (bp) [def, draw=none, below right= of dh]  {$b_{new,2}$};
    \node (blinded) [def, draw=none, below right=of cp]{\includegraphics[width=0.15\textwidth]{blinded.pdf}};
    \node (exchange) [node distance=4em and 0.5em, draw, below =of blinded]{Exchange};

    \tikzstyle{C} = [color=black, line width=1pt]

    \draw [<-, C] (t) -- (dice) node [midway, above, sloped] (TextNode) {};
    \draw [<-, C] (dh) -- (d) node [midway, above, sloped] (TextNode) {};
    \draw [<-, C] (dh) -- (t) node [midway, above, sloped] (TextNode) {};
    \draw [<-, C] (cp) -- (dh) node [midway, above, sloped] (TextNode) {};
    \draw [<-, C] (bp) -- (dh) node [midway, above, sloped] (TextNode) {};
    \draw [<-, C] (blinded) -- (cp) node [midway, above, sloped] (TextNode) {};
    \draw [<-, C] (blinded) -- (bp) node [midway, above, sloped] (TextNode) {};
    \draw [<-, C] (exchange) -- (blinded) node [midway, above, sloped] (TextNode) {{\small transmit}};
  \end{tikzpicture}
  \end{minipage}
  \begin{minipage}{4cm}
  \begin{tikzpicture}
   \tikzstyle{def} = [node distance= 1.5em and 0.5em, inner sep=0em, outer sep=.3em];
    \node (t) [def, draw=none] at (0,0) {$t_3$};
    \node (dice) [def, draw=none, above = of t]{\includegraphics[width=0.2\textwidth]{dice.pdf}};
    \node (dh) [def, draw=none, below left=of b]{\includegraphics[width=0.2\textwidth]{ct.pdf}};
    \node (d) [def, draw=none, above left= of dh]  {$c_{old}$};
    \node (cp) [def, draw=none, below left= of dh]  {$c_{new,3}$};
    \node (bp) [def, draw=none, below right= of dh]  {$b_{new,3}$};
    \node (blinded) [def, draw=none, below right=of cp]{\includegraphics[width=0.15\textwidth]{blinded.pdf}};
    \node (exchange) [node distance=4em and 0.5em, draw, below =of blinded]{Exchange};

    \tikzstyle{C} = [color=black, line width=1pt]

    \draw [<-, C] (t) -- (dice) node [midway, above, sloped] (TextNode) {};
    \draw [<-, C] (dh) -- (d) node [midway, above, sloped] (TextNode) {};
    \draw [<-, C] (dh) -- (t) node [midway, above, sloped] (TextNode) {};
    \draw [<-, C] (cp) -- (dh) node [midway, above, sloped] (TextNode) {};
    \draw [<-, C] (bp) -- (dh) node [midway, above, sloped] (TextNode) {};
    \draw [<-, C] (blinded) -- (cp) node [midway, above, sloped] (TextNode) {};
    \draw [<-, C] (blinded) -- (bp) node [midway, above, sloped] (TextNode) {};
    \draw [<-, C] (exchange) -- (blinded) node [midway, above, sloped] (TextNode) {{\small transmit}};
  \end{tikzpicture}
  \end{minipage}
\end{frame}


\begin{frame}{Exchange: Choose!}
   \begin{center}
    \item Exchange sends back random $\gamma \in \{ 1, 2, 3 \}$ to the customer.
    \end{center}
\end{frame}


\begin{frame}{Customer: Reveal}
   \begin{enumerate}
   \item If $\gamma = 1$, send $t_2$, $t_3$ to exchange
   \item If $\gamma = 2$, send $t_1$, $t_3$ to exchange
   \item If $\gamma = 3$, send $t_1$, $t_2$ to exchange
  \end{enumerate}
\end{frame}


\begin{frame}{Exchange: Verify ($\gamma = 2$)}
  \begin{minipage}{4cm}
  \begin{tikzpicture}
   \tikzstyle{def} = [node distance= 1.5em and 0.5em, inner sep=0em, outer sep=.3em];
    \node (h) [def, draw=none] at (0,0) {$t_1$};
    \node (dh) [def, draw=none, below left=of b]{\includegraphics[width=0.2\textwidth]{ct.pdf}};
    \node (d) [def, draw=none, above left= of dh]  {$C_{old}$};
    \node (cp) [def, draw=none, below left= of dh]  {$c_{new,1}$};
    \node (bp) [def, draw=none, below right= of dh]  {$b_{new,1}$};
    \node (blinded) [def, draw=none, below right=of cp]{\includegraphics[width=0.15\textwidth]{blinded.pdf}};

    \tikzstyle{C} = [color=black, line width=1pt]

    \draw [<-, C] (dh) -- (d) node [midway, above, sloped] (TextNode) {};
    \draw [<-, C] (dh) -- (h) node [midway, above, sloped] (TextNode) {};
    \draw [<-, C] (cp) -- (dh) node [midway, above, sloped] (TextNode) {};
    \draw [<-, C] (bp) -- (dh) node [midway, above, sloped] (TextNode) {};
    \draw [<-, C] (blinded) -- (cp) node [midway, above, sloped] (TextNode) {};
    \draw [<-, C] (blinded) -- (bp) node [midway, above, sloped] (TextNode) {};
  \end{tikzpicture}
  \end{minipage}
  \begin{minipage}{4cm}
 \
  \end{minipage}
  \begin{minipage}{4cm}
  \begin{tikzpicture}
   \tikzstyle{def} = [node distance= 1.5em and 0.5em, inner sep=0em, outer sep=.3em];
    \node (h) [def, draw=none] at (0,0) {$t_3$};
    \node (dh) [def, draw=none, below left=of b]{\includegraphics[width=0.2\textwidth]{ct.pdf}};
    \node (d) [def, draw=none, above left= of dh]  {$C_{old}$};
    \node (cp) [def, draw=none, below left= of dh]  {$c_{new,3}$};
    \node (bp) [def, draw=none, below right= of dh]  {$b_{new,3}$};
    \node (blinded) [def, draw=none, below right=of cp]{\includegraphics[width=0.15\textwidth]{blinded.pdf}};

    \tikzstyle{C} = [color=black, line width=1pt]

    \draw [<-, C] (dh) -- (d) node [midway, above, sloped] (TextNode) {};
    \draw [<-, C] (dh) -- (h) node [midway, above, sloped] (TextNode) {};
    \draw [<-, C] (cp) -- (dh) node [midway, above, sloped] (TextNode) {};
    \draw [<-, C] (bp) -- (dh) node [midway, above, sloped] (TextNode) {};
    \draw [<-, C] (blinded) -- (cp) node [midway, above, sloped] (TextNode) {};
    \draw [<-, C] (blinded) -- (bp) node [midway, above, sloped] (TextNode) {};
  \end{tikzpicture}
  \end{minipage}
\end{frame}


\begin{frame}{Exchange: Blind sign change (RSA)}
   \begin{minipage}{6cm}
    \begin{enumerate}
    \item Take $f_{new,\gamma}'$.
    \item Compute $s' := f_{new,\gamma}'^d \mod n$.
    \item Send signature $s'$.
    \end{enumerate}
   \end{minipage}
  \begin{minipage}{6cm}
  \begin{tikzpicture}
   \tikzstyle{def} = [node distance= 2em and 0.5em, inner sep=0em, outer sep=.3em];
    \node (hammer) [def, draw=none] at (0,0) {\includegraphics[width=0.15\textwidth]{hammer.pdf}};
    \node (signed) [def, draw=none, below left=of hammer]{\includegraphics[width=0.2\textwidth]{sign.pdf}};
    \node (blinded) [def, draw=none, above left=of signed]{\includegraphics[width=0.15\textwidth]{blinded.pdf}};
    \node (customer) [node distance=4em and 0.5em, draw, below =of signed]{Customer};
    \tikzstyle{C} = [color=black, line width=1pt]

    \draw [<-, C] (signed) -- (hammer) node [midway, above, sloped] (TextNode) {};
    \draw [<-, C] (signed) -- (blinded) node [midway, above, sloped] (TextNode) {};
    \draw [<-, C] (customer) -- (signed) node [midway, above, sloped] (TextNode) {{\small transmit}};
  \end{tikzpicture}
  \end{minipage}
\end{frame}


\begin{frame}{Customer: Unblind change (RSA)}
  \begin{minipage}{6cm}
   \begin{enumerate}
    \item Receive $s'$.
    \item Compute $s := s' b_{new,\gamma}^{-1} \mod n$.
    \end{enumerate}
   \end{minipage}
  \begin{minipage}{6cm}
  \begin{tikzpicture}
   \tikzstyle{def} = [node distance= 2em and 0.5em, inner sep=0em, outer sep=.3em];
    \node (b) [def, draw=none] at (0,0) {$b_{new,\gamma}$};
    \node (coin) [def, draw=none, below left=of b]{\includegraphics[width=0.2\textwidth]{coin.pdf}};
    \node (signed) [def, draw=none, above left=of coin]{\includegraphics[width=0.15\textwidth]{sign.pdf}};
    \tikzstyle{C} = [color=black, line width=1pt]

    \draw [<-, C] (coin) -- (b) node [midway, above, sloped] (TextNode) {};
    \draw [<-, C] (coin) -- (signed) node [midway, above, sloped] (TextNode) {};
  \end{tikzpicture}
  \end{minipage}
\end{frame}


\begin{frame}{Exchange: Allow linking change}
  \begin{minipage}{7cm}
    \begin{center}
    Given $C_{old}$

    \vspace{1cm}

    return $T_\gamma$, $s := s' b_{new,\gamma}^{-1} \mod n$.
  \end{center}
   \end{minipage}
  \begin{minipage}{5cm}
   \begin{tikzpicture}
    \tikzstyle{def} = [node distance= 3em and 0.5em, inner sep=0.5em, outer sep=.3em];
    \node (co) [def, draw=none] at (0,0) {$C_{old}$};
    \node (T) [def, draw=none, below left=of co]{$T_\gamma$};
    \node (sign) [def, draw=none, below right=of co]{\includegraphics[width=0.15\textwidth]{sign.pdf}};
    \node (customer) [def, draw, below right=of T] {Customer};

    \tikzstyle{C} = [color=black, line width=1pt]

    \draw [<-, C] (T) -- (co) node [midway, above, sloped] (TextNode) {};
    \draw [<-, C] (sign) -- (co) node [midway, above, sloped] (TextNode) {};
    \draw [<-, C] (customer) -- (T) node [midway, above, sloped] (TextNode) {link};
    \draw [<-, C] (customer) -- (sign) node [midway, above, sloped] (TextNode) {link};
  \end{tikzpicture}
  \end{minipage}
\end{frame}


\begin{frame}{Customer: Link (threat!)}
  \begin{minipage}{6.3cm}
   \begin{enumerate}
    \item Have $c_{old}$.
    \item Obtain $T_\gamma$, $s$ from exchange
    \item Compute $X_\gamma = c_{old}T_\gamma$
    \item Derive $c_{new,\gamma}$ and $b_{new,\gamma}$ from $X_\gamma$
    \item Unblind $s := s' b_{new,\gamma}^{-1} \mod n$
  \end{enumerate}

   \end{minipage}
  \begin{minipage}{5.7cm}
  \begin{tikzpicture}
  \tikzstyle{def} = [node distance= 1.5em and 0.5em, inner sep=0em, outer sep=.3em];
    \node (T) [def, draw=none] at (0,0) {$T_\gamma$};
    \node (exchange) [def, inner sep=0.5em, draw, above left=of T] {Exchange};
    \node (signed) [def, draw=none, below left=of T]{\includegraphics[width=0.15\textwidth]{sign.pdf}};
    \node (dh) [def, draw=none, below right=of T]{\includegraphics[width=0.2\textwidth]{ct.pdf}};
    \node (bp) [def, draw=none, below left= of dh]  {$b_{new,\gamma}$};
    \node (co) [def, draw=none, above right= of dh]  {$c_{old}$};
    \node (cp) [def, draw=none, below= of dh]  {$c_{new,\gamma}$};
    \node (coin) [def, draw=none, below left = of bp]{\includegraphics[width=0.2\textwidth]{coin.pdf}};
    \node (psign) [def, node distance=2.5em and 0em, draw=none, below = of cp]{\includegraphics[width=0.2\textwidth]{planchet-sign.pdf}};

    \tikzstyle{C} = [color=black, line width=1pt]

    \draw [<-, C] (dh) -- (co) node [midway, above, sloped] (TextNode) {};
    \draw [<-, C] (dh) -- (T) node [midway, above, sloped] (TextNode) {};
    \draw [<-, C] (cp) -- (dh) node [midway, above, sloped] (TextNode) {};
    \draw [<-, C] (bp) -- (dh) node [midway, above, sloped] (TextNode) {};
    \draw [<-, C] (coin) -- (signed) node [midway, above, sloped] (TextNode) {};
    \draw [<-, C] (coin) -- (bp) node [midway, above, sloped] (TextNode) {};
    \draw [<-, C] (T) -- (exchange) node [midway, above, sloped] (TextNode) {link};
    \draw [<-, C] (signed) -- (exchange) node [midway, below, sloped] (TextNode) {link};
    \draw [<-, C, double] (psign) -- (cp) node [midway, below, sloped] (TextNode) {};
  \end{tikzpicture}
  \end{minipage}
\end{frame}


\begin{frame}{Refresh protocol summary}
  \begin{itemize}
  \item Customer asks exchange to convert old coin to new coin
  \item Protocol ensures new coins can be recovered from old coin
  \item[$\Rightarrow$] New coins are owned by the same entity!
  \end{itemize}
  Thus, the refresh protocol allows:
  \begin{itemize}
  \item To give unlinkable change.
  \item To give refunds to an anonymous customer.
  \item To expire old keys and migrate coins to new ones.
  \item To handle protocol aborts.
  \end{itemize}
  \noindent
  \begin{center}
    \bf
   Transactions via refresh are equivalent to {\em sharing} a wallet.
\end{center}
\end{frame}


