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+\pdfminorversion=3
+\documentclass[fleqn,xcolor={usenames,dvipsnames}]{beamer}
+\usepackage{amsmath}
+\usepackage{multimedia}
+\usepackage[utf8]{inputenc}
+\usepackage{framed,color,ragged2e}
+\usepackage[absolute,overlay]{textpos}
+\definecolor{shadecolor}{rgb}{0.8,0.8,0.8}
+\usetheme{boxes}
+\setbeamertemplate{navigation symbols}{}
+\usepackage{xcolor}
+\usepackage{tikz,eurosym}
+\usepackage[normalem]{ulem}
+\usepackage{listings}
+\usepackage{adjustbox}
+
+% CSS
+\lstdefinelanguage{CSS}{
+  basicstyle=\ttfamily\scriptsize,
+  keywords={color,background-image:,margin,padding,font,weight,display,position,top,left,right,bottom,list,style,border,size,white,space,min,width, transition:, transform:, transition-property, transition-duration, transition-timing-function},
+  sensitive=true,
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+  morecomment=[s]{/*}{*/},
+  morestring=[b]',
+  morestring=[b]",
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+  alsodigit={-}
+}
+
+% JavaScript
+\lstdefinelanguage{JavaScript}{
+  basicstyle=\ttfamily\scriptsize,
+  morekeywords={typeof, new, true, false, catch, function, return, null, catch, switch, var, if, in, while, do, else, case, break},
+  morecomment=[s]{/*}{*/},
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+  morestring=[b]",
+  morestring=[b]'
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+
+\lstdefinelanguage{HTML5}{
+  basicstyle=\ttfamily\scriptsize,
+  language=html,
+  sensitive=true,
+  alsoletter={<>=-},
+  morecomment=[s]{<!-}{-->},
+  tag=[s],
+  otherkeywords={
+  % General
+  >,
+  % Standard tags
+	<!DOCTYPE,
+  </html, <html, <head, <title, </title, <style, </style, <link, </head, <meta, />,
+	% body
+	</body, <body,
+	% Divs
+	</div, <div, </div>,
+	% Paragraphs
+	</p, <p, </p>,
+	% scripts
+	</script, <script,
+  % More tags...
+  <canvas, /canvas>, <svg, <rect, <animateTransform, </rect>, </svg>, <video, <source, <iframe, </iframe>, </video>, <image, </image>
+  },
+  ndkeywords={
+  % General
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+  % HTML attributes
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+  % SVG attributes
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+  % CSS properties
+  margin:, padding:, background-image:, border:, top:, left:, position:, width:, height:,
+	% CSS3 properties
+  transform:, -moz-transform:, -webkit-transform:,
+  animation:, -webkit-animation:,
+  transition:,  transition-duration:, transition-property:, transition-timing-function:,
+  }
+}
+
+\lstdefinelanguage{JavaScript}{
+  basicstyle=\ttfamily\scriptsize,
+  keywords={typeof, new, true, false, catch, function, return, null, catch, switch, var, if, in, while, do, else, case, break, for},
+  keywordstyle=\color{blue}\bfseries,
+  ndkeywords={class, export, boolean, throw, implements, import, this},
+  ndkeywordstyle=\color{darkgray}\bfseries,
+  identifierstyle=\color{black},
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+  comment=[l]{//},
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+  commentstyle=\color{purple}\ttfamily,
+  stringstyle=\color{red}\ttfamily,
+  morestring=[b]',
+  morestring=[b]"
+}
+
+\usetikzlibrary{shapes,arrows}
+\usetikzlibrary{positioning}
+\usetikzlibrary{calc}
+
+\title{GNU Taler}
+%\subtitle{}
+
+\setbeamertemplate{navigation symbols}{\includegraphics[width=1cm]{inria.pdf} \includegraphics[width=2.3cm]{bfh.png} \includegraphics[width=1.6cm]{fub.pdf} \includegraphics[width=0.4cm]{ashoka.png}  \includegraphics[width=0.4cm]{gnu.png} \includegraphics[width=1cm]{logo-2020.jpg} \hfill}
+%\setbeamercovered{transparent=1}
+
+\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]{logo-2020.jpg}
+  \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} \\
+    \href{https://taler-systems.com/}{taler-systems.com}}
+\end{textblock*}
+
+% Substitute based on who is giving the talk!
