[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]
[taler-marketing] branch master updated: prep
|
From: |
gnunet |
|
Subject: |
[taler-marketing] branch master updated: prep |
|
Date: |
Wed, 07 Dec 2022 13:22:05 +0100 |
This is an automated email from the git hooks/post-receive script.
grothoff pushed a commit to branch master
in repository marketing.
The following commit(s) were added to refs/heads/master by this push:
new c82f241 prep
c82f241 is described below
commit c82f2416def906165370bb5975ed54d8b9049463
Author: Christian Grothoff <grothoff@gnunet.org>
AuthorDate: Wed Dec 7 13:21:32 2022 +0100
prep
---
presentations/comprehensive/ethz.tex | 2952 ++++++++++++++++++++++++++++++++++
1 file changed, 2952 insertions(+)
diff --git a/presentations/comprehensive/ethz.tex
b/presentations/comprehensive/ethz.tex
new file mode 100644
index 0000000..2c20f92
--- /dev/null
+++ b/presentations/comprehensive/ethz.tex
@@ -0,0 +1,2952 @@
+\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[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,fit,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]"
+}
+
+\setbeamersize{description width=1em}
+
+\definecolor{blue}{rgb}{0,0,0.7}
+\newcommand{\orange}[1]{{\color{orange}#1}}
+\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}
+
+
+\title{GNU Taler}
+%\subtitle{}
+
+\setbeamertemplate{navigation symbols}{\includegraphics[width=1cm]{inria.pdf}
\includegraphics[width=2.3cm]{bfh.png} \includegraphics[width=1.6cm]{fub.pdf}
\includegraphics[width=0.4cm]{ashoka.png}
\includegraphics[width=0.4cm]{gnu.png}
\includegraphics[width=1cm]{taler-logo-2021-inkscape.pdf} \hfill}
+%\setbeamercovered{transparent=1}
+
+\author[C. Grothoff]{J. Burdges, F. Dold, {\bf C. Grothoff}, M. Stanisci}
+\date{\today}
+\institute{The GNU Project}
+
+
+\begin{document}
+
+\justifying
+
+\begin{frame}
+ \begin{center}
+ \LARGE {\bf GNU}
+
+ \vfill
+% \includegraphics[width=0.66\textwidth]{logo-2017-fr.pdf}
+ \includegraphics[width=0.66\textwidth]{taler-logo-2021-inkscape.pdf}
+ \end{center}
+\begin{textblock*}{6cm}(.5cm,7.7cm) % {block width} (coords)
+ {\Large {\bf \href{https://taler.net/}{taler.net}} \\
+ \href{https://twitter.com/taler}{taler@twitter} \\
+ \href{https://taler-systems.com/}{taler-systems.com}}
+\end{textblock*}
+
+% Substitute based on who is giving the talk!
+ \begin{textblock*}{6cm}(6.7cm,7.7cm) % {block width} (coords)
+ {\hfill {\Large {\bf Florian Dold \&} \\
+ \hfill {\bf Christian Grothoff}} \\
+ \hfill \{dold,grothoff\}@taler.net }
+\end{textblock*}
+
+\end{frame}
+
+
+\begin{frame}{A Social Problem}
+% \vfill
+ This was a question posed to RAND researchers in 1971:
+
+\begin{quote}
+ ``Suppose you were an advisor to the head of the KGB, the Soviet Secret
Police. Suppose you are given the assignment of designing a system for the
surveillance of all citizens and visitors within the boundaries of the USSR.
The system is not to be too obtrusive or obvious. What would be your decision?''
+\end{quote}
+%The result: an electronic funds transfer system that looks
+%strikingly similar today's debit card system.
+\pause
+ \begin{center}
+ \includegraphics[height=2cm]{pics/nsa_spy.jpg}
+ \end{center}
+\vfill
+ \begin{center}
+``I think one of the big things that we need to do, is we need
+to get a way from true-name payments on the Internet. The credit
+card payment system is one of the worst things that happened for the
+user, in terms of being able to divorce their access from their
+identity.'' \hfill --Edward Snowden, IETF 93 (2015)
+\end{center}
+
+\end{frame}
+
+
+
+\section{The Bank's Problem}
+\begin{frame}{The Bank's Problem}
+
+ 3D secure (``verified by visa'') is a nightmare:
+
+ \begin{minipage}{5cm}
+ \begin{itemize}
+ \item Complicated process
+ \item Shifts liability to consumer
+ \item Significant latency
+ \item Can refuse valid requests
+ \item Legal vendors excluded
+ \item No privacy for buyers
+ \end{itemize}
+ \end{minipage}
+ \begin{minipage}{5cm}
+ \includegraphics[width=\textwidth]{illustrations/cc3ds.pdf}
+ \end{minipage}
+ \vfill
+ Online credit card payments will be replaced, but with what?
