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\title{GNU Taler}
\subtitle{Deployment Plan 2021}

\setbeamertemplate{navigation symbols}{\includegraphics[width=1cm]{inria.pdf} \includegraphics[width=0.5cm]{gnu.png} \includegraphics[width=0.5cm]{ashoka.png}\hfill}
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\author{F. Dold, C. Grothoff}
\date{\today}
\institute{Taler Systems SA}


\begin{document}

\justifying

\begin{frame}
  \begin{center}
    \Large {\bf GNU Taler as a CBDC solution}
    \vfill
    \includegraphics[width=0.66\textwidth]{logo-2020.jpg}
    \vfill
  \end{center}
\begin{textblock*}{4cm}(.5cm,6.7cm) % {block width} (coords)
  {\Large {\bf \href{https://taler-systems.com/}{taler-systems.com}} \\
    Twitter: @taler \\
    mail@taler.net }
\end{textblock*}

% Substitute based on who is giving the talk!
 \begin{textblock*}{6cm}(6.7cm,6.7cm) % {block width} (coords)
   {\hfill {\Large {\bf Christian Grothoff} \\
    \hfill {\bf Leon Schumacher} \\
    \hfill {\bf Florian Dold}} \\
    \hfill \{grothoff,schumacher,dold\}@taler.net }
\end{textblock*}

\end{frame}


\section{Introduction}

\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 \emph{not} a speculative asset / ``get-rich-quick scheme''
  \end{itemize}
  \vfill
\end{frame}

\begin{frame}{Taler: Payment System Architecture}
  \vfill
\begin{center}
\includegraphics[width=1\textwidth]{operations.png}
\end{center}
\end{frame}

\begin{frame}{The Taler Software Ecosystem}
  \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 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 Back-office interface for staff
        \item 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
        \item {\bf Anastasis}: Recovery of lost wallets based on secret splitting
      \end{itemize}
  \end{itemize}
\end{frame}


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

\begin{frame}{Taler: Project Status}
\framesubtitle{\url{https://docs.taler.net/}}
\begin{itemize}
    \item Cryptographic protocols and core exchange component are stable
    \item Current focus: Merchant integration, settlement integration, wallet backup
    \item Pilot project at Bern University of Applied Sciences cafeteria
    \item Internal alpha deployment with a commercial bank in progress
  \end{itemize}
\end{frame}


\begin{frame}{Next Steps: Possible Projects and Collaborations}
  \vfill
\begin{center}
\includegraphics[width=1.0\textwidth]{taler-in-use.png}
\end{center}
\end{frame}

\begin{frame}{Area I: System Integration and Partnerships}
  \framesubtitle{\url{https://lists.gnu.org/mailman/listinfo/taler}}
  Pilots with CB and international CB support organizations could:
  \begin{itemize}
    \item Study integration with the underlying RTGS layer:
  \begin{itemize}
    \item Develop standardized operational procedures
    \item Assess transaction performance at scale
    \item Perform cost analysis in CB environment
    \item Assess effort for integration with commercial banks
  \end{itemize}
    \item Analyze regulatory considerations for different legislations
%    \item Building awareness of Taler as a bearer-based retail CBDC
    \item Perform independent security audits of Taler components
    \item Determine and possibly close gaps in the existing solution
  \end{itemize}
\end{frame}


\begin{frame}{Area II: Development/Research Extensions}
  \framesubtitle{\url{https://myoralvillage.org/}}
We have ideas for protocol extensions and ``programmable money'':
  \begin{itemize}
    \item Mediated wallet-to-wallet payments (instead of customer-to-merchant)
    \item Privacy-preserving auctions (trading, currency exchange)
    \item Age-restricted private payments for children (youth protection)
  \end{itemize}
CBs should also consider funding research to improve:
  \begin{itemize}
    \item General digital wallet usability and availability
    \item Accessibility features for illiterate and innumerate users
    \item Projects that facilitate integration at retailers
  \begin{itemize}
    \item Hardware and software support for embedded systems
    \item Integration into off-the-self E-commerce systems
  \end{itemize}
    \item Protocol extensions for automated tax reporting
  \end{itemize}
\end{frame}


