commit 2d1801f1bbb01bc2515e2f99217f7e4c90272f82
parent de5a660dc7a48d4403d0c3c682704096f84dafe0
Author: Christian Grothoff <christian@grothoff.org>
Date: Sun, 5 Mar 2023 19:23:21 +0100
sbb slides
Diffstat:
1 file changed, 3769 insertions(+), 0 deletions(-)
diff --git a/presentations/comprehensive/sbb.tex b/presentations/comprehensive/sbb.tex
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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, Swiss Franc 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 cryptogrpahy
+ \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}
+
+
+\begin{frame}[c]{Example: The Taler Snack Machine\footnote{by M. Boss and D. Hofer}}
+ \framesubtitle{Integration of a MDB/ICP to Taler gateway.\\Implementation of a NFC or QR-Code to Taler wallet interface.}
+ \vfill
+ \begin{figure}
+ \centering
+ \includegraphics[width=1.0\textwidth]{design}
+ \end{figure}
+\end{frame}
+
+
+\begin{frame}[c]{User story: Install App on Android}
+\framesubtitle{\url{https://wallet.taler.net/}}
+ \begin{figure}
+ \includegraphics[width=0.8\textwidth]{download_wallet.png}
+ \end{figure}
+\end{frame}
+
+\begin{frame}{User story: Withdraw e-cash}
+ \begin{figure}
+ \includegraphics[width=0.8\textwidth]{get_taler_coins.png}
+ \end{figure}
+\end{frame}
+
+\begin{frame}{User story: Use machine!}
+ \begin{figure}
+ \includegraphics[width=0.8\textwidth]{get_snacks.png}
+ \end{figure}
+\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}<1-| handout:0>{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}<1-| handout:0>{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}<1-| handout:0>{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}<1-| handout:0>{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}<1-| handout:0>{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}<1-| handout:0>{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}<1-| handout:0>{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}<1-| handout:0>{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=0.18\textwidth]{shop.pdf}};
+ \node (cart) [draw=none, below=of m]{\includegraphics[width=0.18\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}<1-| handout:0>{Merchant Integration: Payment Request}
+% \begin{figure}[p!]
+ \lstset{language=HTML5}
+ \lstinputlisting{figs/taler-402.html}
+% \caption{Sample HTTP response to prompt the wallet to show an offer.}
+% \label{listing:http-contract}
+% \end{figure}
+
+% \begin{figure*}[p!]
+% \lstset{language=HTML5}
+% \lstinputlisting{figs/taler-contract.html}
+% \caption{Sample JavaScript code to prompt the wallet to show an offer.
+% Here, the contract is fetched on-demand from the server.
+% The {\tt taler\_pay()} function needs to be invoked
+% when the user triggers the checkout.}
+% \label{listing:contract}
+% \end{figure*}
+\end{frame}
+
+
+
+\begin{frame}<1-| handout:0>{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}<1-| handout:0>{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}<1-| handout:0>{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 This step requires communication with the exchange.}
+ \end{center}
+ \vfill
+\end{frame}
+
+
+\begin{frame}{Payment processing with Taler}
+ \begin{center}
+ \includegraphics[height=0.9\textheight]{figs/taler-pay.pdf}
+ \end{center}
+\end{frame}
+
+
+\section{Giving Change}
+
+\begin{frame}
+ \vfill
+ \begin{center}
+ {\bf Part IV: Giving Change}
+ \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}<1-| handout:0>{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}<1-| handout:0>{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 0.45em, 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}<1-| handout:0>{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= 1.4em and 0.45em, 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}<1-| handout:0>{Cut-and-Choose}
+ \begin{minipage}{4cm}
+ \begin{tikzpicture}
+ \tikzstyle{def} = [node distance= 1.4em and 0.45em, 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= 1.4em and 0.45em, 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= 1.4em and 0.45em, 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}<1-| handout:0>{Exchange: Choose!}
+ \begin{center}
+ \item Exchange sends back random $\gamma \in \{ 1, 2, 3 \}$ to the customer.
+ \end{center}
+\end{frame}
+
+
+\begin{frame}<1-| handout:0>{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}<1-| handout:0>{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}<1-| handout:0>{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}<1-| handout:0>{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}<1-| handout:0>{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}<1-| handout:0>{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 properties}
+ \begin{itemize}
+ \item Customer asks exchange to convert old coin to new coin
+ \item Protocol ensures new coins can be recovered from old coin
+ \item[$\Rightarrow$] New coins are owned by the same entity!
+ \end{itemize}
+ Thus, the refresh protocol allows:
+ \begin{itemize}
+ \item To give unlinkable change.
+ \item To give refunds to an anonymous customer.
+ \item To expire old keys and migrate coins to new ones.
+ \item To handle protocol aborts.
+ \end{itemize}
+ \noindent
+% \begin{center}
+% { \bf Transactions via refresh are equivalent to {\em sharing} a wallet.}
+% \end{center}
+\end{frame}
+
+
+\section{Illustration of Programmable Money: Age Restrictions}
+
+\begin{frame}
+ \vfill
+ \begin{center}
+ \vfill
+ {\bf Part V:}
+ \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}<1-| handout:0>{Principle of Subsidiarity}
+\begin{center} \Large
+ Functions of government---such as granting and restricting
+ rights---should be performed\\
+ {\it at the lowest level of authority possible},\\
+ as long as they can be performed {\it adequately}.
+\end{center}
+\vfill
+\uncover<2->{
+ For age-restriction, the lowest level of authority is:\\
+ \begin{center}\Large
+ Parents, guardians and caretakers
+ \end{center}
+}
+\end{frame}
+
+
+\begin{frame}{Age restriction design for GNU Taler}
+Design and implementation of an age restriction scheme\\
+with the following goals:
+
+\begin{enumerate}
+\item It ties age restriction to the \textbf{ability to pay} (not to ID's)
+\item maintains \textbf{anonymity of buyers}
+\item maintains \textbf{unlinkability of transactions}
+\item aligns with \textbf{principle of 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}<1-| handout:0>{Formal Function Signatures}
+\small
+Searching for functions \uncover<2->{with the following signatures}
+\begin{align*}
+ &\bf \Commit\uncover<2->{:
+ &(\age, \omega) &\mapsto (\commitment, \pruf)
+ &\scriptstyle \N_\Age \times \Omega &\scriptstyle \to \Commitments\times\Proofs,
+ }
+ \\
+ &\bf \Attest\uncover<3->{:
+ &(\minage, \commitment, \pruf) &\mapsto \attest
+ &\scriptstyle \N_\Age\times\Commitments\times\Proofs &\scriptstyle \to \Attests \cup \{\Nil\},
+ }
+ \\
+ &\bf \Verify\uncover<4->{:
+ &(\minage, \commitment, \attest) &\mapsto b
+ &\scriptstyle \N_\Age\times\Commitments\times\Attests &\scriptstyle \to \Z_2,
+ }
+ \\
+ &\bf \Derive\uncover<5->{:
+ &(\commitment, \pruf, \omega) &\mapsto (\commitment', \pruf', \blinding)
+ &\scriptstyle \Commitments\times\Proofs\times\Omega &\scriptstyle \to \Commitments\times\Proofs\times\Blindings,
+ }
+ \\
+ &\bf \Compare\uncover<6->{:
+ &(\commitment, \commitment', \blinding) &\mapsto b
+ &\scriptstyle \Commitments\times\Commitments\times\Blindings &\scriptstyle \to \Z_2,
+ }
+\end{align*}
+ \uncover<7->{
+ with $\Omega, \Proofs, \Commitments, \Attests, \Blindings$
+ sufficiently large sets.\\[1em]
+ Basic and security requirements are defined later.\\[2em]
+ }
+
+ \scriptsize
+ \uncover<2->{
+ Mnemonics:\\
+ $\Commitments=$ \textit{c$\Commitments$mmitments},
+ $\commitment=$ \textit{Q-mitment} (commitment),
+ $\Proofs=$ \textit{$\Proofs$roofs},
+ }
+ \uncover<3->{
+ $\pruf=$ \textit{$\pruf$roof},\\
+ $\Attests=$ \textit{a$\Attests$testations},
+ $\attest=$ \textit{a$\attest$testation},
+ }
+ \uncover<5->{
+ $\Blindings=$ \textit{$\Blindings$lindings},
+ $\blinding=$ \textit{$\blinding$linding}.