+\end{frame}
+
+
+\begin{frame}{The Bank's Problem}
+\vfill
+ \begin{textblock*}{12cm}(0.5cm,1cm) % {block width} (coords)
+ \begin{itemize}
+ \item Global tech companies push oligopolies
+ \item Privacy and federated finance are at risk
+% \item 30\% fees are conceivable
+ \item Economic sovereignty is in danger
+ \end{itemize}
+\end{textblock*}
+\begin{textblock*}{4cm}(3.5cm,5.2cm) % {block width} (coords)
+ {\includegraphics[width=\textwidth]{../investors/competitor-logos/amazon.png}}
+\end{textblock*}
+\begin{textblock*}{2cm}(7cm,3cm) % {block width} (coords)
+
{\includegraphics[width=\textwidth]{../investors/competitor-logos/alipay.jpeg}}
+\end{textblock*}
+\begin{textblock*}{2cm}(3cm,3.5cm) % {block width} (coords)
+
{\includegraphics[width=\textwidth]{../investors/competitor-logos/paypal.jpeg}}
+\end{textblock*}
+\begin{textblock*}{2cm}(9cm,5cm) % {block width} (coords)
+
{\includegraphics[width=\textwidth]{../investors/competitor-logos/applepay.jpeg}}
+\end{textblock*}
+\begin{textblock*}{2cm}(7.5cm,5.9cm) % {block width} (coords)
+
{\includegraphics[width=\textwidth]{../investors/competitor-logos/samsungpay.jpeg}}
+\end{textblock*}
+\begin{textblock*}{1cm}(9.5cm,6.3cm) % {block width} (coords)
+
{\includegraphics[width=\textwidth]{../investors/competitor-logos/android_pay.png}}
+\end{textblock*}
+\vfill
+\end{frame}
+
+
+\begin{frame}{The Distraction: Bitcoin}
+
+\begin{itemize}
+\item Unregulated payment system and currency:
+\item[] $\Rightarrow$ lack of regulation is a feature!
+\item Implemented in free software
+\item Decentralised peer-to-peer system \pause
+\item Decentralised banking requires solving Byzantine consensus
+\item Creative solution: tie initial accumulation to solving consensus \pause
+\item[] $\Rightarrow$ Proof-of-work advances ledger
+\item[] $\Rightarrow$ Very expensive banking
+\end{itemize}
+\end{frame}
+
+
+\begin{frame}
+ \frametitle{\includegraphics[height=0.5cm]{pics/bitcoin.png}?}
+ \framesubtitle{Background: \url{https://blockchain.com/charts/}}
+ \centering
+\noindent
+\includegraphics[width=\textwidth]{pics/btc-transaction-cost.png}
+
+Current average transaction value: $\approx$ 1000 USD
+\end{frame}
+
+
+\begin{frame}
+ \frametitle{\includegraphics[height=0.5cm]{pics/zerocoin.png}?}
+
+Cryptography is rather primitive:
+\begin{center}
+ {\bf All Bitcoin transactions are public and linkable!}
+\end{center}
+
+\begin{itemize}
+\item[] $\Rightarrow$ no privacy guarantees
+\item[] $\Rightarrow$ enhanced with ``laundering'' services
+\end{itemize}
+ZeroCoin, CryptoNote (Monero) and ZeroCash (ZCash) offer anonymity.
+\end{frame}
+
+
+\begin{frame}
+ \vfill
+\begin{center}
+{\bf Do you want to have a libertarian economy?}
+\end{center}
+ \vfill
+\begin{center}
+{\bf Do you want to live under total surveillance?}
+\end{center}
+\vfill
+\end{frame}
+
+
+
+\begin{frame}{The Bank of International Settlements}
+ \begin{center}
+ \movie[%scale=0.6,
+ autostart,
+ poster]
+ {
+
\includegraphics[height=0.6\textwidth,width=0.8\textwidth]{white.png}
+ }
+ {bis-cbdc.mp4}
+ \end{center}
+\end{frame}
+
+
+\begin{frame}{The Emergency Act of Canada\footnote{Speech by Premier Kenney,
Alberta, February 2022}}
+ \begin{center}
+ \movie[%scale=0.6,
+ autostart,
+ poster]
+ {
+
\includegraphics[height=0.6\textwidth,width=0.8\textwidth]{ca.png}
+ }
+ {emergencyact.mp4}
+
+ {\tiny \url{https://www.youtube.com/watch?v=NehMAj492SA} (2'2022)}
+ \end{center}
+\end{frame}
+
+
+
+\begin{frame}{GNU Taler}
+ \vfill
+ \begin{center}
+ {\huge {\bf Digital} cash, made \textbf{socially responsible}.}
+ \end{center}
+ \vfill
+ \begin{center}
+ \includegraphics[scale=0.3]{taler-logo-2021-inkscape.pdf}
+ \end{center}
+ \vfill
+ \begin{center}
+ Privacy-Preserving, Practical, Taxable, Free Software, Efficient
+ \end{center}
+ \vfill
+ \vfill
+\ %
+\end{frame}
+
+
+\section{What is Taler?}
+\begin{frame}{What is Taler?}
+ \framesubtitle{\url{https://taler.net/en/features.html}} \noindent
+Taler is
+ \vfill
+ \begin{itemize}
+ \item a Free/Libre software \emph{payment system} infrastructure project
+ \item ... with a surrounding software ecosystem
+ \item ... and a company (Taler Systems S.A.) and community that wants to
deploy it
+ as widely as possible.
