marketing

2023-boj.tex (62357B)

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 \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]{taler-logo-2021-inkscape.pdf} \hfill}
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 \author[C. Grothoff]{J. Burdges, {\bf F. Dold, C. Grothoff, M. Stanisci}}
 \date{\today}
 \institute{Taler Systems SA \& The GNU Project}
 
 
 \begin{document}
 
 \justifying
 
 \begin{frame}
   \begin{center}
     \LARGE {\bf GNU} \\
     \vspace{0.3cm}
 %    \includegraphics[width=0.66\textwidth]{logo-2017-fr.pdf}
     \includegraphics[width=0.66\textwidth]{taler-logo-2021-inkscape.pdf}
     \vfill
   \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*}{8cm}(4.7cm,6.7cm) % {block width} (coords)
    {\hfill {{\bf Dr. Emmanuel Benoist} \\
     \hfill {\bf Dr. Florian Dold} \\
     \hfill {\bf Dr. Andreas Habegger} \\
     \hfill {\bf Dr. Christian Grothoff} \\ }
     \hfill \{benoist,dold,habegger,grothoff\}@taler.net }
 \end{textblock*}
 
 \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}{Agenda}
 \tableofcontents
 \end{frame}
 
 
 \section{Introduction}
 
 \begin{frame}
   \vfill
   \begin{center}
     {\bf Part I: Introduction}
   \end{center}
   \vfill
 \end{frame}
 
 
 \section{What is Taler?}
 \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
     \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 combinable with a DLT back-end if requested
   \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} (but {\em available} under a commercial license).}
   \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} in associated services.}
 \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}{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}{Active collaborations}
 {\tiny
   \begin{description}
   \item {\bf Berner Fachhochschule:} \\ Snack machine \& blockchain integration \& scalability
   \item {\bf Technische Universit\"at Eindhoven:} \\ Post-quantum cryptography
   \item {\bf Freie Universit\"at Berlin:} \\ Programmability \& embedded systems
   \item {\bf The GNU Project}: \\ Integration into FLOSS software
   \item {\bf Code Blau GmbH}: \\ Independent auditor development
   \item {\bf Fraunhofer Gesellschaft}: \\ Identity management \& SSI \& wallet-to-wallet communication
   \item {\bf Fiscaltrust GmbH}: \\ Point-of-sale integration ({\bf 
 new})
   \item {\bf Bank of International Settlements}: \\ Participation in Point Zero Forum 2023 ({\bf new})
   \item {\bf Oesterreichische Nationalbank AG}: \\ Joint presentation 
 proposal for Re:publica 2023 ({\bf new})
   \end{description}
   }
 \end{frame}
 
 
 \begin{frame}{Launch Timeline}
   \begin{description}
     \item[2022] Internal deployment at BFH
     \item[Q1'2023] Deployment using Bitcoin at BFH (running, but not yet announced)
     \item[Q2-3'2023] Deployment of local currency Netzbon in Basel
     \item[Q3'2023] Public deployment of eCHF stablecoin in Switzerland, cleared by FINMA
     \item[2024] German bank executes ``new product process'' for launch in Eurozone
   \end{description}
 \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{Component Zoo}
 
 \begin{frame}
   \vfill
   \begin{center}
     {\bf Part II: 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
       \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})
         \item {\bf Anastasis}: Recovery of lost wallets based on secret splitting
       \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}
   Scalability: 28'500 transactions/second measured % in BS-thesis
   in 2022 using two servers on Grid5000. Likely several times
   higher today (but we did not re-measure recently).
 \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}{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}{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}{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}{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}{LibEuFin}
   LibEuFin is a standalone project that provides adapters to bank account
   access APIs.
 
   \begin{itemize}
     \item LibEuFin provides both a generic access layer and 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 tested with German bank GLS business account and real Euros
   \end{itemize}
   \vfill
   \begin{itemize}
     \item \texttt{libeufin-nexus} is the main service
     \item Almost all configuration (except DB credentials)
       is stored in the database and managed via a RESTful HTTP API
     \item \texttt{libeufin-sandbox} implements a toy EBICS host for protocol
       testing
     \item \texttt{libeufin-cli} is client for the HTTP API (only implements a subset
       of available functionality)
   \end{itemize}
 \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}{Depolymerization}
   Depolymerization is a bridge between GNU Taler and blockchains,
   making Taler a layer 2 system for crypto-currencies (like Lightning).
 