+ }
+\end{frame}
+
+\begin{frame}<1-| handout:0>{Age restriction}
+ \framesubtitle{Naïve scheme}
+ \begin{center}
+ \begin{tikzpicture}[scale=.85]
+ \node[circle,minimum size=20pt,fill=black!15] at ( 60:4) (Exchange) {$\Exchange$};
+ \node[circle,minimum size=20pt,fill=black!15] at ( 0:0) (Client) {$\Child$};
+ \node[circle,minimum size=20pt,fill=black!15] at ( 0:4) (Merchant) {$\Merchant$};
+ \node[circle,minimum size=20pt,fill=blue!15] at (140:3) (Guardian) {$\Guardian$};
+
+ \draw[->] (Guardian) to [out=50,in=130, loop] node[above]
+ {$\Commit$} (Guardian);
+ \draw[->,blue] (Client) to [out=-125,in=-190, loop] node[below,left]
+ {\blue{$\Attest$}} (Client);
+ \draw[->,blue] (Merchant) to [out=50,in=130, loop] node[above]
+ {\blue{$\Verify$}} (Merchant);
+ \draw[->,orange] (Client) to [out=-35,in=-100, loop] node[below]
+ {\orange{$\Derive$}} (Client);
+ \draw[->,orange] (Exchange) to [out=50,in=130, loop] node[above]
+ {\orange{$\Compare$}} (Exchange);
+
+ \draw[orange,|->] (Client) to node[sloped,above,align=left]
+ {\orange{\scriptsize }} (Exchange);
+ \draw[blue,|->] (Client) to node[sloped, above]
+ {\blue{\scriptsize }} (Merchant);
+ \draw[,|->] (Guardian) to node[above,sloped,align=left]
+ {{\scriptsize }} (Client);
+ \end{tikzpicture}
+ \end{center}
+\end{frame}
+
+\begin{frame}<1-| handout:0>{Achieving Unlinkability}
+ \begin{columns}
+ \column{3cm}
+ \begin{center}
+ \fontsize{8pt}{9pt}\selectfont
+ \begin{tikzpicture}[scale=.65]
+ \node[circle,minimum size=20pt,fill=black!15] at ( 60:4) (Exchange) {$\Exchange$};
+ \node[circle,minimum size=20pt,fill=black!15] at ( 0:0) (Client) {$\Child$};
+
+ \draw[->,orange] (Client) to [out=-35,in=-100, loop] node[below]
+ {\orange{$\footnotesize \Derive()$}} (Client);
+ \draw[->,orange] (Exchange) to [out=50,in=130, loop] node[above]
+ {\orange{$\footnotesize \Compare()$}} (Exchange);
+
+ \draw[orange,|->] (Client) to node[sloped,above,align=left]
+ {\orange{\tiny \uncover<2->{$(\commitment_i,\commitment_{i+1})$}}} (Exchange);
+ \end{tikzpicture}
+ \end{center}
+
+ \column{9cm}
+ Simple use of $\Derive()$ and $\Compare()$ is problematic.
+
+ \begin{itemize}
+ \item<2-> Calling $\Derive()$ iteratively generates sequence
+ $(\commitment_0, \commitment_1, \dots)$ of commitments.
+ \item<2-> Exchange calls $\Compare(\commitment_i, \commitment_{i+1}, .)$
+ \item[$\implies$]\uncover<3->{\bf Exchange identifies sequence}
+ \item[$\implies$]\uncover<3->{\bf Unlinkability broken}
+ \end{itemize}
+ \end{columns}
+\end{frame}
+
+\begin{frame}<1-| handout:0>{Achieving Unlinkability}
+ Define cut\&choose protocol \orange{$\DeriveCompare$},
+ using $\Derive()$ and $\Compare()$.\\[0.5em]
+ \uncover<2->{
+ Sketch:
+ \small
+ \begin{enumerate}
+ \item $\Child$ derives commitments $(\commitment_1,\dots,\commitment_\kappa)$
+ from $\commitment_0$ \\
+ by calling $\Derive()$ with blindings $(\beta_1,\dots,\beta_\kappa)$
+ \item $\Child$ calculates $h_0:=H\left(H(\commitment_1, \beta_1)||\dots||H(\commitment_\kappa, \beta_\kappa)\right)$
+ \item $\Child$ sends $\commitment_0$ and $h_0$ to $\Exchange$
+ \item $\Exchange$ chooses $\gamma \in \{1,\dots,\kappa\}$ randomly
+ \item $\Child$ reveals $h_\gamma:=H(\commitment_\gamma, \beta_\gamma)$ and all $(\commitment_i, \beta_i)$, except $(\commitment_\gamma, \beta_\gamma)$
+ \item $\Exchange$ compares $h_0$ and
+ $H\left(H(\commitment_1, \beta_1)||...||h_\gamma||...||H(\commitment_\kappa, \beta_\kappa)\right)$\\
+ and evaluates $\Compare(\commitment_0, \commitment_i, \beta_i)$.
+ \end{enumerate}
+ \vfill
+ Note: Scheme is similar to the {\it refresh} protocol in GNU Taler.
+ }
+\end{frame}
+
+\begin{frame}<1-| handout:0>{Achieving Unlinkability}
+ With \orange{$\DeriveCompare$}
+ \begin{itemize}
+ \item $\Exchange$ learns nothing about $\commitment_\gamma$,
+ \item trusts outcome with $\frac{\kappa-1}{\kappa}$ certainty,
+ \item i.e. $\Child$ has $\frac{1}{\kappa}$ chance to cheat.
+ \end{itemize}
+ \vfill
+ Note: Still need Derive and Compare to be defined.
+\end{frame}
+
+\begin{frame}<1-| handout:0>{Refined scheme}
+
+ \begin{tikzpicture}[scale=.8]
+ \node[circle,minimum size=25pt,fill=black!15] at ( 0:0) (Client) {$\Child$};
+ \node[circle,minimum size=25pt,fill=black!15] at ( 60:5) (Exchange) {$\Exchange$};
+ \node[circle,minimum size=25pt,fill=black!15] at ( 0:5) (Merchant) {$\Merchant$};
+ \node[circle,minimum size=25pt,fill=blue!15] at (130:3) (Guardian) {$\Guardian$};
+
+ \draw[orange,<->] (Client) to node[sloped,below,align=center]
+ {\orange{$\DeriveCompare$}} (Exchange);
+ \draw[blue,->] (Client) to node[sloped, below]
+ {\blue{$(\attest_\minage, \commitment)$}} (Merchant);
+
+ \draw[->] (Guardian) to [out=150,in=70, loop] node[above]
+ {$\Commit(\age)$} (Guardian);
+ \draw[->] (Guardian) to node[below,sloped]
+ {($\commitment$, $\pruf_\age$)} (Client);
+ \draw[->,blue] (Client) to [out=-50,in=-130, loop] node[below]
+ {\blue{$\Attest(\minage, \commitment, \pruf_{\age})$}} (Client);
+ \draw[->,blue] (Merchant) to [out=-50,in=-130, loop] node[below]
+ {\blue{$\Verify(\minage, \commitment, \attest_{\minage})$}} (Merchant);
+ \end{tikzpicture}
+\end{frame}
+
+% \begin{frame}{Achieving Unlinkability}
+% \scriptsize
+% $\DeriveCompare : \Commitments\times\Proofs\times\Omega \to \{0,1\}$\\
+% \vfill
+% $\DeriveCompare(\commitment, \pruf, \omega) =$
+% \begin{itemize}
+% \it
+% \itemsep0.5em
+% \item[$\Child$:]
+% \begin{enumerate}
+% \scriptsize
+% \itemsep0.3em
+% \item for all $i \in \{1,\dots,\kappa\}:
+% (\commitment_i,\pruf_i,\beta_i) \leftarrow \Derive(\commitment, \pruf, \omega + i)$
+% \item $h \leftarrow \Hash\big(\Hash(\commitment_1,\beta_1)\parallel\dots\parallel\Hash(\commitment_\kappa,\beta_\kappa) \big)$
+% \item send $(\commitment, h)$ to $\Exchange$
+% \end{enumerate}
+% \item[$\Exchange$:]
+% \begin{enumerate}
+% \setcounter{enumi}{4}
+% \scriptsize
+% \itemsep0.3em
+% \item save $(\commitment, h)$ \label{st:hash}
+% \item $\gamma \drawfrom \{1,\dots ,\kappa\}$
+% \item send $\gamma$ to $\Child$
+% \end{enumerate}
+% \item[$\Child$:]
+% \begin{enumerate}
+% \setcounter{enumi}{7}
+%
+% \scriptsize
+% \itemsep0.3em
+% \item $h'_\gamma \leftarrow \Hash(\commitment_\gamma, \beta_\gamma)$
+% \item $\mathbf{E}_\gamma \leftarrow \big[(\commitment_1,\beta_1),\dots,
+% (\commitment_{\gamma-1}, \beta_{\gamma-1}),
+% \Nil,
+% (\commitment_{\gamma+1}, \beta_{\gamma+1}),