+ \end{itemize}
+ \vfill
+\noindent
+ However, Taler is
+ \begin{itemize}
+ \item \emph{not} a currency
+ \item \emph{not} a long-term store of value
+ \item \emph{not} a network or instance of a system
+ \item \emph{not} decentralized
+ \item \emph{not} based on proof-of-work or proof-of-stake
+ \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}
+ % TODO: replace with simplified NEW architecture picture!
+\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}{Use Case: Journalism}
+ Today:
+ \begin{itemize}
+ \item Corporate structure % ($\Rightarrow$ filter)
+ \item Advertising primary revenue % ($\Rightarrow$ dependence)
+ \item Tracking readers critical for business success
+ \item Journalism and marketing hard to distinguish
+ \end{itemize}\vfill\pause
+ With GNU Taler:
+ \begin{itemize}
+ \item One-click micropayments per article
+ \item Hosting requires no expertise % (no PCI DSS)
+ \item Reader-funded reporting separated from marketing
+ \item Readers can remain anonymous
+ \end{itemize}
+\end{frame}
+
+
+\begin{frame}{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}{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}{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}
+
+
+\section{Age restrictions}
+
+\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}{Age restriction design for GNU Taler}
+Design and implementation of an age restriction scheme\\
+with the following goals:
+
+\begin{enumerate}
+\item It ties age restriction to the \textbf{ability to pay} (not to ID's)
+\item maintains \textbf{anonymity of buyers}
+\item maintains \textbf{unlinkability of transactions}
+\item aligns with \textbf{principle of subsidiartiy}
+\item is \textbf{practical and efficient}
+\end{enumerate}
+
+\end{frame}
+
+
+\begin{frame}{Age restriction}
+ \framesubtitle{Assumptions and scenario}
+
+ \begin{columns}
+ \column{7.5cm}
+ \begin{itemize}
+ \item<1-> Assumption: Checking accounts are under control of
eligible adults/guardians.
+ \item<2-> \textit{Guardians} \textbf{commit} to an maximum age
+ \item<3-> \textit{Minors} \textbf{attest} their adequate age
+ \item<4-> \textit{Merchants} \textbf{verify} the attestations
+ \item<5-> Minors \textbf{derive} age commitments from existing
ones
+ \item<6-> \textit{Exchanges} \textbf{compare} the derived age
commitments
+ \end{itemize}
+ \column{5cm}
+ \uncover<7->
+ {
+ \begin{center}
+ \fontsize{7pt}{7pt}\selectfont
+ \begin{tikzpicture}[scale=.5]
+ \node[circle,minimum size=15pt,fill=black!15] at ( 60:4)
(Exchange) {$\Exchange$};
+ \node[circle,minimum size=15pt,fill=black!15] at ( 0:0)
(Client) {$\Child$};
+ \node[circle,minimum size=15pt,fill=black!15] at ( 0:4)
(Merchant) {$\Merchant$};
+ \node[circle,minimum size=15pt,fill=blue!15] at (140:3)
(Guardian) {$\Guardian$};
+
+ \draw[->] (Guardian) to [out=50,in=130, loop] node[above]
+ {$\Commit$} (Guardian);
+ \draw[->,blue] (Client) to [out=-125,in=-190, loop]
node[below,left]
+ {\blue{$\Attest$}} (Client);
+ \draw[->,blue] (Merchant) to [out=50,in=130, loop] node[above]
+ {\blue{$\Verify$}} (Merchant);
+ \draw[->,orange] (Client) to [out=-35,in=-100, loop]
node[below]
+ {\orange{$\Derive$}} (Client);
+ \draw[->,orange] (Exchange) to [out=50,in=130, loop] node[above]
+ {\orange{$\Compare$}} (Exchange);
+
+ \draw[orange,|->] (Client) to node[sloped,above,align=left]
+ {\orange{\scriptsize }} (Exchange);
+ \draw[blue,|->] (Client) to node[sloped, above]
+ {\blue{\scriptsize }} (Merchant);
+ \draw[,|->] (Guardian) to node[above,sloped,align=left]
+ {{\scriptsize }} (Client);
+ \end{tikzpicture}
+ \end{center}
+ }
+ \end{columns}
+ \vfill
+ \uncover<7->{Note: Scheme is independent of payment service protocol.}
+\end{frame}
+
+
+\begin{frame}{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}
+
+
+\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}
+
+
+\begin{frame}{Reminder: GNU Taler Fundamentals}
+ \begin{center}
+ \begin{tikzpicture}[scale=.55]
+ \node[circle,fill=black!10] at (3, 4) (Exchange) {$\Exchange$};
+ \node[circle,fill=black!10] at (0, 0) (Customer) {$\Customer$};
+ \node[circle,fill=black!10] at (6, 0) (Merchant) {$\Merchant$};
+
+ \draw[<->] (Customer) to [out=65,in=220] node[sloped,above]
{\sf withdraw} (Exchange);
+ \draw[<->] (Customer) to [out=45,in=240] node[sloped,below]
{\sf refresh} (Exchange);
+ \draw[<->] (Customer) to node[sloped, below] {\sf purchase}
(Merchant);
+ \draw[<->] (Merchant) to node[sloped, above] {\sf deposit}
(Exchange);
+ \end{tikzpicture}
+ \end{center}
+
+ \vfill
+ \begin{itemize}
+ \item Coins are public-/private key-pairs $(C_p, c_s)$.