\section{Attacks \& Defenses}

\begin{frame}
  \vfill
  \begin{center}
    \vfill
    {\bf Attacks \& Defenses}
    \vfill
  \end{center}
  \vfill
\end{frame}


\begin{frame}{Key management}
Taler has many types of keys:
\begin{itemize}
\item Coin keys
\item Denomination keys
\item Online message signing keys
\item Offline key signing keys
\item Merchant keys
\item Auditor key
\item Security module keys
\item Transfer keys
\item Wallet keys
\item {\em TLS keys, DNSSEC keys}
\end{itemize}
\end{frame}


\begin{frame}{Offline keys}
Both exchange and auditor use offline keys.
\begin{itemize}
\item Those keys must be backed up and remain highly confidential!
\item We recommend that computers that have ever had access to those
      keys to NEVER again go online.
\item We recommend using a Raspberry Pi for offline key operations.
      Store it in a safe under multiple locks and keys.
\item Apply full-disk encryption on offline-key signing systems.
\item Have 3--5 full-disk backups of offline-key signing systems.
\end{itemize}
\begin{center}
\includegraphics[scale=0.1]{pi.png}
\end{center}
\end{frame}


\begin{frame}{Online keys}
The exchange needs RSA and EdDSA keys to be available for online signing.
\begin{itemize}
\item Knowledge of these private keys will allow an adversary to
      mint digital cash, possibly resulting in huge financial losses
      (eventually, this will be detected by the auditor, but only
       after some financial losses have been irrevocably incurred).
\item The corresponding public keys are certified using
      Taler's public key infrastructure (which uses offline-only keys).
\end{itemize}
\begin{center}
\includegraphics[width=0.5\textwidth]{taler-diagram-signatures.png}
\end{center}
\vfill
{\tt taler-exchange-offline} can also be used to {\bf revoke} the
online signing keys, if we find they have been compromised.
\vfill
\end{frame}


\begin{frame}{Protecting online keys}
The exchange needs RSA and EdDSA keys to be available for online signing.
\begin{itemize}
\item {\tt taler-exchange-secmod-rsa} and {\tt taler-exchange-secmod-eddsa}
      are the only processes that must have access to the private keys.
\item The secmod processes should run under a different UID, but share
      the same GID with the exchange.
\item The secmods generate the keys, allow {\tt taler-exchange-httpd} to sign with
      them, and eventually delete the private keys.
\item Communication between secmods and {\tt taler-exchange-httpd} is via
      a UNIX domain socket.
\item Online private keys are stored on disk (not in database!) and should
      NOT be backed up (RAID should suffice). If disk is lost, we can always
      create fresh replacement keys!
\end{itemize}
\end{frame}


\begin{frame}{Online keys}
\begin{itemize}
\item The exchange needs $d$ and $w$ to be available for online signing.
\item The corresponding public keys $W$ and $(e,n)$ are certified using
      Taler's public key infrastructure (which uses offline-only keys).
\end{itemize}
\begin{center}
\includegraphics[width=0.5\textwidth]{taler-diagram-signatures.png}
\end{center}
\vfill
\begin{center}
{\bf What happens if those private keys are compromised?}
\end{center}
\vfill
\end{frame}


\begin{frame}{Denomination key $(e,n)$ compromise}
\begin{itemize}
\item An attacker who learns $d$ can sign an arbitrary number of illicit coins
      into existence and deposit them.
\item Auditor and exchange can detect this once the total number of deposits
      (illicit and legitimate) exceeds the number of legitimate coins the
      exchange created.
\item At this point, $(e,n)$ is {\em revoked}.  Users of {\em unspent}
      legitimate coins reveal $b$ from their withdrawal operation and
      obtain a {\em refund}.
\item The financial loss of the exchange is {\em bounded} by the number of
      legitimate coins signed with $d$.
\item[$\Rightarrow$] Taler frequently rotates denomination signing keys and
      deletes $d$ after the signing period of the respective key expires.
\end{itemize}
\begin{center}
\includegraphics[width=0.5\textwidth]{taler-diagram-denom-expiration.png}
\end{center}
\end{frame}


\begin{frame}{Online signing key $W$ compromise}
\begin{itemize}
\item An attacker who learns $w$ can sign deposit confirmations.
\item Attacker sets up two (or more) merchants and customer(s) which double-spend
      legitimate coins at both merchants.
\item The merchants only deposit each coin once at the exchange and get paid once.
\item The attacker then uses $w$ to fake deposit confirmations for the double-spent
      transactions.
\item The attacker uses the faked deposit confirmations to complain to the auditor
      that the exchange did not honor the (faked) deposit confirmations.
\end{itemize}
The auditor can then detect the double-spending, but cannot tell who is to blame,
and (likely) would presume an evil exchange, forcing it to pay both merchants.
\end{frame}


\begin{frame}{Detecting online signing key $W$ compromise}
\begin{itemize}
\item Merchants are required to {\em probabilistically} report
      signed deposit confirmations to the auditor.
\item Auditor can thus detect exchanges not reporting signed
      deposit confirmations.
\item[$\Rightarrow$] Exchange can rekey if illicit key use is detected,
      then only has to honor deposit confirmations it already provided
      to the auditor {\em and} those without proof of double-spending
      {\em and} those merchants reported to the auditor.
\item[$\Rightarrow$] Merchants that do not participate in reporting
      to the auditor risk their deposit permissions being voided in
      cases of an exchange's private key being compromised.
\end{itemize}
\end{frame}


\begin{frame}{Warranting deposit safety}
  Exchange has {\em another} online signing key $W = wG$:
  \begin{center}
    Sends $EdDSA_w(M,H(D),FDH(C))$ to the merchant.
  \end{center}
  This signature means that $M$ was the {\em first} to deposit
  $C$ and that the exchange thus must pay $M$.
  \vfill
  \begin{center}
     Without this, an evil exchange could renege on the deposit
     confirmation and claim double-spending if a coin were
     deposited twice, and then not pay either merchant!
  \end{center}
\end{frame}


\begin{frame}{Database}
The exchange needs the database to detect double spending.
\begin{itemize}
\item Loss of the database will allow technically skilled people
      to double-spend their digital cash, possibly resulting in
      significant financial losses.
\item The database contains total amounts customers withdrew and
      merchants received, so sensitive private banking data. It
      must thus not become public.
\item The auditor must have a (current) copy. Asynchronous replication
      should be sufficient.  This copy can also serve as an
      additional (off-site?) backup.
\end{itemize}
\end{frame}


\begin{frame}{taler-exchange-wirewatch}
{\tt taler-exchange-wirewatch} needs credentials to access data about
incoming wire transfers from the Nexus.
\begin{itemize}
\item This tool should run as a separate UID and GID (from
      {\tt taler-exchange-httpd}).
\item It must have access to the Postgres database (SELECT + INSERT).
\item Its configuration file contains the credentials to talk to Nexus.
\item[$\Rightarrow$] Configuration should be separate from {\tt taler-exchange-httpd}.
\end{itemize}
\end{frame}


\begin{frame}{taler-exchange-transfer}
Only {\tt taler-exchange-transfer} needs credentials to initiate wire
transfers using the Nexus.
\begin{itemize}
\item This tool should run as a separate UID and GID (from
      {\tt taler-exchange-httpd}).
\item It must have access to the Postgres database (SELECT + INSERT).
\item Its configuration file contains the credentials to talk to Nexus.
\item[$\Rightarrow$] Configuration should be separate from {\tt taler-exchange-httpd}.
\end{itemize}
\end{frame}


\begin{frame}{libeufin-nexus}
libeufin-nexus has to be able to interact with the escrow account of the exchange.
\begin{itemize}
\item It must have the private keys to sign EBICS/FinTS messages.
\item It also has its own local database.
\item The Nexus user and database should be kept separate from
      the other exchange users and the Taler exchange database.
\end{itemize}
\end{frame}


\begin{frame}{Hardware}
General notions:
\begin{itemize}
\item Platforms with disabled Intel ME \& disabled remote administration are safer.
\item VMs are not a security mechanism. Side-channel attacks abound. Avoid running any
      Taler component in a virtual machine ``for security''.
\end{itemize}
\end{frame}


\begin{frame}{Operating system}
General notions:
\begin{itemize}
\item It should be safe to run the different Taler components (including Nginx, Nexus
      and Postgres) all on the same physical hardware (under different UIDs/GIDs).
      We would separate them onto different physical machines during scale-out, but not
      necessarily for ``basic'' security.
\item Limiting and auditing system administrator access will be crucial.
\item We recommend to {\bf not} use any anti-virus.
\item We recommend using a well-supported GNU/Linux operating system (such as
      Debian or Ubuntu).
\end{itemize}
\end{frame}


\begin{frame}{Network}
\begin{itemize}
\item We recommend to {\bf not} use any host-based firewall.
      Taler components can use UNIX domain sockets (or bind to localhost).
\item A network-based
      firewall is not required, but as long as TCP 80/443 are open Taler should
      work fine.
\item Any firewall must be configured to permit connection to Auditor
      for database synchronization.
\item We recommend running the Taler exchange behind an Nginx or Apache
      proxy for TLS termination.
\item We recommend using static IP address configurations (IPv4 and IPv6).
\item We recommend using DNSSEC with DANE in addition to TLS certificates.
\item We recommend auditing the TLS setup using \url{https://observatory.mozilla.org}.
\end{itemize}
\end{frame}




\section{Offline payments}

\begin{frame}
  \vfill
  \begin{center}
    {\bf Offline payments}
  \end{center}
  \vfill
\end{frame}


\begin{frame}{Requirements: Online vs. Offline Digital Currencies}
\framesubtitle{\url{https://taler.net/papers/euro-bearer-online-2021.pdf}}
\begin{itemize}
    \item Offline capabilities are sometimes cited as a requirement for digital payment solutions
    \item All implementations must either use restrictive hardware elements and/or introduce
      counterparty risk.
    \item[$\Rightarrow$] Permanent offline features weaken a digital payment solution (privacy, security)
    \item[$\Rightarrow$] Introduces unwarranted competition for physical cash (endangers emergency-preparedness).
  \end{itemize}
  We recommend a tiered approach:
      \begin{enumerate}
        \item Online-first, bearer-based digital currency with Taler
        \item (Optional:) Limited offline mode for network outages
        \item Physical cash for emergencies (power outage, catastrophic cyber incidents)
      \end{enumerate}
\end{frame}


\begin{frame}{Fully Offline Payments {\bf (WiP)}}
\framesubtitle{\url{https://docs.taler.net/design-documents/030-offline-payments.html}}
Many central banks today demand offline capabilities for digital payment solutions.
\vfill
\noindent
Three possible approaches:
\begin{enumerate}
  \item Trust-based offline payments (has counterparty and/or privacy risks)
  \item Full HSM Taler wallet (has hardware costs)
  \item Light-weight HSM balance register
\end{enumerate}
\vfill
\end{frame}


\begin{frame}{A Scenario}
{God is offline, but customer pays online}
\begin{center}
  \includegraphics[height=0.4\textwidth]{shrine.jpg}
\end{center}
\end{frame}

\begin{frame}{Typical Payment Process}{All equivalent: Twint, PayPal, AliPay, PayTM}
\begin{center}
   \movie[%scale=0.6,
           autostart,
           poster]
           {
               \includegraphics[height=0.3\textwidth,width=0.4\textwidth]{white.png}
           }
           {twint.mkv}

           {\tiny (C) Twint, 2023}
\end{center}
\end{frame}


\begin{frame}{Secure Payment ...}{Everything green?}
\begin{center}
  \includegraphics[height=0.3\textwidth]{paymentTwint-screen_25.png}
\end{center}
\end{frame}

\begin{frame}{Exploit ``Code''}{Programming optional}
\begin{center}
  \includegraphics[height=0.3\textwidth]{paymentTwint-screen.png}
\end{center}
\end{frame}

\begin{frame}{``Customers'' {\em love} Twint ...}{Daily non-business for shops}
\begin{center}
  \includegraphics[height=0.3\textwidth]{paymentTwint-screen_50.png}
\end{center}
\end{frame}


\begin{frame}{Partially Offline Payments with GNU Taler\footnote{Joint work with Emmanuel Benoist, Priscilla Huang and Sebastian Marchano}}

\begin{center}
\resizebox{8cm}{7cm}{
\begin{sequencediagram}
    \newinst{pos}{\shortstack{PoS \\
      \\ \begin{tikzpicture}
        \node [fill=gray!20,draw=black,thick ,align=center] {PoS key \\ PoS ID};
      \end{tikzpicture}
    }}
    \newinst[2]{customer}{\shortstack{Customer \\
      \\ \begin{tikzpicture}
        \node [fill=gray!20,draw=black,thick ,align=center] {Digital \\ Wallet};
      \end{tikzpicture}
    }}
    \newinst[2]{backend}{\shortstack{Merchant Backend \\
       \\ \begin{tikzpicture}[shape aspect=.5]
        \tikzset{every node/.style={cylinder, shape border rotate=90, draw,fill=gray!25}}
        \node at (1.5,0) {\shortstack{{\tiny PoS key} \\ {\tiny PoS ID}}};
       \end{tikzpicture}
    }}
    \postlevel
    \mess[0]{pos}{PoS ID}{customer}
    \begin{sdblock}{optional}{}
      \begin{callself}{customer}{Amount}{}
      \end{callself}
    \end{sdblock}
    \prelevel
    \prelevel
    \prelevel
    \prelevel
    \prelevel
    \begin{sdblock}{optional}{}
      \begin{callself}{pos}{Amount}{}
      \end{callself}
    \end{sdblock}
    \postlevel
    \mess[0]{customer}{PoS ID, [Amount]?}{backend}
    \mess[0]{backend}{Contract}{customer}
    \postlevel
    \mess[0]{customer}{Payment}{backend}
    \begin{callself}{pos}{OTP(PoS key)}{}
    \end{callself}
    \prelevel
    \prelevel
    \begin{callself}{backend}{OTP(PoS key)}{}
    \end{callself}
    \mess[0]{backend}{OTP code}{customer}
    \postlevel
    \mess[0]{customer}{OTP code}{pos}
\end{sequencediagram}
}
\end{center}
\end{frame}



\section{Programmable money: Age restrictions}

\begin{frame}
  \vfill
  \begin{center}
    {\bf Programmable money: Age restrictions}
  \end{center}
  \vfill
\end{frame}


\begin{frame}{Age restriction in E-commerce}

	\begin{description}
		\item[Problem:]~\\[1em]
			Verification of minimum age requirements in e-commerce.\\[2em]

		\item[Common solutions:]

\begin{tabular}{l<{\onslide<2->}c<{\onslide<3->}cr<{\onslide}}
	& \blue{Privacy} & \tikzmark{topau} \blue{Ext. authority}& \\[\medskipamount]
	1. ID Verification     & bad   & required & \\[\medskipamount]
	2. Restricted Accounts & bad   & required & \\[\medskipamount]
	3. Attribute-based     & good  & required &\tikzmark{bottomau} \\[\medskipamount]
\end{tabular}
	\end{description}

