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 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 101 102 103 104 105 106 107 108 109 110 111 112 113 114 115 116 117 118 119 120 121 122 123 124 125 126 127 128 129 130 131 132 133 134 135 136 137 138 139 140 141 142 143 144 145 146 147 148 149 150 151 152 153 154 155 156 157 158 159 160 161 162 163 164 165 166 167 168 169 170 171 172 173 174 175 176 177 178 179 180 181 182 183 184 185 186 187 188 189 190 191 192 193 194 195 196 197 198 199 200 201 202 203 204 205 206 207 208 209 210 211 212 213 214 215 216 217 218 219 220 221 222 223 224 225 226 227 228 229 230 231 232 233 234 235 236 237 238 239 240 241 242 243 244 245 246 247  \chapter{Future Work}\label{chapter:future-work} We now discuss future work that builds upon the results presented so far. \subsection*{Standard Model} Our current instantiation of the Taler protocol relies heavily on hash functions. Since the result by Canetti and others \cite{canetti2004random} about the theoretical impossibility of securely instantiating protocols that rely on the random oracle assumption for their security, a vast amount of literature has been devoted to find instantiations of interesting protocols in the standard model \cite{koblitz2015random}. The Taler protocol syntax could likely be also instantiated securely in the standard model, based existing on blind signature schemes in the standard model. The trade-off however is that while removing the random oracle assumption, typically other less well known assumptions must be made. \subsection*{Post-Quantum security} The possibility of post-quantum computers breaking the security of established cryptographic primitives has lately received a lot of attention from cryptographers. While currently most schemes with post-quantum security are impractical, it might be worthwhile to further investigate their application to e-cash, based on existing work such as \cite{zhang2018new}. \subsection*{Applications to network incentives} Some peer-to-peer networking protocols (such as onion routing \cite{dingledine2004tor}) do not have inherent incentives and rely on volunteers to provide infrastructure. In future work, we want to look at adding incentives in the form of Taler payments to a peer-to-peer networking platform such as GNUnet. \subsection*{Smart(er) Contracts and Auctions} Contract terms in Taler are relatively limited. There are some interesting secure multiparty computations, such as privacy-preserving auctions \cite{brandt2006obtain} that could be offered by exchanges as a fixed smart contract. This would allow a full privacy-preserving auction platform, as current auction protocols only output the winner of a privacy-preserving auction but do not address the required anonymous payments. \subsection*{Backup and Sync}\label{sec:future-work-backup-sync} Synchronization of wallets between multiple devices is a useful feature, but a na\"ive implementation endangers privacy. A carefully designed protocol for backup and synchronization must make sure that the hosting service for the wallet's data cannot collaborate with the exchange and merchants to deanonymize users or transactions. Thus when spending coins for a payment, devices should not have to synchronously talk to their backup/sync provider. This creates the challenge of allocating the total available balance to individual devices in a way that fits the customer's spending pattern, and only require synchronous communication at fixed intervals or when really necessary to re-allocate coins. Another possible approach might be to use Private Information Retrieval (PIR) \cite{goldberg2007improving} to access backup and synchronization information. \subsection*{Machine-Verified Proofs} We currently model only a subset of the GNU Taler protocol formally, and proofs are handwritten and verified by humans. A tool such as CryptoVerif \cite{blanchet2007cryptoverif} can allow a higher coverage and computer-checked proofs, and would allow protocol changes to be validated in shorter time. \subsection*{Coin Restrictions / Taler for Children''} By designating certain denominations for different purposes, GNU Taler could be used to implement a very simple form of anonymous credentials \cite{paquin2011u,camenisch2004signature}, which then could be used to implement a Taler wallet specifically aimed at children, in order to teach them responsible and autonomous spending behavior, while granting them privacy and at the same time preventing them from making age-inappropriate purchases online, as the discretion of parents. %\subsection*{gnunet-blockchain / deployment of the full stack payment system} %=> no, talk more about integration with real banks, KYC % %\subsection*{P2P payments} % %\subsection*{NFC Wallet} % %\subsection*{large, scalable deployment} %I.e. sharding, db replication, load balancer(s) % %\subsection*{Hardware security module for exchange} % %\subsection*{Bitcoin/Blockchain integration} % %\subsection*{UX study and improvements} %(including tracking/planning of spending) % %\subsection*{News Distribution} \chapter{Conclusion}\label{chapter:conclusion} % sources and inspirations % https://www.bis.org/publ/arpdf/ar2018e5.pdf % https://www.bis.org/publ/qtrpdf/r_qt1709f.pdf % http://andolfatto.blogspot.com/2015/02/fedcoin-on-desirability-of-government.html % mention eKrona/Riksbank % bessere ueberleitung % effizienz! % freie software / commons % scalability of blockchains % expand on some ideas such as naiveity of blockchains % 3 areas for currencies % einleitung: we introduced two systems that solve/address core problems for % banking (consensus, transactions, ...) "geld ist kein selbstzweck" % reason for having money is: ... % central banks are the only tool that we know that works % geld ist politisch, macht, verantwortung, gesellschaften aufbauen oder ruinieren % apolitical solution impossible This book presented GNU Taler, an efficient protocol for value-based electronic payment systems with focus on security and privacy. While we believe our approach to be socially and economically beneficial, a technological impact analysis is in order prior to adopting new systems that have broad economic and socio-political implications. Currencies serve three key functions in society:~\cite{mankiw2010macroeconomics} \begin{enumerate} \item As a unit for measurement of value, \item a medium of exchange, and \item a store of value. \end{enumerate} How do the various methods measure up to these requirements? \section{Cryptocurrencies vs. Central-Bank-Issued Currencies} \begin{figure} \centering \includegraphics[width=\textwidth]{diagrams/bitcoin-market-price.png} \caption[Historical market price of Bitcoin.]