commit 88c6f8f498da59ea813858a7de72814ad29dc5f4
parent 1612630809c6bc5ac39a5805c67bbd26081b75dc
Author: Christian Grothoff <christian@grothoff.org>
Date: Fri, 4 Nov 2022 10:31:00 +0100
rx
Diffstat:
9 files changed, 2932 insertions(+), 0 deletions(-)
diff --git a/presentations/comprehensive/definitions.tex b/presentations/comprehensive/definitions.tex
@@ -0,0 +1,90 @@
+\newcommand{\blue}[1]{{\color{blue}#1}}
+\newcommand{\red}[1]{{\color{red}#1}}
+\newcommand{\Guardian}{\mathcal{G}}
+\newcommand{\Child}{\mathcal{C}}
+\newcommand{\Customer}{\mathcal{C}}
+\newcommand{\Merchant}{\mathcal{M}}
+\newcommand{\Exchange}{\mathcal{E}}
+
+\newcommand{\Commit}{\mathsf{Commit}}
+\newcommand{\Attest}{\mathsf{Attest}}
+\newcommand{\Verify}{\mathsf{Verify}}
+\newcommand{\Derive}{\mathsf{Derive}}
+\newcommand{\DeriveCompare}{\mathsf{DeriveCompare_\kappa}}
+\newcommand{\Compare}{\mathsf{Compare}}
+\newcommand{\AgeVer}{\mathsf{AgeVer}}
+
+\newcommand{\HashF}{\mathsf{H}}
+\newcommand{\Hash}{\mathsf{H}}
+\newcommand{\Block}{\mathbb{B}}
+\newcommand{\Pub}{\mathsf{Pub}}
+\newcommand{\Sign}{\mathsf{Sig}}
+\newcommand{\Ver}{\mathsf{Ver}}
+\newcommand{\Encoding}{\mathsf{Encoding}}
+\newcommand{\ECDSA}{\mathsf{ECDSA}}
+\newcommand{\Null}{\mathcal{O}}
+\newcommand{\EC}{\mathrm{ec}}
+\newcommand{\Curve}{\mathsf{Curve25519}}
+\newcommand{\SHA}{\mathsf{SHA256}}
+\newcommand{\SHAF}{\mathsf{SHA252}}
+\newcommand{\FDH}{\mathsf{FDH}}
+
+\newcommand{\negl}{\epsilon}
+
+\newcommand{\rand}{\mathsf{rand}}
+\newcommand{\age}{\mathsf{a}}
+\newcommand{\Age}{\mathsf{M}}
+\newcommand{\bage}{\mathsf{b}}
+\newcommand{\minage}{\mathsf{m}}
+\newcommand{\attest}{\mathsf{T}}
+\newcommand{\commitment}{\mathsf{Q}}
+\newcommand{\pruf}{\mathsf{P}}
+\newcommand{\Vcommitment}{\vec{\mathsf{Q}}}
+\newcommand{\Vpruf}{\vec{\mathsf{P}}}
+\newcommand{\blinding}{\beta}
+
+\newcommand{\ZN}{\mathbb{Z}_N}
+\newcommand{\Z}{\mathbb{Z}}
+\newcommand{\N}{\mathbb{N}}
+\newcommand{\A}{\mathbb{A}}
+\newcommand{\E}{\mathbb{E}}
+\newcommand{\F}{\mathbb{F}}
+\newcommand{\seck}{\mathsf{s}}
+\newcommand{\pubk}{\mathsf{P}}
+\renewcommand{\H}{\mathbb{H}}
+\newcommand{\K}{\mathbb{K}}
+\newcommand{\Proofs}{\mathbb{P}}
+\newcommand{\Commitments}{\mathbb{O}}
+\newcommand{\Attests}{\mathbb{T}}
+\newcommand{\Blindings}{\mathbb{B}}
+\newcommand{\Nil}{\perp}
+
+\newcommand{\p}{\mathsf{p}}
+\newcommand{\com}{\mathsf{com}}
+\newcommand{\prf}{\mathsf{prf}}
+
+\newcommand{\Adv}{\mathcal{A}}
+\newcommand{\PPT}{\mathfrak{A}}
+\newcommand{\Probability}{\mathrm{Pr}}
+\newcommand{\Algorithm}{f}
+\renewcommand{\Game}[1]{G_\Adv^\mathsf{#1}}
+
+\DeclareMathOperator{\Image}{Im}
+\DeclareMathOperator{\Mod}{mod}
+
+\newcommand{\Encode}[1]{\overbracket[0.5pt][2pt]{\,#1\,}}
+\newcommand{\Decode}[1]{\underbracket[0.5pt][3pt]{\,#1\,}}
+\newcommand{\FDHg}[1]{[#1]_g\,}
+\newcommand{\logg}{{\breve{g}}}
+
+
+\newcommand{\drawfrom}{\xleftarrow{\$}}
+\newcommand\Exists{%
+ \mathop{\lower0.75ex\hbox{\ensuremath{%
+ \mathlarger{\mathlarger{\mathlarger{\mathlarger{\exists}}}}}}}%
+ \limits}
+
+\newcommand\Forall{%
+ \mathop{\lower0.75ex\hbox{\ensuremath{%
+ \mathlarger{\mathlarger{\mathlarger{\mathlarger{\forall}}}}}}}%
+ \limits}
diff --git a/presentations/comprehensive/fed.tex b/presentations/comprehensive/fed.tex
@@ -0,0 +1,1943 @@
+\pdfminorversion=3
+\documentclass[fleqn,xcolor={usenames,dvipsnames}]{beamer}
+\usepackage{amsmath}
+\usepackage{multimedia}
+\usepackage[utf8]{inputenc}
+\usepackage{framed,color,ragged2e}
+\usepackage[absolute,overlay]{textpos}
+\definecolor{shadecolor}{rgb}{0.8,0.8,0.8}
+\usetheme{boxes}
+\setbeamertemplate{navigation symbols}{}
+\usepackage{xcolor}
+\usepackage{tikz,eurosym}
+\usepackage[normalem]{ulem}
+\usepackage{listings}
+\usepackage{adjustbox}
+
+% CSS
+\lstdefinelanguage{CSS}{
+ basicstyle=\ttfamily\scriptsize,
+ keywords={color,background-image:,margin,padding,font,weight,display,position,top,left,right,bottom,list,style,border,size,white,space,min,width, transition:, transform:, transition-property, transition-duration, transition-timing-function},
+ sensitive=true,
+ morecomment=[l]{//},
+ morecomment=[s]{/*}{*/},
+ morestring=[b]',
+ morestring=[b]",
+ alsoletter={:},
+ alsodigit={-}
+}
+
+% JavaScript
+\lstdefinelanguage{JavaScript}{
+ basicstyle=\ttfamily\scriptsize,
+ morekeywords={typeof, new, true, false, catch, function, return, null, catch, switch, var, if, in, while, do, else, case, break},
+ morecomment=[s]{/*}{*/},
+ morecomment=[l]//,
+ morestring=[b]",
+ morestring=[b]'
+}
+
+\lstdefinelanguage{HTML5}{
+ basicstyle=\ttfamily\scriptsize,
+ language=html,
+ sensitive=true,
+ alsoletter={<>=-},
+ morecomment=[s]{<!-}{-->},
+ tag=[s],
+ otherkeywords={
+ % General
+ >,
+ % Standard tags
+ <!DOCTYPE,
+ </html, <html, <head, <title, </title, <style, </style, <link, </head, <meta, />,
+ % body
+ </body, <body,
+ % Divs
+ </div, <div, </div>,
+ % Paragraphs
+ </p, <p, </p>,
+ % scripts
+ </script, <script,
+ % More tags...
+ <canvas, /canvas>, <svg, <rect, <animateTransform, </rect>, </svg>, <video, <source, <iframe, </iframe>, </video>, <image, </image>
+ },
+ ndkeywords={
+ % General
+ =,
+ % HTML attributes
+ charset=, src=, id=, width=, height=, style=, type=, rel=, href=,
+ % SVG attributes
+ fill=, attributeName=, begin=, dur=, from=, to=, poster=, controls=, x=, y=, repeatCount=, xlink:href=,
+ % CSS properties
+ margin:, padding:, background-image:, border:, top:, left:, position:, width:, height:,
+ % CSS3 properties
+ transform:, -moz-transform:, -webkit-transform:,
+ animation:, -webkit-animation:,
+ transition:, transition-duration:, transition-property:, transition-timing-function:,
+ }
+}
+
+\lstdefinelanguage{JavaScript}{
+ basicstyle=\ttfamily\scriptsize,
+ keywords={typeof, new, true, false, catch, function, return, null, catch, switch, var, if, in, while, do, else, case, break, for},
+ keywordstyle=\color{blue}\bfseries,
+ ndkeywords={class, export, boolean, throw, implements, import, this},
+ ndkeywordstyle=\color{darkgray}\bfseries,
+ identifierstyle=\color{black},
+ sensitive=false,
+ comment=[l]{//},
+ morecomment=[s]{/*}{*/},
+ commentstyle=\color{purple}\ttfamily,
+ stringstyle=\color{red}\ttfamily,
+ morestring=[b]',
+ morestring=[b]"
+}
+
+\usetikzlibrary{shapes,arrows}
+\usetikzlibrary{positioning}
+\usetikzlibrary{calc}
+
+\title{GNU Taler}
+%\subtitle{}
+
+\setbeamertemplate{navigation symbols}{\includegraphics[width=1cm]{inria.pdf} \includegraphics[width=0.5cm]{gnu.png} \includegraphics[width=0.5cm]{ashoka.png}\hfill}
+%\setbeamercovered{transparent=1}
+
+\author[C. Grothoff]{L. Schumacher, F. Dold, {\bf C. Grothoff}, M. Stanisci}
+\date{\today}
+\institute{The GNU Project}
+
+
+\begin{document}
+
+\justifying
+
+\begin{frame}
+ \begin{center}
+ \LARGE {\bf GNU}
+
+ \vfill
+% \includegraphics[width=0.66\textwidth]{logo-2017-fr.pdf}
+ \includegraphics[width=0.66\textwidth]{logo-2020.jpg}
+ \end{center}
+\begin{textblock*}{6cm}(.5cm,7.7cm) % {block width} (coords)
+ {\Large {\bf \href{https://taler.net/}{taler.net}} \\
+ \href{https://twitter.com/taler}{taler@twitter} \\
+ \href{https://taler-systems.com/}{taler-systems.com}}
+\end{textblock*}
+
+% Substitute based on who is giving the talk!