\uncover<4->{
	\begin{tikzpicture}[overlay,remember picture]
	\draw[orange,thick,rounded corners]
		($(pic cs:topau) +(0,0.5)$) rectangle ($(pic cs:bottomau) -(0.3, 0.2)$);
	\end{tikzpicture}
	\begin{center}
	\bf Principle of Subsidiarity is violated
	\end{center}
}
\end{frame}


\begin{frame}{Principle of Subsidiarity}
\begin{center} \Large
	Functions of government---such as granting and restricting
	rights---should be performed\\
	{\it at the lowest level of authority possible},\\
	as long as they can be performed {\it adequately}.
\end{center}
\vfill
\uncover<2->{
	For age-restriction, the lowest level of authority is:\\
	\begin{center}\Large
	Parents, guardians and caretakers
	\end{center}
}
\end{frame}


\begin{frame}{Age restriction design for GNU Taler}
Design and implementation of an age restriction scheme\\
with the following goals:

\begin{enumerate}
\item It ties age restriction to the \textbf{ability to pay} (not to ID's)
\item maintains \textbf{anonymity of buyers}
\item maintains \textbf{unlinkability of transactions}
\item aligns with \textbf{principle of subsidiartiy}
\item is \textbf{practical and efficient}
\end{enumerate}

\end{frame}


\begin{frame}{Age restriction}
	\framesubtitle{Assumptions and scenario}

	\begin{columns}
		\column{7.5cm}
	\begin{itemize}
		\item<1-> Assumption: Checking accounts are under control of eligible adults/guardians.
		\item<2-> \textit{Guardians} \textbf{commit} to an maximum age
		\item<3-> \textit{Minors} \textbf{attest} their adequate age
		\item<4-> \textit{Merchants} \textbf{verify} the attestations
		\item<5-> Minors \textbf{derive} age commitments from existing ones
		\item<6-> \textit{Exchanges} \textbf{compare} the derived age commitments
	\end{itemize}
		\column{5cm}
		\uncover<7->
		{
		\begin{center}
		\fontsize{7pt}{7pt}\selectfont
	\begin{tikzpicture}[scale=.5]
		\node[circle,minimum size=15pt,fill=black!15] at ( 60:4) (Exchange) {$\Exchange$};
		\node[circle,minimum size=15pt,fill=black!15] at (  0:0) (Client) {$\Child$};
		\node[circle,minimum size=15pt,fill=black!15] at (  0:4) (Merchant) {$\Merchant$};
		\node[circle,minimum size=15pt,fill=blue!15]  at (140:3) (Guardian) {$\Guardian$};

		\draw[->] (Guardian)   to [out=50,in=130, loop] node[above]
			{$\Commit$} (Guardian);
		\draw[->,blue] (Client)   to [out=-125,in=-190, loop] node[below,left]
			{\blue{$\Attest$}} (Client);
		\draw[->,blue] (Merchant) to [out=50,in=130, loop] node[above]
			{\blue{$\Verify$}} (Merchant);
		\draw[->,orange] (Client)   to [out=-35,in=-100, loop] node[below]
			{\orange{$\Derive$}} (Client);
		\draw[->,orange] (Exchange) to [out=50,in=130, loop] node[above]
			{\orange{$\Compare$}} (Exchange);

		\draw[orange,|->] (Client)   to node[sloped,above,align=left]
			{\orange{\scriptsize }} (Exchange);
		\draw[blue,|->] (Client)   to node[sloped, above]
			{\blue{\scriptsize }} (Merchant);
		\draw[,|->] (Guardian) to node[above,sloped,align=left]
			{{\scriptsize }} (Client);
	\end{tikzpicture}
		\end{center}
		}
	\end{columns}
	\vfill
	\uncover<7->{Note: Scheme is independent of payment service protocol.}
\end{frame}


\begin{frame}{Formal Function Signatures}
\small
Searching for functions \uncover<2->{with the following signatures}
\begin{align*}
	&\bf \Commit\uncover<2->{:
		&(\age, \omega) &\mapsto (\commitment, \pruf)
		&\scriptstyle \N_\Age \times \Omega &\scriptstyle \to \Commitments\times\Proofs,
		}
	\\
	&\bf \Attest\uncover<3->{:
		&(\minage, \commitment, \pruf) &\mapsto \attest
		&\scriptstyle \N_\Age\times\Commitments\times\Proofs &\scriptstyle \to \Attests \cup \{\Nil\},
		}
	\\
	&\bf \Verify\uncover<4->{:
		&(\minage, \commitment, \attest) &\mapsto b
		&\scriptstyle \N_\Age\times\Commitments\times\Attests &\scriptstyle \to \Z_2,
		}
	\\
	&\bf \Derive\uncover<5->{:
		&(\commitment, \pruf, \omega) &\mapsto (\commitment', \pruf', \blinding)
		&\scriptstyle \Commitments\times\Proofs\times\Omega &\scriptstyle \to \Commitments\times\Proofs\times\Blindings,
		}
	\\
	&\bf \Compare\uncover<6->{:
		&(\commitment, \commitment', \blinding) &\mapsto b
		&\scriptstyle \Commitments\times\Commitments\times\Blindings &\scriptstyle \to \Z_2,
		}
\end{align*}
	\uncover<7->{
		with $\Omega, \Proofs, \Commitments, \Attests, \Blindings$
		sufficiently large sets.\\[1em]
		Basic and security requirements are defined later.\\[2em]
	}

		\scriptsize
	\uncover<2->{
		Mnemonics:\\
		$\Commitments=$ \textit{c$\Commitments$mmitments},
		$\commitment=$ \textit{Q-mitment} (commitment),
		$\Proofs=$ \textit{$\Proofs$roofs},
	}
	\uncover<3->{
		$\pruf=$ \textit{$\pruf$roof},\\
		$\Attests=$ \textit{a$\Attests$testations},
		$\attest=$ \textit{a$\attest$testation},
	}
	\uncover<5->{
		$\Blindings=$ \textit{$\Blindings$lindings},
		$\blinding=$ \textit{$\blinding$linding}.
	}
\end{frame}

\begin{frame}{Age restriction}
	\framesubtitle{Naïve scheme}
	\begin{center}
	\begin{tikzpicture}[scale=.85]
		\node[circle,minimum size=20pt,fill=black!15] at ( 60:4) (Exchange) {$\Exchange$};
		\node[circle,minimum size=20pt,fill=black!15] at (  0:0) (Client) {$\Child$};
		\node[circle,minimum size=20pt,fill=black!15] at (  0:4) (Merchant) {$\Merchant$};
		\node[circle,minimum size=20pt,fill=blue!15]  at (140:3) (Guardian) {$\Guardian$};

		\draw[->] (Guardian)   to [out=50,in=130, loop] node[above]
			{$\Commit$} (Guardian);
		\draw[->,blue] (Client)   to [out=-125,in=-190, loop] node[below,left]
			{\blue{$\Attest$}} (Client);
		\draw[->,blue] (Merchant) to [out=50,in=130, loop] node[above]
			{\blue{$\Verify$}} (Merchant);
		\draw[->,orange] (Client)   to [out=-35,in=-100, loop] node[below]
			{\orange{$\Derive$}} (Client);
		\draw[->,orange] (Exchange) to [out=50,in=130, loop] node[above]
			{\orange{$\Compare$}} (Exchange);

		\draw[orange,|->] (Client)   to node[sloped,above,align=left]
			{\orange{\scriptsize }} (Exchange);
		\draw[blue,|->] (Client)   to node[sloped, above]
			{\blue{\scriptsize }} (Merchant);
		\draw[,|->] (Guardian) to node[above,sloped,align=left]
			{{\scriptsize }} (Client);
	\end{tikzpicture}
	\end{center}
\end{frame}

\begin{frame}{Achieving Unlinkability}
	\begin{columns}
		\column{3cm}
		\begin{center}
		\fontsize{8pt}{9pt}\selectfont
		\begin{tikzpicture}[scale=.65]
			\node[circle,minimum size=20pt,fill=black!15] at ( 60:4) (Exchange) {$\Exchange$};
			\node[circle,minimum size=20pt,fill=black!15] at (  0:0) (Client) {$\Child$};

			\draw[->,orange] (Client)   to [out=-35,in=-100, loop] node[below]
				{\orange{$\footnotesize \Derive()$}} (Client);
			\draw[->,orange] (Exchange) to [out=50,in=130, loop] node[above]
				{\orange{$\footnotesize \Compare()$}} (Exchange);

			\draw[orange,|->] (Client)   to node[sloped,above,align=left]
				{\orange{\tiny \uncover<2->{$(\commitment_i,\commitment_{i+1})$}}} (Exchange);
		\end{tikzpicture}
		\end{center}

		\column{9cm}
	Simple use of $\Derive()$ and $\Compare()$ is problematic.

	\begin{itemize}
		\item<2-> Calling $\Derive()$ iteratively generates sequence
			$(\commitment_0, \commitment_1, \dots)$ of commitments.
		\item<2-> Exchange calls $\Compare(\commitment_i, \commitment_{i+1}, .)$
		\item[$\implies$]\uncover<3->{\bf Exchange identifies sequence}
		\item[$\implies$]\uncover<3->{\bf Unlinkability broken}
	\end{itemize}
	\end{columns}
\end{frame}

\begin{frame}{Achieving Unlinkability}
	Define cut\&choose protocol \orange{$\DeriveCompare$},
	using $\Derive()$ and $\Compare()$.\\[0.5em]
	\uncover<2->{
	Sketch:
	\small
	\begin{enumerate}
		\item $\Child$ derives commitments $(\commitment_1,\dots,\commitment_\kappa)$
			from $\commitment_0$ \\
			by calling $\Derive()$ with blindings $(\beta_1,\dots,\beta_\kappa)$
		\item $\Child$ calculates $h_0:=H\left(H(\commitment_1, \beta_1)||\dots||H(\commitment_\kappa, \beta_\kappa)\right)$
		\item $\Child$ sends $\commitment_0$ and $h_0$ to $\Exchange$
		\item $\Exchange$ chooses $\gamma \in \{1,\dots,\kappa\}$ randomly
		\item $\Child$ reveals $h_\gamma:=H(\commitment_\gamma, \beta_\gamma)$ and all $(\commitment_i, \beta_i)$, except $(\commitment_\gamma, \beta_\gamma)$
		\item $\Exchange$ compares $h_0$ and
			$H\left(H(\commitment_1, \beta_1)||...||h_\gamma||...||H(\commitment_\kappa, \beta_\kappa)\right)$\\
			and evaluates $\Compare(\commitment_0, \commitment_i, \beta_i)$.
	\end{enumerate}
	\vfill
	Note: Scheme is similar to the {\it refresh} protocol in GNU Taler.
	}
\end{frame}

\begin{frame}{Achieving Unlinkability}
	With \orange{$\DeriveCompare$}
	\begin{itemize}
		\item $\Exchange$ learns nothing about $\commitment_\gamma$,
		\item trusts outcome with $\frac{\kappa-1}{\kappa}$ certainty,
		\item i.e. $\Child$ has $\frac{1}{\kappa}$ chance to cheat.
	\end{itemize}
	\vfill
	Note: Still need Derive and Compare to be defined.
\end{frame}

\begin{frame}{Refined scheme}

	\begin{tikzpicture}[scale=.8]
		\node[circle,minimum size=25pt,fill=black!15] at (  0:0) (Client)   {$\Child$};
		\node[circle,minimum size=25pt,fill=black!15] at ( 60:5) (Exchange) {$\Exchange$};
		\node[circle,minimum size=25pt,fill=black!15] at (  0:5) (Merchant) {$\Merchant$};
		\node[circle,minimum size=25pt,fill=blue!15]  at (130:3) (Guardian) {$\Guardian$};

		\draw[orange,<->] (Client)   to node[sloped,below,align=center]
			{\orange{$\DeriveCompare$}} (Exchange);
		\draw[blue,->] (Client)   to node[sloped, below]
			{\blue{$(\attest_\minage, \commitment)$}} (Merchant);

		\draw[->] (Guardian)   to [out=150,in=70, loop] node[above]
			{$\Commit(\age)$} (Guardian);
		\draw[->] (Guardian)   to node[below,sloped]
			{($\commitment$, $\pruf_\age$)} (Client);
		\draw[->,blue] (Client)   to [out=-50,in=-130, loop] node[below]
			{\blue{$\Attest(\minage, \commitment, \pruf_{\age})$}} (Client);
		\draw[->,blue] (Merchant) to [out=-50,in=-130, loop] node[below]
			{\blue{$\Verify(\minage, \commitment, \attest_{\minage})$}} (Merchant);
	\end{tikzpicture}
\end{frame}

 \begin{frame}{Achieving Unlinkability}
 	\scriptsize
 	$\DeriveCompare : \Commitments\times\Proofs\times\Omega \to \{0,1\}$\\
 	\vfill
 	$\DeriveCompare(\commitment, \pruf, \omega) =$
 \begin{itemize}
 \it
 	\itemsep0.5em
 	\item[$\Child$:]
 		\begin{enumerate}
 				\scriptsize
 			\itemsep0.3em
 			\item for all $i \in \{1,\dots,\kappa\}:
 				(\commitment_i,\pruf_i,\beta_i) \leftarrow \Derive(\commitment, \pruf, \omega + i)$
 			\item $h \leftarrow \Hash\big(\Hash(\commitment_1,\beta_1)\parallel\dots\parallel\Hash(\commitment_\kappa,\beta_\kappa) \big)$
 			\item send $(\commitment, h)$ to $\Exchange$
 		\end{enumerate}
 	\item[$\Exchange$:]
 		\begin{enumerate}
 			\setcounter{enumi}{4}
 				\scriptsize
 			\itemsep0.3em
 			\item save $(\commitment, h)$ \label{st:hash}
 			\item $\gamma \drawfrom \{1,\dots ,\kappa\}$
 			\item send $\gamma$ to $\Child$
 		\end{enumerate}
 	\item[$\Child$:]
 		\begin{enumerate}
 			\setcounter{enumi}{7}