+ \item Exchange blindly signs $\FDH(C_p)$ with denomination key
$d_p$
+ \item Verification:
+ \begin{eqnarray*}
+ 1 &\stackrel{?}{=}&
+ \mathsf{SigCheck}\big(\FDH(C_p), D_p, \sigma_p\big)
+ \end{eqnarray*}
+ \scriptsize($D_p$ = public key of denomination and $\sigma_p$ =
signature)
+
+ \end{itemize}
+\end{frame}
+
+\begin{frame}{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}
+
+
+\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}
+
+
+\begin{frame}{Blockchain based cryptocurrencies}
+ \begin{tikzpicture}[remember picture,overlay]
+ \node (N1)[above right=5mm and 25mm of current page.center]
{\includegraphics[width=34mm]{media/news1.png}};
+ \node (N0)[below=-3mm of N1]
{\includegraphics[width=34mm]{media/news0.png}};
+ \node (N2)[below left=-26mm and -2.5mm of N1]
{\includegraphics[width=34mm]{media/news2.png}};
+ \end{tikzpicture}
+ \begin{block}{Biggest cryptocurrencies}
+ \begin{itemize}
+ \item \textbf{BTC} Bitcoin
+ \item \textbf{ETH} Ethereum
+ \end{itemize}
+ \end{block}
+ \begin{block}{Common blockchain limitations}
+ \begin{itemize}
+ \item \textbf{Delay} block and confirmation delay
+ \item \textbf{Cost} transaction fees
+ \item \textbf{Scalability} limited amount of transaction per second
+ \item \textbf{Ecological impact} computation redundancy
+ \item \textbf{Privacy}
+ \item \textbf{Regulatory risk}
+ \end{itemize}
+ \end{block}
+\end{frame}
+
+\begin{frame}{Taler}{Architecture}
+ \begin{columns}
+ \column{0.5\paperwidth}
+ \begin{tikzpicture}[
+ rect/.style={circle, draw=black},
+ sym/.style={-stealth, shorten >= 2pt, shorten <= 2pt}
+ ]
+ % Taler payment system
+ \node[rect](1) {Exchange};
+ \node[rect,below left=1.5cm and 0.7cm of 1](2) {Customer};
+ \node[rect,below right=1.5cm and 0.7cm of 1](3) {Merchant};
+
+ \draw[sym] (1) -- node [midway, above, sloped] {\tiny Withdraw
coins} (2);
+ \draw[sym] (2) -- node [midway, above, sloped] {\tiny Spend coins}
(3);
+ \draw[sym] (3) -- node [midway, above, sloped] {\tiny Deposit
coins} (1);
+
+ % Settlement layer
+ \node[left=2cm of 1](E1){};
+ \node[right=2cm of 1](E2){};
+ \draw[sym] (E1) -- node [midway, above] {\tiny Deposit money} (1);
+ \draw[sym] (1) -- node [midway, above] {\tiny Withdraw money} (E2);
+
+ % Auditor
+ \node[above= of 1](A){Auditor};
+ \draw[sym] (A) -- node [midway, right] {\tiny Verify} (1);
+
+ % Separator
+ \node[below=1mm of E1] (S1S) {};
+ \node[below=1mm of E2] (S1E) {};
+ \node[above=6mm of E1] (S2S) {};
+ \node[above=6mm of E2] (S2E) {};
+
+ \draw[dotted] (S1S) -- (S1E);
+ \draw[dotted] (S2S) -- (S2E);
+
+ \node[below right=-2mm and -1.5mm of S2S] {\tiny{\emph{Settlement
Layer}}};
+ \node[below right=-2mm and -1.5mm of S1S] {\tiny{\emph{Taler
payment system}}};
+ \end{tikzpicture}
+ \column{0.47\paperwidth}
+ \begin{block}{Settlement layer}
+ \begin{itemize}
+ \item This work, Blockchain!