+ \begin{textblock*}{6cm}(6.7cm,7.7cm) % {block width} (coords)
+   {%\hfill {\Large {\bf Florian Dold \&} \\
+    \hfill {\bf Christian Grothoff}} \\
+    \hfill grothoff@taler.net
+\end{textblock*}
+
+\end{frame}
+
+
+
+\section{What is GNU Taler?}
+\begin{frame}{What is GNU Taler?}
+  \begin{center}
+Taler is an electronic instant payment system.
+  \end{center}
+  \begin{itemize}
+  \item Uses electronic coins stored in {\bf wallets} on customer's device
+  \item Like {\bf cash}
+  \item Pay in {\bf existing currencies} (i.e. EUR, USD, BTC), \\
+    or use it to create new {\bf regional currencies}
+  \end{itemize}
+  \vfill
+  \pause
+ \noindent
+ However, Taler is
+  \begin{itemize}
+    \item \emph{not} a currency
+    \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
+    \item \emph{not} a speculative asset / ``get-rich-quick scheme''
+  \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 {... must 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}
+\frametitle{Architecture of Taler}
+\begin{center}
+  \includegraphics[width=1\textwidth]{operations.png}
+\end{center}
+\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}
+
+
+\section{Impact}
+\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}{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}
+
+
+\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}
+
+
+\section{Project status}
+\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}{Taler: Project Status}
+\begin{itemize}
+    \item Protocols and core exchange component are stable
+    \item Current focus: Merchant integration, settlement integration, wallet backup, 100k TPS scalability
+    \item Pilot project at Bern University of Applied Sciences cafeteria
+    \item Internal alpha deployment with a commercial bank in progress
+  \end{itemize}
+  \vfill
+\begin{center}
+\includegraphics[width=0.7\textwidth]{taler-in-use.png}
+\end{center}
+\end{frame}
+
+
+\begin{frame}{Ongoing development}
+We have ideas for protocol extensions and ``programmable money'':
+  \begin{itemize}
+    \item Mediated wallet-to-wallet payments (instead of customer-to-merchant)
+    \item Age-restricted private payments for children (youth protection)
+    \item Privacy-preserving auctions (trading, currency exchange)
+  \end{itemize}
+We also are looking into:
+  \begin{itemize}
+    \item General digital wallet usability and availability
+    \item Accessibility features for illiterate and innumerate users
+  \end{itemize}
+\end{frame}
+
+
+\begin{frame}{How to support?}
+  \begin{description}
+    \item[Join:] {\small \url{https://lists.gnu.org/mailman/listinfo/taler}}, \\
+                 \url{irc://irc.freenode.net/\#taler}
+    \item[Develop:] \url{https://bugs.taler.net/}, \url{https://git.taler.net/}
+    \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[Invest:] \url{https://taler-systems.com/}
+  \end{description}
+  We are looking for:
+  \begin{itemize}
+    \item European media organizations (50+ staff, 2+M annual turnover)
+          for pilot Taler integration paid by \url{https://stadiem.eu/} project
+          ({\bf urgent})!
+    \item Commercial banks to offer Taler to their customers
+    \item Central banks to operate Taler as a CBDC
+  \end{itemize}
+\end{frame}
+
+
+\begin{frame}
+\frametitle{Do you have any questions?}
+\vfill
+References:
+{\tiny
+  \begin{enumerate}
+ \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}
+}
+\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}?}
+
+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}{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}
+
+
+\section{How does it work?}
+\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) at (0,0) {\includegraphics[width=0.2\textwidth]{shop.pdf}};
+    \node (cart) [draw=none, below=of m]{\includegraphics[width=0.2\textwidth]{cart.pdf}};
+    \node (merchant) [node distance=4em and 0.5em, draw, below =of cart]{Merchant};
+    \tikzstyle{C} = [color=black, line width=1pt];
+    \draw [<-, C] (cart) -- (origin) node [midway, above, sloped] (TextNode) {};
+    \draw [<-, C] (merchant) -- (cart) node [midway, above, sloped] (TextNode) {{\small transmit}};
+  \end{tikzpicture}
+  \end{center}
+\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}{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}
+
+
+
+
+
+
+
+
+\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}