 				\scriptsize
 			\itemsep0.3em
 			\item $h'_\gamma \leftarrow \Hash(\commitment_\gamma, \beta_\gamma)$
 			\item $\mathbf{E}_\gamma \leftarrow \big[(\commitment_1,\beta_1),\dots,
 				(\commitment_{\gamma-1}, \beta_{\gamma-1}),
 				\Nil,
 				(\commitment_{\gamma+1}, \beta_{\gamma+1}),
 				\dots,(\commitment_\kappa, \beta_\kappa)\big]$
 			\item send $(\mathbf{E}_\gamma, h'_\gamma)$ to $\Exchange$
 		\end{enumerate}
 	\item[$\Exchange$:]
 		\begin{enumerate}
 			\setcounter{enumi}{10}
 				\scriptsize
 			\itemsep0.3em
 			\item for all $i \in \{1,\dots,\kappa\}\setminus\{\gamma\}: h_i \leftarrow \Hash(\mathbf{E}_\gamma[i])$
 			\item if $h \stackrel{?}{\neq} \HashF(h_1\|\dots\|h_{\gamma-1}\|h'_\gamma\|h_{\gamma+1}\|\dots\|h_{\kappa-1})$ return 0
 			\item for all $i \in \{1,\dots,\kappa\}\setminus\{\gamma\}$:
 				if $0 \stackrel{?}{=} \Compare(\commitment,\commitment_i, \beta_i)$ return $0$
 			\item return 1
 		\end{enumerate}
 \end{itemize}
 \end{frame}

\begin{frame}{Basic Requirements}

	Candidate functions
	\[ (\Commit, \Attest, \Verify, \Derive, \Compare) \]
	must first meet \textit{basic} requirements:

	\begin{itemize}
		\item Existence of attestations
		\item Efficacy of attestations
		\item Derivability of commitments and attestations
	\end{itemize}
\end{frame}

\begin{frame}{Basic Requirements}
	\framesubtitle{Formal Details}

	\begin{description}
		\item[Existence of attestations]
			{\scriptsize
			\begin{align*}
				\Forall_{\age\in\N_\Age \atop \omega \in \Omega}:
				\Commit(\age, \omega) =: (\commitment, \pruf)
				\implies
				\Attest(\minage, \commitment, \pruf) =
				\begin{cases}
					\attest \in \Attests, \text{ if } \minage \leq \age\\
					\Nil \text{ otherwise}
				\end{cases}
			\end{align*}}
		\item[Efficacy of attestations]
			{\scriptsize
			\begin{align*}
				\Verify(\minage, \commitment, \attest) = \
				\begin{cases}
					1, \text{if } \Exists_{\pruf \in \Proofs}: \Attest(\minage, \commitment, \pruf) = \attest\\
					0 \text{ otherwise}
				\end{cases}
			\end{align*}}

			{\scriptsize
			\begin{align*}
				\forall_{n \leq \age}: \Verify\big(n, \commitment, \Attest(n, \commitment, \pruf)\big) = 1.
			\end{align*}}
		\item[etc.]
	\end{description}
\end{frame}

\begin{frame}{Requirements}
	\framesubtitle{Details}

	\begin{description}
		\item[Derivability of commitments and proofs:]~\\[0.1em]
		{\scriptsize
		Let \begin{align*}
			\age & \in\N_\Age,\,\, \omega_0, \omega_1 \in\Omega\\
			(\commitment_0, \pruf_0) & \leftarrow \Commit(\age, \omega_0),\\
			(\commitment_1, \pruf_1, \blinding) & \leftarrow  \Derive(\commitment_0, \pruf_0, \omega_1).
		\end{align*}
		We require
		\begin{align*}
			\Compare(\commitment_0, \commitment_1, \blinding) = 1 \label{req:comparity}
		\end{align*}
		and for all $n\leq\age$:
		\begin{align*}
					\Verify(n, \commitment_1, \Attest(n, \commitment_1, \pruf_1)) &%
					=
					\Verify(n, \commitment_0,  \Attest(n, \commitment_0,  \pruf_0))
		\end{align*}}
	\end{description}
\end{frame}

\begin{frame}{Security Requirements}
	Candidate functions must also meet \textit{security} requirements.
	Those are defined via security games:
	\begin{itemize}
		\item Game: Age disclosure by commitment or attestation
		\item[$\leftrightarrow$] Requirement: Non-disclosure of age
			\vfill

		\item Game: Forging attestation
		\item[$\leftrightarrow$] Requirement: Unforgeability of
			minimum age
			\vfill

		\item Game: Distinguishing derived commitments and attestations
		\item[$\leftrightarrow$] Requirement: Unlinkability of
			commitments and attestations

	\end{itemize}
	\vfill

	Meeting the security requirements means that adversaries can win
	those games only with negligible advantage.
	\vfill
	Adversaries are arbitrary polynomial-time algorithms, acting on all
	relevant input.
\end{frame}

\begin{frame}{Security Requirements}
	\framesubtitle{Simplified Example}

	\begin{description}
		\item[Game $\Game{FA}(\lambda)$---Forging an attest:]~\\
	{\small
	\begin{enumerate}
		\item $ (\age, \omega)	\drawfrom	\N_{\Age-1}\times\Omega $
		\item $ (\commitment, \pruf)	\leftarrow	\Commit(\age, \omega) $
		\item $ (\minage, \attest) \leftarrow \Adv(\age, \commitment, \pruf)$
		\item Return 0 if $\minage \leq \age$
		\item Return $\Verify(\minage,\commitment,\attest)$
	\end{enumerate}
	}
	\vfill
	\item[Requirement: Unforgeability of minimum age]
		{\small
	\begin{equation*}
		\Forall_{\Adv\in\PPT(\N_\Age\times\Commitments\times\Proofs\to \N_\Age\times\Attests)}:
		\Probability\Big[\Game{FA}(\lambda) = 1\Big] \le \negl(\lambda)
	\end{equation*}
	}
	\end{description}
\end{frame}


\begin{frame}{Solution: Instantiation with ECDSA}
%	\framesubtitle{Definition of Commit}

	\begin{description}
		\item[To Commit to age (group) $\age \in \{1,\dots,\Age\}$]~\\
		\begin{enumerate}
			\item<2-> Guardian generates ECDSA-keypairs, one per age (group):
				\[\langle(q_1, p_1),\dots,(q_\Age,p_\Age)\rangle\]
			\item<3-> Guardian then \textbf{drops} all private keys
				$p_i$ for $i > \age$:
				\[\Big \langle(q_1, p_1),\dots,
					(q_\age, p_\age),
					(q_{\age +1}, \red{\Nil}),\dots,
					(q_\Age, \red{\Nil})\Big\rangle\]

				\begin{itemize}
					\item $\Vcommitment := (q_1, \dots, q_\Age)$ is the \textit{Commitment},
					\item $\Vpruf_\age := (p_1, \dots, p_\age, \Nil,\dots,\Nil)$ is the \textit{Proof}
				\end{itemize}
				\vfill
			\item<4-> Guardian gives child $\langle \Vcommitment, \Vpruf_\age \rangle$
				\vfill
		\end{enumerate}
	\end{description}
\end{frame}

\begin{frame}{Instantiation with ECDSA}
	\framesubtitle{Definitions of Attest and Verify}

	Child has
	\begin{itemize}
		\item ordered public-keys $\Vcommitment = (q_1, \dots, q_\Age) $,
		\item (some) private-keys $\Vpruf = (p_1, \dots, p_\age, \Nil, \dots, \Nil)$.
	\end{itemize}
	\begin{description}
		\item<2->[To \blue{Attest} a minimum age $\blue{\minage} \leq \age$:]~\\
			Sign a message with ECDSA using private key $p_\blue{\minage}$
	\end{description}

	\vfill

	\uncover<3->{
	Merchant gets
	\begin{itemize}
		\item ordered public-keys $\Vcommitment = (q_1, \dots, q_\Age) $
		\item Signature $\sigma$
	\end{itemize}
	\begin{description}
		\item<4->[To \blue{Verify} a minimum age $\minage$:]~\\
			Verify the ECDSA-Signature $\sigma$ with public key $q_\minage$.
	\end{description}
	}
	\vfill
\end{frame}

\begin{frame}{Instantiation with ECDSA}
	\framesubtitle{Definitions of Derive and Compare}
	Child has
	$\Vcommitment = (q_1, \dots, q_\Age) $ and
	$\Vpruf = (p_1, \dots, p_\age, \Nil, \dots, \Nil)$.
	\begin{description}
		\item<2->[To \blue{Derive} new $\Vcommitment'$ and $\Vpruf'$:]
			Choose random $\beta\in\Z_g$ and calculate
			\small
			\begin{align*}
				\Vcommitment' &:= \big(\beta * q_1,\ldots,\beta * q_\Age\big),\\
				\Vpruf' &:= \big(\beta p_1,\ldots,\beta p_\age,\Nil,\ldots,\Nil\big)
			\end{align*}
			Note: $ (\beta p_i)*G = \beta*(p_i*G)  = \beta*q_i$\\
			\scriptsize $\beta*q_i$ is scalar multiplication on the elliptic curve.
	\end{description}

		\vfill
	\uncover<3->{
		Exchange gets $\Vcommitment = (q_1,\dots,q_\Age)$, $\Vcommitment' = (q_1', \dots, q_\Age')$ and $\beta$
	\begin{description}
		\item[To \blue{Compare}, calculate:]
			\small
		$(\beta * q_1, \ldots , \beta * q_\Age) \stackrel{?}{=} (q'_1,\ldots, q'_\Age)$
	\end{description}
	\vfill
	}
\end{frame}

\begin{frame}{Instantiation with ECDSA}

	Functions
	(Commit, Attest, Verify, Derive, Compare)\\
	as defined in the instantiation with ECDSA\\[0.5em]
	\begin{itemize}
		\item meet the basic requirements,\\[0.5em]
		\item also meet all security requirements.\\
		Proofs by security reduction, details are in the paper.
	\end{itemize}

\end{frame}


\begin{frame}{Instantiation with ECDSA}
 	\framesubtitle{Full definitions}
 	\scriptsize

 \begin{align*}
 	\Commit_{E,\FDHg{\cdot}}(\age, \omega) &:= \Big\langle
 		\overbrace{(q_1,\ldots,q_\Age)}^{= \Vcommitment},\;
 		\overbrace{(p_1,\ldots,p_\age, \Nil,\ldots,\Nil)}^{= \Vpruf \text{, length }\Age}
 		\Big\rangle\\
 	\Attest_{E,\HashF}(\bage, \Vcommitment, \Vpruf) &:=
 		\begin{cases}
 			\attest_\bage := \Sign_{E,\HashF}\big(\bage,\Vpruf[\bage]\big) & \text{if } \Vpruf[\bage] \stackrel{?}{\neq} \Nil\\
 			\Nil & \text{otherwise}
 		\end{cases}\\
 %
 	\Verify_{E,\HashF}(\bage, \Vcommitment, \attest) &:= \Ver_{E,\HashF}(\bage, \Vcommitment[\bage], \attest)\\
 %
 	\Derive_{E, \FDHg{\cdot}}(\Vcommitment, \Vpruf, \omega) &:=
 		\Big\langle(\beta * q_1,\ldots,\beta * q_\Age),
 		     (\beta p_1,\ldots,\beta p_\age,\Nil,\ldots,\Nil), \beta \Big\rangle \\
 		     & \text{ with } \beta := \FDHg{\omega} \text{ and multiplication } \beta p_i \text{ modulo } g \nonumber\\
 %
 	\Compare_E(\Vcommitment, \Vcommitment', \beta)	&:=
 		\begin{cases}
 			1 & \text{if } (\beta * q_1, \ldots , \beta * q_\Age) \stackrel{?}{=} (q'_1,\ldots, q'_\Age)\\
 			0 & \text{otherwise}
 		\end{cases}
 \end{align*}
\end{frame}


\begin{frame}{Reminder: GNU Taler Fundamentals}
	\begin{center}
	\begin{tikzpicture}[scale=.55]
		\node[circle,fill=black!10] at (3, 4) (Exchange) {$\Exchange$};
		\node[circle,fill=black!10] at (0, 0) (Customer) {$\Customer$};
		\node[circle,fill=black!10] at (6, 0) (Merchant) {$\Merchant$};

		\draw[<->] (Customer)   to [out=65,in=220] node[sloped,above] {\sf withdraw} (Exchange);
		\draw[<->] (Customer)   to [out=45,in=240] node[sloped,below] {\sf refresh} (Exchange);
		\draw[<->] (Customer)   to node[sloped, below] {\sf purchase} (Merchant);
		\draw[<->] (Merchant) to node[sloped, above] {\sf deposit} (Exchange);
	\end{tikzpicture}
	\end{center}

	\vfill
	\begin{itemize}
		\item Coins are public-/private key-pairs $(C_p, c_s)$.
		\item Exchange blindly signs $\FDH(C_p)$ with denomination key $d_p$
		\item Verification:
		\begin{eqnarray*}
			1  &\stackrel{?}{=}&
			\mathsf{SigCheck}\big(\FDH(C_p), D_p, \sigma_p\big)
		\end{eqnarray*}
		\scriptsize($D_p$ = public key of denomination and $\sigma_p$ = signature)

	\end{itemize}
\end{frame}

\begin{frame}{Integration with GNU Taler}
	\framesubtitle{Binding age restriction to coins}

	To bind an age commitment $\commitment$ to a coin $C_p$, instead of
	signing $\FDH(C_p)$, $\Exchange$ now blindly signs
	\begin{center}
		$\FDH(C_p, \orange{H(\commitment)})$
	\end{center}

	\vfill
	Verfication of a coin now requires $H(\commitment)$, too:
	\begin{center}
		$1  \stackrel{?}{=}
		\mathsf{SigCheck}\big(\FDH(C_p, \orange{H(\commitment)}), D_p, \sigma_p\big)$
	\end{center}
	\vfill
\end{frame}

\begin{frame}{Integration with GNU Taler}
	\framesubtitle{Integrated schemes}
	\fontsize{8pt}{9pt}\selectfont
	\begin{tikzpicture}[scale=.9]
		\node[circle,minimum size=25pt,fill=black!15] at (  0:0) (Client)   {$\Child$};
		\node[circle,minimum size=25pt,fill=black!15] at ( 60:5) (Exchange) {$\Exchange$};
		\node[circle,minimum size=25pt,fill=black!15] at (  0:5) (Merchant) {$\Merchant$};
		\node[circle,minimum size=25pt,fill=blue!15]  at (130:3) (Guardian) {$\Guardian$};

		\draw[<->] (Guardian)   to  node[sloped,above,align=center]
			{{\sf withdraw}\orange{, using}\\ $\FDH(C_p\orange{, H(\commitment)})$} (Exchange);
		\draw[<->] (Client)   to node[sloped,below,align=center]
			{{\sf refresh} \orange{ + }\\ \orange{$\DeriveCompare$}} (Exchange);
		\draw[<->] (Client)   to node[sloped, below]
			{{\sf purchase} \blue{+ $(\attest_\minage, \commitment)$}} (Merchant);
		\draw[<->] (Merchant) to node[sloped, above]
			{{\sf deposit} \orange{+ $H(\commitment)$}} (Exchange);

		\draw[->] (Guardian)   to [out=70,in=150, loop] node[above]
			{$\Commit(\age)$} (Guardian);
		\draw[->] (Guardian)   to node[below,sloped]
			{($\commitment$, $\pruf_\age$)} (Client);
		\draw[->,blue] (Client)   to [out=-50,in=-130, loop] node[below]
			{\blue{$\Attest(\minage, \commitment, \pruf_{\age})$}} (Client);
		\draw[->,blue] (Merchant) to [out=-50,in=-130, loop] node[below]
			{\blue{$\Verify(\minage, \commitment, \attest_{\minage})$}} (Merchant);
	\end{tikzpicture}
\end{frame}

\begin{frame}{Instantiation with Edx25519}
	Paper also formally defines another signature scheme: Edx25519.\\[1em]

	\begin{itemize}
		\item Scheme already in use in GNUnet,
		\item based on EdDSA (Bernstein et al.),
		\item generates compatible signatures and
		\item allows for key derivation from both, private and public keys, independently.
	\end{itemize}~\\[1em]

	Current implementation of age restriction in GNU Taler uses Edx25519.
\end{frame}


\begin{frame}{Age Restrictions based on KYC}
 Subsidiarity requires bank accounts being owned by adults.
			\begin{itemize}
			\item Scheme can be adapted to case where minors have bank accounts
				\begin{itemize}
					\item Assumption: banks provide minimum age
						information during bank
						transactions.
					\item Child and Exchange execute a variant of
						the cut\&choose protocol.
				\end{itemize}
			\end{itemize}
\end{frame}

\begin{frame}{Discussion}
	\begin{itemize}
		\item Our solution can in principle be used with any token-based payment scheme
		\item GNU Taler best aligned with our design goals (security, privacy and efficiency)
		\item Subsidiarity requires bank accounts being owned by adults
			\begin{itemize}
			\item Scheme can be adapted to case where minors have bank accounts
				\begin{itemize}
					\item Assumption: banks provide minimum age
						information during bank
						transactions.
					\item Child and Exchange execute a variant of
						the cut\&choose protocol.
				\end{itemize}
			\end{itemize}
		\item Our scheme offers an alternative to identity management systems (IMS)
	\end{itemize}
\end{frame}
\begin{frame}{Related Work}
	\begin{itemize}
		\item Current privacy-perserving systems all based on attribute-based credentials (Koning et al., Schanzenbach et al., Camenisch et al., Au et al.)
		\item Attribute-based approach lacks support:
			\begin{itemize}
				\item Complex for consumers and retailers
				\item Requires trusted third authority
			\end{itemize}
		\vfill
		\item Other approaches tie age-restriction to ability to pay ("debit cards for kids")
			\begin{itemize}
				\item Advantage: mandatory to payment process
				\item Not privacy friendly
			\end{itemize}
	\end{itemize}
\end{frame}

\begin{frame}{Conclusion}
	Age restriction is a technical, ethical and legal challenge.