   \begin{itemize}
     \item Currently implemented for Bitcoin and Ethereum
           crypto-currencies, with the DLTs as the ``RTGS''
     \item Provides same API to Exchange as LibEuFin
 %   \item Transaction rate and speed limited by the underlying blockchain
     \item Implemented in Rust
   \end{itemize}
   \begin{center}
       \url{https://bitcoin.ice.bfh.ch/}
   \end{center}
 \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{Basic Cryptography}
 
 \begin{frame}
   \vfill
   \begin{center}
     {\bf Part III: Basic Cryptography}
   \end{center}
   \vfill
 \end{frame}
 
 
 
 
 \begin{frame}{How does it work?}
 We use a few well established and tested 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}{Withdrawing coins on the Web}
   \begin{center}
     \includegraphics[height=0.9\textheight]{figs/taler-withdraw.pdf}
   \end{center}
 \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}
 
 
 \section{Illustration of Programmable Money: Age Restrictions}
 
 \begin{frame}
   \vfill
   \begin{center}
     \vfill
     {\bf Part IV:}
     \vfill
     {\bf Illustration of Programmable Money}
     \vfill
     {\bf Zero-knowledge Age Restrictions}
     \vfill
   \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}{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 subsidiarity}
 \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}{Age Restrictions based on KYC}
 %		\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)
  Subsidiarity requires bank accounts being owned by adults
 			\begin{itemize}
 			\item Scheme can be adapted to case where minors have bank accounts
       		\item Assumption: banks provide minimum age
 						information during bank
 						transactions.
 			\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 age restriction scheme offers a solution that is
 	\begin{itemize}
 		\item based on subsidiarity
 		\item privacy-preserving
 		\item efficient
 		\item an alternative to IMS
 	\end{itemize}
     Other types of programmability (escrow, auctions) are under development.
 \end{frame}
 
 
 \section{Integration considerations}
 
 \begin{frame}
   \vfill
   \begin{center}
     {\bf Part V: Integration considerations}
   \end{center}
   \vfill
 \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}{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}{Partially Offline Payments with GNU Taler}
 We have filed for a patent to address situations where only the merchant is offline:
 \begin{enumerate}
   \item Customer pays by scanning static QR code and entering amount on mobile phone.
   \item Merchant confirms payment by checking simple unique numeric confirmation code.
   \item[$\Rightarrow$] Allows for very simple, cheap and secure merchant on-boarding.
      Makes integration with existing PoS vendors optional.
 \end{enumerate}
 \begin{center}
        {\bf Needed Point-of-sale hardware costs only $\approx$ \EUR{10}}
 \end{center}
 \vfill
 \end{frame}
 
 
 \section{Blockchain Integration}
 
 \begin{frame}
   \vfill
   \begin{center}
     {\bf Part VI: Blockchain Integration}
   \end{center}
   \vfill
   Antoine d’Aligny, Emmanuel Benoist and Christian Grothoff: ``{\em Project Depolymerization: Tokenization of Blockchains}''. {\bf 4th Conference on Blockchain Research \& Applications for Innovative Networks and Services}, 2022
   \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}{Layer 2 solutions: Taler vs. Lightning}
 
 \begin{minipage}{5.5cm}
 {\bf Taler:}
 \begin{itemize}
 \item[\checkmark] can be used with any currency or asset
 \item[\checkmark] can make payments instantly between any two parties
 \item[\checkmark] has income transparency and can accommodate KYC, AML and CFT
 \item[\checkmark] has cryptographic privacy protections
 \item[\checkmark] can be used immediately to make instant payments
 \item[\checkmark] uses one or more central exchange service providers
 \end{itemize}
 \end{minipage}
 \hfill
 \begin{minipage}{5.5cm}
 {\bf Lightning:}
 \begin{itemize}
 \item[$\times$] only works with Bitcoin
 \item[$\times$] requires payment route establishment, which can fail
 \item[$\times$] cannot enforce regulatory requirements
 \item[$\times$] requires money to be locked in payment channels
 \item[$\times$] requires expensive Bitcoin node or trusted service to transact
 \item[$\times$] claims to be decentralized, but uses few and centralized nodes in practice
 \end{itemize}
 \end{minipage}
 \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}{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 contracts}
         \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 likely for another
     reorganization of a similar scope to happen again.
     Depolymerizer learns from reorganizations by increasing its confirmation delay.
 \end{frame}
 