{Historical market price (in USD) of Bitcoin across major exchanges (Source:~\url{https://blockchain.com}).} \label{fig:volatility} \end{figure} Cryptocurrencies generally fail to achieve the required stability to serve as a reasonable unit of measurement (Figure~\ref{fig:volatility}). The volatility of cyptocurrencies is caused by a combination of a lack of institutions that could intervene to dampen fluctuations and a comparatively limited liquidity in the respective markets. The latter is exacerbated by the limited ability of decentralized cryptocurrencies to handle large transaction volumes, despite their extreme levels of resource consumption. As a result, the utility of decentralized cryptocurrencies is limited to highly speculative investments and to the facilitation of criminal transactions. With respect to privacy, completely decentralized cryptocurrencies provide either too much or too little anonymity. Transparent cryptocurrencies create the spectre of discriminatory pricing, while especially for privacy-enhanced cryptocurrencies the lack of regulation creates an attractive environment for fraud and criminal activity from tax evasion to financing of terrorism. These problems are easily addressed by combining the register (or ledger) with a central bank providing a regulatory framework and monetary policy, including anti-money-laundering and know-your-customer enforcement. \section{Electronic Payments} Day-to-day payments using registers are expensive and inconvenient. Using a register requires users to {\em identify} themselves to {\em authorize} transactions, and the use of register-based banking systems tends to be more expensive than the direct exchange of physical cash. However, with the ongoing digitalization of daily life where a significant number of transactions is realized over networks, some form of electronic payments remain inevitable. The current alternative to (centrally banked) electronic cash are a payment systems under full control of oligopoly companies such as Google, Apple, Facebook or Visa. The resulting oligopolies are anti-competitive. In addition to excessive fees, they sometimes even refuse to process payments with certain types of legal businesses, which then are often ruined due to lack of alternatives. Combining payment services with companies where the core business model is advertising is also particularly damaging for privacy. Finally, the sheer size of these companies creates systemic risks, just as their global scale creates challenges for regulation. As GNU Taler is free software, even without backing by a central bank, Taler would not suffer from these drawbacks arising from the use of proprietary technology. Furthermore, Taler-style electronic cash comes with some unique benefits: \begin{itemize} \item improved income transparency compared to cash and traditional Chaum-style e-cash, \item anonymity for payers, \item avoidance of enticement towards consumer debt --- especially compared to credit cards, and \item support of new business models and Internet security mechanisms which require (anonymous) micro-transactions. \end{itemize} Central banks are carefully considering what might be the right technology to implement an electronic version of their centrally banked currency, and with Taler we hope to address most of their concerns. Nevertheless, all electronic payment systems, including Taler even when backed by central-bank-issued currencies, come with their own inherent set of risks:~\cite{riksbank2017riksbank} \begin{itemize} \item increased risk of a bank run: in a banking crisis, as it is easier to withdraw large amounts of digital cash quickly --- even from remote locations; \item increased volatility due to foreign holdings that would not be as easily possible with physical cash; \item increased risk of theft and disruption: while physical cash can also be stolen (and likely with much less effort), it is difficult to transport in volume~\cite{force2015money}, the risk is increased with computers because attacks scale \cite{hammer2018billion}, and generally many small incidents are socially preferable over a tiny number of very large-scale incidents; and \item unavailability in crisis situations without electricity and Internet connectivity. \end{itemize} We believe that in the case of Taler, some of the risks mentioned above can be mitigated: \begin{itemize} \item Volatility due to foreign holdings and the resulting increased risk of bank runs can be reduced by putting limits on the amount of electronic coins that customers can withdraw. Limiting the validity periods of coins is another method that can help disincentivize the use of Taler as a value store. \item The use of open standards and reference implementations enables white-hat security research around GNU Taler, which together with good operational security procedures and the possibility of competing providers should reduce the risks from attacks. \item GNU Taler can co-exist with physical cash, and might even help revive the use of cash if it succeeds in reducing credit card use online thereby eliminating a key reason for people to have credit cards. \end{itemize} Unlike cryptocurrencies, Taler does not prescribe a solution for monetary policy or just taxation, as we believe these issues need to be subject to continuous political debate and cannot be solved'' by simplistic algorithms. What we offer to society is an open and free (as in free speech) system with mechanisms to audit merchants' income, instead of proprietary systems controlled by a few oligopoly companies.