	Existing solutions are
	\begin{itemize}
		\item without strong protection of privacy or
		\item based on identity management systems (IMS)
	\end{itemize}
	\vfill

	Our scheme offers a solution that is
	\begin{itemize}
		\item based on subsidiarity
		\item privacy preserving
		\item efficient
		\item an alternative to IMS
	\end{itemize}
\end{frame}


\section{Software development \& deployment}

\begin{frame}
  \vfill
  \begin{center}
    {\bf Software development \& deployment}
  \end{center}
  \vfill
\end{frame}


\begin{frame}{Development Infrastructure}
  \begin{itemize}
    \item Borg: incremental backup
    \item Buildbot: CI/CD ({\url https://buildbot.taler.net/})
    \item Davical: Caldav group calendar
    \item Docker: virtualization, packaging
    \item Git/Gitolite: distributed version control ({\url https://git.taler.net/})
    \item Mailman: public e-mail lists ({\url taler@gnu.org/})
    \item Mantis: bug tracker ({\url https://bugs.taler.net/})
    \item Mattermost: messaging, process management ({\url https://mattermost.taler.net/})
    \item Sphinx: documentation generation (HTML, PDF, info, man) ({\url https://docs.taler.net/})
    \item Weblate: collaborative AI-supported internationalization ({\url https://weblate.taler.net/})
    \end{itemize}
\end{frame}


\begin{frame}{Development Tools}
  \begin{itemize}
    \item Coverity: static analysis (C/C++) ({\url https://scan.coverity.com/})
    \item GNU recutils: constant registration ({\url https://gana.gnunet.org/})
    \item Twister: fault injection
    \item Valgrind: dynamic analysis (C/C++)
    \item zzuf: fuzzing
  \end{itemize}
\end{frame}


\begin{frame}{Cryptographic dependencies}
  \begin{itemize}
    \item libargon2
    \item libgcrypt
    \item libsodium
  \end{itemize}
\end{frame}


\begin{frame}{Additional dependencies}
  \begin{itemize}
    \item libsqlite3
    \item libpq / Postgres
    \item libjansson
    \item libcurl
    \item libunistring
    \item {\bf GNU libmicrohttpd}
    \item {\bf GNUnet}
  \end{itemize}
\end{frame}


\begin{frame}
\frametitle{High-level Deployment Recipe}
\dots as a bank
\begin{enumerate}
  \item Create an escrow bank account for the exchange with EBICS access
  \item Provision offline signing machine
  \item Provision two PostgreSQL databases (for libeufin-nexus and exchange)
  \item Provision user-facing exchange service and secmod processes
  \item Provision libeufin-nexus (connected to escrow account and providing
    an internal API to the exchange)
  \item Test using the ``taler-wallet-cli``
\end{enumerate}
\end{frame}


\begin{frame}{Exchange escrow account access}
The Taler exchange needs to communicate with a core banking system \dots
\begin{itemize}
  \item to query for transactions into the exchange's escrow account
  \item to initiate payments of aggregated Taler deposits to merchants
\end{itemize}

In a Taler deployment, the \emph{Taler Wire Gateway} provides an API to the exchange
for Taler-specific access to the Exchange's escrow account.  Multiple implementations
of the Taler Wire Gateway exist:

\begin{itemize}
  \item libeufin-bank, a self-contained play money demo bank
  \item libeufin-nexus, an adapter to EBICS and other protocols
  \item Depolymerizer, an adapter to blockchains
\end{itemize}
\end{frame}


\begin{frame}{libeufin-nexus setup overview}
\framesubtitle{\url{https://docs.taler.net/libeufin/}}
  \begin{itemize}
    \item Obtain EBICS subscriber configuration (host URL, host ID, user ID,
      partner ID) for the bank account
    \item Create and back up the key material for the bank connection (contains
      EBICS subscriber configuration and private keys)
    \item Export key letter and activate subscriber in the EBICS host (physical mail)
    \item Confirm connection is active
    \item Set up scheduled tasks for ingesting new transactions / sending payment initiations
  \end{itemize}
\end{frame}


\begin{frame}{libeufin-nexus limitations at GLS Bank}
  The GLS accounts with EBICS access that we have access to have some limitations:
  \begin{itemize}
    \item SEPA instant credit transfers are not supported yet (by the bank)
    \item Erroneous payment initiations are accepted by the GLS EBICS host,
      but an error message is later sent only by paper mail (and not reported
      by the CRZ download request)
    \item Limited access to transaction history (3 months)
  \end{itemize}
\end{frame}



\section{Performance}

\begin{frame}
  \vfill
  \begin{center}
    {\bf Performance}\footnote{Joint work with Marco Boss}
  \end{center}
  \vfill
\end{frame}


\begin{frame} \frametitle{Performance} \framesubtitle{Other Payment Systems}
  \centering
  \begin{minipage}{0.32\textwidth}
    \centering
    \visible<1->{Bitcoin}\\\vspace{1em}
    \only<1>{? TPS}
    \only<2->{4 TPS}\\
    \visible<3->{
      \vspace{2em}
      \includegraphics[width=0.2\linewidth]{performance.pdf}\hspace{1em}
      \includegraphics[width=0.2\linewidth]{mining.pdf}
    }
  \end{minipage}
  \begin{minipage}{0.32\textwidth}
    \centering
    \visible<4->{PayPal\\\vspace{1em}193 TPS}\\
    \visible<5->{
      \vspace{2em}
      \includegraphics[width=0.2\linewidth]{privacy.pdf}
    }
  \end{minipage}
  \begin{minipage}{0.32\textwidth}
    \centering
    \visible<6->{Visa\\\vspace{1em}1'667 TPS}\\
    \visible<7->{
      \vspace{2em}
      \includegraphics[width=0.2\textwidth]{privacy.pdf}
    }
  \end{minipage}
  \let\thefootnote\relax\footnote{
    \tiny
    [06.22]\hspace{3em}
    - \href{https://www.researchgate.net/publication/330585021_Analysis_of_the_Possibilities_for_Improvement_of_BlockChain_Technology}{Researchgate}
  }
\end{frame}

\begin{frame} \frametitle{Performance} \framesubtitle{CBDC Projects}
  \centering
  \begin{minipage}{0.32\textwidth}
    \centering
    \visible<1->{e-Krona (Sweden)\\\vspace{1em}100 TPS}\\
    \visible<2->{
      \vspace{2em}
      \includegraphics[width=0.2\linewidth]{performance.pdf}\hspace{1em}
      \includegraphics[width=0.2\linewidth]{privacy.pdf}
    }
  \end{minipage}
  \begin{minipage}{0.32\textwidth}
    \centering
    \visible<3->{e-CNY (China)\\\vspace{1em}10'000 TPS}\\
    \visible<4->{
      \vspace{2em}
      \includegraphics[width=0.2\linewidth]{privacy.pdf}
    }
  \end{minipage}
  \begin{minipage}{0.32\textwidth}
    \centering
    \visible<5->{Project Hamilton (MIT)\\\vspace{1em}1'700'000 TPS}\\
    \visible<6->{
      \vspace{2em}
      \includegraphics[width=0.2\textwidth]{privacy.pdf}\hspace{1em}
      \includegraphics[width=0.2\textwidth]{applicable.pdf}
    }
  \end{minipage}
  \let\thefootnote\relax\footnote{
    \tiny
    [06.22]\hspace{3em}
    - \href{https://www.bostonfed.org/-/media/Documents/Project-Hamilton/Project-Hamilton-Phase-1-Whitepaper.pdf}{Bostonfed}
    \hspace{3em}- \href{https://www.atlanticcouncil.org/blogs/econographics/a-report-card-on-chinas-central-bank-digital-currency-the-e-cny/}{Atlatic Council}
    \hspace{3em}- \href{https://www.riksbank.se/globalassets/media/rapporter/e-krona/2022/e-krona-pilot-phase-2.pdf}{Riksbank}
  }
\end{frame}

\note{
  \href{https://www.cleanpng.com/png-computer-icons-privacy-policy-personally-identifia-956152/}{Privacy}
  \href{https://icon-library.com/png/116428.html}{Applicability}
  \href{https://www.pinclipart.com/pindetail/ibToJmx_vector-performance-icon-clipart/}{Performance}
  \href{https://www.clipartmax.com/middle/m2H7G6G6G6b1b1b1_icon-for-mining-clipart-mining-computer-icons-clip-miner-icon-png/}{Mining}
}


\begin{frame}[fragile]\frametitle{Grid'5000}
  \begin{multicols*}{2}
  \null \vfill
  \begin{itemize}
    \item <1-> Large-scale flexible testbed
    \item <2-> 800 nodes with total 15'000 cores
    \item <3-> Bare metal deployments
    \item <4-> Fully customizable software stack
  \end{itemize}
  \vfill \null
  \columnbreak
  \begin{center}
    \includegraphics[width=0.5\linewidth]{g5k.pdf}\\
    \vspace{2em}
    \includegraphics[width=0.9\linewidth]{g5k-map.jpg}
  \end{center}
  \end{multicols*}
\end{frame}
\note{
  Flexible Grossversuchsanlage
}

\begin{frame}\frametitle{Platform Access}
  \framesubtitle{jFed - Java Based GUI and CLI}
  \begin{figure}
    \centering
    \includegraphics[width=0.7\textwidth]{jfed.png}
  \end{figure}
\end{frame}

%\subsection{Setup}
%\subsectionpage

\begin{frame}\frametitle{Architecture}
  \begin{figure}
    \centering
    \includegraphics[width=0.8\textwidth]{arch.pdf}
  \end{figure}
\end{frame}

\begin{frame}\frametitle{Allocate an Experiment}
  \only<1>{\begin{figure}
    \includegraphics[width=0.9\textwidth]{flow-1}
  \end{figure}
  }
  \only<2>{\begin{figure}
    \includegraphics[width=0.9\textwidth]{flow-2}
  \end{figure}
  }
  \only<3>{\begin{figure}
    \includegraphics[width=0.9\textwidth]{flow-3}
  \end{figure}
  }
  \only<4>{\begin{figure}
    \includegraphics[width=0.9\textwidth]{flow-4}
  \end{figure}
  }
\end{frame}


\begin{frame}\frametitle{Horizontal Distribution}
  \begin{figure}
    \centering
    \includegraphics[width=0.75\linewidth]{dist-hori-more}
  \end{figure}
\end{frame}


\begin{frame}[fragile] \frametitle{Dashboard}
  \begin{center}
  \movie[poster, width=0.75\textwidth, showcontrols]{
    \includegraphics[width=0.75\textwidth]{white.png}}
    {dashboards.mp4}
  \end{center}
\end{frame}


\section{Blockchain integration: Project Depolymerization}

\begin{frame}
  \vfill
  \begin{center}
    {\bf Blockchain integration: Project Depolymerization}
  \end{center}
  \vfill
\end{frame}


\begin{frame}{Blockchain based cryptocurrencies}
    \begin{tikzpicture}[remember picture,overlay]
        \node (N1)[above right=5mm and 25mm of current page.center] {\includegraphics[width=34mm]{media/news1.png}};
        \node (N0)[below=-3mm of N1] {\includegraphics[width=34mm]{media/news0.png}};
        \node (N2)[below left=-26mm and -2.5mm of N1] {\includegraphics[width=34mm]{media/news2.png}};
    \end{tikzpicture}
    \begin{block}{Biggest cryptocurrencies}
        \begin{itemize}
            \item \textbf{BTC} Bitcoin
            \item \textbf{ETH} Ethereum
        \end{itemize}
    \end{block}
    \begin{block}{Common blockchain limitations}
        \begin{itemize}
            \item \textbf{Delay} block and confirmation delay
            \item \textbf{Cost} transaction fees
            \item \textbf{Scalability} limited amount of transaction per second
            \item \textbf{Ecological impact} computation redundancy
            \item \textbf{Privacy}
            \item \textbf{Regulatory risk}
        \end{itemize}
    \end{block}
\end{frame}

\begin{frame}{Taler}{Architecture}
    \begin{columns}
        \column{0.5\paperwidth}
        \begin{tikzpicture}[
                rect/.style={circle, draw=black},
                sym/.style={-stealth, shorten >= 2pt, shorten <= 2pt}
            ]
            % Taler payment system
            \node[rect](1) {Exchange};
            \node[rect,below left=1.5cm and 0.7cm of 1](2) {Customer};
            \node[rect,below right=1.5cm and 0.7cm of 1](3) {Merchant};

            \draw[sym] (1) -- node [midway, above, sloped] {\tiny Withdraw coins} (2);
            \draw[sym] (2) -- node [midway, above, sloped] {\tiny Spend coins} (3);
            \draw[sym] (3) -- node [midway, above, sloped] {\tiny Deposit coins} (1);

