BRAINS2022 presentation draft

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+\documentclass{beamer}
+
+\usepackage{tikz}
+\usepackage{ragged2e}
+
+\apptocmd{\frame}{}{\justifying}{}
+
+\usetikzlibrary{positioning,fit,patterns}
+
+\title{Depolymerization}
+\subtitle{Integrating GNU Taler with blockchain-based cryptocurrencies}
+\author{Antoine d'Aligny}
+\institute{Bern University of Applied Sciences}
+\date{\today}
+\titlegraphic{\includegraphics[width=2.5cm]{../docs/media/taler.png}}
+
+
+\begin{document}
+
+\begin{frame}
+    \titlepage
+\end{frame}
+
+\begin{frame}{GNU Taler}{Overview}
+    \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}{Taler payment system}
+            \begin{itemize}
+                \item Tokenization
+                \item Blind signatures (privacy)
+            \end{itemize}
+        \end{block}
+        \begin{block}{Settlement layer}
+            \begin{itemize}
+                \item Designed for fiat currencies
+                \item  BTC and ETH ?
+            \end{itemize}
+        \end{block}
+    \end{columns}
+\end{frame}
+
+\begin{frame}{GNU 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]{../docs/media/taler.png}};
+    
+        %% Depolymerization
+        \node[right=12mm of D] {\small{Credit}};
+        \node[right=55mm of D] {\small{Debit}};
+        \draw[dashed,-stealth] (1.north) |- (off.west);
+        \draw[dashed,-stealth] (off.east) -| (6.north);
+    
+        %% Separation
+        \draw[dotted] (-2.1,-1.3) -- (9,-1.3);
+        \draw[dotted] (-2.1,-2.5) -- (9,-2.5);
+    \end{tikzpicture}
+    \end{center}
+\end{frame}
+
+\begin{frame}{Blockchain-based cryptocurrencies}{What is a blockchain?}
+    \begin{center}
+        \begin{tikzpicture}[
+                block/.style={rectangle,draw=black,fill=black!10,minimum size=7mm},
+                ar/.style={-stealth},
+                nconf/.style={dotted},
+            ]
+            \node[block](1){};
+            \node[block,right=5mm of 1](2){$T_0$};
+            \node[block,right=5mm of 2](3){};
+            \node[block,right=5mm of 3](4){$T_1$};
+            \node[block,right=5mm of 4](5){};
+            \node[block,right=5mm of 5](6){};
+            \node[block,nconf,right=5mm of 6](7){$T_2$};
+            \draw[ar] (1) -- (2);
+            \draw[ar] (2) -- (3);
+            \draw[ar] (3) -- (4);
+            \draw[ar] (4) -- (5);
+            \draw[ar] (5) -- (6);
+            \draw[ar] (6) -- (7);
+        \end{tikzpicture}
+    \end{center}
+    \begin{center}
+    Append-only database composed of a list of linked blocks
+    \end{center}
+\end{frame}
+
+\begin{frame}{Blockchain-based cryptocurrencies}{Distributed Ledger}
+    \begin{center}
+        \begin{tikzpicture}[
+                block/.style={rectangle,draw=black,fill=black!10,minimum size=7mm},
+                ar/.style={-stealth},
+                pink/.style={draw=teal, fill=teal!10},
+                tan/.style={draw=orange, fill=orange!10},
+                blue/.style={draw=magenta, fill=magenta!10},
+                node/.style={circle, minimum size=7mm},
+            ]
+            % Common
+            \node[block](2){};
+            \node[block,right=5mm of 2](3){};
+            \node[block,pink,right=5mm of 3](4){};
+            \draw[ar] (2) -- (3);
+            \draw[ar] (3) -- (4);
+
+            % Center
+            \node[block,pink,right=5mm of 4](5){B};
+            \draw[ar] (4) -- (5);
+            
+            % Top
+            \node[block,blue,above=7mm of 5](5t){A};
+            \draw[ar] (4.east) -- (5t.west);
+
+            % Bottom
+            \node[block,tan,below=7mm of 4](4b){};
+            \node[block,tan,below=7mm of 5](5b){C};
+            \draw[ar] (3.east) -- (4b.west);
+            \draw[ar] (4b) -- (5b);
+
+            % Network
+            \node[node,blue,above left=0.1cm and 2cm of 2](A) {A};
+            \node[node,pink,below left=1cm and 0.45cm of A](B) {B};
+            \node[node,tan,below right=1cm and 0.45cm of A](C) {C};
+            \draw (A) -- (B);
+            \draw (B) -- (C);
+            \draw (C) -- (A);
+        \end{tikzpicture}
+        \begin{center}
+            Peer-to-peer network of nodes reaching consensus
+        \end{center}
+    \end{center}
+\end{frame}
+
+\begin{frame}{Blockchain-based cryptocurrencies}{Consensus protocol}
+    \begin{itemize}
+        \item Enforce a delay between blocks
+        \item Require coordinated effort to create new block
+        \item The longest chain is the consensus 
+    \end{itemize}
+    \begin{columns}
+        \column{0.4\paperwidth}
+        \begin{block}{\textbf{BTC} Bitcoin}
+            \begin{itemize}
+                \item Proof of Work
+                \item new block every 10 min
+                \item 4 txs per sec
+            \end{itemize}
+        \end{block}
+        \column{0.4\paperwidth}
+        \begin{block}{\textbf{ETH} Ethereum}
+            \begin{itemize}
+                \item Proof of Stake
+                \item new block every 12 sec
+                \item 14 txs per sec
+            \end{itemize}
+        \end{block}
+    \end{columns}
+\end{frame}
+
+
+\begin{frame}{Distributed Ledger limitations}{Chain reorganization}
+    \begin{center}
+        \begin{tikzpicture}[
+                block/.style={rectangle,draw=black,fill=black!10,minimum size=7mm},
+                nconf/.style={dotted,fill=none},
+                replaced/.style={pattern=north east lines,draw=black!60,fill opacity=0.3},