+ \end{itemize}
+ \end{block}
+ \begin{block}{Taler payment system}
+ \begin{itemize}
+ \item Realtime transactions, 1 RTT
+ \item Scalable microtransactions
+ \item Blind signatures (privacy)
+ \end{itemize}
+ \end{block}
+
+ \end{columns}
+\end{frame}
+
+\begin{frame}{Taler}{Blockchain settlement layer}
+ \begin{center}
+ \begin{tikzpicture}[
+ rect/.style={rectangle, draw=black, minimum width=30mm},
+ sym/.style={stealth-stealth, shorten >= 2pt, shorten <= 2pt},
+ block/.style={rectangle,draw=black,fill=black!10,minimum
size=7mm},
+ ]
+
+ %% Architecture
+ \node(Tt){Taler};
+ \node[rect,below=0cm of Tt](Tc){Exchange};
+ \node[rect,fit={(Tt) (Tc)}](T){};
+
+ \node[rect,below=7mm of Tc](D) {\textbf{Depolymerization}};
+
+ \node[rect,below=7mm of D](Bc){Node};
+ \node[below=0cm of Bc](Bt){Blockchain};
+ \node[rect,fit={(Bt) (Bc)}](B){};
+
+ \draw[sym] (T) -- (D);
+ \draw[sym] (D) -- (B);
+
+ %% Blockchain
+ \node[block,right=8mm of B] (1){};
+ \node[block,right=4mm of 1] (2){};
+ \node[block,right=4mm of 2] (3){};
+ \node[block,right=4mm of 3] (4){};
+ \node[block,right=4mm of 4] (5){};
+ \node[block,right=4mm of 5] (6){};
+ \draw[-stealth] (1) -- (2);
+ \draw[-stealth] (2) -- (3);
+ \draw[-stealth] (3) -- (4);
+ \draw[-stealth] (4) -- (5);
+ \draw[-stealth] (5) -- (6);
+
+ \node[left=4mm of 1] (S){};
+ \node[right=4mm of 6] (E){};
+ \draw[-stealth] (S) -- (1);
+ \draw[-stealth] (6) -- (E);
+
+ %% Taler
+ \node[block, below right=-7.5mm and 20.5mm of T] (off){Off-chain
transactions};
+ \node[above=-0.5mm of off]
{\includegraphics[height=7mm]{media/taler.png}};
+
+ %% Depolymerization
+ \node[right=11mm of D] {\small{Credit}};
+ \node[right=50mm of D] {\small{Debit}};
+ \draw[dashed,-stealth] (1.north) |- (off.west);
+ \draw[dashed,-stealth] (off.east) -| (6.north);
+ \end{tikzpicture}
+ \end{center}
+\end{frame}
+
+\begin{frame}{Challenges}
+ \begin{block}{Taler Metadata}
+ \begin{itemize}
+ \item Metadata are required to link a wallet to credits and
+ allow merchant to link deposits to debits
+ \item Putting metadata in blockchain transactions can be tricky
+ \end{itemize}
+ \end{block}
+ \begin{block}{Blockchain based cryptocurrencies}
+ \begin{itemize}
+ \item Blockchain transactions lack finality (fork)
+ \item Transactions can be stuck for a long time (mempool)
+ \end{itemize}
+ \end{block}
+\end{frame}
+
+\begin{frame}{Blockchain challenges}{Chain reorganization}
+ \begin{center}
+ \begin{tikzpicture}[
+ block/.style={rectangle,draw=black,fill=black!10,minimum
size=7mm},
+ ar/.style={-stealth}
+ ]
+ % Common
+ \node[block](1){};
+ \node[block,right=5mm of 1](2){$D_0$};
+ \node[block,right=5mm of 2](3){};
+ \draw[ar] (1) -- (2);
+ \draw[ar] (2) -- (3);
+
+ % Current
+ \node [block,right=5mm of 3](4){};
+ \node[block,right=5mm of 4](5){};
+ \node[block,right=5mm of 5](6){$D_1$};
+ \draw[ar] (3) -- (4);
+ \draw[ar] (4) -- (5);
+ \draw[ar] (5) -- (6);
+
+ % Fork
+ \node [block,above=7mm of 4](4p){};
+ \node[block,right=5mm of 4p](5p){$D_2$};
+ \node[block,right=5mm of 5p](6p){};
+ \node[block,right=5mm of 6p](7p){};
+ \draw[ar] (3.east) -- (4p.west);
+ \draw[ar] (4p) -- (5p);
+ \draw[ar] (5p) -- (6p);
+ \draw[ar] (6p) -- (7p);
+
+ % Indication
+ \node [right=5mm of 7p]{\emph{fork}};
+ \node [right=17mm of 6]{\emph{active}};
+ \end{tikzpicture}
+ \end{center}
+ A fork is when concurrent blockchain states coexist. Nodes will follow
+ the longest chain, replacing recent blocks if necessary during a
+ blockchain reorganization. If a deposit transaction disappears from the
+ blockchain, an irrevocable withdraw transactions would no longer be backed
+ by credit.