            % Settlement layer
            \node[left=2cm of 1](E1){};
            \node[right=2cm of 1](E2){};
            \draw[sym] (E1) -- node [midway, above] {\tiny Deposit money} (1);
            \draw[sym] (1) -- node [midway, above] {\tiny Withdraw money} (E2);

            % Auditor
            \node[above= of 1](A){Auditor};
            \draw[sym] (A) -- node [midway, right] {\tiny Verify} (1);

            % Separator
            \node[below=1mm of E1] (S1S) {};
            \node[below=1mm of E2] (S1E) {};
            \node[above=6mm of E1] (S2S) {};
            \node[above=6mm of E2] (S2E) {};

            \draw[dotted] (S1S) -- (S1E);
            \draw[dotted] (S2S) -- (S2E);

            \node[below right=-2mm and -1.5mm of S2S] {\tiny{\emph{Settlement Layer}}};
            \node[below right=-2mm and -1.5mm of S1S] {\tiny{\emph{Taler payment system}}};
        \end{tikzpicture}
        \column{0.47\paperwidth}
        \begin{block}{Settlement layer}
            \begin{itemize}
                \item For Depolymerization: Blockchain!
            \end{itemize}
        \end{block}
        \begin{block}{Taler payment system}
            \begin{itemize}
                \item Realtime transactions, 1 RTT
                \item Scalable microtransactions
                \item Blind signatures (privacy)
            \end{itemize}
        \end{block}

    \end{columns}
\end{frame}

\begin{frame}{Taler}{Blockchain settlement layer}
    \begin{center}
        \begin{tikzpicture}[
                rect/.style={rectangle, draw=black, minimum width=30mm},
                sym/.style={stealth-stealth, shorten >= 2pt, shorten <= 2pt},
                block/.style={rectangle,draw=black,fill=black!10,minimum size=7mm},
            ]

            %% Architecture
            \node(Tt){Taler};
            \node[rect,below=0cm of Tt](Tc){Exchange};
            \node[rect,fit={(Tt) (Tc)}](T){};

            \node[rect,below=7mm of Tc](D) {\textbf{Depolymerization}};

            \node[rect,below=7mm of D](Bc){Node};
            \node[below=0cm of Bc](Bt){Blockchain};
            \node[rect,fit={(Bt) (Bc)}](B){};

            \draw[sym] (T) -- (D);
            \draw[sym] (D) -- (B);

            %% Blockchain
            \node[block,right=8mm of B] (1){};
            \node[block,right=4mm of 1] (2){};
            \node[block,right=4mm of 2] (3){};
            \node[block,right=4mm of 3] (4){};
            \node[block,right=4mm of 4] (5){};
            \node[block,right=4mm of 5] (6){};
            \draw[-stealth] (1) -- (2);
            \draw[-stealth] (2) -- (3);
            \draw[-stealth] (3) -- (4);
            \draw[-stealth] (4) -- (5);
            \draw[-stealth] (5) -- (6);

            \node[left=4mm of 1] (S){};
            \node[right=4mm of 6] (E){};
            \draw[-stealth] (S) -- (1);
            \draw[-stealth] (6) -- (E);

            %% Taler
            \node[block, below right=-7.5mm and 20.5mm of T] (off){Off-chain transactions};
            \node[above=-0.5mm of off] {\includegraphics[height=7mm]{taler-logo-2021-inkscape.pdf}};

            %% Depolymerization
            \node[right=11mm of D] {\small{Credit}};
            \node[right=50mm of D] {\small{Debit}};
            \draw[dashed,-stealth] (1.north) |- (off.west);
            \draw[dashed,-stealth] (off.east) -| (6.north);
        \end{tikzpicture}
    \end{center}
\end{frame}

\begin{frame}{Challenges}
    \begin{block}{Taler Metadata}
        \begin{itemize}
            \item Metadata are required to link a wallet to credits and
                  allow merchant to link deposits to debits
            \item Putting metadata in blockchain transactions can be tricky
        \end{itemize}
    \end{block}
    \begin{block}{Blockchain based cryptocurrencies}
        \begin{itemize}
            \item Blockchain transactions lack finality (fork)
            \item Transactions can be stuck for a long time (mempool)
        \end{itemize}
    \end{block}
\end{frame}

\begin{frame}{Blockchain challenges}{Chain reorganization}
    \begin{center}
        \begin{tikzpicture}[
                block/.style={rectangle,draw=black,fill=black!10,minimum size=7mm},
                ar/.style={-stealth}
            ]
            % Common
            \node[block](1){};
            \node[block,right=5mm of 1](2){$D_0$};
            \node[block,right=5mm of 2](3){};
            \draw[ar] (1) -- (2);
            \draw[ar] (2) -- (3);

            % Current
            \node [block,right=5mm of 3](4){};
            \node[block,right=5mm of 4](5){};
            \node[block,right=5mm of 5](6){$D_1$};
            \draw[ar] (3) -- (4);
            \draw[ar] (4) -- (5);
            \draw[ar] (5) -- (6);

            % Fork
            \node [block,above=7mm of 4](4p){};
            \node[block,right=5mm of 4p](5p){$D_2$};
            \node[block,right=5mm of 5p](6p){};
            \node[block,right=5mm of 6p](7p){};
            \draw[ar] (3.east) -- (4p.west);
            \draw[ar] (4p) -- (5p);
            \draw[ar] (5p) -- (6p);
            \draw[ar] (6p) -- (7p);

            % Indication
            \node [right=5mm of 7p]{\emph{fork}};
            \node [right=17mm of 6]{\emph{active}};
        \end{tikzpicture}
    \end{center}
    A fork is when concurrent blockchain states coexist. Nodes will follow
    the longest chain, replacing recent blocks if necessary during a
    blockchain reorganization. If a deposit transaction disappears from the
    blockchain, an irrevocable withdraw transactions would no longer be backed
    by credit.
\end{frame}

\begin{frame}{Blockchain challenges}{Stuck transactions}
    We want confirmed debits within a limited time frame.
    \begin{figure}
        \centering
        \only<1> {
            \begin{tikzpicture}[
                    dot/.style={circle,fill,inner sep=1pt,}
                ]
                \node (I) {\includegraphics[width=\textwidth]{media/fee.png}};
                \node [below left=-2.5mm and -1.5cm of I] (Tx) {\small Tx};
                \node [dot,above=8.4mm of Tx](D) {};
                \draw [dotted,thick] (Tx) -- (D);
                \node [left=-4.5cm of Tx] (C) {\small conf};
                \node [dot,above=8.4mm of C](D1) {};
                \draw [dotted,thick] (C) -- (D1);
            \end{tikzpicture}
        }
        \only<2> {
            \includegraphics[width=\textwidth]{media/fee_var.png}
            \caption{Bitcoin average transaction fee over 6 months {\tiny (ychart)}}
        }
    \end{figure}
    \only<1>{When we trigger a debit with a fee too small, it may not be
        confirmed in a timely fashion.}
    \only<2>{However, transaction fees are unpredictable.}
\end{frame}


\begin{frame}{Depolymerization}{Architecture}
    \begin{center}
        \begin{tikzpicture}[
                rect/.style={rectangle, draw=black, minimum height=6mm, minimum width=28mm},
                sym/.style={stealth-stealth, shorten >= 2pt, shorten <= 2pt}
            ]
            \node[rect](1) {Taler Exchange};
            \node[rect,below=of 1](2) {Wire Gateway};
            \node[rect,right=of 2](3) {PostgreSQL};
            \node[rect,right=of 3](4) {DLT Adapter};
            \node[rect,above=of 4](5) {DLT Full Node};

            \draw[sym] (1) -- node [midway,right] {\tiny HTTP} (2);
            \draw[sym] (2) -- node [midway,above] {\tiny SQL} (3);
            \draw[sym] (3) -- node [midway,above] {\tiny SQL} (4);
            \draw[sym] (4) -- node [midway,left ] {\tiny RPC} (5);


            \node[above= 2mm of 1]{\small{\emph{Wire Gateway API}}};
            \node[above= 2mm of 5]{\small{\emph{DLT specific}}};
            \node[above=22mm of 3](T) {};
            \draw[dotted] (3) -- (T);
        \end{tikzpicture}
    \end{center}
    \begin{itemize}
        \item Common database to store transactions state and communicate
              with notifications
        \item Wire Gateway for Taler API compatibility
        \item DLT specific adapter
    \end{itemize}
\end{frame}

\begin{frame}{Storing metadata}{Bitcoin}
    \begin{block}{Bitcoin - Credit}
        \begin{itemize}
            \item Transactions from code
            \item Only 32B + URI
            \item \textbf{OP\_RETURN}
        \end{itemize}
    \end{block}
    \begin{block}{Bitcoin - Debit}
        \begin{itemize}
            \item Transactions from common wallet software
            \item Only 32B
            \item \textbf{Fake Segwit Addresses}
        \end{itemize}
    \end{block}
\end{frame}
\begin{frame}{Storing metadata}{Ethereum}
    \begin{block}{Smart contract ?}
        \begin{itemize}
            \item Logs in smart contract is the recommend way {\tiny (ethereum.org)}
            \item Expensive (additional storage and execution fees)
            \item Avoidable attack surface (error prone)
        \end{itemize}
    \end{block}
    \begin{block}{Custom input format}
        Use input data in transactions, usually used to call smart contract, to
        store our metadata.
    \end{block}
\end{frame}

\begin{frame}{Handling blockchain reorganization}
    \begin{center}
        \begin{tikzpicture}[
                block/.style={rectangle,draw=black,fill=black!10,minimum size=7mm},
                conf/.style={draw=black!60!green,fill=black!60!green!10},
                nconf/.style={dotted},
                err/.style={draw=black!60!red,fill=black!60!red!10},
                ar/.style={-stealth}
            ]
            % Common
            \node[block,conf](1){};
            \node[block,conf,right=5mm of 1](2){$D_0$};
            \node[block,conf,right=5mm of 2](3){};
            \draw[ar] (1) -- (2);
            \draw[ar] (2) -- (3);

            % Current
            \only<1>{
                \node [block,nconf,right=5mm of 3](4){};
            }
            \only<2->{
                \node [block,conf,right=5mm of 3](4){\only<3>{$D_3$}};
            }
            \node[block,nconf,right=5mm of 4](5){};
            \node[block,nconf,right=5mm of 5](6){$D_1$};
            \draw[ar] (3) -- (4);
            \draw[ar] (4) -- (5);
            \draw[ar] (5) -- (6);

            % Fork
            \only<-2>{
                \node [block,nconf,above=7mm of 4](4p){};
            }
            \only<3>{
                \node [block,dashed,err,above=7mm of 4](4p){$D_3'$};
            }
            \node[block,nconf,right=5mm of 4p](5p){$D_2$};
            \node[block,nconf,right=5mm of 5p](6p){};
            \node[block,nconf,right=5mm of 6p](7p){};
            \draw[ar] (3.east) -- (4p.west);
            \draw[ar] (4p) -- (5p);
            \draw[ar] (5p) -- (6p);
            \draw[ar] (6p) -- (7p);

            % Indication
            \node [right=5mm of 7p]{\emph{fork}};
            \node [right=17mm of 6]{\emph{active}};
        \end{tikzpicture}
    \end{center}
    \only<1>{As small reorganizations are common, Satoshi already recommended to
        apply a confirmation delay to handle most disturbances and attacks.}
    \only<2>{If a reorganization longer than the confirmation delay happens,
        but it did not remove credits, Depolymerizer is safe and automatically
        resumes.}
    \only<3>{If a fork removed a confirmed debit, an attacker may create a
        conflicting transaction. Depolymerizer suspends operation until lost
        credits reappear.}
\end{frame}

\begin{frame}{Adaptive confirmation}
    \begin{center}
        \begin{tikzpicture}[
                block/.style={rectangle,draw=black,fill=black!10,minimum size=7mm},
                conf/.style={draw=black!60!green,fill=black!60!green!10},
                nconf/.style={dotted},
                conft/.style={text=black!60!green},
                confl/.style={draw=black!60!green},
                ar/.style={-stealth}
            ]
            % Common
            \node(0){};
            \node[block,conf,right=5mm of 0](1){};
            \node[block,conf,right=5mm of 1](2){};
            \draw[ar] (0) -- (1);
            \draw[ar] (1) -- (2);

            % Current
            \node[block,conf,right=5mm of 2](3){};
            \node[block,nconf,right=5mm of 3](4){};
            \node[block,nconf,right=5mm of 4](5){};
            \node[block,nconf,right=5mm of 5](6){};
            \draw[ar] (2) -- (3);
            \draw[ar] (3) -- (4);
            \draw[ar] (4) -- (5);
            \draw[ar] (5) -- (6);

            % Fork
            \node[block,nconf,above=7mm of 3](3p){};
            \node[block,nconf,right=5mm of 3p](4p){};
            \node[block,nconf,right=5mm of 4p](5p){};
            \node[block,nconf,right=5mm of 5p](6p){};
            \node[block,nconf,right=5mm of 6p](7p){};
            \draw[ar] (2.east) -- (3p.west);
            \draw[ar] (3p) -- (4p);
            \draw[ar] (4p) -- (5p);
            \draw[ar] (5p) -- (6p);
            \draw[ar] (6p) -- (7p);

            % Indication
            \node[right=5mm of 7p]{\emph{fork}};
            \node[right=17mm of 6]{\emph{active}};

            % Confirmation
            \path (0) -- (1) node[conft,midway, below=6mm] (M) {Max};
            \path (2) -- (3) node[conft,midway, below=6mm] (N) {New};
            \path (3) -- (4) node[conft,midway, below=6mm] (I) {Initial};
            \node[above=25mm of M] (Mp) {};
            \node[above=25mm of N] (Np) {};
            \node[above=25mm of I] (Ip) {};
            \draw[confl,thick,dotted](M) -- (Mp);
            \draw[confl](N) -- (Np);
            \draw[confl,thick,dotted](I) -- (Ip);
        \end{tikzpicture}
    \end{center}
    If we experience a reorganization once, its dangerously likely for another
    one of a similar scope to happen again. Depolymerizer learns from reorganizations
    by increasing its confirmation delay.
\end{frame}



\begin{frame}{DLT Adapter}{Architecture}
    \begin{block}{Event system}
        \begin{itemize}
            \item \textbf{Watcher} watch and notify for new blocks with credits
            \item \textbf{Wire Gateway} notify requested debits
            \item \textbf{Worker} operates on notifications updating state
        \end{itemize}
    \end{block}
\end{frame}


\begin{frame}{DLT Adapter state machine}
    \begin{columns}
        \column{0.5\paperwidth}
        \begin{figure}
            \begin{tikzpicture}[
                    rect/.style={rectangle, draw=black, minimum height=6mm, minimum width=50mm},
                ]