+ \begin{textblock*}{6cm}(6.7cm,7.7cm) % {block width} (coords)
+ {\hfill {\Large {\bf Leon Schumacher \&} \\
+ \hfill {\bf Christian Grothoff}} \\
+ \hfill \{schumacher,grothoff\}@taler.net }
+\end{textblock*}
+
+\end{frame}
+
+
+\section{What is Taler?}
+\begin{frame}{What is Taler?}
+ \begin{center}
+Taler is an electronic instant payment system.
+ \end{center}
+ \begin{itemize}
+ \item Uses electronic coins stored in {\bf wallets} on customer's device
+ \item Like {\bf cash}
+ \item Pay in {\bf existing currencies} (i.e. EUR, USD, BTC), \\
+ or use it to create new {\bf regional currencies}
+ \end{itemize}
+ \vfill
+ \pause
+ \noindent
+ However, Taler is
+ \begin{itemize}
+ \item \emph{not} a currency
+ \item \emph{not} a long-term store of value
+ \item \emph{not} a network or instance of a system
+ \item \emph{not} decentralized
+ \item \emph{not} based on proof-of-work or proof-of-stake
+ \item \emph{not} a speculative asset / ``get-rich-quick scheme''
+ \end{itemize}
+\end{frame}
+
+
+\begin{frame}{Design principles}
+ \framesubtitle{https://taler.net/en/principles.html}
+GNU Taler must ...
+\begin{enumerate}
+ \item {... be implemented as {\bf free software}.}
+ \item {... protect the {\bf privacy of buyers}.}
+ \item {... must enable the state to {\bf tax income} and crack down on
+ illegal business activities.}
+ \item {... prevent payment fraud.}
+ \item {... only {\bf disclose the minimal amount of information
+ necessary}.}
+ \item {... be usable.}
+ \item {... be efficient.}
+ \item {... avoid single points of failure.}
+ \item {... foster {\bf competition}.}
+\end{enumerate}
+\end{frame}
+
+
+\begin{frame}
+\frametitle{Taler Overview}
+\begin{center}
+\begin{tikzpicture}
+ \tikzstyle{def} = [node distance= 5em and 6.5em, inner sep=1em, outer sep=.3em];
+ \node (origin) at (0,0) {};
+ \node (exchange) [def,above=of origin,draw]{Exchange};
+ \node (customer) [def, draw, below left=of origin] {Customer};
+ \node (merchant) [def, draw, below right=of origin] {Merchant};
+ \node (auditor) [def, draw, above right=of origin]{Auditor};
+% \node (regulator) [def, draw, above=of auditor]{CSSF};
+
+ \tikzstyle{C} = [color=black, line width=1pt]
+
+ \draw [<-, C] (customer) -- (exchange) node [midway, above, sloped] (TextNode) {withdraw coins};
+ \draw [<-, C] (exchange) -- (merchant) node [midway, above, sloped] (TextNode) {deposit coins};
+ \draw [<-, C] (merchant) -- (customer) node [midway, above, sloped] (TextNode) {spend coins};
+ \draw [<-, C] (exchange) -- (auditor) node [midway, above, sloped] (TextNode) {verify};
+% \draw [<-, C] (regulator) -- (auditor) node [midway, above, sloped] (TextNode) {report};
+
+\end{tikzpicture}
+\end{center}
+\end{frame}
+
+\begin{frame}{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}
+\frametitle{Architecture of Taler}
+\begin{center}
+ \includegraphics[width=1\textwidth]{operations.png}
+\end{center}
+\end{frame}
+
+
+\begin{frame}{Taler: Unique Regulatory Features for Central Banks}
+ \framesubtitle{\url{https://www.snb.ch/en/mmr/papers/id/working_paper_2021_03}}
+ \begin{itemize}
+ \item Central bank issues digital coins equivalent to issuing cash \\
+ $\Rightarrow$ monetary policy remains under CB control
+ \item Architecture with consumer accounts at commercial banks \\
+ $\Rightarrow$ no competition for commercial banking (S\&L) \\
+ $\Rightarrow$ CB does not have to manage KYC, customer support
+ \item Withdrawal limits and denomination expiration \\
+ $\Rightarrow$ protects against bank runs and hoarding
+ \item Income transparency and possibility to set fees \\
+ $\Rightarrow$ additional insights into economy and new policy options
+ \item Revocation protocols and loss limitations \\
+ $\Rightarrow$ exit strategy and handles catastrophic security incidents
+ \item Privacy by cryptographic design not organizational compliance \\
+ $\Rightarrow$ CB cannot be forced to facilitate mass-surveillance
+ \end{itemize}
+\end{frame}
+
+
+\begin{frame}{Usability of Taler}
+ \vfill
+ \begin{center}
+ \url{https://demo.taler.net/}
+ \end{center}
+ \begin{enumerate}
+ \item Install browser extension.
+ \item Visit the {\tt bank.demo.taler.net} to withdraw coins.
+ \item Visit the {\tt shop.demo.taler.net} to spend coins.
+ \end{enumerate}
+ \vfill
+\end{frame}
+
+
+\begin{frame}{How does it work?}
+We use a few ancient constructions:
+ \begin{itemize}
+ \item Cryptographic hash function (1989)
+ \item Blind signature (1983)
+ \item Schnorr signature (1989)
+ \item Diffie-Hellman key exchange (1976)
+ \item Cut-and-choose zero-knowledge proof (1985)
+ \end{itemize}
+But of course we use modern instantiations.
+\end{frame}
+
+
+\begin{frame}{Definition: Taxability}
+ We say Taler is taxable because:
+ \begin{itemize}
+ \item Merchant's income is visible from deposits.
+ \item Hash of contract is part of deposit data.
+ \item State can trace income and enforce taxation.
+ \end{itemize}\pause
+ Limitations:
+ \begin{itemize}
+ \item withdraw loophole
+ \item {\em sharing} coins among family and friends
+ \end{itemize}
+\end{frame}
+
+
+\begin{frame}{Exchange setup: Create a denomination key (RSA)}
+ \begin{minipage}{6cm}
+ \begin{enumerate}
+ \item Pick random primes $p,q$.
+ \item Compute $n := pq$, $\phi(n) = (p-1)(q-1)$
+ \item Pick small $e < \phi(n)$ such that
+ $d := e^{-1} \mod \phi(n)$ exists.
+ \item Publish public key $(e,n)$.
+ \end{enumerate}
+ \end{minipage}
+ \begin{minipage}{6cm}
+ \begin{tikzpicture}
+ \tikzstyle{def} = [node distance=1em and 1em, inner sep=0em, outer sep=.3em];
+ \node (origin) at (0,0) {\includegraphics[width=0.2\textwidth]{dice.pdf}};
+ \node (primes) [draw=none, below = of origin] at (0,0) {$(p, q)$};
+ \node (seal) [def, draw=none, below left=of primes]{\includegraphics[width=0.15\textwidth]{seal.pdf}};
+ \node (hammer) [def, draw=none, below right=of primes]{\includegraphics[width=0.15\textwidth]{hammer.pdf}};
+
+ \tikzstyle{C} = [color=black, line width=1pt]
+
+ \draw [<-, C] (primes) -- (origin) node [midway, above, sloped] (TextNode) {};
+ \draw [<-, C] (seal) -- (primes) node [midway, above, sloped] (TextNode) {};
+ \draw [<-, C] (hammer) -- (primes) node [midway, above, sloped] (TextNode) {};
+ \end{tikzpicture}
+% \includegraphics[width=0.4\textwidth]{seal.pdf}
+ \end{minipage}
+\end{frame}
+
+
+\begin{frame}{Merchant: Create a signing key (EdDSA)}
+ \begin{minipage}{6cm}
+ \begin{itemize}
+ \item pick random $m \mod o$ as private key
+ \item $M = mG$ public key
+ \end{itemize}
+ \end{minipage}
+ \begin{minipage}{6cm}
+ \begin{tikzpicture}
+ \tikzstyle{def} = [node distance= 1em and 1em, inner sep=0em, outer sep=.3em];
+ \node (origin) at (0,0) {\includegraphics[width=0.2\textwidth]{dice.pdf}};
+ \node (m) [draw=none, below = of origin] at (0,0) {$m$};
+ \node (seal) [draw=none, below=of m]{M};
+ \tikzstyle{C} = [color=black, line width=1pt]
+
+ \draw [<-, C] (m) -- (origin) node [midway, above, sloped] (TextNode) {};
+ \draw [<-, C] (seal) -- (primes) node [midway, above, sloped] (TextNode) {};
+ \end{tikzpicture}
+ \end{minipage}
+ \parbox[t]{3cm}{{\bf Capability:} $m \Rightarrow$ }
+ \raisebox{\dimexpr-\height+\baselineskip}{\includegraphics[width=0.1\textwidth]{merchant-sign.pdf}}
+\end{frame}
+
+
+\begin{frame}{Customer: Create a planchet (EdDSA)}
+ \begin{minipage}{8cm}
+ \begin{itemize}
+ \item Pick random $c \mod o$ private key
+ \item $C = cG$ public key
+ \end{itemize}
+ \end{minipage}
+ \begin{minipage}{4cm}
+ \begin{tikzpicture}
+ \tikzstyle{def} = [node distance= 1em and 1em, inner sep=0em, outer sep=.3em];
+ \node (origin) at (0,0) {\includegraphics[width=0.2\textwidth]{dice.pdf}};
+ \node (c) [draw=none, below = of origin] at (0,0) {$c$};
+ \node (planchet) [draw=none, below=of c]{\includegraphics[width=0.4\textwidth]{planchet.pdf}};
+ \tikzstyle{C} = [color=black, line width=1pt]
+
+ \draw [<-, C] (c) -- (origin) node [midway, above, sloped] (TextNode) {};
+ \draw [<-, C] (planchet) -- (c) node [midway, above, sloped] (TextNode) {};
+ \end{tikzpicture}
+ \end{minipage}
+ \parbox[t]{3cm}{{\bf Capability:} $c \Rightarrow$ }
+ \raisebox{\dimexpr-\height+\baselineskip}{\includegraphics[width=0.1\textwidth]{planchet-sign.pdf}}
+\end{frame}
+
+
+\begin{frame}{Customer: Blind planchet (RSA)}
+ \begin{minipage}{6cm}
+ \begin{enumerate}
+ \item Obtain public key $(e,n)$
+ \item Compute $f := FDH(C)$, $f < n$.