+% \dots,(\commitment_\kappa, \beta_\kappa)\big]$
+% \item send $(\mathbf{E}_\gamma, h'_\gamma)$ to $\Exchange$
+% \end{enumerate}
+% \item[$\Exchange$:]
+% \begin{enumerate}
+% \setcounter{enumi}{10}
+% \scriptsize
+% \itemsep0.3em
+% \item for all $i \in \{1,\dots,\kappa\}\setminus\{\gamma\}: h_i \leftarrow \Hash(\mathbf{E}_\gamma[i])$
+% \item if $h \stackrel{?}{\neq} \HashF(h_1\|\dots\|h_{\gamma-1}\|h'_\gamma\|h_{\gamma+1}\|\dots\|h_{\kappa-1})$ return 0
+% \item for all $i \in \{1,\dots,\kappa\}\setminus\{\gamma\}$:
+% if $0 \stackrel{?}{=} \Compare(\commitment,\commitment_i, \beta_i)$ return $0$
+% \item return 1
+% \end{enumerate}
+% \end{itemize}
+% \end{frame}
+
+\begin{frame}<1-| handout:0>{Basic Requirements}
+
+ Candidate functions
+ \[ (\Commit, \Attest, \Verify, \Derive, \Compare) \]
+ must first meet \textit{basic} requirements:
+
+ \begin{itemize}
+ \item Existence of attestations
+ \item Efficacy of attestations
+ \item Derivability of commitments and attestations
+ \end{itemize}
+\end{frame}
+
+\begin{frame}<1-| handout:0>{Basic Requirements}
+ \framesubtitle{Formal Details}
+
+ \begin{description}
+ \item[Existence of attestations]
+ {\scriptsize
+ \begin{align*}
+ \Forall_{\age\in\N_\Age \atop \omega \in \Omega}:
+ \Commit(\age, \omega) =: (\commitment, \pruf)
+ \implies
+ \Attest(\minage, \commitment, \pruf) =
+ \begin{cases}
+ \attest \in \Attests, \text{ if } \minage \leq \age\\
+ \Nil \text{ otherwise}
+ \end{cases}
+ \end{align*}}
+ \item[Efficacy of attestations]
+ {\scriptsize
+ \begin{align*}
+ \Verify(\minage, \commitment, \attest) = \
+ \begin{cases}
+ 1, \text{if } \Exists_{\pruf \in \Proofs}: \Attest(\minage, \commitment, \pruf) = \attest\\
+ 0 \text{ otherwise}
+ \end{cases}
+ \end{align*}}
+
+ {\scriptsize
+ \begin{align*}
+ \forall_{n \leq \age}: \Verify\big(n, \commitment, \Attest(n, \commitment, \pruf)\big) = 1.
+ \end{align*}}
+ \item[etc.]
+ \end{description}
+\end{frame}
+
+%\begin{frame}{Requirements}
+% \framesubtitle{Details}
+%
+% \begin{description}
+% \item[Derivability of commitments and proofs:]~\\[0.1em]
+% {\scriptsize
+% Let \begin{align*}
+% \age & \in\N_\Age,\,\, \omega_0, \omega_1 \in\Omega\\
+% (\commitment_0, \pruf_0) & \leftarrow \Commit(\age, \omega_0),\\
+% (\commitment_1, \pruf_1, \blinding) & \leftarrow \Derive(\commitment_0, \pruf_0, \omega_1).
+% \end{align*}
+% We require
+% \begin{align*}
+% \Compare(\commitment_0, \commitment_1, \blinding) = 1 \label{req:comparity}
+% \end{align*}
+% and for all $n\leq\age$:
+% \begin{align*}
+% \Verify(n, \commitment_1, \Attest(n, \commitment_1, \pruf_1)) &%
+% =
+% \Verify(n, \commitment_0, \Attest(n, \commitment_0, \pruf_0))
+% \end{align*}}
+% \end{description}
+%\end{frame}
+
+\begin{frame}<1-| handout:0>{Security Requirements}
+ Candidate functions must also meet \textit{security} requirements.
+ Those are defined via security games:
+ \begin{itemize}
+ \item Game: Age disclosure by commitment or attestation
+ \item[$\leftrightarrow$] Requirement: Non-disclosure of age
+ \vfill
+
+ \item Game: Forging attestation
+ \item[$\leftrightarrow$] Requirement: Unforgeability of
+ minimum age
+ \vfill
+
+ \item Game: Distinguishing derived commitments and attestations
+ \item[$\leftrightarrow$] Requirement: Unlinkability of
+ commitments and attestations
+
+ \end{itemize}
+ \vfill
+
+ Meeting the security requirements means that adversaries can win
+ those games only with negligible advantage.
+ \vfill
+ Adversaries are arbitrary polynomial-time algorithms, acting on all
+ relevant input.
+\end{frame}
+
+\begin{frame}<1-| handout:0>{Security Requirements}
+ \framesubtitle{Simplified Example}
+
+ \begin{description}
+ \item[Game $\Game{FA}(\lambda)$---Forging an attest:]~\\
+ {\small
+ \begin{enumerate}
+ \item $ (\age, \omega) \drawfrom \N_{\Age-1}\times\Omega $
+ \item $ (\commitment, \pruf) \leftarrow \Commit(\age, \omega) $
+ \item $ (\minage, \attest) \leftarrow \Adv(\age, \commitment, \pruf)$
+ \item Return 0 if $\minage \leq \age$
+ \item Return $\Verify(\minage,\commitment,\attest)$
+ \end{enumerate}
+ }
+ \vfill
+ \item[Requirement: Unforgeability of minimum age]
+ {\small
+ \begin{equation*}
+ \Forall_{\Adv\in\PPT(\N_\Age\times\Commitments\times\Proofs\to \N_\Age\times\Attests)}:
+ \Probability\Big[\Game{FA}(\lambda) = 1\Big] \le \negl(\lambda)
+ \end{equation*}
+ }
+ \end{description}
+\end{frame}
+
+
+\begin{frame}<1-| handout:0>{Solution: Instantiation with ECDSA}
+% \framesubtitle{Definition of Commit}
+
+ \begin{description}
+ \item[To Commit to age (group) $\age \in \{1,\dots,\Age\}$]~\\
+ \begin{enumerate}
+ \item<2-> Guardian generates ECDSA-keypairs, one per age (group):
+ \[\langle(q_1, p_1),\dots,(q_\Age,p_\Age)\rangle\]
+ \item<3-> Guardian then \textbf{drops} all private keys
+ $p_i$ for $i > \age$:
+ \[\Big \langle(q_1, p_1),\dots,
+ (q_\age, p_\age),
+ (q_{\age +1}, \red{\Nil}),\dots,
+ (q_\Age, \red{\Nil})\Big\rangle\]
+
+ \begin{itemize}
+ \item $\Vcommitment := (q_1, \dots, q_\Age)$ is the \textit{Commitment},
+ \item $\Vpruf_\age := (p_1, \dots, p_\age, \Nil,\dots,\Nil)$ is the \textit{Proof}
+ \end{itemize}
+ \vfill
+ \item<4-> Guardian gives child $\langle \Vcommitment, \Vpruf_\age \rangle$
+ \vfill
+ \end{enumerate}
+ \end{description}
+\end{frame}
+
+\begin{frame}<1-| handout:0>{Instantiation with ECDSA}
+ \framesubtitle{Definitions of Attest and Verify}
+
+ Child has
+ \begin{itemize}
+ \item ordered public-keys $\Vcommitment = (q_1, \dots, q_\Age) $,
+ \item (some) private-keys $\Vpruf = (p_1, \dots, p_\age, \Nil, \dots, \Nil)$.
+ \end{itemize}
+ \begin{description}
+ \item<2->[To \blue{Attest} a minimum age $\blue{\minage} \leq \age$:]~\\
+ Sign a message with ECDSA using private key $p_\blue{\minage}$
+ \end{description}
+
+ \vfill
+
+ \uncover<3->{
+ Merchant gets
+ \begin{itemize}
+ \item ordered public-keys $\Vcommitment = (q_1, \dots, q_\Age) $
+ \item Signature $\sigma$
+ \end{itemize}
+ \begin{description}
+ \item<4->[To \blue{Verify} a minimum age $\minage$:]~\\
+ Verify the ECDSA-Signature $\sigma$ with public key $q_\minage$.
+ \end{description}
+ }
+ \vfill
+\end{frame}
+
+\begin{frame}<1-| handout:0>{Instantiation with ECDSA}
+ \framesubtitle{Definitions of Derive and Compare}
+ Child has
+ $\Vcommitment = (q_1, \dots, q_\Age) $ and
+ $\Vpruf = (p_1, \dots, p_\age, \Nil, \dots, \Nil)$.