                \node[rect](wo1) {Wait for notifications};
                \node[rect, below=4mm of wo1](wo2) {Reconcile local DB with DLT};
                \node[rect, below=4mm of wo2](wo3) {Trigger debits};
                \node[rect, below=4mm of wo3](wo4) {Reissue stuck debits};
                \node[rect, below=4mm of wo4](wo5) {Bounce malformed credits};
                \draw[-stealth] (wo1) -- (wo2);
                \draw[-stealth] (wo2) -- (wo3);
                \draw[-stealth] (wo3) -- (wo4);
                \draw[-stealth] (wo4) -- (wo5);
                \draw[-stealth] (wo5) .. controls ([xshift=-0.4cm] wo5.west) and ([xshift=-0.4cm] wo1.west) .. (wo1);
            \end{tikzpicture}
            \caption{Worker loop}
        \end{figure}
        \column{0.47\paperwidth}
        \begin{block}{DLT reconcialisation}
            \begin{itemize}
                \item List new and removed transactions since last reconciliation
                \item Check for confirmed credits removal
                \item Register new credits
                \item Recover lost debits
            \end{itemize}
        \end{block}
    \end{columns}
\end{frame}

\begin{frame}{Related work}
    \begin{block}{Centralization - Coinbase off-chain sending}
        \begin{itemize}
            \item [$+$] Fast and cheap: off chain transaction
            \item [$-$] Trust in Coinbase: privacy, security \& transparency
        \end{itemize}
    \end{block}
    \begin{block}{Layering - Lightning Network}
        \begin{itemize}
            \item [$+$] Fast and cheap: off-chain transactions
            \item [$-$] Requires setting up bidirectional payment channels
            \item [$-$] Fraud attempts are mitigated via a complex penalty system
        \end{itemize}
    \end{block}
\end{frame}

\begin{frame}{Conclusion}
    Blockchains can be used as a settlement layer for GNU Taler
    with Depolymerizer.

    \begin{itemize}
        \item [$-$] Trust exchange operator or auditors
        \item [$+$] Fast and cheap
        \item [$+$] Realtime, ms latency
        \item [$+$] Linear scalability
        \item [$+$] Ecological
        \item [$+$] Privacy when it can, transparency when it must (avoid tax evasion and money laundering)
    \end{itemize}
Future work:
    \begin{itemize}
        \item  Universal auditability, using sharded transactions history
        \item  Smarter analysis, update confirmation delay based on currency network behavior
        \item  Multisig by multiple operator for transactions validation
    \end{itemize}
\end{frame}


\section{Future Work \& Conclusion}

\begin{frame}
  \vfill
  \begin{center}
    {\bf Future Work \& Conclusion}
  \end{center}
  \vfill
\end{frame}


\begin{frame}{Taler: Project Status}
\framesubtitle{\url{https://docs.taler.net/}}
\begin{itemize}
    \item Cryptographic protocols and core exchange component are stable
    \item Pilot project at Bern University of Applied Sciences cafeteria
    \item Regional currency projects in Switzerland preparing for launch
    \item Internal alpha deployment with GLS Bank (Germany)
  \end{itemize}
\end{frame}


\begin{frame}{Competitor comparison}
  \begin{center} \small
    \begin{tabular}{l||c|c|c|c|c}
                & Cash     & Bitcoin    & Zerocoin  & Creditcard & GNU Taler  \\ \hline \hline
    Online      &$-$$-$$-$ &   ++       &    ++     &     +      &   +++      \\ \hline
    Offline     & +++      &   $-$$-$   &    $-$$-$ &     +      &   $+$$+$   \\ \hline
    Trans. cost & +        & $-$$-$$-$  & $-$$-$$-$ &     $-$    &   ++       \\ \hline
    Speed       & +        & $-$$-$$-$  & $-$$-$$-$ &     o      &   ++       \\ \hline
    Taxation    & $-$      &   $-$$-$   & $-$$-$$-$ &    +++     &  +++       \\ \hline
    Payer-anon  &  ++      &   o        &    ++     &  $-$$-$$-$ &  +++       \\ \hline
    Payee-anon  & ++       &   o        &    ++     &  $-$$-$$-$ &  $-$$-$$-$ \\ \hline
    Security    &  $-$     &   o        &    o      &    $-$$-$  &  ++        \\ \hline
    Conversion  & +++      &  $-$$-$$-$ & $-$$-$$-$ &    +++     &  +++       \\ \hline
    Libre       &  $-$     &  +++       &    +++    & $-$ $-$ $-$&  +++       \\
  \end{tabular}
  \end{center}
\end{frame}


\begin{frame}{Active collaborations}
  \begin{description}
  \item {\bf Freie Universit\"at Berlin:} \\ Programmability \& embedded systems
  \item {\bf The GNU Project}: \\ Integration into FLOSS software
  \item {\bf Fraunhofer Gesellschaft}: \\ Identity management \& SSI \& wallet-to-wallet communication
  \item {\bf NGI TALER}: \\ 11 partners deploying GNU Taler across Europe
  \end{description}
\end{frame}


\begin{frame}{NGI TALER PILOT}
\framesubtitle{\url{https://taler.net/en/consortium.html}}

 \begin{itemize}
    \item EU Project started December 2023 to deploy GNU Taler
    \item 3 financial institutions (GLS Bank, Magnet Bank, Visual Vest),
          2 academic institutions (Berner FH, TU Eindhoven), 3 SMEs (Taler Systems SA, Code Blau GmbH, Petit Singularites), 3 non-profits (NLnet Foundation, E-Seniors Association, Homo Digitalis)
    \item $\approx$ \EUR 5M budget over 3 years
    \item Objective: {\bf Deploy GNU Taler in Europe}
  \end{itemize}
\end{frame}


\begin{frame}{Key NGI PILOT Activities}
  \begin{itemize}
    \item Integration (core banking, online publishers, e-commerce, public transportation)
    \item Compliant (establish compliance processes at each bank)
    \item Availability (packaging, porting to more platforms, browsers)
    \item Hardware support (offline payments, vending machines)
    \item Security audits of code and design
    \item Accessible (old people, children, blind users)
    \item Future-proof (post-quantum, standardized)
    \item Widely known and used (community building, open calls)
  \end{itemize}
\end{frame}


\begin{frame}{Launch Timeline}
  \begin{description}
    \item[Q2'2022] Internal deployment at BFH
    \item[Q1'2023] Deployment using Bitcoin at BFH (running, but not announced)
    \item[Q2'2024] Deployment of local currency Netzbon in Basel
    \item[Q3'2024] Public deployment of eCHF stablecoin in Switzerland, cleared by FINMA
    \item[Q1'2025] GLS bank launches in Eurozone
    \item[Q3'2025] Magnet bank launches in Hungary
  \end{description}
\end{frame}


\begin{frame}{Other ongoing developments}
  \begin{itemize}
    \item Privacy-preserving auctions (trading, currency exchange) ({\tt oezguer@taler.net})
    \item Hardware and software support for embedded systems ({\tt mikolai@taler.net})
    \item GNU Name System registry with GNU Taler payments ({\tt schanzen@gnunet.org})
    \item Performance improvements for RSA in FLOSS crypto libraries (NLnet project)
    \item Parallel verification of RSA signatures on GPUs (libgpuverify.git)
    \item Tax-deductable receipts for donations to charities (donau.git)
    \item Unlinkable anonymous subscriptions and discount tokens (merchant.git, branch)
    \item Support for illiterate and innumerate users\footnote{Background: \url{https://myoralvillage.org/}}
          (not yet funded)
  \end{itemize}
\end{frame}


\begin{frame}{How to support?}
  \begin{description}
    \item[Join:] {\small \url{https://lists.gnu.org/mailman/listinfo/taler}}
    \item[Develop:] \url{https://bugs.taler.net/}, \url{https://git.taler.net/}
    \item[Apply:] \url{https://nlnet.nl/propose}, \url{https://nlnet.nl/taler}
    \item[Translate:] \url{https://weblate.taler.net/}, \url{translation-volunteer@taler.net}
    \item[Integrate:] \url{https://docs.taler.net/}
    \item[Donate:] \url{https://gnunet.org/ev}
    \item[Partner:] \url{https://taler-systems.com/}
  \end{description}
\end{frame}


\begin{frame}{Conclusion}
  \begin{center}
    {\bf  What can we do?}
   \end{center}
  \vfill
\begin{itemize}
 \item{Suffer mass-surveillance enabled by credit card oligopolies with high fees, and}
 \item{Engage in arms race with deliberately unregulatable blockchains}
% \item{Enjoy the ``benefits'' of cash \\
%  \hfill  \includegraphics[height=0.3\textheight]{atm-rupee.jpg} \hfill}
\end{itemize}
\vfill
\begin{center}
  {\bf OR}
\end{center}
\vfill
\begin{itemize}
 \item{Establish free software alternative balancing social goals!}
\end{itemize}
\vfill
\end{frame}


\begin{frame}
\frametitle{Do you have any questions?}
\vfill
References:
{\tiny
  \begin{enumerate}
 \item{Özgür Kesim, Christian Grothoff, Florian Dold and Martin Schanzenbach.
      {\em Zero-Knowledge Age Restriction for GNU Taler}.
       {\bf 27th European Symposium on Research in Computer Security (ESORICS), 2022}.}
 \item{David Chaum, Christian Grothoff and Thomas Moser.
       {\em How to issue a central bank digital currency}.
       {\bf SNB Working Papers, 2021}.}
 \item{Christian Grothoff, Bart Polot and Carlo von Loesch.
       {\em The Internet is broken: Idealistic Ideas for Building a GNU Network}.
       {\bf W3C/IAB Workshop on Strengthening the Internet Against Pervasive Monitoring (STRINT)}, 2014.}
 \item{Jeffrey Burdges, Florian Dold, Christian Grothoff and Marcello Stanisci.
       {\em Enabling Secure Web Payments with GNU Taler}.
       {\bf SPACE 2016}.}
 \item{Florian Dold, Sree Harsha Totakura, Benedikt M\"uller, Jeffrey Burdges and Christian Grothoff.
       {\em Taler: Taxable Anonymous Libre Electronic Reserves}.
       Available upon request. 2016.}
 \item{Eli Ben-Sasson, Alessandro Chiesa, Christina Garman, Matthew Green, Ian Miers, Eran Tromer and Madars Virza.
       {\em Zerocash: Decentralized Anonymous Payments from Bitcoin}.
       {\bf IEEE Symposium on Security \& Privacy, 2016}.}
 \item{David Chaum, Amos Fiat and Moni Naor.
       {\em Untraceable electronic cash}.
       {\bf Proceedings on Advances in Cryptology, 1990}.}
  \item{Phillip Rogaway.
       {\em The Moral Character of Cryptographic Work}.
       {\bf Asiacrypt}, 2015.} \label{bib:rogaway}
\end{enumerate}
}
\begin{center}
  {\bf Let money facilitate trade; but ensure capital serves society.}
\end{center}
\end{frame}


\begin{frame}{Rights}
  \begin{itemize}
    \item GNUnet e.V. shared copyrights of their AGPLv3+ licensed code with Taler Systems SA
    \item Taler Systems SA holds copyrights to entire GNU Taler code base (AGPLv3+, GPLv3+,
      dual-licensing exclusive domain of Taler Systems SA)
    \item Taler Systems SA applied for patent on offline payment solution
    \item Taler Systems SA holds trademark on ``Taler''.
    \item FSF holds trademark on ``GNU'', we are authorized to use ``GNU Taler''.
    \item Taler Systems SA owns {\tt taler.net} and {\tt taler-systems.com}.
  \end{itemize}
\end{frame}


\end{document}




\begin{frame}{Taler {\tt /withdraw/sign}}
% Customer withdrawing coins with blind signatures
% \bigskip
  \begin{figure}[th]
    \begin{minipage}[b]{0.45\linewidth}
      \begin{center}
        \begin{tikzpicture}[scale = 0.4,
            transform shape,
            msglabel/.style    = { text = Black, yshift = .3cm,
                                   sloped, midway },
            okmsg/.style       = { ->, color = MidnightBlue, thick,
                                   >=stealth },
            rstmsg/.style      = { ->, color = BrickRed, thick,
                                   >=stealth }
          ]
          \node[draw = MidnightBlue,
            fill = CornflowerBlue,
            minimum width = .3cm,
            minimum height = 10cm
          ] (h1) at (-4, 0) {};
          \node[draw = MidnightBlue,
            fill = CornflowerBlue,
            minimum width = .3cm,
            minimum height = 10cm
          ] (h2) at (4, 0) {};
          \node[above = 0cm of h1] {Wallet};
          \node[above = 0cm of h2] {Exchange};

          \path[->, color = MidnightBlue, very thick, >=stealth]
            (-5, 4.5) edge
            node[rotate=90, text = Black, yshift = .3cm] {Time}
            (-5, -4.5);
          \path[okmsg, dashed]
             ($(h1.east)+(0, 4.0)+(0, -1.0)$) edge
             node[msglabel] {SEPA(RK,A)}
             ($(h2.west)+(0, 3.5)+(0, -1.0)$);
          \path[okmsg]
            ($(h1.east)+(0, -1.0)$) edge
            node[msglabel] {POST {\tt /withdraw/sign} $S_{RK}(DK, B_b(C))$}
            ($(h2.west)+(0, -1.5)$);
          \path[okmsg]
            ($(h2.west)+(0, -2.0)$) edge
            node[msglabel] {200 OK: $S_{DK}(B_b(C))$)}
            ($(h1.east)+(0, -2.5)$);
          \path[rstmsg]
            ($(h2.west)+(0, -3.5)$) edge
            node[msglabel] {402 PAYMENT REQUIRED: $S_{RK}(DK, B_b(C))$)}
            ($(h1.east)+(0, -4)$);
          \node at (5.3, 0) {};
        \end{tikzpicture}
      \end{center}
      Result: $\langle c, S_{DK}(C) \rangle$.
    \end{minipage}
    \hspace{0.5cm}
    \begin{minipage}[b]{0.45\linewidth}
      \tiny
      \begin{description}
      \item[$A$] Some amount, $A \ge A_{DK}$
      \item[$RK$] Reserve key
      \item[$DK$] Denomination key
      \item[$b$] Blinding factor
      \item[$B_b()$] RSA-FDH blinding % DK supressed
      \item[$C$] Coin public key $C := cG$
      \item[$S_{RK}()$] EdDSA signature
      \item[$S_{DK}()$] RSA-FDH signature
      \end{description}
    \end{minipage}
  \end{figure}
\end{frame}


\begin{frame}[t]{Taler {\tt /deposit}}
Merchant and exchange see only the public coin $\langle C, S_{DK}(C) \rangle$.
\bigskip
  \begin{figure}[th]
    \begin{minipage}[b]{0.45\linewidth}
      \begin{center}
        \begin{tikzpicture}[scale = 0.4,
            transform shape,
            msglabel/.style    = { text = Black, yshift = .3cm,
                                   sloped, midway },
            okmsg/.style       = { ->, color = MidnightBlue, thick,
                                   >=stealth },
            rstmsg/.style      = { ->, color = BrickRed, thick,
                                   >=stealth }
          ]
          \node[draw = MidnightBlue,
            fill = CornflowerBlue,
            minimum width = .3cm,
            minimum height = 10cm
          ] (h1) at (-4, 0) {};
          \node[draw = MidnightBlue,
            fill = CornflowerBlue,
            minimum width = .3cm,
            minimum height = 10cm
          ] (h2) at (4, 0) {};
          \node[above = 0cm of h1] {Merchant};
          \node[above = 0cm of h2] {Exchange};