+ \item Pick blinding factor $b \in \mathbb Z_n$
+ \item Transmit $f' := f b^e \mod n$
+ \end{enumerate}
+ \end{minipage}
+ \begin{minipage}{6cm}
+ \begin{tikzpicture}
+ \tikzstyle{def} = [node distance= 2em and 0.5em, inner sep=0em, outer sep=.3em];
+ \node (origin) at (0,0) {\includegraphics[width=0.2\textwidth]{dice.pdf}};
+ \node (b) [def, draw=none, below = of origin] at (0,-0.2) {$b$};
+ \node (blinded) [def, draw=none, below right=of b]{\includegraphics[width=0.2\textwidth]{blinded.pdf}};
+ \node (planchet) [def, draw=none, above right=of blinded]{\includegraphics[width=0.15\textwidth]{planchet.pdf}};
+ \node (exchange) [node distance=4em and 0.5em, draw, below =of blinded]{Exchange};
+ \tikzstyle{C} = [color=black, line width=1pt]
+
+ \draw [<-, C] (b) -- (origin) node [midway, above, sloped] (TextNode) {};
+ \draw [<-, C] (blinded) -- (planchet) node [midway, above, sloped] (TextNode) {};
+ \draw [<-, C] (blinded) -- (b) node [midway, above, sloped] (TextNode) {};
+ \draw [<-, C] (exchange) -- (blinded) node [midway, above, sloped] (TextNode) {{\small transmit}};
+ \end{tikzpicture}
+ \end{minipage}
+\end{frame}
+
+
+\begin{frame}{Exchange: Blind sign (RSA)}
+ \begin{minipage}{6cm}
+ \begin{enumerate}
+ \item Receive $f'$.
+ \item Compute $s' := f'^d \mod n$.
+ \item Send signature $s'$.
+ \end{enumerate}
+ \end{minipage}
+ \begin{minipage}{6cm}
+ \begin{tikzpicture}
+ \tikzstyle{def} = [node distance= 2em and 0.5em, inner sep=0em, outer sep=.3em];
+ \node (hammer) [def, draw=none] at (0,0) {\includegraphics[width=0.15\textwidth]{hammer.pdf}};
+ \node (signed) [def, draw=none, below left=of hammer]{\includegraphics[width=0.2\textwidth]{sign.pdf}};
+ \node (blinded) [def, draw=none, above left=of signed]{\includegraphics[width=0.15\textwidth]{blinded.pdf}};
+ \node (customer) [node distance=4em and 0.5em, draw, below =of signed]{Customer};
+ \tikzstyle{C} = [color=black, line width=1pt]
+
+ \draw [<-, C] (signed) -- (hammer) node [midway, above, sloped] (TextNode) {};
+ \draw [<-, C] (signed) -- (blinded) node [midway, above, sloped] (TextNode) {};
+ \draw [<-, C] (customer) -- (signed) node [midway, above, sloped] (TextNode) {{\small transmit}};
+ \end{tikzpicture}
+ \end{minipage}
+\end{frame}
+
+
+\begin{frame}{Customer: Unblind coin (RSA)}
+ \begin{minipage}{6cm}
+ \begin{enumerate}
+ \item Receive $s'$.
+ \item Compute $s := s' b^{-1} \mod n$ % \\
+ % ($(f')^d = (f b^e)^d = f^d b$).
+ \end{enumerate}
+ \end{minipage}
+ \begin{minipage}{6cm}
+ \begin{tikzpicture}
+ \tikzstyle{def} = [node distance= 2em and 0.5em, inner sep=0em, outer sep=.3em];
+ \node (b) [def, draw=none] at (0,0) {$b$};
+ \node (coin) [def, draw=none, below left=of b]{\includegraphics[width=0.2\textwidth]{coin.pdf}};
+ \node (signed) [def, draw=none, above left=of coin]{\includegraphics[width=0.15\textwidth]{sign.pdf}};
+ \tikzstyle{C} = [color=black, line width=1pt]
+
+ \draw [<-, C] (coin) -- (b) node [midway, above, sloped] (TextNode) {};
+ \draw [<-, C] (coin) -- (signed) node [midway, above, sloped] (TextNode) {};
+ \end{tikzpicture}
+ \end{minipage}
+\end{frame}
+
+\begin{frame}{Withdrawing coins on the Web}
+ \begin{center}
+ \includegraphics[height=0.9\textheight]{figs/taler-withdraw.pdf}
+ \end{center}
+\end{frame}
+
+
+\begin{frame}{Customer: Build shopping cart}
+ \begin{center}
+ \begin{tikzpicture}
+ \tikzstyle{def} = [node distance= 1em and 1em, inner sep=0em, outer sep=.3em];
+ \node (origin) at (0,0) {\includegraphics[width=0.2\textwidth]{shop.pdf}};
+ \node (cart) [draw=none, below=of m]{\includegraphics[width=0.2\textwidth]{cart.pdf}};
+ \node (merchant) [node distance=4em and 0.5em, draw, below =of cart]{Merchant};
+ \tikzstyle{C} = [color=black, line width=1pt];
+ \draw [<-, C] (cart) -- (origin) node [midway, above, sloped] (TextNode) {};
+ \draw [<-, C] (merchant) -- (cart) node [midway, above, sloped] (TextNode) {{\small transmit}};
+ \end{tikzpicture}
+ \end{center}
+\end{frame}
+
+
+\begin{frame}{Merchant: Propose contract (EdDSA)}
+ \begin{minipage}{6cm}
+ \begin{enumerate}
+ \item Complete proposal $D$.
+ \item Send $D$, $EdDSA_m(D)$
+ \end{enumerate}
+ \end{minipage}
+ \begin{minipage}{6cm}
+ \begin{tikzpicture}
+ \tikzstyle{def} = [node distance=2em and 0.5em, inner sep=0em, outer sep=.3em];
+ \node (cart) [def, draw=none] at (0,0) {\includegraphics[width=0.15\textwidth]{cart.pdf}};
+ \node (proposal) [def, draw=none, below right=of cart]{\includegraphics[width=0.5\textwidth]{merchant_propose.pdf}};
+ \node (customer) [node distance=4em and 0.5em, draw, below =of proposal]{Customer};
+ \tikzstyle{C} = [color=black, line width=1pt];
+ \node (sign) [def, draw=none, above right=of proposal] {$m$};
+ \tikzstyle{C} = [color=black, line width=1pt]
+
+ \draw [<-, C] (proposal) -- (sign) node [midway, above, sloped] (TextNode) {};
+ \draw [<-, C] (proposal) -- (cart) node [midway, above, sloped] (TextNode) {};
+ \draw [<-, C] (customer) -- (proposal) node [midway, above, sloped] (TextNode) {{\small transmit}};
+ \end{tikzpicture}
+ \end{minipage}
+\end{frame}
+
+
+\begin{frame}{Customer: Spend coin (EdDSA)}
+ \begin{minipage}{6cm}
+ \begin{enumerate}
+ \item Receive proposal $D$, $EdDSA_m(D)$.
+ \item Send $s$, $C$, $EdDSA_c(D)$
+ \end{enumerate}
+ \end{minipage}
+ \begin{minipage}{6cm}
+ \begin{tikzpicture}
+ \tikzstyle{def} = [node distance=2em and 0.4em, inner sep=0em, outer sep=.3em];
+ \node (proposal) [def, draw=none] at (0,0) {\includegraphics[width=0.15\textwidth]{merchant_propose.pdf}};
+ \node (contract) [def, draw=none, below right=of cart]{\includegraphics[width=0.3\textwidth]{contract.pdf}};
+ \node (c) [def, draw=none, above=of contract] {$c$};
+ \node (merchant) [node distance=4em and 0.5em, draw, below=of contract]{Merchant};
+ \node (coin) [def, draw=none, right=of contract]{\includegraphics[width=0.2\textwidth]{coin.pdf}};
+ \tikzstyle{C} = [color=black, line width=1pt]
+
+ \draw [<-, C] (contract) -- (c) node [midway, above, sloped] (TextNode) {};
+ \draw [<-, C] (contract) -- (proposal) node [midway, above, sloped] (TextNode) {};
+ \draw [<-, C] (merchant) -- (contract) node [midway, above, sloped] (TextNode) {{\small transmit}};
+ \draw [<-, C] (merchant) -- (coin) node [midway, below, sloped] (TextNode) {{\small transmit}};
+ \end{tikzpicture}
+ \end{minipage}
+\end{frame}
+
+
+\begin{frame}{Merchant and Exchange: Verify coin (RSA)}
+ \begin{minipage}{6cm}
+ \begin{equation*}
+ s^e \stackrel{?}{\equiv} FDH(C) \mod n
+ \end{equation*}
+ \end{minipage}
+ \begin{minipage}{6cm}
+ \begin{minipage}{0.2\textwidth}
+ \includegraphics[width=\textwidth]{coin.pdf}
+ \end{minipage}
+ $\stackrel{?}{\Leftrightarrow}$
+ \begin{minipage}{0.2\textwidth}
+ \includegraphics[width=\textwidth]{seal.pdf}
+ \end{minipage}
+ \end{minipage}
+ \vfill
+ The exchange does not only verify the signature, but also
+ checks that the coin was not double-spent.