+ \begin{description}
+ \item<2->[To \blue{Derive} new $\Vcommitment'$ and $\Vpruf'$:]
+ Choose random $\beta\in\Z_g$ and calculate
+ \small
+ \begin{align*}
+ \Vcommitment' &:= \big(\beta * q_1,\ldots,\beta * q_\Age\big),\\
+ \Vpruf' &:= \big(\beta p_1,\ldots,\beta p_\age,\Nil,\ldots,\Nil\big)
+ \end{align*}
+ Note: $ (\beta p_i)*G = \beta*(p_i*G) = \beta*q_i$\\
+ \scriptsize $\beta*q_i$ is scalar multiplication on the elliptic curve.
+ \end{description}
+
+ \vfill
+ \uncover<3->{
+ Exchange gets $\Vcommitment = (q_1,\dots,q_\Age)$, $\Vcommitment' = (q_1', \dots, q_\Age')$ and $\beta$
+ \begin{description}
+ \item[To \blue{Compare}, calculate:]
+ \small
+ $(\beta * q_1, \ldots , \beta * q_\Age) \stackrel{?}{=} (q'_1,\ldots, q'_\Age)$
+ \end{description}
+ \vfill
+ }
+\end{frame}
+
+\begin{frame}<1-| handout:0>{Instantiation with ECDSA}
+
+ Functions
+ (Commit, Attest, Verify, Derive, Compare)\\
+ as defined in the instantiation with ECDSA\\[0.5em]
+ \begin{itemize}
+ \item meet the basic requirements,\\[0.5em]
+ \item also meet all security requirements.\\
+ Proofs by security reduction, details are in the paper.
+ \end{itemize}
+
+\end{frame}
+
+
+% \begin{frame}{Instantiation with ECDSA}
+% \framesubtitle{Full definitions}
+% \scriptsize
+%
+% \begin{align*}
+% \Commit_{E,\FDHg{\cdot}}(\age, \omega) &:= \Big\langle
+% \overbrace{(q_1,\ldots,q_\Age)}^{= \Vcommitment},\;
+% \overbrace{(p_1,\ldots,p_\age, \Nil,\ldots,\Nil)}^{= \Vpruf \text{, length }\Age}
+% \Big\rangle\\
+% \Attest_{E,\HashF}(\bage, \Vcommitment, \Vpruf) &:=
+% \begin{cases}
+% \attest_\bage := \Sign_{E,\HashF}\big(\bage,\Vpruf[\bage]\big) & \text{if } \Vpruf[\bage] \stackrel{?}{\neq} \Nil\\
+% \Nil & \text{otherwise}
+% \end{cases}\\
+% %
+% \Verify_{E,\HashF}(\bage, \Vcommitment, \attest) &:= \Ver_{E,\HashF}(\bage, \Vcommitment[\bage], \attest)\\
+% %
+% \Derive_{E, \FDHg{\cdot}}(\Vcommitment, \Vpruf, \omega) &:=
+% \Big\langle(\beta * q_1,\ldots,\beta * q_\Age),
+% (\beta p_1,\ldots,\beta p_\age,\Nil,\ldots,\Nil), \beta \Big\rangle \\
+% & \text{ with } \beta := \FDHg{\omega} \text{ and multiplication } \beta p_i \text{ modulo } g \nonumber\\
+% %
+% \Compare_E(\Vcommitment, \Vcommitment', \beta) &:=
+% \begin{cases}
+% 1 & \text{if } (\beta * q_1, \ldots , \beta * q_\Age) \stackrel{?}{=} (q'_1,\ldots, q'_\Age)\\
+% 0 & \text{otherwise}
+% \end{cases}
+% \end{align*}
+% \end{frame}
+
+
+\begin{frame}<1-| handout:0>{Reminder: GNU Taler Fundamentals}
+ \begin{center}
+ \begin{tikzpicture}[scale=.55]
+ \node[circle,fill=black!10] at (3, 4) (Exchange) {$\Exchange$};
+ \node[circle,fill=black!10] at (0, 0) (Customer) {$\Customer$};
+ \node[circle,fill=black!10] at (6, 0) (Merchant) {$\Merchant$};
+
+ \draw[<->] (Customer) to [out=65,in=220] node[sloped,above] {\sf withdraw} (Exchange);
+ \draw[<->] (Customer) to [out=45,in=240] node[sloped,below] {\sf refresh} (Exchange);
+ \draw[<->] (Customer) to node[sloped, below] {\sf purchase} (Merchant);
+ \draw[<->] (Merchant) to node[sloped, above] {\sf deposit} (Exchange);
+ \end{tikzpicture}
+ \end{center}
+
+ \vfill
+ \begin{itemize}
+ \item Coins are public-/private key-pairs $(C_p, c_s)$.
+ \item Exchange blindly signs $\FDH(C_p)$ with denomination key $d_p$
+ \item Verification:
+ \begin{eqnarray*}
+ 1 &\stackrel{?}{=}&
+ \mathsf{SigCheck}\big(\FDH(C_p), D_p, \sigma_p\big)
+ \end{eqnarray*}
+ \scriptsize($D_p$ = public key of denomination and $\sigma_p$ = signature)
+
+ \end{itemize}
+\end{frame}
+
+\begin{frame}<1-| handout:0>{Integration with GNU Taler}
+ \framesubtitle{Binding age restriction to coins}
+
+ To bind an age commitment $\commitment$ to a coin $C_p$, instead of
+ signing $\FDH(C_p)$, $\Exchange$ now blindly signs
+ \begin{center}
+ $\FDH(C_p, \orange{H(\commitment)})$
+ \end{center}
+
+ \vfill
+ Verfication of a coin now requires $H(\commitment)$, too:
+ \begin{center}
+ $1 \stackrel{?}{=}
+ \mathsf{SigCheck}\big(\FDH(C_p, \orange{H(\commitment)}), D_p, \sigma_p\big)$
+ \end{center}
+ \vfill
+\end{frame}
+
+\begin{frame}<1-| handout:0>{Integration with GNU Taler}
+ \framesubtitle{Integrated schemes}
+ \fontsize{8pt}{9pt}\selectfont
+ \begin{tikzpicture}[scale=.9]
+ \node[circle,minimum size=25pt,fill=black!15] at ( 0:0) (Client) {$\Child$};
+ \node[circle,minimum size=25pt,fill=black!15] at ( 60:5) (Exchange) {$\Exchange$};
+ \node[circle,minimum size=25pt,fill=black!15] at ( 0:5) (Merchant) {$\Merchant$};
+ \node[circle,minimum size=25pt,fill=blue!15] at (130:3) (Guardian) {$\Guardian$};
+
+ \draw[<->] (Guardian) to node[sloped,above,align=center]
+ {{\sf withdraw}\orange{, using}\\ $\FDH(C_p\orange{, H(\commitment)})$} (Exchange);
+ \draw[<->] (Client) to node[sloped,below,align=center]
+ {{\sf refresh} \orange{ + }\\ \orange{$\DeriveCompare$}} (Exchange);
+ \draw[<->] (Client) to node[sloped, below]
+ {{\sf purchase} \blue{+ $(\attest_\minage, \commitment)$}} (Merchant);
+ \draw[<->] (Merchant) to node[sloped, above]
+ {{\sf deposit} \orange{+ $H(\commitment)$}} (Exchange);
+
+ \draw[->] (Guardian) to [out=70,in=150, loop] node[above]
+ {$\Commit(\age)$} (Guardian);
+ \draw[->] (Guardian) to node[below,sloped]
+ {($\commitment$, $\pruf_\age$)} (Client);
+ \draw[->,blue] (Client) to [out=-50,in=-130, loop] node[below]
+ {\blue{$\Attest(\minage, \commitment, \pruf_{\age})$}} (Client);
+ \draw[->,blue] (Merchant) to [out=-50,in=-130, loop] node[below]
+ {\blue{$\Verify(\minage, \commitment, \attest_{\minage})$}} (Merchant);
+ \end{tikzpicture}
+\end{frame}
+
+
+\begin{frame}<1-| handout:0>{Instantiation with Edx25519}
+ Paper also formally defines another signature scheme: Edx25519.\\[1em]
+
+ \begin{itemize}
+ \item Scheme already in use in GNUnet,
+ \item based on EdDSA (Bernstein et al.),
+ \item generates compatible signatures and
+ \item allows for key derivation from both, private and public keys, independently.
+ \end{itemize}~\\[1em]
+
+ Current implementation of age restriction in GNU Taler uses Edx25519.