\begin{frame}
\frametitle{Do you have any questions?}

\vfill
Further reading:
{\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{Jeffrey Burdges, Florian Dold, Christian Grothoff and Marcello Stanisci.
       {\em Enabling Secure Web Payments with GNU Taler}.
       {\bf SPACE 2016}.}
 \item{Florian Dold, Sree Harsha Totakura, Benedikt M\"uller, Jeffrey Burdges and Christian Grothoff.
       {\em Taler: Taxable Anonymous Libre Electronic Reserves}.
       Available upon request. 2016.}
 \item{Eli Ben-Sasson, Alessandro Chiesa, Christina Garman, Matthew Green, Ian Miers, Eran Tromer and Madars Virza.
       {\em Zerocash: Decentralized Anonymous Payments from Bitcoin}.
       {\bf IEEE Symposium on Security \& Privacy, 2016}.}
  \item{Phillip Rogaway.
       {\em The Moral Character of Cryptographic Work}.
       {\bf Asiacrypt}, 2015.} \label{bib:rogaway}
 \item{Christian Grothoff, Bart Polot and Carlo von Loesch.
       {\em The Internet is broken: Idealistic Ideas for Building a GNU Network}.
       {\bf W3C/IAB Workshop on Strengthening the Internet Against Pervasive Monitoring (STRINT)}, 2014.}
 \item{David Chaum, Amos Fiat and Moni Naor.
       {\em Untraceable electronic cash}.
       {\bf Proceedings on Advances in Cryptology, 1990}.}
\end{enumerate}
}
\begin{center}
  {\bf Let money facilitate trade; but ensure capital serves society.}
\end{center}
\end{frame}


\begin{frame}{Backup Slides}
\end{frame}



\begin{frame}{Taler Protocol}
  \framesubtitle{\url{https://taler.net/papers/thesis-dold-phd-2019.pdf}}
  \begin{itemize}
    \item Core concept:  Blind-signed tokens of value
    \item Based on stable, proven cryptographic constructions and primitives
      (Chaum-style eCash with extension)
    \item Concrete protocol with focus on data minimization
      and transparency/auditability through digital signatures.
    \item Documented in detail in a PhD thesis by Dold
    \item Exchange implementation has undergone external security audit
  \end{itemize}
\end{frame}



%\begin{frame}{CBDC is a Hot Topic}
%  \vfill
%\begin{center}
%\includegraphics[width=1.0\textwidth]{cbdc-news.png}
%\end{center}
%\end{frame}

\begin{frame}{CBDC Initiatives and Taler}
  Many initiatives are currently at the level of requirements discussion:
  \begin{minipage}{7cm}
  \begin{itemize}
    \item ECB: Report on a Digital Euro / Eurosystem report on the public consultation on a Digital Euro
    \item Bank of England: Just initiated a task force
  \end{itemize}
  \end{minipage}
  \begin{minipage}{5cm}
  \includegraphics[width=0.8\textwidth]{cbdc-news.png}
  \end{minipage}
  \vfill
  Taler can serve as the foundation for a \emph{bearer-based} \emph{retail} CBDC.
  \begin{itemize}
    \item Taler replicates physical cash rather than bank deposits
    \item Taler has unique design principles and regulatory features that align with CBDC
      requirements
    \item ECB survey has identified privacy as a primary requirement of end users
  \end{itemize}
\end{frame}

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



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

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

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


\begin{frame}
\frametitle{Taler: Merchant Details}
\begin{center}
\begin{tikzpicture}
 \tikzstyle{def} = [node distance= 5em and 4.5em, inner sep=1em, outer sep=.3em];
 \node (origin) at (0,0) {};
 \node (frontend) [def,above=of origin,draw]{merchant-frontend};
 \node (backend) [def,below=of frontend,draw]{taler-backend};
 \node (postgres) [def, draw, below left=of backend] {Postgres};
 \node (sqlite) [def, draw, below=of backend] {Sqlite};
 \node (alt) [def, draw, below right=of backend] {...};

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

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


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


\section{Integration with the core banking system}

\begin{frame}
  \vfill
  \begin{center}
    {\bf Part II: Integration with the core banking system}
  \end{center}
  \vfill
\end{frame}

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

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

\begin{itemize}
  \item a self-contained play money demo bank
  \item LibEuFin, an adapter to EBICS and other protocols
\end{itemize}