+\end{frame}
+
+\begin{frame}{Blockchain challenges}{Stuck transactions}
+ We want confirmed debits within a limited time frame.
+ \begin{figure}
+ \centering
+ \only<1> {
+ \begin{tikzpicture}[
+ dot/.style={circle,fill,inner sep=1pt,}
+ ]
+ \node (I) {\includegraphics[width=\textwidth]{media/fee.png}};
+ \node [below left=-2.5mm and -1.5cm of I] (Tx) {\small Tx};
+ \node [dot,above=8.4mm of Tx](D) {};
+ \draw [dotted,thick] (Tx) -- (D);
+ \node [left=-4.5cm of Tx] (C) {\small conf};
+ \node [dot,above=8.4mm of C](D1) {};
+ \draw [dotted,thick] (C) -- (D1);
+ \end{tikzpicture}
+ }
+ \only<2> {
+ \includegraphics[width=\textwidth]{media/fee_var.png}
+ \caption{Bitcoin average transaction fee over 6 months {\tiny
(ychart)}}
+ }
+ \end{figure}
+ \only<1>{When we trigger a debit with a fee too small, it may not be
+ confirmed in a timely fashion.}
+ \only<2>{However, transaction fees are unpredictable.}
+\end{frame}
+
+
+\begin{frame}{Depolymerization}{Architecture}
+ \begin{center}
+ \begin{tikzpicture}[
+ rect/.style={rectangle, draw=black, minimum height=6mm,
minimum width=28mm},
+ sym/.style={stealth-stealth, shorten >= 2pt, shorten <= 2pt}
+ ]
+ \node[rect](1) {Taler Exchange};
+ \node[rect,below=of 1](2) {Wire Gateway};
+ \node[rect,right=of 2](3) {PostgreSQL};
+ \node[rect,right=of 3](4) {DLT Adapter};
+ \node[rect,above=of 4](5) {DLT Full Node};
+
+ \draw[sym] (1) -- node [midway,right] {\tiny HTTP} (2);
+ \draw[sym] (2) -- node [midway,above] {\tiny SQL} (3);
+ \draw[sym] (3) -- node [midway,above] {\tiny SQL} (4);
+ \draw[sym] (4) -- node [midway,left ] {\tiny RPC} (5);
+
+
+ \node[above= 2mm of 1]{\small{\emph{Wire Gateway API}}};
+ \node[above= 2mm of 5]{\small{\emph{DLT specific}}};
+ \node[above=22mm of 3](T) {};
+ \draw[dotted] (3) -- (T);
+ \end{tikzpicture}
+ \end{center}
+ \begin{itemize}
+ \item Common database to store transactions state and communicate
+ with notifications
+ \item Wire Gateway for Taler API compatibility
+ \item DLT specific adapter
+ \end{itemize}
+\end{frame}
+
+\begin{frame}{Storing metadata}{Bitcoin}
+ \begin{block}{Bitcoin - Credit}
+ \begin{itemize}
+ \item Transactions from code
+ \item Only 32B + URI
+ \item \textbf{OP\_RETURN}
+ \end{itemize}
+ \end{block}
+ \begin{block}{Bitcoin - Debit}
+ \begin{itemize}
+ \item Transactions from common wallet software
+ \item Only 32B
+ \item \textbf{Fake Segwit Addresses}
+ \end{itemize}
+ \end{block}
+\end{frame}
+\begin{frame}{Storing metadata}{Ethereum}
+ \begin{block}{Smart contract ?}
+ \begin{itemize}
+ \item Logs in smart contract is the recommend way {\tiny
(ethereum.org)}
+ \item Expensive (additional storage and execution fees)
+ \item Avoidable attack surface (error prone)
+ \end{itemize}
+ \end{block}
+ \begin{block}{Custom input format}
+ Use input data in transactions, usually used to call smart contract, to
+ store our metadata.