+\end{frame}
+
+
+\begin{frame}{Age Restrictions based on KYC}
+% \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
+ \begin{itemize}
+ \item Assumption: banks provide minimum age
+ information during bank
+ transactions.
+ \item Child and Exchange execute a variant of
+ the cut\&choose protocol.
+ \end{itemize}
+ \end{itemize}
+\end{frame}
+
+
+\begin{frame}<1-| handout:0>{Related Work}
+ \begin{itemize}
+ \item Current privacy-perserving systems all based on attribute-based credentials (Koning et al., Schanzenbach et al., Camenisch et al., Au et al.)
+ \item Attribute-based approach lacks support:
+ \begin{itemize}
+ \item Complex for consumers and retailers
+ \item Requires trusted third authority
+ \end{itemize}
+ \vfill
+ \item Other approaches tie age-restriction to ability to pay ("debit cards for kids")
+ \begin{itemize}
+ \item Advantage: mandatory to payment process
+ \item Not privacy friendly
+ \end{itemize}
+ \end{itemize}
+\end{frame}
+
+\begin{frame}{Conclusion}
+ Age restriction is a technical, ethical and legal challenge.
+
+ Existing solutions are
+ \begin{itemize}
+ \item without strong protection of privacy or
+ \item based on identity management systems (IMS)
+ \end{itemize}
+ \vfill
+
+ Our 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{Measures against Advanced Attacks}
+
+\begin{frame}
+ \vfill
+ \begin{center}
+ {\bf Part VI: Measures against Advanced Attacks}
+ \end{center}
+ \vfill
+\end{frame}
+
+
+\begin{frame}{Warranting deposit safety}
+ Exchange has online signing key $W = wG$:
+ \begin{center}
+ Sends $EdDSA_w(M,H(D),FDH(C))$ to the merchant.
+ \end{center}
+ This signature means that $M$ was the {\em first} to deposit
+ $C$ and that the exchange thus must pay $M$.
+ \vfill
+ \begin{center}
+ Without this, a malicious exchange could renege on the deposit
+ confirmation and claim double-spending if a coin were
+ deposited twice, and then not pay either merchant!
+ \end{center}
+\end{frame}
+
+
+\begin{frame}{Key management}
+Taler has many types of keys:
+\begin{itemize}
+\item Coin keys
+\item Denomination keys
+\item Online message signing keys
+\item Offline key signing keys
+\item Merchant keys
+\item Auditor key
+\item Security module keys
+\item Transfer keys
+\item Wallet keys
+\item {\em TLS keys, DNSSEC keys}
+\end{itemize}
+\end{frame}
+
+
+\begin{frame}{Offline keys}
+Both exchange and auditor use offline keys.
+\begin{itemize}
+\item Those keys must be backed up and remain highly confidential!
+\item We recommend that computers that have ever had access to those
+ keys to NEVER again go online.
+\item We recommend using a Raspberry Pi for offline key operations.
+ Store it in a safe under multiple locks and keys.
+\item Apply full-disk encryption on offline-key signing systems.
+\item Have 3--5 full-disk backups of offline-key signing systems.
+\end{itemize}
+\begin{center}
+\includegraphics[scale=0.1]{pi.png}
+\end{center}
+\end{frame}
+
+
+\begin{frame}{Protecting online keys}
+The exchange needs keys to be available for online signing.
+\begin{itemize}
+\item {\tt taler-exchange-secmod-\{cs,eddsa,rsa\}}
+ are the only processes that must have access to the private keys.
+\item The secmod processes should run under a different UID, but share
+ the same GID with the exchange.
+\item The secmods generate the keys, allow {\tt taler-exchange-httpd} to sign with
+ them, and eventually delete the private keys.
+\item Communication between secmods and {\tt taler-exchange-httpd} is via
+ a UNIX domain socket.
+\item Online private keys are stored on disk (not in database!) and should
+ NOT be backed up (RAID should suffice). If disk is lost, we can always
+ create fresh replacement keys!
+\end{itemize}
+\end{frame}
+
+
+\begin{frame}{Online keys}
+\begin{center}
+\includegraphics[width=0.9\textwidth]{taler-diagram-signatures.png}
+\end{center}
+\end{frame}
+
+\begin{frame}{Online keys}
+The exchange needs keys to be available for online signing:
+\begin{itemize}
+\item Knowledge of these private keys will allow an adversary to
+ mint digital cash, possibly resulting in financial losses
+% (eventually, this will be detected by the auditor, but only
+% after some financial losses have been irrevocably incurred).
+\item The corresponding public keys are certified using
+ Taler's public key infrastructure (which uses offline-only keys).
+\end{itemize}
+\vfill
+{\tt taler-exchange-offline} can also be used to {\bf revoke} the
+online signing keys, if we find they have been compromised.
+\vfill
+\end{frame}
+
+
+\begin{frame}{Online keys}
+\begin{itemize}
+\item The exchange needs $d$ and $w$ to be available for online signing.
+\item The corresponding public keys $W$ and $(e,n)$ are certified using
+ Taler's public key infrastructure (which uses offline-only keys).
+\end{itemize}
+\vfill
+\begin{center}
+{\bf What happens if those private keys are compromised?}
+\end{center}
+\vfill
+\end{frame}
+
+
+\begin{frame}{Denomination key $(e,n)$ compromise}
+\begin{itemize}
+\item An attacker who learns $d$ can sign an arbitrary number of illicit coins
+ into existence and deposit them.
+\item Auditor and exchange can detect this once the total number of deposits
+ (illicit and legitimate) exceeds the number of legitimate coins the
+ exchange created.
+\item At this point, $(e,n)$ is {\em revoked}. Users of {\em unspent}
+ legitimate coins reveal $b$ from their withdrawal operation and
+ obtain a {\em refund}.
+\item The financial loss of the exchange is {\em bounded} by the number of
+ legitimate coins signed with $d$.
+\item[$\Rightarrow$] Taler frequently rotates denomination signing keys and
+ deletes $d$ after the signing period of the respective key expires.
+\end{itemize}
+\begin{center}
+\includegraphics[width=0.5\textwidth]{taler-diagram-denom-expiration.png}
+\end{center}
+\end{frame}
+
+
+\begin{frame}{Online signing key $W$ compromise}
+\begin{itemize}
+\item An attacker who learns $w$ can sign deposit confirmations.
+\item Attacker sets up two (or more) merchants and customer(s) which double-spend
+ legitimate coins at both merchants.
+\item The merchants only deposit each coin once at the exchange and get paid once.
+\item The attacker then uses $w$ to fake deposit confirmations for the double-spent
+ transactions.
+\item The attacker uses the faked deposit confirmations to complain to the auditor
+ that the exchange did not honor the (faked) deposit confirmations.
+\end{itemize}
+The auditor can then detect the double-spending, but cannot tell who is to blame,
+and (likely) would presume a malicious exchange, forcing it to pay both merchants.
+\end{frame}
+
+
+\begin{frame}{Detecting online signing key $W$ compromise}
+\begin{itemize}
+\item Merchants are required to {\em probabilistically} report
+ signed deposit confirmations to the auditor.
+\item Auditor can thus detect exchanges not reporting signed
+ deposit confirmations.
+\item[$\Rightarrow$] Exchange can rekey if illicit key use is detected,
+ then only has to honor deposit confirmations it already provided
+ to the auditor {\em and} those without proof of double-spending
+ {\em and} those merchants reported to the auditor.
+\item[$\Rightarrow$] Merchants that do not participate in reporting
+ to the auditor risk their deposit permissions being voided in
+ cases of an exchange's private key being compromised.
+\end{itemize}
+\end{frame}
+
+
+\begin{frame}{Database}
+The exchange needs the database to detect double spending.
+\begin{itemize}
+\item Loss of the database will allow technically skilled people
+ to double-spend their digital cash, possibly resulting in
+ significant financial losses.
+\item The database contains total amounts customers withdrew and
+ merchants received, so sensitive private banking data. It
+ must thus not become public.
+\item The auditor must have a (current) copy. Asynchronous replication
+ should be sufficient. This copy can also serve as an
+ additional (off-site?) backup.
+\end{itemize}
+\begin{center}
+ The database can also be replaced with a DLT if customer
+ requires it.