\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 Taler Wire Gateway API for the exchange
    \item currently, only EBICS 2.5 is supported
    \item other APIs such as FinTS or PSD2-style XS2A APIs can be added
      without requiring changes to the Exchange
    \item tested with a GLS business account
  \end{itemize}
\end{frame}

\begin{frame}{LibEuFin Concepts}
  \begin{itemize}
    \item A LibEuFin \emph{bank connection} is a set of credentials and parameters
      to talk to the bank's account access API.
    \item A LibEuFin \emph{bank account} is the information about a bank
      account (balances, transactions, payment initiations) stored locally
      within the LibEuFin service.  A LibEuFin bank account has a default Bank
      Connection that is used to communicate with the bank's API.
    \item A \emph{facade} provides a domain-specific access layer to bank accounts
      and connections.  The \emph{Taler Wire Gateway Facade} implements the
      API required by the Taler exchange and translates it to operations on the
      underlying account/connection.
  \end{itemize}
\end{frame}

\begin{frame}{LibEuFin Tooling}
  \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}{LibEuFin Setup Overview}
  \begin{itemize}
    \item Obtain EBICS subscriber configuration (host URL, host ID, user ID,
      partner ID) for the Exchange's escrow account
    \item Deploy the LibEuFin Nexus service
    \item Create a new LibEuFin bank connection (of type \texttt{ebics})
    \item Export and back up the key material for the bank connection (contains
      EBICS subscriber configuration and private keys)
    \item Send subscriber initialization to the EBICS host (electronically)
    \item Export key letter and activate subscriber in the EBICS host (manually)
    \item Synchronize the bank connection
    \item Import the account into LibEuFin
    \item Create a Taler Wire Gateway facade
    \item Set up scheduled tasks for ingesting new transactions / sending payment
      initiations
  \end{itemize}
\end{frame}



\begin{frame}{LibEuFin Implementation Limitations}
  \begin{itemize}
    \item LibEuFin is less stable than other Taler components, and future
      updates might contain breaking changes (tooling, APIs and database
      schema)
    \item Error handling and recovery is still rather primitive
    \item The Taler Wire Gateway does not yet implement automatic return
      transactions when transactions with a malformed subject (i.e.  no reserve
      public key) are received
  \end{itemize}
\end{frame}

\begin{frame}{LibEuFin EBICS Limitations}
  The GLS accounts with EBICS access that we have access to have some limitations:
  \begin{itemize}
    \item SEPA Instant Credit Transfers aren't supported yet
    \item Erroneous payment initiations are accepted by the GLS EBICS host,
      but an error message is later sent only by paper mail (and not reported
      by the CRZ download request)
    \item Limited access to transaction history (3 months)
  \end{itemize}
\end{frame}

\begin{frame}[fragile]{LibEuFin Setup Guide}
\vfill
\begin{center}
\url{https://docs.taler.net/libeufin/nexus-tutorial.html}
\end{center}
\vfill
\end{frame}


\section{Operator security considerations}

\begin{frame}
  \vfill
  \begin{center}
    {\bf Part III: Operator security considerations}
  \end{center}
  \vfill
\end{frame}


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


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


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


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


\begin{frame}{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 also not become public.
\item The auditor must have a (current) copy. Asynchronous replication
      is considered sufficient.  This copy could also be used as an
      additional (off-site?) backup.
\end{itemize}
\end{frame}


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


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


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


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


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


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


\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}{Customer-to-customer payments}
  \begin{itemize}
  \item Customer-to-bank account will be easy.
  \item Wallet-to-wallet transactions require withdraw ($\Rightarrow$ customer authorization!)
  \end{itemize}
\end{frame}


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

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

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


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

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


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

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


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

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


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

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


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

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

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


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


\begin{frame}{Merchant Integration: Wallet Detection}
 \lstset{language=JavaScript}
 \lstinputlisting{figs/taler-presence-js.html}
% \caption{Sample code to detect the Taler wallet. Allowing the
%  Web site to detect the presence of the wallet leaks one bit
%  of information about the user. The above logic also works
%  if the wallet is installed while the page is open.}
%  \label{listing:presence}
\end{frame}


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

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


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


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

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


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

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


\begin{frame}{Merchant and Exchange: Verify coin (RSA)}
   \begin{minipage}{6cm}
 \begin{equation*}
   s^e \stackrel{?}{\equiv} FDH(C) \mod n
   \end{equation*}
   \end{minipage}
  \begin{minipage}{6cm}
  \begin{minipage}{0.2\textwidth}
    \includegraphics[width=\textwidth]{coin.pdf}
  \end{minipage}
  $\stackrel{?}{\Leftrightarrow}$
  \begin{minipage}{0.2\textwidth}
    \includegraphics[width=\textwidth]{seal.pdf}
  \end{minipage}
  \end{minipage}
\end{frame}


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


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


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

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


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

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

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


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

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

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


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

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

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

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

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

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

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


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


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


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

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

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

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

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


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

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


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

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


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

    \vspace{1cm}

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

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

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


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

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

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

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


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


\end{document}