+ \vfill
+ \pause
+ \begin{center}
+ {\bf Taler is an online payment system.}
+ \end{center}
+ \vfill
+\end{frame}
+
+
+\begin{frame}{Requirements: Online vs. Offline Digital Currencies}
+\framesubtitle{\url{https://taler.net/papers/euro-bearer-online-2021.pdf}}
+\begin{itemize}
+ \item Offline capabilities are sometimes cited as a requirement for digital payment solutions
+ \item All implementations must either use restrictive hardware elements and/or introduce
+ counterparty risk.
+ \item[$\Rightarrow$] Permanent offline features weaken a digital payment solution (privacy, security)
+ \item[$\Rightarrow$] Introduces unwarranted competition for physical cash (endangers emergency-preparedness).
+ \end{itemize}
+ We recommend a tiered approach:
+ \begin{enumerate}
+ \item Online-first, bearer-based digital currency with Taler
+ \item (Optional:) Limited offline mode for network outages
+ \item Physical cash for emergencies (power outage, catastrophic cyber incidents)
+ \end{enumerate}
+\end{frame}
+
+
+\begin{frame}{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}
+
+
+
+\begin{frame}{Warranting deposit safety}
+ Exchange has {\em another} online signing key $W = wG$:
+ \begin{center}
+ Sends $EdDSA_w(M,H(D),FDH(C))$ to the merchant.
+ \end{center}
+ This signature means that $M$ was the {\em first} to deposit
+ $C$ and that the exchange thus must pay $M$.
+ \vfill
+ \begin{center}
+ Without this, an evil exchange could renege on the deposit
+ confirmation and claim double-spending if a coin were
+ deposited twice, and then not pay either merchant!
+ \end{center}
+\end{frame}
+
+
+\begin{frame}{Online keys}
+\begin{itemize}
+\item The exchange needs $d$ and $w$ to be available for online signing.
+\item The corresponding public keys $W$ and $(e,n)$ are certified using
+ Taler's public key infrastructure (which uses offline-only keys).
+\end{itemize}
+\begin{center}
+\includegraphics[width=0.5\textwidth]{taler-diagram-signatures.png}
+\end{center}
+\vfill
+\begin{center}
+{\bf What happens if those private keys are compromised?}
+\end{center}
+\vfill
+\end{frame}
+
+
+\begin{frame}{Denomination key $(e,n)$ compromise}
+\begin{itemize}
+\item An attacker who learns $d$ can sign an arbitrary number of illicit coins
+ into existence and deposit them.
+\item Auditor and exchange can detect this once the total number of deposits
+ (illicit and legitimate) exceeds the number of legitimate coins the
+ exchange created.
+\item At this point, $(e,n)$ is {\em revoked}. Users of {\em unspent}
+ legitimate coins reveal $b$ from their withdrawal operation and
+ obtain a {\em refund}.
+\item The financial loss of the exchange is {\em bounded} by the number of
+ legitimate coins signed with $d$.
+\item[$\Rightarrow$] Taler frequently rotates denomination signing keys and
+ deletes $d$ after the signing period of the respective key expires.
+\end{itemize}
+\begin{center}
+\includegraphics[width=0.5\textwidth]{taler-diagram-denom-expiration.png}
+\end{center}
+\end{frame}
+
+
+\begin{frame}{Online signing key $W$ compromise}
+\begin{itemize}
+\item An attacker who learns $w$ can sign deposit confirmations.
+\item Attacker sets up two (or more) merchants and customer(s) which double-spend
+ legitimate coins at both merchants.
+\item The merchants only deposit each coin once at the exchange and get paid once.
+\item The attacker then uses $w$ to fake deposit confirmations for the double-spent
+ transactions.
+\item The attacker uses the faked deposit confirmations to complain to the auditor
+ that the exchange did not honor the (faked) deposit confirmations.
+\end{itemize}
+The auditor can then detect the double-spending, but cannot tell who is to blame,
+and (likely) would presume an evil exchange, forcing it to pay both merchants.
+\end{frame}
+
+
+\begin{frame}{Detecting online signing key $W$ compromise}
+\begin{itemize}
+\item Merchants are required to {\em probabilistically} report
+ signed deposit confirmations to the auditor.
+\item Auditor can thus detect exchanges not reporting signed
+ deposit confirmations.
+\item[$\Rightarrow$] Exchange can rekey if illicit key use is detected,
+ then only has to honor deposit confirmations it already provided
+ to the auditor {\em and} those without proof of double-spending
+ {\em and} those merchants reported to the auditor.
+\item[$\Rightarrow$] Merchants that do not participate in reporting
+ to the auditor risk their deposit permissions being voided in
+ cases of an exchange's private key being compromised.
+\end{itemize}
+\end{frame}
+
+
+
+
+\section{Competitor analysis}
+\begin{frame}{Competitor comparison}
+ \begin{center} \small
+ \begin{tabular}{l||c|c|c|c|c}
+ & Cash & Bitcoin & Zerocoin & Creditcard & GNU Taler \\ \hline \hline
+ Online &$-$$-$$-$ & ++ & ++ & + & +++ \\ \hline
+ Offline & +++ & $-$$-$ & $-$$-$ & + & $-$$-$ \\ \hline
+ Trans. cost & + & $-$$-$$-$ & $-$$-$$-$ & $-$ & ++ \\ \hline
+ Speed & + & $-$$-$$-$ & $-$$-$$-$ & o & ++ \\ \hline
+ Taxation & $-$ & $-$$-$ & $-$$-$$-$ & +++ & +++ \\ \hline
+ Payer-anon & ++ & o & ++ & $-$$-$$-$ & +++ \\ \hline
+ Payee-anon & ++ & o & ++ & $-$$-$$-$ & $-$$-$$-$ \\ \hline
+ Security & $-$ & o & o & $-$$-$ & ++ \\ \hline
+ Conversion & +++ & $-$$-$$-$ & $-$$-$$-$ & +++ & +++ \\ \hline
+ Libre & $-$ & +++ & +++ & $-$ $-$ $-$ & +++ \\
+ \end{tabular}
+ \end{center}
+\end{frame}
+
+
+\begin{frame}{Taler: Project Status}
+\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}
+ Pilots with banking 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 banking 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{Background: \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}
+Central banks 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}{How to support?}
+ \begin{description}
+ \item[Join:] {\small \url{https://lists.gnu.org/mailman/listinfo/taler}}, \\
+ \url{irc://irc.freenode.net/\#taler}
+ \item[Develop:] \url{https://bugs.taler.net/}, \url{https://git.taler.net/}
+ \item[Translate:] \url{https://weblate.taler.net/}, \url{translation-volunteer@taler.net}
+ \item[Integrate:] \url{https://docs.taler.net/}
+ \item[Donate:] \url{https://gnunet.org/ev}
+ \item[Invest:] \url{https://taler-systems.com/}
+ \end{description}
+\end{frame}
+
+
+\begin{frame}
+\frametitle{Do you have any questions?}
+\vfill
+References:
+{\tiny
+ \begin{enumerate}
+ \item{David Chaum, Christian Grothoff and Thomas Moser.
+ {\em How to issue a central bank digital currency}.
+ {\bf SNB Working Papers, 2021}.}
+ \item{Christian Grothoff, Bart Polot and Carlo von Loesch.
+ {\em The Internet is broken: Idealistic Ideas for Building a GNU Network}.
+ {\bf W3C/IAB Workshop on Strengthening the Internet Against Pervasive Monitoring (STRINT)}, 2014.}
+ \item{Jeffrey Burdges, Florian Dold, Christian Grothoff and Marcello Stanisci.
+ {\em Enabling Secure Web Payments with GNU Taler}.
+ {\bf SPACE 2016}.}
+ \item{Florian Dold, Sree Harsha Totakura, Benedikt M\"uller, Jeffrey Burdges and Christian Grothoff.
+ {\em Taler: Taxable Anonymous Libre Electronic Reserves}.
+ Available upon request. 2016.}
+ \item{Eli Ben-Sasson, Alessandro Chiesa, Christina Garman, Matthew Green, Ian Miers, Eran Tromer and Madars Virza.
+ {\em Zerocash: Decentralized Anonymous Payments from Bitcoin}.
+ {\bf IEEE Symposium on Security \& Privacy, 2016}.}
+ \item{David Chaum, Amos Fiat and Moni Naor.
+ {\em Untraceable electronic cash}.
+ {\bf Proceedings on Advances in Cryptology, 1990}.}
+ \item{Phillip Rogaway.
+ {\em The Moral Character of Cryptographic Work}.