+                err/.style={draw=black!60!red,fill=black!60!red!10,dashed},
+                ar/.style={-stealth}
+            ]
+            % Common
+            \node[block](2){};
+            \node[block,right=5mm of 2](3){};
+            \draw[ar] (2) -- (3);
+
+            % Current
+            \only<-2>{
+                \node[block,right=5mm of 3](4){};
+                \node[block,right=5mm of 4](5){$T$};
+                \node[block,nconf,right=5mm of 5](6){};
+                \draw[ar] (5) -- (6);
+            }
+            \only<3->{
+                \node[block,replaced,right=5mm of 3](4){};
+                \node[block,replaced,right=5mm of 4](5){$T$};
+            }
+            \draw[ar] (3) -- (4);
+            \draw[ar] (4) -- (5);
+          
+
+            \only<2->{
+                % Fork
+                \node[block,above=7mm of 4](4p){};
+                \only<-3>{
+                    \node[block,right=5mm of 4p](5p){};
+                }
+                \only<4->{
+                    \node[block,err,right=5mm of 4p](5p){$T'$};
+                }
+                \node[block,right=5mm of 5p](6p){};
+                \node[block,nconf,right=5mm of 6p](7p){
+                    \only<3>{$T$}
+                };
+                \draw[ar] (3.east) -- (4p.west);
+                \draw[ar] (4p) -- (5p);
+                \draw[ar] (5p) -- (6p);
+                \draw[ar] (6p) -- (7p);
+            }
+
+            % Indication
+            \only<2->{
+                \node [right=2mm of 7p]{\emph{
+                    \only<2>{
+                        new
+                    }
+                    \only<3->{
+                        active
+                    }
+                }};
+            }
+            \node [right=14mm of 6]{\emph{
+                \only<-2>{
+                    active
+                }
+                \only<3->{
+                    fork
+                }
+            }};
+        \end{tikzpicture}
+    \end{center}
+    \begin{center}
+        \only<1>{
+            We have a chain with an important transaction
+        }
+        \only<2>{
+            We may discover a new longer chain
+        }
+        \only<3>{
+            The important transaction is now pending
+        }
+        \only<4>{
+            A conflict can invalidate the transaction
+        }
+    \end{center}
+\end{frame}
+
+\begin{frame}{Distributed Ledger limitations}{Confirmation delay}
+    \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](2){};
+            \node[block,conf,right=5mm of 2](3){$T_0$};
+            \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>{$T_3$}};
+            }
+            \node[block,nconf,right=5mm of 4](5){};
+            \node[block,nconf,right=5mm of 5](6){$T_1$};
+            \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){$T_3'$};
+            }
+            \node[block,nconf,right=5mm of 4p](5p){$T_2$};
+            \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{new}};
+            \node [right=17mm of 6]{\emph{active}};
+        \end{tikzpicture}
+    \end{center}
+    \begin{center}
+        \only<1>{Confirmation delay}
+        \only<2>{Handling harmless reorganizations}
+        \only<3>{Handling harmful reorganizations}
+    \end{center}
+\end{frame}
+
+\begin{frame}{Distributed Ledger limitations}{Stuck transactions}
+    \begin{block}{Transaction fees}
+        \begin{itemize}
+            \item Block production incentive
+            \item Variable by design
+        \end{itemize}
+    \end{block}
+    \begin{center}
+        \begin{tikzpicture}[
+            dot/.style={circle,fill,inner sep=1pt,}
+        ]
+        \node (I) {\includegraphics[width=\textwidth]{../docs/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}
+    \end{center}
+    \begin{center}
+        Stuck transactions caused by transaction fees being a market-based value
+    \end{center}
+\end{frame}
+
+\begin{frame}{Incompatible amount}
+    GNU Taler amount follow RFC 8905:  $2^{53}$ unit and 8 decimal digits
+    \begin{block}{\textbf{BTC} Bitcoin}
+        \begin{itemize}
+            \item 1 satoshi = $10^{-8}$ BTC
+            \item capped at 21 000 000 BTC
+            \item Fit perfectly
+        \end{itemize}
+    \end{block}
+    \begin{block}{\textbf{ETH} Ethereum}
+        \begin{itemize}
+            \item 1 wei = $10^{-18}$ ETH
+            \item actually more than 120 580 000 without cap
+            \item average fee: 21 000 * 14 = 294 000 Gwei or 0.000295 ETH
+            \item truncated at $10^{-8}$ ETH it can still contain any amount you can send without losing money 
+        \end{itemize}
+    \end{block}
+\end{frame}
+
+
+\begin{frame}{Conclusion}
+    Distributed Ledgers can be used as a settlement layer for GNU Taler
+    with Depolymerizer.
+
+    \begin{itemize}
+        \item [$-$] Trust exchange operators 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}
+\end{frame}
+
+\begin{frame}{Conclusion}{Future work}
+    \begin{itemize}
+        \item  Wallet-to-wallet payments are WIP
+        \item  Legal concerns around known-your-customer with cryptocurrencies
+    \end{itemize}
+
+    \begin{block}{Availability}
+    An experimental public Depolymerization exchange is available at
+    \url{https://bitcoin.ice.bfh.ch/}. To use the service, one only needs
+    to install a GNU Taler wallet from \url{https://wallet.taler.net/} and
+    instruct it to use this exchange to withdraw coins.
+    \end{block}
+\end{frame}
+
+\end{document}