+\end{center}
+\end{frame}
+
+
+\section{Component Architecture}
+
+\begin{frame}
+ \vfill
+ \begin{center}
+ {\bf Part VII: Component Architecture}
+ \end{center}
+ \vfill
+\end{frame}
+
+
+\begin{frame}[fragile]{Taler: Bank Perspective}
+\begin{adjustbox}{max totalsize={.9\textwidth}{.7\textheight},center}
+\begin{tikzpicture}
+ \tikzstyle{def} = [node distance= 5em and 6.5em, inner sep=1em, outer sep=.3em];
+ \node (origin) at (0,0) {};
+ \node (exchange) [def,above=of origin,draw]{Exchange};
+ \node (nexus) [def, draw, below right=of exchange] {Nexus};
+ \node (corebanking) [def, draw, below left=of nexus] {Core Banking};
+ \node (nginx) [def, draw, above=of exchange]{Nginx};
+ \node (postgres) [def, draw, below left=of exchange]{Postgres};
+ \node (postgres-nexus) [def, draw, below right=of nexus]{Postgres};
+
+ \tikzstyle{C} = [color=black, line width=1pt]
+
+ \draw [<-, C] (exchange) -- (nginx) node [midway, above, sloped] (TextNode) {REST API};
+ \draw [<-, C] (postgres) -- (exchange) node [midway, above, sloped] (TextNode) {SQL};
+ \draw [<-, C] (postgres-nexus) -- (nexus) node [midway, above, sloped] (TextNode) {SQL};
+ \draw [<-, C] (nexus) -- (exchange) node [midway, above, sloped] (TextNode) {Internal REST API};
+ \draw [<-, C] (corebanking) -- (nexus) node [midway, above, sloped] (TextNode) {EBICS/FinTS};
+
+\end{tikzpicture}
+\end{adjustbox}
+\end{frame}
+
+
+\begin{frame}{Taler: Exchange Architecture}
+\begin{center}
+\begin{tikzpicture}
+ \tikzstyle{def} = [node distance=2em and 2.5em, inner sep=1em, outer sep=.3em];
+ \node (origin) at (0,0) {};
+ \node (httpd) [def,above=of origin,draw]{httpd};
+ \node (secmod-rsa) [def, draw, right=of httpd] {secmod-rsa};
+ \node (secmod-eddsa) [def, draw, left=of httpd] {secmod-eddsa};
+ \node (postgres) [def, draw, below=of httpd]{Postgres};
+ \node (aggregator) [def, draw, right=of postgres]{aggregator};
+ \node (transfer) [def, draw, below left=of postgres]{transfer};
+ \node (wirewatch) [def, draw, below right=of postgres]{wirewatch};
+ \node (nexus) [def, draw, below=of postgres]{Nexus};
+
+ \tikzstyle{C} = [color=black, line width=1pt]
+
+ \draw [<->, C] (httpd) -- (postgres) node [midway, above, sloped] (TextNode) {};
+ \draw [<->, C] (httpd) -- (secmod-rsa) node [midway, above, sloped] (TextNode) {};
+ \draw [<->, C] (httpd) -- (secmod-eddsa) node [midway, above, sloped] (TextNode) {};
+ \draw [<->, C] (aggregator) -- (postgres) node [midway, above, sloped] (TextNode) {};
+ \draw [<->, C] (wirewatch) -- (postgres) node [midway, above, sloped] (TextNode) {};
+ \draw [<->, C] (transfer) -- (postgres) node [midway, above, sloped] (TextNode) {};
+ \draw [->, C] (transfer) -- (nexus) node [midway, above, sloped] (TextNode) {};
+ \draw [<-, C] (wirewatch) -- (nexus) node [midway, above, sloped] (TextNode) {};
+\end{tikzpicture}
+\end{center}
+\end{frame}
+
+
+\begin{frame}
+\frametitle{Taler: Auditor Perspective}
+\begin{center}
+\begin{tikzpicture}
+ \tikzstyle{def} = [node distance=2em and 2.5em, inner sep=1em, outer sep=.3em];
+ \node (origin) at (0,0) {};
+ \node (httpd) [def,above left=of origin,draw]{auditor-httpd};
+ \node (report) [def,above right=of origin,draw]{auditor-report};
+ \node (postgres-A) [def, draw, below=of origin] {Postgres (Auditor)};
+ \node (postgres-E) [def, draw, below=of postgres-A] {Postgres (Exchange)};
+
+ \tikzstyle{C} = [color=black, line width=1pt]
+
+ \draw [->, C] (postgres-E) -- (postgres-A) node [midway, above, sloped] (TextNode) {sync};
+ \draw [<->, C] (httpd) -- (postgres-A) node [midway, above, sloped] (TextNode) {};
+ \draw [<->, C] (report) -- (postgres-A) node [midway, above, sloped] (TextNode) {};
+\end{tikzpicture}
+\end{center}
+\end{frame}
+
+
+\begin{frame}
+\frametitle{Taler: Merchant Perspective}
+\begin{center}
+\begin{tikzpicture}
+ \tikzstyle{def} = [node distance= 3.5em and 2em, inner sep=1em, outer sep=.3em];
+ \node (origin) at (0,0) {};
+ \node (backend) [def,above=of origin,draw]{{\tiny taler-merchant-httpd}};
+ \node (frontend) [def,above left=of backend,draw]{{\tiny E-commerce Frontend}};
+ \node (backoffice) [def,above right=of
+backend,draw]{\tiny Backoffice};
+ \node (postgres) [def, draw, below left=of backend] {\tiny Postgres};
+ \node (sqlite) [def, draw, below=of backend] {\tiny Sqlite};
+ \node (alt) [def, draw, below right=of backend] {...};
+
+ \tikzstyle{C} = [color=black, line width=1pt]
+
+ \draw [->, C] (frontend) -- (backend) node [midway, above, sloped]
+(TextNode) {\tiny REST API};
+ \draw [->, C] (backoffice) -- (backend) node [midway, above, sloped]
+(TextNode) {\tiny REST API};
+ \draw [<->, C] (backend) -- (postgres) node [midway, above, sloped]
+(TextNode) {\tiny SQL};
+ \draw [<->, C] (backend) -- (sqlite) node [midway, above, sloped]
+(TextNode) {\tiny SQL};
+ \draw [<->, C] (backend) -- (alt) node [midway, above, sloped]
+(TextNode) {\tiny SQL};
+\end{tikzpicture}
+\end{center}
+\end{frame}
+
+
+\begin{frame}
+\frametitle{Taler: Wallet Architecture}
+\begin{center}
+\begin{tikzpicture}
+ \tikzstyle{def} = [node distance= 5em and 4.5em, inner sep=1em, outer sep=.3em];
+ \node (origin) at (0,0) {};
+ \node (gui) [def,above=of origin,draw]{wallet-gui};
+ \node (core) [def,below=of gui,draw]{wallet-core};
+ \node (sync) [def, draw, below left=of core] {Sync};
+ \node (taler) [def, draw, below right=of core] {Taler};
+ \node (anastasis) [def, draw, below=of core] {Anastasis};
+
+ \tikzstyle{C} = [color=black, line width=1pt]
+ \draw [<->, C] (gui) -- (core) node [midway, above, sloped] (TextNode) {};
+ \draw [<->, C] (core) -- (sync) node [midway, above, sloped] (TextNode) {Backup};
+ \draw [<->, C] (core) -- (taler) node [midway, above, sloped] (TextNode) {Payment};
+ \draw [<->, C] (core) -- (anastasis) node [midway, above, sloped] (TextNode) {Key Escrow};
+\end{tikzpicture}
+\end{center}
+\end{frame}
+
+
+\begin{frame}[t]{Software architecture for the Taler Snack Machine}
+ \framesubtitle{Code at \url{https://git.taler.net/taler-mdb}}
+\begin{figure}
+ \centering
+ \includegraphics[width=.9\textwidth]{software_stack}
+ \end{figure}
+\end{frame}
+
+
+
+\section{Integration considerations}
+
+\begin{frame}
+ \vfill
+ \begin{center}
+ {\bf Part VIII: 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 \pause
+Largely implemented, only UI support missing. Expected to ship in Q1'2023.