+ {\bf Asiacrypt}, 2015.} \label{bib:rogaway}
+\end{enumerate}
+}
+\begin{center}
+ {\bf Let money facilitate trade; but ensure capital serves society.}
+\end{center}
+\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}
+\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}
+
+
+\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}
+
+
+\section{Integration considerations}
+
+\begin{frame}
+ \vfill
+ \begin{center}
+ {\bf Part IV: 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}
+
+
+
+\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}
diff --git a/presentations/comprehensive/images/bfh.png b/presentations/comprehensive/images/bfh.png
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diff --git a/presentations/comprehensive/images/fraunhofer.png b/presentations/comprehensive/images/fraunhofer.png
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diff --git a/presentations/comprehensive/images/fub.pdf b/presentations/comprehensive/images/fub.pdf
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diff --git a/presentations/comprehensive/researchexchange.tex b/presentations/comprehensive/researchexchange.tex
@@ -0,0 +1,778 @@
+%\pdfminorversion=3
+\documentclass[fleqn,xcolor={usenames,dvipsnames}]{beamer}
+\include{setup}
+
+\title{Zero-Knowledge Age Restriction for GNU Taler}
+%\subtitle{}
+
+\setbeamertemplate{navigation symbols}{%
+ \includegraphics[width=1.6cm]{images/fub.pdf}
+ \hspace{1em}
+ \includegraphics[width=2.3cm]{images/bfh.png}
+ \includegraphics[width=1cm]{images/taler-logo-2020.jpg}
+ \includegraphics[width=1cm]{images/fraunhofer.png}
+ \hfill%
+}
+%\setbeamercovered{transparent=1}
+
+\author{Özgür Kesim, Christian Grothoff,\\ Florian Dold, Martin Schanzenbach}
+\institute{FU Berlin, BFH, Taler Systems SA, Fraunhofer AISEC}
+
+
+\begin{document}
+
+
+\section{Introduction}
+
+\begin{frame}{Age restriction in E-commerce}
+
+ \begin{description}
+ \item[Problem:]~\\[1em]
+ Verification of minimum age requirements in e-commerce.\\[2em]
+
+ \item[Common solutions:]
+
+\begin{tabular}{l<{\onslide<2->}c<{\onslide<3->}cr<{\onslide}}
+ & \blue{Privacy} & \tikzmark{topau} \blue{Ext. authority}& \\[\medskipamount]
+ 1. ID Verification & bad & required & \\[\medskipamount]
+ 2. Restricted Accounts & bad & required & \\[\medskipamount]
+ 3. Attribute-based & good & required &\tikzmark{bottomau} \\[\medskipamount]
+\end{tabular}
+ \end{description}
+
+\uncover<4->{
+ \begin{tikzpicture}[overlay,remember picture]
+ \draw[orange,thick,rounded corners]
+ ($(pic cs:topau) +(0,0.5)$) rectangle ($(pic cs:bottomau) -(0.3, 0.2)$);
+ \end{tikzpicture}
+ \begin{center}
+ \bf Principle of Subsidiarity is violated
+ \end{center}
+}
+\end{frame}
+
+
+\begin{frame}{Principle of Subsidiarity}
+\begin{center} \Large
+ Functions of government---such as granting and restricting
+ rights---should be performed\\
+ {\it at the lowest level of authority possible},\\
+ as long as they can be performed {\it adequately}.
+\end{center}
+\vfill
+\uncover<2->{
+ For age-restriction, the lowest level of authority is:\\
+ \begin{center}\Large
+ Parents, guardians and caretakers
+ \end{center}
+}
+\end{frame}
+
+\begin{frame}{Our contribution}
+Design and implementation of an age restriction scheme\\
+with the following goals:
+
+\begin{enumerate}
+\item It ties age restriction to the \textbf{ability to pay} (not to ID's)
+\item maintains \textbf{anonymity of buyers}
+\item maintains \textbf{unlinkability of transactions}
+\item aligns with \textbf{principle of subsidiartiy}
+\item is \textbf{practical and efficient}
+\end{enumerate}
+
+\end{frame}
+
+
+\section{Age Restriction}
+
+\begin{frame}{Age restriction}
+ \framesubtitle{Assumptions and scenario}
+
+ \begin{columns}
+ \column{7.5cm}
+ \begin{itemize}
+ \item<1-> Assumption: Checking accounts are under control of eligible adults/guardians.
+ \item<2-> \textit{Guardians} \textbf{commit} to an maximum age
+ \item<3-> \textit{Minors} \textbf{attest} their adequate age
+ \item<4-> \textit{Merchants} \textbf{verify} the attestations
+ \item<5-> Minors \textbf{derive} age commitments from existing ones
+ \item<6-> \textit{Exchanges} \textbf{compare} the derived age commitments
+ \end{itemize}
+ \column{5cm}
+ \uncover<7->
+ {
+ \begin{center}
+ \fontsize{7pt}{7pt}\selectfont
+ \begin{tikzpicture}[scale=.5]
+ \node[circle,minimum size=15pt,fill=black!15] at ( 60:4) (Exchange) {$\Exchange$};
+ \node[circle,minimum size=15pt,fill=black!15] at ( 0:0) (Client) {$\Child$};
+ \node[circle,minimum size=15pt,fill=black!15] at ( 0:4) (Merchant) {$\Merchant$};
+ \node[circle,minimum size=15pt,fill=blue!15] at (140:3) (Guardian) {$\Guardian$};
+
+ \draw[->] (Guardian) to [out=50,in=130, loop] node[above]
+ {$\Commit$} (Guardian);
+ \draw[->,blue] (Client) to [out=-125,in=-190, loop] node[below,left]
+ {\blue{$\Attest$}} (Client);
+ \draw[->,blue] (Merchant) to [out=50,in=130, loop] node[above]
+ {\blue{$\Verify$}} (Merchant);
+ \draw[->,orange] (Client) to [out=-35,in=-100, loop] node[below]
+ {\orange{$\Derive$}} (Client);
+ \draw[->,orange] (Exchange) to [out=50,in=130, loop] node[above]
+ {\orange{$\Compare$}} (Exchange);
+
+ \draw[orange,|->] (Client) to node[sloped,above,align=left]
+ {\orange{\scriptsize }} (Exchange);
+ \draw[blue,|->] (Client) to node[sloped, above]
+ {\blue{\scriptsize }} (Merchant);
+ \draw[,|->] (Guardian) to node[above,sloped,align=left]
+ {{\scriptsize }} (Client);
+ \end{tikzpicture}
+ \end{center}
+ }
+ \end{columns}
+ \vfill
+ \uncover<7->{Note: Scheme is independent of payment service protocol.}
+\end{frame}
+
+
+\begin{frame}{Formal Function Signatures}
+\small
+Searching for functions \uncover<2->{with the following signatures}
+\begin{align*}
+ &\bf \Commit\uncover<2->{:
+ &(\age, \omega) &\mapsto (\commitment, \pruf)
+ &\scriptstyle \N_\Age \times \Omega &\scriptstyle \to \Commitments\times\Proofs,
+ }
+ \\
+ &\bf \Attest\uncover<3->{:
+ &(\minage, \commitment, \pruf) &\mapsto \attest
+ &\scriptstyle \N_\Age\times\Commitments\times\Proofs &\scriptstyle \to \Attests \cup \{\Nil\},
+ }
+ \\
+ &\bf \Verify\uncover<4->{:
+ &(\minage, \commitment, \attest) &\mapsto b
+ &\scriptstyle \N_\Age\times\Commitments\times\Attests &\scriptstyle \to \Z_2,
+ }
+ \\
+ &\bf \Derive\uncover<5->{:
+ &(\commitment, \pruf, \omega) &\mapsto (\commitment', \pruf', \blinding)
+ &\scriptstyle \Commitments\times\Proofs\times\Omega &\scriptstyle \to \Commitments\times\Proofs\times\Blindings,
+ }
+ \\
+ &\bf \Compare\uncover<6->{:
+ &(\commitment, \commitment', \blinding) &\mapsto b
+ &\scriptstyle \Commitments\times\Commitments\times\Blindings &\scriptstyle \to \Z_2,
+ }
+\end{align*}
+ \uncover<7->{
+ with $\Omega, \Proofs, \Commitments, \Attests, \Blindings$
+ sufficiently large sets.\\[1em]
+ Basic and security requirements are defined later.\\[2em]
+ }
+
+ \scriptsize
+ \uncover<2->{
+ Mnemonics:\\
+ $\Commitments=$ \textit{c$\Commitments$mmitments},
+ $\commitment=$ \textit{Q-mitment} (commitment),
+ $\Proofs=$ \textit{$\Proofs$roofs},
+ }
+ \uncover<3->{
+ $\pruf=$ \textit{$\pruf$roof},\\
+ $\Attests=$ \textit{a$\Attests$testations},
+ $\attest=$ \textit{a$\attest$testation},
+ }
+ \uncover<5->{
+ $\Blindings=$ \textit{$\Blindings$lindings},
+ $\blinding=$ \textit{$\blinding$linding}.
+ }
+\end{frame}
+
+\begin{frame}{Age restriction}
+ \framesubtitle{Naïve scheme}
+ \begin{center}
+ \begin{tikzpicture}[scale=.85]
+ \node[circle,minimum size=20pt,fill=black!15] at ( 60:4) (Exchange) {$\Exchange$};
+ \node[circle,minimum size=20pt,fill=black!15] at ( 0:0) (Client) {$\Child$};
+ \node[circle,minimum size=20pt,fill=black!15] at ( 0:4) (Merchant) {$\Merchant$};
+ \node[circle,minimum size=20pt,fill=blue!15] at (140:3) (Guardian) {$\Guardian$};
+
+ \draw[->] (Guardian) to [out=50,in=130, loop] node[above]
+ {$\Commit$} (Guardian);
+ \draw[->,blue] (Client) to [out=-125,in=-190, loop] node[below,left]
+ {\blue{$\Attest$}} (Client);
+ \draw[->,blue] (Merchant) to [out=50,in=130, loop] node[above]
+ {\blue{$\Verify$}} (Merchant);
+ \draw[->,orange] (Client) to [out=-35,in=-100, loop] node[below]
+ {\orange{$\Derive$}} (Client);
+ \draw[->,orange] (Exchange) to [out=50,in=130, loop] node[above]
+ {\orange{$\Compare$}} (Exchange);
+
+ \draw[orange,|->] (Client) to node[sloped,above,align=left]
+ {\orange{\scriptsize }} (Exchange);
+ \draw[blue,|->] (Client) to node[sloped, above]
+ {\blue{\scriptsize }} (Merchant);
+ \draw[,|->] (Guardian) to node[above,sloped,align=left]
+ {{\scriptsize }} (Client);
+ \end{tikzpicture}
+ \end{center}
+\end{frame}
+
+\begin{frame}{Achieving Unlinkability}
+ \begin{columns}
+ \column{3cm}
+ \begin{center}
+ \fontsize{8pt}{9pt}\selectfont
+ \begin{tikzpicture}[scale=.65]
+ \node[circle,minimum size=20pt,fill=black!15] at ( 60:4) (Exchange) {$\Exchange$};
+ \node[circle,minimum size=20pt,fill=black!15] at ( 0:0) (Client) {$\Child$};
+
+ \draw[->,orange] (Client) to [out=-35,in=-100, loop] node[below]
+ {\orange{$\footnotesize \Derive()$}} (Client);
+ \draw[->,orange] (Exchange) to [out=50,in=130, loop] node[above]
+ {\orange{$\footnotesize \Compare()$}} (Exchange);
+
+ \draw[orange,|->] (Client) to node[sloped,above,align=left]
+ {\orange{\tiny \uncover<2->{$(\commitment_i,\commitment_{i+1})$}}} (Exchange);
+ \end{tikzpicture}
+ \end{center}
+
+ \column{9cm}
+ Simple use of $\Derive()$ and $\Compare()$ is problematic.