 
 \begin{frame}{Project Depolymerization Summary}
     Taler can be used as a layer 2 for existing
     crypto-currencies and stablecoins with Depolymerizer:
 
     \begin{itemize}
         \item [$-$] Trust exchange operator or auditors
         \item [$+$] Fast and cheap
         \item [$+$] Realtime: transactions with milliseconds of latency
         \item [$+$] Linear scalability
         \item [$+$] Ecological
         \item [$+$] Privacy when it can, transparency when it must (avoid tax evasion and money laundering)
     \end{itemize}
 \end{frame}
 
 
 \section{Conclusion}
 
 \begin{frame}
   \vfill
   \begin{center}
     {\bf Part VII: Conclusion}
   \end{center}
   \vfill
 \end{frame}
 
 
 \begin{frame}{Feature 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}{Summary of Taler Solution}
 
 \begin{enumerate}
 \item {\bf Frictionless one click}, cash-like digital payments \& instant
 confirmation for all fiat- and crypto-currencies and for all users,
 unbanked, young and old.
 \item With {\bf income-transparency on the recipient side} is
 crime-preventing \& Taler coins are as secure as cash without
 counterfeits. No ID theft.
 \item {\bf Guaranteed privacy} for spender \& data minimization: payment
 requires/shares no personal information.
 \item {\bf No bank account needed}
 \item Highly efficient in power, processing \& storage, extremely low
 cost \& {\bf highly scalable} to 100’000 transactions/sec.
 \item Economically viable (sub-cent) instant {\bf micro-transactions} for
 e-commerce, Internet of Things, metaverse, machine2machine, $\ldots$
 \end{enumerate}
 \end{frame}
 
 
 \begin{frame}{Collaboration with BoJ}
 \framesubtitle{Discussion Proposal}
 Taler Systems SA can provide:
 \begin{itemize}
 \item Integration with Japanese banking APIs
 \item Sandbox setup for evaluation by BoJ
 \item Support in maintenance \& roll-outs
 \end{itemize}
 Additionally:
 \begin{itemize}
 \item I will have a sabbatical in Fall/Winter 2023
 \item Objective is to visit financial institutions to present
       contemporary capabilities and gather requirements for
       further research
 \item I would be happy to spend part of it in Japan
 \end{itemize}
 \end{frame}
 
 
 
 \begin{frame}{Do you have any questions?}
   \framesubtitle{\url{https://taler.net/en/bibliography.html}}
   \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{Martin Summer and Hannes Hermanky.
        {\em A digital euro and the future of cash}.
        {\bf Monetary Policy \& The Economy Q1-Q2/22}.}
  \item{Antoine d’Aligny, Emmanuel Benoist, Florian Dold, Christian Grothoff, Özgür Kesim and Martin Schanzenbach.
        {\em Who comes after us? The correct mindset for designing a Central Bank Digital Currency}.
        {\bf SUERF Policy Notes 279/2022}.}
  \item{Florian Dold.
        {\em GNU Taler}.
        {\bf University of Rennes 1, PhD Thesis, 2019}.}
  \item{Christian Grothoff and Alex Pentland.
        {\em Digital cash and privacy: What are the alternatives to Libra?}.
        {\bf MIT Media Lab, 2019}.}
  \item{Jeffrey Burdges, Florian Dold, Christian Grothoff and Marcello Stanisci.
        {\em Enabling Secure Web Payments with GNU Taler}.
        {\bf SPACE 2016}.}
  \item{Özgür Kesim, Christian Grothoff, Florian Dold and Martin Schanzenbach.
        {\em Zero-Knowledge Age Restriction for GNU Taler}.
        {\bf ESORICS, 2022}.}
  \item{Gian Demarmels and Lucien Heuzeveldt.
        {\em Adding Schnorr's Blind Signature in Taler}.
        {\bf BFH, Bachelor's Thesis, 2022}.}
  \item{Marco Boss.
        {\em GNU Taler Scalability}.
        {\bf BFH, Bachelor's Thesis, 2022}.}
 \end{enumerate}
 }
 \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}