+\end{frame}
+
+
+\section{Blockchain Integration}
+
+\begin{frame}
+ \vfill
+ \begin{center}
+ {\bf Part IX: 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}<1-| handout:0>{Taler}{Architecture}
+ \begin{columns}
+ \column{0.5\paperwidth}
+ \begin{tikzpicture}[
+ rect/.style={circle, draw=black},
+ sym/.style={-stealth, shorten >= 2pt, shorten <= 2pt}
+ ]
+ % Taler payment system
+ \node[rect](1) {Exchange};
+ \node[rect,below left=1.5cm and 0.7cm of 1](2) {Customer};
+ \node[rect,below right=1.5cm and 0.7cm of 1](3) {Merchant};
+
+ \draw[sym] (1) -- node [midway, above, sloped] {\tiny Withdraw coins} (2);
+ \draw[sym] (2) -- node [midway, above, sloped] {\tiny Spend coins} (3);
+ \draw[sym] (3) -- node [midway, above, sloped] {\tiny Deposit coins} (1);
+
+ % Settlement layer
+ \node[left=2cm of 1](E1){};
+ \node[right=2cm of 1](E2){};
+ \draw[sym] (E1) -- node [midway, above] {\tiny Deposit money} (1);
+ \draw[sym] (1) -- node [midway, above] {\tiny Withdraw money} (E2);
+
+ % Auditor
+ \node[above= of 1](A){Auditor};
+ \draw[sym] (A) -- node [midway, right] {\tiny Verify} (1);
+
+ % Separator
+ \node[below=1mm of E1] (S1S) {};
+ \node[below=1mm of E2] (S1E) {};
+ \node[above=6mm of E1] (S2S) {};
+ \node[above=6mm of E2] (S2E) {};
+
+ \draw[dotted] (S1S) -- (S1E);
+ \draw[dotted] (S2S) -- (S2E);
+
+ \node[below right=-2mm and -1.5mm of S2S] {\tiny{\emph{Settlement Layer}}};
+ \node[below right=-2mm and -1.5mm of S1S] {\tiny{\emph{Taler payment system}}};
+ \end{tikzpicture}
+ \column{0.47\paperwidth}
+ \begin{block}{Settlement layer}
+ \begin{itemize}
+ \item RTGS $\equiv$ Blockchain!
+ \end{itemize}
+ \end{block}
+ \begin{block}{Taler payment system}
+ \begin{itemize}
+ \item Realtime transactions, 1 RTT
+ \item Scalable microtransactions
+ \item Blind signatures (privacy)
+ \end{itemize}
+ \end{block}
+
+ \end{columns}
+\end{frame}
+
+\begin{frame}{Taler}{Blockchain settlement layer}
+ \begin{center}
+ \begin{tikzpicture}[
+ rect/.style={rectangle, draw=black, minimum width=30mm},
+ sym/.style={stealth-stealth, shorten >= 2pt, shorten <= 2pt},
+ block/.style={rectangle,draw=black,fill=black!10,minimum size=7mm},
+ ]
+
+ %% Architecture
+ \node(Tt){Taler};
+ \node[rect,below=0cm of Tt](Tc){Exchange};
+ \node[rect,fit={(Tt) (Tc)}](T){};
+
+ \node[rect,below=7mm of Tc](D) {\textbf{Depolymerization}};
+
+ \node[rect,below=7mm of D](Bc){Node};
+ \node[below=0cm of Bc](Bt){Blockchain};
+ \node[rect,fit={(Bt) (Bc)}](B){};
+
+ \draw[sym] (T) -- (D);
+ \draw[sym] (D) -- (B);
+
+ %% Blockchain
+ \node[block,right=8mm of B] (1){};
+ \node[block,right=4mm of 1] (2){};
+ \node[block,right=4mm of 2] (3){};
+ \node[block,right=4mm of 3] (4){};
+ \node[block,right=4mm of 4] (5){};
+ \node[block,right=4mm of 5] (6){};
+ \draw[-stealth] (1) -- (2);
+ \draw[-stealth] (2) -- (3);
+ \draw[-stealth] (3) -- (4);
+ \draw[-stealth] (4) -- (5);
+ \draw[-stealth] (5) -- (6);
+
+ \node[left=4mm of 1] (S){};
+ \node[right=4mm of 6] (E){};
+ \draw[-stealth] (S) -- (1);
+ \draw[-stealth] (6) -- (E);
+
+ %% Taler
+ \node[block, below right=-7.5mm and 20.5mm of T] (off){Off-chain transactions};
+ \node[above=-0.5mm of off] {\includegraphics[height=7mm]{taler-logo-2021-inkscape.pdf}};
+
+ %% Depolymerization
+ \node[right=11mm of D] {\small{Credit}};
+ \node[right=50mm of D] {\small{Debit}};
+ \draw[dashed,-stealth] (1.north) |- (off.west);
+ \draw[dashed,-stealth] (off.east) -| (6.north);
+ \end{tikzpicture}
+ \end{center}
+\end{frame}
+
+\begin{frame}<1-| handout:0>{Challenges}
+ \begin{block}{Taler Metadata}
+ \begin{itemize}
+ \item Metadata are required to link a wallet to credits and
+ allow merchant to link deposits to debits
+ \item Putting metadata in blockchain transactions can be tricky
+ \end{itemize}
+ \end{block}
+ \begin{block}{Blockchain based cryptocurrencies}
+ \begin{itemize}
+ \item Blockchain transactions lack finality (fork)
+ \item Transactions can be stuck for a long time (mempool)
+ \end{itemize}
+ \end{block}
+\end{frame}
+
+\begin{frame}<1-| handout:0>{Blockchain challenges}{Chain reorganization}
+ \begin{center}
+ \begin{tikzpicture}[
+ block/.style={rectangle,draw=black,fill=black!10,minimum size=7mm},
+ ar/.style={-stealth}
+ ]
+ % Common
+ \node[block](1){};
+ \node[block,right=5mm of 1](2){$D_0$};
+ \node[block,right=5mm of 2](3){};
+ \draw[ar] (1) -- (2);
+ \draw[ar] (2) -- (3);
+
+ % Current
+ \node [block,right=5mm of 3](4){};
+ \node[block,right=5mm of 4](5){};
+ \node[block,right=5mm of 5](6){$D_1$};
+ \draw[ar] (3) -- (4);
+ \draw[ar] (4) -- (5);
+ \draw[ar] (5) -- (6);
+
+ % Fork
+ \node [block,above=7mm of 4](4p){};
+ \node[block,right=5mm of 4p](5p){$D_2$};
+ \node[block,right=5mm of 5p](6p){};
+ \node[block,right=5mm of 6p](7p){};
+ \draw[ar] (3.east) -- (4p.west);
+ \draw[ar] (4p) -- (5p);
+ \draw[ar] (5p) -- (6p);
+ \draw[ar] (6p) -- (7p);
+
+ % Indication
+ \node [right=5mm of 7p]{\emph{fork}};
+ \node [right=17mm of 6]{\emph{active}};
+ \end{tikzpicture}
+ \end{center}
+ A fork is when concurrent blockchain states coexist. Nodes will follow
+ the longest chain, replacing recent blocks if necessary during a
+ blockchain reorganization. If a deposit transaction disappears from the
+ blockchain, an irrevocable withdraw transactions would no longer be backed
+ by credit.
+\end{frame}
+
+\begin{frame}<1-| handout:0>{Blockchain challenges}{Stuck transactions}
+ We want confirmed debits within a limited time frame.