+
+ \begin{itemize}
+ \item<2-> Calling $\Derive()$ iteratively generates sequence
+ $(\commitment_0, \commitment_1, \dots)$ of commitments.
+ \item<2-> Exchange calls $\Compare(\commitment_i, \commitment_{i+1}, .)$
+ \item[$\implies$]\uncover<3->{\bf Exchange identifies sequence}
+ \item[$\implies$]\uncover<3->{\bf Unlinkability broken}
+ \end{itemize}
+ \end{columns}
+\end{frame}
+
+\begin{frame}{Achieving Unlinkability}
+ Define cut\&choose protocol \orange{$\DeriveCompare$},
+ using $\Derive()$ and $\Compare()$.\\[0.5em]
+ \uncover<2->{
+ Sketch:
+ \small
+ \begin{enumerate}
+ \item $\Child$ derives commitments $(\commitment_1,\dots,\commitment_\kappa)$
+ from $\commitment_0$ \\
+ by calling $\Derive()$ with blindings $(\beta_1,\dots,\beta_\kappa)$
+ \item $\Child$ calculates $h_0:=H\left(H(\commitment_1, \beta_1)||\dots||H(\commitment_\kappa, \beta_\kappa)\right)$
+ \item $\Child$ sends $\commitment_0$ and $h_0$ to $\Exchange$
+ \item $\Exchange$ chooses $\gamma \in \{1,\dots,\kappa\}$ randomly
+ \item $\Child$ reveals $h_\gamma:=H(\commitment_\gamma, \beta_\gamma)$ and all $(\commitment_i, \beta_i)$, except $(\commitment_\gamma, \beta_\gamma)$
+ \item $\Exchange$ compares $h_0$ and
+ $H\left(H(\commitment_1, \beta_1)||...||h_\gamma||...||H(\commitment_\kappa, \beta_\kappa)\right)$\\
+ and evaluates $\Compare(\commitment_0, \commitment_i, \beta_i)$.
+ \end{enumerate}
+ \vfill
+ Note: Scheme is similar to the {\it refresh} protocol in GNU Taler.
+ }
+\end{frame}
+
+\begin{frame}{Achieving Unlinkability}
+ With \orange{$\DeriveCompare$}
+ \begin{itemize}
+ \item $\Exchange$ learns nothing about $\commitment_\gamma$,
+ \item trusts outcome with $\frac{\kappa-1}{\kappa}$ certainty,
+ \item i.e. $\Child$ has $\frac{1}{\kappa}$ chance to cheat.
+ \end{itemize}
+ \vfill
+ Note: Still need Derive and Compare to be defined.
+\end{frame}
+
+\begin{frame}{Refined scheme}
+
+ \begin{tikzpicture}[scale=.8]
+ \node[circle,minimum size=25pt,fill=black!15] at ( 0:0) (Client) {$\Child$};
+ \node[circle,minimum size=25pt,fill=black!15] at ( 60:5) (Exchange) {$\Exchange$};
+ \node[circle,minimum size=25pt,fill=black!15] at ( 0:5) (Merchant) {$\Merchant$};
+ \node[circle,minimum size=25pt,fill=blue!15] at (130:3) (Guardian) {$\Guardian$};
+
+ \draw[orange,<->] (Client) to node[sloped,below,align=center]
+ {\orange{$\DeriveCompare$}} (Exchange);
+ \draw[blue,->] (Client) to node[sloped, below]
+ {\blue{$(\attest_\minage, \commitment)$}} (Merchant);
+
+ \draw[->] (Guardian) to [out=150,in=70, loop] node[above]
+ {$\Commit(\age)$} (Guardian);
+ \draw[->] (Guardian) to node[below,sloped]
+ {($\commitment$, $\pruf_\age$)} (Client);
+ \draw[->,blue] (Client) to [out=-50,in=-130, loop] node[below]
+ {\blue{$\Attest(\minage, \commitment, \pruf_{\age})$}} (Client);
+ \draw[->,blue] (Merchant) to [out=-50,in=-130, loop] node[below]
+ {\blue{$\Verify(\minage, \commitment, \attest_{\minage})$}} (Merchant);
+ \end{tikzpicture}
+\end{frame}
+
+% \begin{frame}{Achieving Unlinkability}
+% \scriptsize
+% $\DeriveCompare : \Commitments\times\Proofs\times\Omega \to \{0,1\}$\\
+% \vfill
+% $\DeriveCompare(\commitment, \pruf, \omega) =$
+% \begin{itemize}
+% \it
+% \itemsep0.5em
+% \item[$\Child$:]
+% \begin{enumerate}
+% \scriptsize
+% \itemsep0.3em
+% \item for all $i \in \{1,\dots,\kappa\}:
+% (\commitment_i,\pruf_i,\beta_i) \leftarrow \Derive(\commitment, \pruf, \omega + i)$
+% \item $h \leftarrow \Hash\big(\Hash(\commitment_1,\beta_1)\parallel\dots\parallel\Hash(\commitment_\kappa,\beta_\kappa) \big)$
+% \item send $(\commitment, h)$ to $\Exchange$
+% \end{enumerate}
+% \item[$\Exchange$:]
+% \begin{enumerate}
+% \setcounter{enumi}{4}
+% \scriptsize
+% \itemsep0.3em
+% \item save $(\commitment, h)$ \label{st:hash}
+% \item $\gamma \drawfrom \{1,\dots ,\kappa\}$
+% \item send $\gamma$ to $\Child$
+% \end{enumerate}
+% \item[$\Child$:]
+% \begin{enumerate}
+% \setcounter{enumi}{7}
+%
+% \scriptsize
+% \itemsep0.3em
+% \item $h'_\gamma \leftarrow \Hash(\commitment_\gamma, \beta_\gamma)$
+% \item $\mathbf{E}_\gamma \leftarrow \big[(\commitment_1,\beta_1),\dots,
+% (\commitment_{\gamma-1}, \beta_{\gamma-1}),
+% \Nil,
+% (\commitment_{\gamma+1}, \beta_{\gamma+1}),
+% \dots,(\commitment_\kappa, \beta_\kappa)\big]$
+% \item send $(\mathbf{E}_\gamma, h'_\gamma)$ to $\Exchange$
+% \end{enumerate}
+% \item[$\Exchange$:]
+% \begin{enumerate}
+% \setcounter{enumi}{10}
+% \scriptsize
+% \itemsep0.3em
+% \item for all $i \in \{1,\dots,\kappa\}\setminus\{\gamma\}: h_i \leftarrow \Hash(\mathbf{E}_\gamma[i])$
+% \item if $h \stackrel{?}{\neq} \HashF(h_1\|\dots\|h_{\gamma-1}\|h'_\gamma\|h_{\gamma+1}\|\dots\|h_{\kappa-1})$ return 0
+% \item for all $i \in \{1,\dots,\kappa\}\setminus\{\gamma\}$:
+% if $0 \stackrel{?}{=} \Compare(\commitment,\commitment_i, \beta_i)$ return $0$
+% \item return 1
+% \end{enumerate}
+% \end{itemize}
+% \end{frame}
+
+\begin{frame}{Basic Requirements}
+
+ Candidate functions
+ \[ (\Commit, \Attest, \Verify, \Derive, \Compare) \]
+ must first meet \textit{basic} requirements:
+
+ \begin{itemize}
+ \item Existence of attestations
+ \item Efficacy of attestations
+ \item Derivability of commitments and attestations
+ \end{itemize}
+\end{frame}
+
+\begin{frame}{Basic Requirements}
+ \framesubtitle{Formal Details}
+
+ \begin{description}
+ \item[Existence of attestations]
+ {\scriptsize
+ \begin{align*}
+ \Forall_{\age\in\N_\Age \atop \omega \in \Omega}:
+ \Commit(\age, \omega) =: (\commitment, \pruf)
+ \implies
+ \Attest(\minage, \commitment, \pruf) =
+ \begin{cases}
+ \attest \in \Attests, \text{ if } \minage \leq \age\\
+ \Nil \text{ otherwise}
+ \end{cases}
+ \end{align*}}
+ \item[Efficacy of attestations]
+ {\scriptsize
+ \begin{align*}
+ \Verify(\minage, \commitment, \attest) = \
+ \begin{cases}
+ 1, \text{if } \Exists_{\pruf \in \Proofs}: \Attest(\minage, \commitment, \pruf) = \attest\\
+ 0 \text{ otherwise}
+ \end{cases}
+ \end{align*}}
+
+ {\scriptsize
+ \begin{align*}
+ \forall_{n \leq \age}: \Verify\big(n, \commitment, \Attest(n, \commitment, \pruf)\big) = 1.
+ \end{align*}}
+ \item[etc.]
+ \end{description}
+\end{frame}
+
+%\begin{frame}{Requirements}
+% \framesubtitle{Details}
+%
+% \begin{description}
+% \item[Derivability of commitments and proofs:]~\\[0.1em]
+% {\scriptsize
+% Let \begin{align*}
+% \age & \in\N_\Age,\,\, \omega_0, \omega_1 \in\Omega\\
+% (\commitment_0, \pruf_0) & \leftarrow \Commit(\age, \omega_0),\\
+% (\commitment_1, \pruf_1, \blinding) & \leftarrow \Derive(\commitment_0, \pruf_0, \omega_1).