          \path[->, color = MidnightBlue, very thick, >=stealth]
            (-5, 4.5) edge
            node[rotate=90, text = Black, yshift = .3cm] {Time}
            (-5, -4.5);
          \path[->, color = MidnightBlue, thick, >=stealth]
            ($(h1.east)+(0,3)$) edge
            node[text = Black, yshift = .3cm, sloped] {POST {\tt /deposit} $S_{DK}(C), S_{c}(D)$}
            ($(h2.west)+(0,2)$);
          \path[->, color = MidnightBlue, thick, >=stealth]
            ($(h2.west)+(0,0.5)$) edge
            node[text = Black, yshift = .3cm, sloped] {200 OK: $S_{SK}(S_{c}(D))$}
            ($(h1.east)+(0,-0.5)$);
          \path[rstmsg]
            ($(h2.west)+(0, -2.5)$) edge
            node[msglabel] {409 CONFLICT: $S_{c}(D')$}
            ($(h1.east)+(0, -3.5)$);
          \node at (5.3, 0) {};
        \end{tikzpicture}
      \end{center}
    \end{minipage}
    \hspace{0.5cm}
    \begin{minipage}[b]{0.45\linewidth}
      \tiny
      \begin{description}
      \item[$DK$] Denomination key
      \item[$S_{DK}()$] RSA-FDH signature using $DK$
      \item[$c$] Private coin key, $C := cG$.
      \item[$S_{C}()$] EdDSA signature using $c$
      \item[$D$] Deposit details
      \item[$SK$] Exchange's signing key
      \item[$S_{SK}()$] EdDSA signature using $SK$
      \item[$D'$] Conficting deposit details $D' \not= D$
      \end{description}
    \end{minipage}
  \end{figure}
\end{frame}


\begin{frame}{Taler {\tt /refresh/melt}}
  \begin{figure}[th]
    \begin{minipage}[b]{0.45\linewidth}
      \begin{center}
	\begin{tikzpicture}[scale = 0.4,
            transform shape,
            msglabel/.style    = { text = Black, yshift = .3cm,
                                   sloped, midway },
            okmsg/.style       = { ->, color = MidnightBlue, thick,
                                   >=stealth },
            rstmsg/.style      = { ->, color = BrickRed, thick,
                                   >=stealth }
	  ]
	  \node[draw = MidnightBlue,
	    fill = CornflowerBlue,
	    minimum width = .3cm,
	    minimum height = 10cm
	  ] (h1) at (-4, 0) {};
	  \node[draw = MidnightBlue,
	    fill = CornflowerBlue,
	    minimum width = .3cm,
	    minimum height = 10cm
	  ] (h2) at (4, 0) {};
	  \node[above = 0cm of h1] {Customer};
	  \node[above = 0cm of h2] {Exchange};

	  \path[->, color = MidnightBlue, very thick, >=stealth]
	    (-5, 4.5) edge
	    node[rotate=90, text = Black, yshift = .3cm] {Time}
	    (-5, -4.5);
	  \path[->, color = MidnightBlue, thick, >=stealth]
	    ($(h1.east)+(0,3)$) edge
	    node[text = Black, yshift = .3cm, sloped] {POST {\tt /refresh/melt} $S_{DK}(C), S_c({\cal DK}, {\cal T},{\cal B})$}
	    ($(h2.west)+(0,2)$);
	  \path[->, color = MidnightBlue, thick, >=stealth]
	    ($(h2.west)+(0,0.5)$) edge
	    node[text = Black, yshift = .3cm, sloped] {200 OK: $S_{SK}(H({\cal T}, {\cal B}),\gamma)$}
	    ($(h1.east)+(0,-0.5)$);
	  \path[rstmsg]
	    ($(h2.west)+(0, -2.5)$) edge
	    node[msglabel] {409 CONFLICT: $S_{C}(X), \ldots$}
	    ($(h1.east)+(0, -3.5)$);
	  \node at (5.3, 0) {};
	\end{tikzpicture}
      \end{center}
    \end{minipage}
    \hspace{0.5cm}
    \begin{minipage}[b]{0.45\linewidth}
      \tiny
      \begin{description}
      \item[$\kappa$] System-wide security parameter, usually 3.
      \\ \smallskip
      \item[$\cal DK$] $:= [DK^{(i)}]_i$ \\ List of denomination keys \\
      $D + \sum_i A_{DK^{(i)}} < A_{DK}$
      \item[$t_j$] Random scalar for $j<\kappa$
      \item[${\cal T}$] $:= [T_j]_\kappa$ where $T_j = t_j G$
      \item[$k_j$] $:= c T_j = t_j C$ is an ECDHE
      \item[$b_j^{(i)}$] $:= KDF_b(k_j,i)$ % blinding factor
      \item[$c_j^{(i)}$] $:= KDF_c(k_j,i)$ % coin secret keys
      \item[$C_j^{(i)}$] $: = c_j^{(i)} G$ % new coin publics % keys
      \item[${\cal B}$] $:= [H( \beta_j )]_\kappa$ where \\
         $\beta_j := \left[ B_{b_j^{(i)}}(C_j^{(i)}) \right]_i$
      \\ \smallskip
      \item[$\gamma$] Random value in $[0,\kappa)$
%      \\ \smallskip
%      \item[$X$] Deposit or refresh
      \end{description}
    \end{minipage}
  \end{figure}
\end{frame}


\begin{frame}{Taler {\tt /refresh/reveal}}
  \begin{figure}[th]
    \begin{minipage}[b]{0.45\linewidth}
      \begin{center}
	\begin{tikzpicture}[scale = 0.4,
            transform shape,
            msglabel/.style    = { text = Black, yshift = .3cm,
                                   sloped, midway },
            okmsg/.style       = { ->, color = MidnightBlue, thick,
                                   >=stealth },
            rstmsg/.style      = { ->, color = BrickRed, thick,
                                   >=stealth }
	  ]
	  \node[draw = MidnightBlue,
	    fill = CornflowerBlue,
	    minimum width = .3cm,
	    minimum height = 10cm
	  ] (h1) at (-4, 0) {};
	  \node[draw = MidnightBlue,
	    fill = CornflowerBlue,
	    minimum width = .3cm,
	    minimum height = 10cm
	  ] (h2) at (4, 0) {};
	  \node[above = 0cm of h1] {Customer};
	  \node[above = 0cm of h2] {Exchange};

	  \path[->, color = MidnightBlue, very thick, >=stealth]
	    (-5, 4.5) edge
	    node[rotate=90, text = Black, yshift = .3cm] {Time}
	    (-5, -4.5);
	  \path[->, color = MidnightBlue, thick, >=stealth]
	    ($(h1.east)+(0,3)$) edge
	    node[text = Black, yshift = .3cm, sloped] {POST {\tt /refresh/reveal} $H({\cal T}, {\cal B}), {\tilde{\cal T}}, \beta_\gamma$}
	    ($(h2.west)+(0,2)$);
	  \path[->, color = MidnightBlue, thick, >=stealth]
	    ($(h2.west)+(0,0.5)$) edge
	    node[text = Black, yshift = .3cm, sloped] {200 OK: $\cal S$}
	    ($(h1.east)+(0,-0.5)$);
	  \path[rstmsg]
	    ($(h2.west)+(0, -2.5)$) edge
	    node[msglabel] {400 BAD REQUEST: $Z$}
	    ($(h1.east)+(0, -3.5)$);
	  \node at (5.3, 0) {};
	\end{tikzpicture}
      \end{center}
    \end{minipage}
    \hspace{0.5cm}
    \begin{minipage}[b]{0.45\linewidth}
      \tiny
      \begin{description}
      \item[$\cal DK$] $:= [DK^{(i)}]_i$
      \item[$t_j$] .. \\ \smallskip

      \item[$\tilde{\cal T}$] $:= [t_j | j \in \kappa, j \neq \gamma]$ \\ \smallskip

      \item[$k_\gamma$] $:= c T_\gamma = t_\gamma C$
      \item[$b_\gamma^{(i)}$] $:= KDF_b(k_\gamma,i)$
      \item[$c_\gamma^{(i)}$] $:= KDF_c(k_\gamma,i)$
      \item[$C_\gamma^{(i)}$] $: = c_\gamma^{(i)} G$

      \item[$B_\gamma^{(i)}$] $:= B_{b_\gamma^{(i)}}(C_\gamma^{(i)})$
      \item[$\beta_\gamma$] $:= \big[ B_\gamma^{(i)} \big]_i$
      \item[$\cal S$] $:= \left[ S_{DK^{(i)}}( B_\gamma^{(i)} ) \right]_i$ \\ \smallskip

      \item[$Z$] Cut-and-choose missmatch information
      \end{description}
    \end{minipage}
  \end{figure}
\end{frame}


\begin{frame}{Taler {\tt /refresh/link}}
  \begin{figure}[th]
    \begin{minipage}[b]{0.45\linewidth}
      \begin{center}
	\begin{tikzpicture}[scale = 0.4,
            transform shape,
            msglabel/.style    = { text = Black, yshift = .3cm,
                                   sloped, midway },
            okmsg/.style       = { ->, color = MidnightBlue, thick,
                                   >=stealth },
            rstmsg/.style      = { ->, color = BrickRed, thick,
                                   >=stealth }
	  ]
	  \node[draw = MidnightBlue,
	    fill = CornflowerBlue,
	    minimum width = .3cm,
	    minimum height = 10cm
	  ] (h1) at (-4, 0) {};
	  \node[draw = MidnightBlue,
	    fill = CornflowerBlue,
	    minimum width = .3cm,
	    minimum height = 10cm
	  ] (h2) at (4, 0) {};
	  \node[above = 0cm of h1] {Customer};
	  \node[above = 0cm of h2] {Exchagne};

	  \path[->, color = MidnightBlue, very thick, >=stealth]
	    (-5, 4.5) edge
	    node[rotate=90, text = Black, yshift = .3cm] {Time}
	    (-5, -4.5);
	  \path[->, color = MidnightBlue, thick, >=stealth]
	    ($(h1.east)+(0,3)$) edge
	    node[text = Black, yshift = .3cm, sloped] {POST {\tt /refresh/link} $C$}
	    ($(h2.west)+(0,2)$);
	  \path[->, color = MidnightBlue, thick, >=stealth]
	    ($(h2.west)+(0,0.5)$) edge
	    node[text = Black, yshift = .3cm, sloped] {200 OK: $T_\gamma$}
	    ($(h1.east)+(0,-0.5)$);
	  \path[rstmsg]
	    ($(h2.west)+(0, -2.5)$) edge
	    node[msglabel] {404 NOT FOUND}
	    ($(h1.east)+(0, -3.5)$);
	  \node at (5.3, 0) {};
	\end{tikzpicture}
      \end{center}
    \end{minipage}
    \hspace{0.5cm}
    \begin{minipage}[b]{0.45\linewidth}
      \tiny
      \begin{description}
      \item[$C$] Old coind public key \\ \smallskip
      \item[$T_\gamma$] Linkage data $\cal L$ at $\gamma$
      \end{description}
    \end{minipage}
  \end{figure}
\end{frame}


\begin{frame}{Operational security}
  \begin{center}
    \resizebox{\textwidth}{!}{
\begin{tikzpicture}[
  font=\sffamily,
  every matrix/.style={ampersand replacement=\&,column sep=2cm,row sep=2cm},
  source/.style={draw,thick,rounded corners,fill=green!20,inner sep=.3cm},
  process/.style={draw,thick,circle,fill=blue!20},
  sink/.style={source,fill=green!20},
  datastore/.style={draw,very thick,shape=datastore,inner sep=.3cm},
  dots/.style={gray,scale=2},
  to/.style={->,>=stealth',shorten >=1pt,semithick,font=\sffamily\footnotesize},
  every node/.style={align=center}]

  % Position the nodes using a matrix layout
  \matrix{
    \node[source] (wallet) {Wallet};
      \& \node[process] (browser) {Browser};
      \& \node[process] (shop) {Web shop};
      \& \node[sink] (backend) {Taler backend}; \\
  };

  % Draw the arrows between the nodes and label them.
  \draw[to] (browser) to[bend right=50] node[midway,above] {(4) signed contract}
      node[midway,below] {(signal)} (wallet);
  \draw[to] (wallet) to[bend right=50] node[midway,above] {(signal)}
      node[midway,below] {(5) signed coins} (browser);
  \draw[<->] (browser) -- node[midway,above] {(3,6) custom}
      node[midway,below] {(HTTPS)} (shop);
  \draw[to] (shop) to[bend right=50] node[midway,above] {(HTTPS)}
      node[midway,below] {(1) proposed contract / (7) signed coins} (backend);
  \draw[to] (backend) to[bend right=50] node[midway,above] {(2) signed contract / (8) confirmation}
      node[midway,below] {(HTTPS)} (shop);
\end{tikzpicture}
}
\end{center}
\end{frame}


\begin{frame}{Use Cases: Refugee Camps}
  Today:
  \begin{itemize}
  \item Non-bankable
  \item Direct distribution of goods to population
  \item Limited economic activity in camps
  \item High level of economic dependence
  \end{itemize}\vfill\pause
  With GNU Taler:
  \begin{itemize}
  \item Local currency issued as basic income backed by aid
  \item Taxation possible based on economic status
  \item Local governance enabled by local taxes
  \item Increased economic independence and political participation
  \end{itemize}
\end{frame}


\begin{frame}{Use Case: Anti-Spam}
  \framesubtitle{Background: \url{https://pep.security/}}
  Today, p$\equiv$p provides authenticated encryption for e-mail:
  \begin{itemize}
    \item Free software
    \item Easy to use opportunistic encryption
    \item Available for Outlook, Android, Enigmail
    \item Spies \& spam filters can no longer inspect content
  \end{itemize}\vfill\pause
  With GNU Taler:
  \begin{itemize}
    \item Peer-to-peer payments via e-mail
    \item If unsolicited sender, hide messages from user \&
          automatically request payment from sender
    \item Sender can attach payment to be moved to inbox
    \item Receiver may grant refund to sender
  \end{itemize}
\end{frame}