+ \begin{figure}
+ \centering
+ \only<1> {
+ \begin{tikzpicture}[
+ dot/.style={circle,fill,inner sep=1pt,}
+ ]
+ \node (I) {\includegraphics[width=\textwidth]{media/fee.png}};
+ \node [below left=-2.5mm and -1.5cm of I] (Tx) {\small Tx};
+ \node [dot,above=8.4mm of Tx](D) {};
+ \draw [dotted,thick] (Tx) -- (D);
+ \node [left=-4.5cm of Tx] (C) {\small conf};
+ \node [dot,above=8.4mm of C](D1) {};
+ \draw [dotted,thick] (C) -- (D1);
+ \end{tikzpicture}
+ }
+ \only<2> {
+ \includegraphics[width=\textwidth]{media/fee_var.png}
+ \caption{Bitcoin average transaction fee over 6 months {\tiny (ychart)}}
+ }
+ \end{figure}
+ \only<1>{When we trigger a debit with a fee too small, it may not be
+ confirmed in a timely fashion.}
+ \only<2>{However, transaction fees are unpredictable.}
+\end{frame}
+
+
+\begin{frame}{Depolymerization}{Architecture}
+ \begin{center}
+ \begin{tikzpicture}[
+ rect/.style={rectangle, draw=black, minimum height=6mm, minimum width=28mm},
+ sym/.style={stealth-stealth, shorten >= 2pt, shorten <= 2pt}
+ ]
+ \node[rect](1) {Taler Exchange};
+ \node[rect,below=of 1](2) {Wire Gateway};
+ \node[rect,right=of 2](3) {PostgreSQL};
+ \node[rect,right=of 3](4) {DLT Adapter};
+ \node[rect,above=of 4](5) {DLT Full Node};
+
+ \draw[sym] (1) -- node [midway,right] {\tiny HTTP} (2);
+ \draw[sym] (2) -- node [midway,above] {\tiny SQL} (3);
+ \draw[sym] (3) -- node [midway,above] {\tiny SQL} (4);
+ \draw[sym] (4) -- node [midway,left ] {\tiny RPC} (5);
+
+
+ \node[above= 2mm of 1]{\small{\emph{Wire Gateway API}}};
+ \node[above= 2mm of 5]{\small{\emph{DLT specific}}};
+ \node[above=22mm of 3](T) {};
+ \draw[dotted] (3) -- (T);
+ \end{tikzpicture}
+ \end{center}
+ \begin{itemize}
+ \item Common database to store transactions state and communicate
+ with notifications
+ \item Wire Gateway for Taler API compatibility
+ \item DLT specific adapter
+ \end{itemize}
+\end{frame}
+
+\begin{frame}{Storing metadata}{Bitcoin}
+ \begin{block}{Bitcoin - Credit}
+ \begin{itemize}
+ \item Transactions from code
+ \item Only 32B + URI
+ \item \textbf{OP\_RETURN}
+ \end{itemize}
+ \end{block}
+ \begin{block}{Bitcoin - Debit}
+ \begin{itemize}
+ \item Transactions from common wallet software
+ \item Only 32B
+ \item \textbf{Fake Segwit Addresses}
+ \end{itemize}
+ \end{block}
+\end{frame}
+\begin{frame}{Storing metadata}{Ethereum}
+ \begin{block}{Smart 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}<1-| handout:0>{DLT Adapter}{Architecture}
+ \begin{block}{Event system}
+ \begin{itemize}
+ \item \textbf{Watcher} watch and notify for new blocks with credits
+ \item \textbf{Wire Gateway} notify requested debits
+ \item \textbf{Worker} operates on notifications updating state
+ \end{itemize}
+ \end{block}
+\end{frame}
+
+
+\begin{frame}<1-| handout:0>{DLT Adapter state machine}
+ \begin{columns}
+ \column{0.5\paperwidth}
+ \begin{figure}
+ \begin{tikzpicture}[
+ rect/.style={rectangle, draw=black, minimum height=6mm, minimum width=50mm},
+ ]
+
+ \node[rect](wo1) {Wait for notifications};
+ \node[rect, below=4mm of wo1](wo2) {Reconcile local DB with DLT};
+ \node[rect, below=4mm of wo2](wo3) {Trigger debits};
+ \node[rect, below=4mm of wo3](wo4) {Reissue stuck debits};
+ \node[rect, below=4mm of wo4](wo5) {Bounce malformed credits};
+ \draw[-stealth] (wo1) -- (wo2);
+ \draw[-stealth] (wo2) -- (wo3);
+ \draw[-stealth] (wo3) -- (wo4);
+ \draw[-stealth] (wo4) -- (wo5);
+ \draw[-stealth] (wo5) .. controls ([xshift=-0.4cm] wo5.west) and ([xshift=-0.4cm] wo1.west) .. (wo1);
+ \end{tikzpicture}
+ \caption{Worker loop}
+ \end{figure}
+ \column{0.47\paperwidth}
+ \begin{block}{DLT reconcialisation}
+ \begin{itemize}
+ \item List new and removed transactions since last reconciliation
+ \item Check for confirmed credits removal
+ \item Register new credits
+ \item Recover lost debits
+ \end{itemize}
+ \end{block}
+ \end{columns}
+\end{frame}
+
+\begin{frame}<1-| handout:0>{Related work}
+ \begin{block}{Centralization - Coinbase off-chain sending}
+ \begin{itemize}
+ \item [$+$] Fast and cheap: off chain transaction
+ \item [$-$] Trust in Coinbase: privacy, security \& transparency
+ \end{itemize}
+ \end{block}
+ \begin{block}{Layering - Lightning Network}
+ \begin{itemize}
+ \item [$+$] Fast and cheap: off-chain transactions
+ \item [$-$] Requires setting up bidirectional payment channels
+ \item [$-$] Fraud attempts are mitigated via a complex penalty system
+ \end{itemize}
+ \end{block}
+\end{frame}
+
+\begin{frame}{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}
+%Future work:
+% \begin{itemize}
+% \item Universal auditability, using sharded transactions history
+% \item Smarter analysis, update confirmation delay based on currency network behavior
+% \item Multisig by multiple operator for transactions validation
+% \end{itemize}
+\end{frame}
+
+
+\section{Conclusion}
+
+\begin{frame}
+ \vfill
+ \begin{center}
+ {\bf Part X: Conclusion}
+ \end{center}
+ \vfill
+\end{frame}
+
+
+\begin{frame}{Trust Earned on Multiple Levels}
+\begin{itemize}
+\item Free/Libre software with published external code reviews.
+\item The Swiss National Bank published white paper:
+ ``How to issue a CBDC'' on their website based on Taler technology.
+\item Taler endorsed by the Austrian National Bank in their Q2
+ publication as potential {\em Digital Euro} solution.
+\item The EU Commission has issued a Seal of Excellence to Taler
+ Systems SA
+\item FINMA Switzerland had no objections to launch (planned for Q3'2023)
+\end{itemize}
+\end{frame}
+
+
+\begin{frame}{Features we did NOT discuss in-depth}
+ \begin{description}
+ \item[{\bf taler-exchange-secmod-cs:}] Clause-Schnorr blind signature support
+ \item[{\bf Fakebank:}] high-performance in-memory RTGS emulator
+ \item[{\bf libbrandt:}] Escrow-based programmability extensions (e.g. for auctions)
+ \item[{\bf twister}:] Man-in-the-middle fault-injection for testing
+ \item[{\bf mch}:] Taler for embedded devices ({\bf WiP})
+ \end{description}
+\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}<1-| handout:0>{Development Infrastructure}
+ \begin{itemize}
+ \item Borg: incremental backup
+ \item Buildbot: CI/CD
+ \item Davical: Caldav group calendar
+ \item Docker: virtualization, packaging
+ \item Git/Gitolite: distributed version control
+ \item Mailman: public e-mail lists
+ \item Mantis: bug tracker
+ \item Mattermost: messaging, process management
+ \item Sphinx: documentation generation (HTML, PDF, info, man)
+ \item Weblate: collaborative AI-supported internationalization
+ \end{itemize}
+\end{frame}
+
+
+\begin{frame}<1-| handout:0>{Development Tools}
+ \begin{itemize}
+ \item Coverity: static analysis (C/C++)
+ \item GNU recutils: constant registration
+ \item Twister: fault injection
+ \item Valgrind: dynamic analysis (C/C++)
+ \item zzuf: fuzzing
+ \end{itemize}
+\end{frame}
+
+
+\begin{frame}<1-| handout:0>{Cryptographic dependencies}
+ \begin{itemize}
+ \item libargon2
+ \item libgcrypt
+ \item libsodium
+ \end{itemize}
+\end{frame}
+
+
+\begin{frame}<1-| handout:0>{Additional dependencies}
+ \begin{itemize}
+ \item libsqlite3
+ \item libpq / Postgres
+ \item libjansson
+ \item libcurl
+ \item libunistring
+ \item {\bf GNU libmicrohttpd}
+ \item {\bf GNUnet}
+ \end{itemize}
+\end{frame}
+
+
+\begin{frame}{Rights}
+ \begin{itemize}
+ \item GNUnet e.V. shared copyrights of their AGPLv3+ licensed code with Taler Systems SA
+ \item Taler Systems SA holds copyrights to entire GNU Taler code base (AGPLv3+, GPLv3+,
+ dual-licensing exclusive domain of Taler Systems SA)
+ \item Taler Systems SA applied for patent on offline payment solution
+ \item Taler Systems SA holds trademark on ``Taler''.
+ \item FSF holds trademark on ``GNU'', we are authorized to use ``GNU Taler''.
+ \item Taler Systems SA owns {\tt taler.net} and {\tt taler-systems.com}.
+ \end{itemize}
+\end{frame}
+
+
+\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 BFH \& SBB}
+\framesubtitle{Discussion Proposal}
+{\small
+With an SBB machine, we could:
+\begin{itemize}
+\item Pay for SBB tickets with GNU Taler
+\item[$\Rightarrow$] SBB would receive money in regular bank account
+\item[$\Rightarrow$] Lower costs than with CC / physical cash
+\item[$\Rightarrow$] Customers would pay with privacy
+\item Convert physical cash to digital cash
+\item[$\Rightarrow$] SBB would need Taler ``reserve'' to withdraw from
+\item[$\Rightarrow$] Regulatory requirements (SMS, withdraw limits)
+\item[$\Rightarrow$] SBB would likely want to charge service fees
+\end{itemize}
+Steps:
+\begin{enumerate}
+\item SBB provides BFH with access \& documentation
+\item BFH researchers and students would do integration
+\item Pilot machine could likely be set up around here
+\item SBB would have final decision on any rollout
+\end{enumerate}
+}
+\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}