+ \end{block}
+\end{frame}
+
+\begin{frame}{Handling blockchain reorganization}
+ \begin{center}
+ \begin{tikzpicture}[
+ block/.style={rectangle,draw=black,fill=black!10,minimum
size=7mm},
+ conf/.style={draw=black!60!green,fill=black!60!green!10},
+ nconf/.style={dotted},
+ err/.style={draw=black!60!red,fill=black!60!red!10},
+ ar/.style={-stealth}
+ ]
+ % Common
+ \node[block,conf](1){};
+ \node[block,conf,right=5mm of 1](2){$D_0$};
+ \node[block,conf,right=5mm of 2](3){};
+ \draw[ar] (1) -- (2);
+ \draw[ar] (2) -- (3);
+
+ % Current
+ \only<1>{
+ \node [block,nconf,right=5mm of 3](4){};
+ }
+ \only<2->{
+ \node [block,conf,right=5mm of 3](4){\only<3>{$D_3$}};
+ }
+ \node[block,nconf,right=5mm of 4](5){};
+ \node[block,nconf,right=5mm of 5](6){$D_1$};
+ \draw[ar] (3) -- (4);
+ \draw[ar] (4) -- (5);
+ \draw[ar] (5) -- (6);
+
+ % Fork
+ \only<-2>{
+ \node [block,nconf,above=7mm of 4](4p){};
+ }
+ \only<3>{
+ \node [block,dashed,err,above=7mm of 4](4p){$D_3'$};
+ }
+ \node[block,nconf,right=5mm of 4p](5p){$D_2$};
+ \node[block,nconf,right=5mm of 5p](6p){};
+ \node[block,nconf,right=5mm of 6p](7p){};
+ \draw[ar] (3.east) -- (4p.west);
+ \draw[ar] (4p) -- (5p);
+ \draw[ar] (5p) -- (6p);
+ \draw[ar] (6p) -- (7p);
+
+ % Indication
+ \node [right=5mm of 7p]{\emph{fork}};
+ \node [right=17mm of 6]{\emph{active}};
+ \end{tikzpicture}
+ \end{center}
+ \only<1>{As small reorganizations are common, Satoshi already recommended
to
+ apply a confirmation delay to handle most disturbances and attacks.}
+ \only<2>{If a reorganization longer than the confirmation delay happens,
+ but it did not remove credits, Depolymerizer is safe and automatically
+ resumes.}
+ \only<3>{If a fork removed a confirmed debit, an attacker may create a
+ conflicting transaction. Depolymerizer suspends operation until lost
+ credits reappear.}
+\end{frame}
+
+\begin{frame}{Adaptive confirmation}
+ \begin{center}
+ \begin{tikzpicture}[
+ block/.style={rectangle,draw=black,fill=black!10,minimum
size=7mm},
+ conf/.style={draw=black!60!green,fill=black!60!green!10},
+ nconf/.style={dotted},
+ conft/.style={text=black!60!green},
+ confl/.style={draw=black!60!green},
+ ar/.style={-stealth}
+ ]
+ % Common
+ \node(0){};
+ \node[block,conf,right=5mm of 0](1){};
+ \node[block,conf,right=5mm of 1](2){};
+ \draw[ar] (0) -- (1);
+ \draw[ar] (1) -- (2);
+
+ % Current
+ \node[block,conf,right=5mm of 2](3){};
+ \node[block,nconf,right=5mm of 3](4){};
+ \node[block,nconf,right=5mm of 4](5){};
+ \node[block,nconf,right=5mm of 5](6){};
+ \draw[ar] (2) -- (3);
+ \draw[ar] (3) -- (4);
+ \draw[ar] (4) -- (5);
+ \draw[ar] (5) -- (6);
+
+ % Fork
+ \node[block,nconf,above=7mm of 3](3p){};
+ \node[block,nconf,right=5mm of 3p](4p){};
+ \node[block,nconf,right=5mm of 4p](5p){};
+ \node[block,nconf,right=5mm of 5p](6p){};
+ \node[block,nconf,right=5mm of 6p](7p){};
+ \draw[ar] (2.east) -- (3p.west);
+ \draw[ar] (3p) -- (4p);
+ \draw[ar] (4p) -- (5p);
+ \draw[ar] (5p) -- (6p);
+ \draw[ar] (6p) -- (7p);
+
+ % Indication
+ \node[right=5mm of 7p]{\emph{fork}};
+ \node[right=17mm of 6]{\emph{active}};
+
+ % Confirmation
+ \path (0) -- (1) node[conft,midway, below=6mm] (M) {Max};
+ \path (2) -- (3) node[conft,midway, below=6mm] (N) {New};
+ \path (3) -- (4) node[conft,midway, below=6mm] (I) {Initial};
+ \node[above=25mm of M] (Mp) {};
+ \node[above=25mm of N] (Np) {};
+ \node[above=25mm of I] (Ip) {};
+ \draw[confl,thick,dotted](M) -- (Mp);
+ \draw[confl](N) -- (Np);
+ \draw[confl,thick,dotted](I) -- (Ip);
+ \end{tikzpicture}
+ \end{center}
+ If we experience a reorganization once, its dangerously likely for another
+ one of a similar scope to happen again. Depolymerizer learns from
reorganizations
+ by increasing its confirmation delay.