+% \end{align*}
+% We require
+% \begin{align*}
+% \Compare(\commitment_0, \commitment_1, \blinding) = 1 \label{req:comparity}
+% \end{align*}
+% and for all $n\leq\age$:
+% \begin{align*}
+% \Verify(n, \commitment_1, \Attest(n, \commitment_1, \pruf_1)) &%
+% =
+% \Verify(n, \commitment_0, \Attest(n, \commitment_0, \pruf_0))
+% \end{align*}}
+% \end{description}
+%\end{frame}
+
+\begin{frame}{Security Requirements}
+ Candidate functions must also meet \textit{security} requirements.
+ Those are defined via security games:
+ \begin{itemize}
+ \item Game: Age disclosure by commitment or attestation
+ \item[$\leftrightarrow$] Requirement: Non-disclosure of age
+ \vfill
+
+ \item Game: Forging attestation
+ \item[$\leftrightarrow$] Requirement: Unforgeability of
+ minimum age
+ \vfill
+
+ \item Game: Distinguishing derived commitments and attestations
+ \item[$\leftrightarrow$] Requirement: Unlinkability of
+ commitments and attestations
+
+ \end{itemize}
+ \vfill
+
+ Meeting the security requirements means that adversaries can win
+ those games only with negligible advantage.
+ \vfill
+ Adversaries are arbitrary polynomial-time algorithms, acting on all
+ relevant input.
+\end{frame}
+
+\begin{frame}{Security Requirements}
+ \framesubtitle{Simplified Example}
+
+ \begin{description}
+ \item[Game $\Game{FA}(\lambda)$---Forging an attest:]~\\
+ {\small
+ \begin{enumerate}
+ \item $ (\age, \omega) \drawfrom \N_{\Age-1}\times\Omega $
+ \item $ (\commitment, \pruf) \leftarrow \Commit(\age, \omega) $
+ \item $ (\minage, \attest) \leftarrow \Adv(\age, \commitment, \pruf)$
+ \item Return 0 if $\minage \leq \age$
+ \item Return $\Verify(\minage,\commitment,\attest)$
+ \end{enumerate}
+ }
+ \vfill
+ \item[Requirement: Unforgeability of minimum age]
+ {\small
+ \begin{equation*}
+ \Forall_{\Adv\in\PPT(\N_\Age\times\Commitments\times\Proofs\to \N_\Age\times\Attests)}:
+ \Probability\Big[\Game{FA}(\lambda) = 1\Big] \le \negl(\lambda)
+ \end{equation*}
+ }
+ \end{description}
+\end{frame}
+
+\section{Solution/Instantiation}
+
+\begin{frame}{Solution: Instantiation with ECDSA}
+% \framesubtitle{Definition of Commit}
+
+ \begin{description}
+ \item[To Commit to age (group) $\age \in \{1,\dots,\Age\}$]~\\
+ \begin{enumerate}
+ \item<2-> Guardian generates ECDSA-keypairs, one per age (group):
+ \[\langle(q_1, p_1),\dots,(q_\Age,p_\Age)\rangle\]
+ \item<3-> Guardian then \textbf{drops} all private keys
+ $p_i$ for $i > \age$:
+ \[\Big \langle(q_1, p_1),\dots,
+ (q_\age, p_\age),
+ (q_{\age +1}, \red{\Nil}),\dots,
+ (q_\Age, \red{\Nil})\Big\rangle\]
+
+ \begin{itemize}
+ \item $\Vcommitment := (q_1, \dots, q_\Age)$ is the \textit{Commitment},
+ \item $\Vpruf_\age := (p_1, \dots, p_\age, \Nil,\dots,\Nil)$ is the \textit{Proof}
+ \end{itemize}
+ \vfill
+ \item<4-> Guardian gives child $\langle \Vcommitment, \Vpruf_\age \rangle$
+ \vfill
+ \end{enumerate}
+ \end{description}
+\end{frame}
+
+\begin{frame}{Instantiation with ECDSA}
+ \framesubtitle{Definitions of Attest and Verify}
+
+ Child has
+ \begin{itemize}
+ \item ordered public-keys $\Vcommitment = (q_1, \dots, q_\Age) $,
+ \item (some) private-keys $\Vpruf = (p_1, \dots, p_\age, \Nil, \dots, \Nil)$.
+ \end{itemize}
+ \begin{description}
+ \item<2->[To \blue{Attest} a minimum age $\blue{\minage} \leq \age$:]~\\
+ Sign a message with ECDSA using private key $p_\blue{\minage}$
+ \end{description}
+
+ \vfill
+
+ \uncover<3->{
+ Merchant gets
+ \begin{itemize}
+ \item ordered public-keys $\Vcommitment = (q_1, \dots, q_\Age) $
+ \item Signature $\sigma$
+ \end{itemize}
+ \begin{description}
+ \item<4->[To \blue{Verify} a minimum age $\minage$:]~\\
+ Verify the ECDSA-Signature $\sigma$ with public key $q_\minage$.
+ \end{description}
+ }
+ \vfill
+\end{frame}
+
+\begin{frame}{Instantiation with ECDSA}
+ \framesubtitle{Definitions of Derive and Compare}
+ Child has
+ $\Vcommitment = (q_1, \dots, q_\Age) $ and
+ $\Vpruf = (p_1, \dots, p_\age, \Nil, \dots, \Nil)$.
+ \begin{description}
+ \item<2->[To \blue{Derive} new $\Vcommitment'$ and $\Vpruf'$:]
+ Choose random $\beta\in\Z_g$ and calculate
+ \small
+ \begin{align*}
+ \Vcommitment' &:= \big(\beta * q_1,\ldots,\beta * q_\Age\big),\\
+ \Vpruf' &:= \big(\beta p_1,\ldots,\beta p_\age,\Nil,\ldots,\Nil\big)
+ \end{align*}
+ Note: $ (\beta p_i)*G = \beta*(p_i*G) = \beta*q_i$\\
+ \scriptsize $\beta*q_i$ is scalar multiplication on the elliptic curve.
+ \end{description}
+
+ \vfill
+ \uncover<3->{
+ Exchange gets $\Vcommitment = (q_1,\dots,q_\Age)$, $\Vcommitment' = (q_1', \dots, q_\Age')$ and $\beta$
+ \begin{description}
+ \item[To \blue{Compare}, calculate:]
+ \small
+ $(\beta * q_1, \ldots , \beta * q_\Age) \stackrel{?}{=} (q'_1,\ldots, q'_\Age)$
+ \end{description}
+ \vfill
+ }
+\end{frame}
+
+\begin{frame}{Instantiation with ECDSA}
+
+ Functions
+ (Commit, Attest, Verify, Derive, Compare)\\
+ as defined in the instantiation with ECDSA\\[0.5em]
+ \begin{itemize}
+ \item meet the basic requirements,\\[0.5em]
+ \item also meet all security requirements.\\
+ Proofs by security reduction, details are in the paper.
+ \end{itemize}
+
+\end{frame}
+
+
+% \begin{frame}{Instantiation with ECDSA}
+% \framesubtitle{Full definitions}
+% \scriptsize
+%
+% \begin{align*}
+% \Commit_{E,\FDHg{\cdot}}(\age, \omega) &:= \Big\langle
+% \overbrace{(q_1,\ldots,q_\Age)}^{= \Vcommitment},\;
+% \overbrace{(p_1,\ldots,p_\age, \Nil,\ldots,\Nil)}^{= \Vpruf \text{, length }\Age}
+% \Big\rangle\\
+% \Attest_{E,\HashF}(\bage, \Vcommitment, \Vpruf) &:=
+% \begin{cases}
+% \attest_\bage := \Sign_{E,\HashF}\big(\bage,\Vpruf[\bage]\big) & \text{if } \Vpruf[\bage] \stackrel{?}{\neq} \Nil\\
+% \Nil & \text{otherwise}
+% \end{cases}\\
+% %
+% \Verify_{E,\HashF}(\bage, \Vcommitment, \attest) &:= \Ver_{E,\HashF}(\bage, \Vcommitment[\bage], \attest)\\
+% %
+% \Derive_{E, \FDHg{\cdot}}(\Vcommitment, \Vpruf, \omega) &:=
+% \Big\langle(\beta * q_1,\ldots,\beta * q_\Age),
+% (\beta p_1,\ldots,\beta p_\age,\Nil,\ldots,\Nil), \beta \Big\rangle \\
+% & \text{ with } \beta := \FDHg{\omega} \text{ and multiplication } \beta p_i \text{ modulo } g \nonumber\\
+% %
+% \Compare_E(\Vcommitment, \Vcommitment', \beta) &:=
+% \begin{cases}
+% 1 & \text{if } (\beta * q_1, \ldots , \beta * q_\Age) \stackrel{?}{=} (q'_1,\ldots, q'_\Age)\\
+% 0 & \text{otherwise}
+% \end{cases}
+% \end{align*}
+% \end{frame}
+
+\section{Integration with GNU Taler}
+
+\begin{frame}{GNU Taler}
+ \framesubtitle{https://www.taler.net}
+ \begin{columns}
+ \column{4cm}
+ \fontsize{8pt}{9pt}\selectfont
+ \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}
+ \column{8cm}
+ \begin{itemize}
+ \item Protocol suite for online payment services
+ \item Based on Chaum's blind signatures
+ % \item Taxable, efficient, free software
+ \item Allows for change and refund (F. Dold)
+ \item Privacy preserving: anonymous and unlinkable payments
+ \end{itemize}
+ \end{columns}
+
+ \vfill
+ \uncover<2->{
+ \begin{itemize}
+ \item Coins are public-/private key-pairs $(C_p, c_s)$.