+\end{frame}
+
+
+
+\begin{frame}{DLT Adapter}{Architecture}
+ \begin{block}{Event system}
+ \begin{itemize}
+ \item \textbf{Watcher} watch and notify for new blocks with credits
+ \item \textbf{Wire Gateway} notify requested debits
+ \item \textbf{Worker} operates on notifications updating state
+ \end{itemize}
+ \end{block}
+\end{frame}
+
+
+\begin{frame}{DLT Adapter state machine}
+ \begin{columns}
+ \column{0.5\paperwidth}
+ \begin{figure}
+ \begin{tikzpicture}[
+ rect/.style={rectangle, draw=black, minimum height=6mm,
minimum width=50mm},
+ ]
+
+ \node[rect](wo1) {Wait for notifications};
+ \node[rect, below=4mm of wo1](wo2) {Reconcile local DB with
DLT};
+ \node[rect, below=4mm of wo2](wo3) {Trigger debits};
+ \node[rect, below=4mm of wo3](wo4) {Reissue stuck debits};
+ \node[rect, below=4mm of wo4](wo5) {Bounce malformed credits};
+ \draw[-stealth] (wo1) -- (wo2);
+ \draw[-stealth] (wo2) -- (wo3);
+ \draw[-stealth] (wo3) -- (wo4);
+ \draw[-stealth] (wo4) -- (wo5);
+ \draw[-stealth] (wo5) .. controls ([xshift=-0.4cm] wo5.west)
and ([xshift=-0.4cm] wo1.west) .. (wo1);
+ \end{tikzpicture}
+ \caption{Worker loop}
+ \end{figure}
+ \column{0.47\paperwidth}
+ \begin{block}{DLT reconcialisation}
+ \begin{itemize}
+ \item List new and removed transactions since last
reconciliation
+ \item Check for confirmed credits removal
+ \item Register new credits
+ \item Recover lost debits
+ \end{itemize}
+ \end{block}
+ \end{columns}
+\end{frame}
+
+\begin{frame}{Related work}
+ \begin{block}{Centralization - Coinbase off-chain sending}
+ \begin{itemize}
+ \item [$+$] Fast and cheap: off chain transaction
+ \item [$-$] Trust in Coinbase: privacy, security \& transparency
+ \end{itemize}
+ \end{block}
+ \begin{block}{Layering - Lightning Network}
+ \begin{itemize}
+ \item [$+$] Fast and cheap: off-chain transactions
+ \item [$-$] Requires setting up bidirectional payment channels
+ \item [$-$] Fraud attempts are mitigated via a complex penalty
system
+ \end{itemize}
+ \end{block}
+\end{frame}
+
+\begin{frame}{Conclusion}
+ Blockchains can be used as a settlement layer for GNU Taler
+ with Depolymerizer.
+
+ \begin{itemize}
+ \item [$-$] Trust exchange operator or auditors
+ \item [$+$] Fast and cheap
+ \item [$+$] Realtime, ms latency
+ \item [$+$] Linear scalability
+ \item [$+$] Ecological
+ \item [$+$] Privacy when it can, transparency when it must (avoid tax
evasion and money laundering)
+ \end{itemize}
+Future work:
+ \begin{itemize}
+ \item Universal auditability, using sharded transactions history
+ \item Smarter analysis, update confirmation delay based on currency
network behavior
+ \item Multisig by multiple operator for transactions validation
+ \end{itemize}
+\end{frame}
+
+
+\section{Conclusion}
+
+
+\begin{frame}{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}{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}{How to support?}
+ \begin{description}
+ \item[Join:] {\small \url{https://lists.gnu.org/mailman/listinfo/taler}},
\\
+ \url{https://libera.chat/\#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}{Conclusion}
+ \begin{center}
+ {\bf What can we do?}
+ \end{center}
+ \vfill
+\begin{itemize}
+ \item{Suffer mass-surveillance enabled by credit card oligopolies with high
fees, and}
+ \item{Engage in arms race with deliberately unregulatable blockchains}
+% \item{Enjoy the ``benefits'' of cash \\
+% \hfill \includegraphics[height=0.3\textheight]{atm-rupee.jpg} \hfill}
+\end{itemize}
+\vfill
+\begin{center}
+ {\bf OR}
+\end{center}
+\vfill
+\begin{itemize}
+ \item{Establish free software alternative balancing social goals!}
+\end{itemize}
+\vfill
+\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}
+
+
+
+
+\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}
--
To stop receiving notification emails like this one, please contact
gnunet@gnunet.org.
| [Prev in Thread] |
Current Thread |
[Next in Thread] |
- [taler-marketing] branch master updated: prep,
gnunet <=