+ \item Exchange blindly signs $\FDH(C_p)$ with denomination key $d_p$
+ \item Verification:
+ \begin{eqnarray*}
+ 1 &\stackrel{?}{=}&
+ \mathsf{SigCheck}\big(\FDH(C_p), D_p, \sigma_p\big)
+ \end{eqnarray*}
+ \scriptsize($D_p$ = public key of denomination and $\sigma_p$ = signature)
+
+ \end{itemize}
+ }
+\end{frame}
+
+\begin{frame}{Integration with GNU Taler}
+ \framesubtitle{Binding age restriction to coins}
+
+ To bind an age commitment $\commitment$ to a coin $C_p$, instead of
+ signing $\FDH(C_p)$, $\Exchange$ now blindly signs
+ \begin{center}
+ $\FDH(C_p, \orange{H(\commitment)})$
+ \end{center}
+
+ \vfill
+ Verfication of a coin now requires $H(\commitment)$, too:
+ \begin{center}
+ $1 \stackrel{?}{=}
+ \mathsf{SigCheck}\big(\FDH(C_p, \orange{H(\commitment)}), D_p, \sigma_p\big)$
+ \end{center}
+ \vfill
+\end{frame}
+
+\begin{frame}{Integration with GNU Taler}
+ \framesubtitle{Integrated schemes}
+ \fontsize{8pt}{9pt}\selectfont
+ \begin{tikzpicture}[scale=.9]
+ \node[circle,minimum size=25pt,fill=black!15] at ( 0:0) (Client) {$\Child$};
+ \node[circle,minimum size=25pt,fill=black!15] at ( 60:5) (Exchange) {$\Exchange$};
+ \node[circle,minimum size=25pt,fill=black!15] at ( 0:5) (Merchant) {$\Merchant$};
+ \node[circle,minimum size=25pt,fill=blue!15] at (130:3) (Guardian) {$\Guardian$};
+
+ \draw[<->] (Guardian) to node[sloped,above,align=center]
+ {{\sf withdraw}\orange{, using}\\ $\FDH(C_p\orange{, H(\commitment)})$} (Exchange);
+ \draw[<->] (Client) to node[sloped,below,align=center]
+ {{\sf refresh} \orange{ + }\\ \orange{$\DeriveCompare$}} (Exchange);
+ \draw[<->] (Client) to node[sloped, below]
+ {{\sf purchase} \blue{+ $(\attest_\minage, \commitment)$}} (Merchant);
+ \draw[<->] (Merchant) to node[sloped, above]
+ {{\sf deposit} \orange{+ $H(\commitment)$}} (Exchange);
+
+ \draw[->] (Guardian) to [out=70,in=150, loop] node[above]
+ {$\Commit(\age)$} (Guardian);
+ \draw[->] (Guardian) to node[below,sloped]
+ {($\commitment$, $\pruf_\age$)} (Client);
+ \draw[->,blue] (Client) to [out=-50,in=-130, loop] node[below]
+ {\blue{$\Attest(\minage, \commitment, \pruf_{\age})$}} (Client);
+ \draw[->,blue] (Merchant) to [out=-50,in=-130, loop] node[below]
+ {\blue{$\Verify(\minage, \commitment, \attest_{\minage})$}} (Merchant);
+ \end{tikzpicture}
+\end{frame}
+
+\begin{frame}{Instantiation with Edx25519}
+ Paper also formally defines another signature scheme: Edx25519.\\[1em]
+
+ \begin{itemize}
+ \item Scheme already in use in GNUnet,
+ \item based on EdDSA (Bernstein et al.),
+ \item generates compatible signatures and
+ \item allows for key derivation from both, private and public keys, independently.
+ \end{itemize}~\\[1em]
+
+ Current implementation of age restriction in GNU Taler uses Edx25519.
+\end{frame}
+
+\section{Discussion, Related Work, Conclusion}
+
+\begin{frame}{Discussion}
+ \begin{itemize}
+ \item Our solution can in principle be used with any token-based payment scheme
+ \item GNU Taler best aligned with our design goals (security, privacy and efficiency)
+ \item Subsidiarity requires bank accounts being owned by adults
+ \begin{itemize}
+ \item Scheme can be adapted to case where minors have bank accounts
+ \begin{itemize}
+ \item Assumption: banks provide minimum age
+ information during bank
+ transactions.
+ \item Child and Exchange execute a variant of
+ the cut\&choose protocol.
+ \end{itemize}
+ \end{itemize}
+ \item Our scheme offers an alternative to identity management systems (IMS)
+ \end{itemize}
+\end{frame}
+\begin{frame}{Related Work}
+ \begin{itemize}
+ \item Current privacy-perserving systems all based on attribute-based credentials (Koning et al., Schanzenbach et al., Camenisch et al., Au et al.)
+ \item Attribute-based approach lacks support:
+ \begin{itemize}
+ \item Complex for consumers and retailers
+ \item Requires trusted third authority
+ \end{itemize}
+ \vfill
+ \item Other approaches tie age-restriction to ability to pay ("debit cards for kids")
+ \begin{itemize}
+ \item Advantage: mandatory to payment process
+ \item Not privacy friendly
+ \end{itemize}
+ \end{itemize}
+\end{frame}
+
+\begin{frame}{Conclusion}
+ Age restriction is a technical, ethical and legal challenge.
+
+ Existing solutions are
+ \begin{itemize}
+ \item without strong protection of privacy or
+ \item based on identity management systems (IMS)
+ \end{itemize}
+ \vfill
+
+ Our scheme offers a solution that is
+ \begin{itemize}
+ \item based on subsidiarity
+ \item privacy preserving
+ \item efficient
+ \item an alternative to IMS
+ \end{itemize}
+\end{frame}
+
+
+\end{document}
diff --git a/presentations/comprehensive/setup.tex b/presentations/comprehensive/setup.tex
@@ -0,0 +1,121 @@
+\usepackage{amsmath}
+\usepackage{multimedia}
+\usepackage[utf8]{inputenc}
+\usepackage{framed,color,ragged2e}
+\usepackage[absolute,overlay]{textpos}
+\definecolor{shadecolor}{rgb}{0.8,0.8,0.8}
+\usetheme{boxes}
+\setbeamertemplate{navigation symbols}{}
+\usepackage{xcolor}
+\usepackage[normalem]{ulem}
+\usepackage{listings}
+\usepackage{adjustbox}
+\usepackage{array}
+\usepackage{bbding}
+\usepackage{relsize}
+\usepackage{graphicx}
+\usepackage{tikz,eurosym,calc}
+\usetikzlibrary{tikzmark}
+\usetikzlibrary{shapes,arrows,arrows.meta}
+\usetikzlibrary{positioning,patterns}
+\usetikzlibrary{calc}
+
+% CSS
+\lstdefinelanguage{CSS}{
+ basicstyle=\ttfamily\scriptsize,
+ keywords={color,background-image:,margin,padding,font,weight,display,position,top,left,right,bottom,list,style,border,size,white,space,min,width, transition:, transform:, transition-property, transition-duration, transition-timing-function},
+ sensitive=true,
+ morecomment=[l]{//},
+ morecomment=[s]{/*}{*/},
+ morestring=[b]',
+ morestring=[b]",
+ alsoletter={:},
+ alsodigit={-}
+}
+
+% JavaScript
+\lstdefinelanguage{JavaScript}{
+ basicstyle=\ttfamily\scriptsize,
+ morekeywords={typeof, new, true, false, catch, function, return, null, catch, switch, var, if, in, while, do, else, case, break},
+ morecomment=[s]{/*}{*/},
+ morecomment=[l]//,
+ morestring=[b]",
+ morestring=[b]'
+}
+
+\lstdefinelanguage{HTML5}{
+ basicstyle=\ttfamily\scriptsize,
+ language=html,
+ sensitive=true,
+ alsoletter={<>=-},
+ morecomment=[s]{<!-}{-->},
+ tag=[s],
+ otherkeywords={
+ % General
+ >,
+ % Standard tags
+ <!DOCTYPE,
+ </html, <html, <head, <title, </title, <style, </style, <link, </head, <meta, />,
+ % body
+ </body, <body,
+ % Divs
+ </div, <div, </div>,
+ % Paragraphs
+ </p, <p, </p>,
+ % scripts
+ </script, <script,
+ % More tags...
+ <canvas, /canvas>, <svg, <rect, <animateTransform, </rect>, </svg>, <video, <source, <iframe, </iframe>, </video>, <image, </image>
+ },
+ ndkeywords={
+ % General
+ =,
+ % HTML attributes
+ charset=, src=, id=, width=, height=, style=, type=, rel=, href=,
+ % SVG attributes
+ fill=, attributeName=, begin=, dur=, from=, to=, poster=, controls=, x=, y=, repeatCount=, xlink:href=,
+ % CSS properties
+ margin:, padding:, background-image:, border:, top:, left:, position:, width:, height:,
+ % CSS3 properties
+ transform:, -moz-transform:, -webkit-transform:,
+ animation:, -webkit-animation:,
+ transition:, transition-duration:, transition-property:, transition-timing-function:,
+ }
+}
+
+\lstdefinelanguage{JavaScript}{
+ basicstyle=\ttfamily\scriptsize,
+ keywords={typeof, new, true, false, catch, function, return, null, catch, switch, var, if, in, while, do, else, case, break, for},
+ keywordstyle=\color{blue}\bfseries,
+ ndkeywords={class, export, boolean, throw, implements, import, this},
+ ndkeywordstyle=\color{darkgray}\bfseries,
+ identifierstyle=\color{black},
+ sensitive=false,
+ comment=[l]{//},
+ morecomment=[s]{/*}{*/},
+ commentstyle=\color{purple}\ttfamily,
+ stringstyle=\color{red}\ttfamily,
+ morestring=[b]',
+ morestring=[b]"
+}
+
+%\usetheme{Dresden}
+\setbeamersize{description width=1em}
+
+\setbeamertemplate{footline}{
+ \hbox{%
+ \begin{beamercolorbox}[wd=\paperwidth,ht=3ex,dp=1.5ex,leftskip=2ex,rightskip=2ex]{page footer}%
+ \usebeamerfont{title in head/foot}%
+ \insertshorttitle \hfill
+ \insertsection \hfill
+ \insertframenumber{} / \inserttotalframenumber
+ \end{beamercolorbox}}%
+}
+
+
+\definecolor{blue}{rgb}{0,0,0.7}
+\newcommand{\orange}[1]{{\color{orange}#1}}
+\newcommand{\TODO}[1]{\orange{TODO: #1}}
+
+
+\input{definitions}
