marketing

en.tex (24762B)

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 \documentclass{article}
 \usepackage[margin=1in]{geometry}
 \usepackage[utf8]{inputenc}
 \usepackage{wrapfig}
 
 \usepackage{textcomp}
 \usepackage{xcolor}
 \usepackage{bytefield}
 \usepackage{tikz}
 \usetikzlibrary{positioning,fit}
 \usepackage{graphicx}
 \usepackage{url}
 \title{Project Depolymerization: \\
   Bitcoin Payments with GNU Taler}
 
 \begin{document}
 
 \author{Antoine d'Aligny \and Emmanuel Benoist \and Christian Grothoff}
 
 \maketitle
 
 \begin{abstract}
   GNU Taler is an electronic payment system implemented as Free Software. The
   Depolymerization of the Bern University of Applied Sciences (BFH) enables
   payments with blockchain-based cryptocurrencies using GNU Taler.  This
   article explains how to use the resulting payment system to efficiently and
   quickly pay with Bitcoin.
 \end{abstract}
 
 \section{Introduction}
 
 Today, popular cryptocurrencies like Bitcoin~\cite{nakamoto2008bitcoin} and
 Ethereum~\cite{ethereum} are not useful for electronic payments in everyday
 life (say to buy bread, pay for a beer or a snack in a vending machine).  For
 real-world payments, on-chain transactions are too slow, too expensive and the
 decentralized nature of blockchains does not allow transaction rates required
 in practice.
 
 GNU Taler is a centralized payment system that allows privacy-preserving
 payments to be settled within milliseconds at minimal cost.  By combining
 the GNU Taler payment system with a blockchain, GNU Taler can address key
 performance and privacy limitations of blockchains.
 Figure~\ref{fig:offchain} shows the resulting architecture where GNU Taler
 serves as a layer-2 solution over a blockchain.
 
 \begin{figure}[hb]
     \begin{center}
         \input{figures/settlement_layer.tex}
     \end{center}
     \caption{Blockchain settlement layer with Depolymerization.}\label{fig:offchain}
 \end{figure}
 
 GNU Taler exchanges are expected to be operated by regulated payment service
 providers that can be trusted because they are audited in real-time.  Today,
 no such a regulated entity exists. To nevertheless enable experimentation with
 the technology, Prof. Benoist is operating an experimental exchange at {\tt
   https://bitcoin.ice.bfh.ch/} as a research project.  While the general
 public is at liberty to use the system, there is no warranty that the
 generated Bitcoin Taler coins will be redeemed as the university reserves the
 right to shut down the system at any time for any reason without enabling
 users to recover their assets.
 
 \section{Withdrawing Bitcoin}
 
 To begin using GNU Taler for Bitcoin payments, users need to install a GNU
 Taler wallet. Wallets are available from {\tt https://wallet.taler.net/}
 for various platforms and as extensions for various browsers.  We will
 largely focus on the user experience for the Android wallet, as in most
 cases the user experience is quite similar for the other implementations.
 
 \subsection{Adding the Bitcoin exchange}
 
 The experimental Bitcoin exchange operated by BFH is not in the default list
 of exchanges supported by a GNU Taler wallet. Thus, users must first add the
 exchange. To do so, we need to open the ``Settings'' dialog, as shown in
 Figures~\ref{fig:mainmenu:hamburger} and~\ref{fig:mainmenu:settings}.
 
 \begin{figure}[h!]
 \begin{center}
 \includegraphics[width=0.3\textwidth]{Screenshots/Screenshot_20221121-210438.png}
 \end{center}
 \caption{To withdraw Bitcoin, you must first select the main menu, using the
   ``hamburger'' icon in the main ``Balances'' view.}
 \label{fig:mainmenu:hamburger}
 \end{figure}
 
 \begin{figure}[h!]
 \begin{center}
 \includegraphics[width=0.3\textwidth]{Screenshots/Screenshot_20221121-210443.png}
 \end{center}
 \caption{In the main menu, select ``Settings''.}
 \label{fig:mainmenu:settings}
 \end{figure}
 
 In the ``Settings'' menu, the ``Exchanges'' entry can then be selected
 to inspect and modify the list of GNU Taler exchanges the wallet is
 configured to use.  To add the BFH exchange, select the ``+''
 button and add the respective URL as shown in Figure~\ref{fig:exchange:add}.
 
 %\begin{figure}
 %\begin{center}
 %\includegraphics[width=0.3\textwidth]{Screenshots/Screenshot_20221121-210448.png}
 %\end{center}
 %\caption{In ``Settings'', select ``Exchanges'' to view the list of Taler exchange providers.}
 %\end{figure}
 
 \begin{figure}[h!]
 \begin{center}
 \includegraphics[width=0.3\textwidth]{Screenshots/Screenshot_20221121-210513.png}
 \end{center}
 \caption{Unless already present, use the ``+'' button to add a Bitcoin exchange. We
   use {\tt bitcoin.ice.bfh.ch} which is the only Depolymerization operator we are
   aware of at this time.}
 \label{fig:exchange:add}
 \end{figure}
 
 The Taler shorthand for the ``mainnet'' Bitcoin currency is ``BITCOINBTC''
 (instead of just ``BTC''), as the Bitcoin ``testnet'' also uses ``BTC'' and
 thus the two currencies might be confused, especially given that within
 GNU Taler, the underlying Bitcoin addresses are not visible.
 
 %\begin{figure}
 %\begin{center}
 %\includegraphics[width=0.3\textwidth]{Screenshots/Screenshot_20221121-210559.png}
 %\end{center}
 %\caption{If done correctly, an exchange for ``BITCOINBTC'' should now be in the list.
 %  GNU Taler uses ``BITCOINBTC'' instead of just ``BTC'' to avoid confusion with
 %  the worthless ``TESTNETBTC''.}
 %\end{figure}
 
 \subsection{Adding BITCOINBTC to the Taler wallet}
 
 To move Bitcoins from a Bitcoin wallet to a Taler wallet,
 one starts with the menu reachable from the three horizontal dots next to the Bitcoin exchange and selects the
 ``Withdraw'' operation (Figure~\ref{fig:menu:withdraw}).
 
 \begin{figure}[h!]
 \begin{center}
 \includegraphics[width=0.3\textwidth]{Screenshots/Screenshot_20221121-210606.png}
 \end{center}
 \caption{By clicking the three dots next to the Bitcoin exchange, we get the menu
   to ``Withdraw'' bitcoins. Here, ``withdraw'' means to transfer BTC from a
   Bitcoin wallet to a Taler wallet.}
 \label{fig:menu:withdraw}
 \end{figure}
 
 Afterwards, the wallet will ask for the amount of BTC
 that the user wants to withdraw (Figure~\ref{fig:withdraw:amount})
 and ask for the user to agree to the terms of service
 of the Taler exchange operator.
 
 \begin{figure}[h!]
 \begin{center}
 \includegraphics[width=0.3\textwidth]{Screenshots/Screenshot_20221121-210626.png}
 \end{center}
 \caption{Specify the amount of (BITCOIN)BTC that you want to withdraw.}
 \label{fig:withdraw:amount}
 \end{figure}
 
 GNU Taler exchange operators can also charge transaction fees,
 just like Bitcoin miners or banks.  GNU Taler wallets show
 applicable fees for every operation before allowing the user
 to proceed with the transaction.  Figure~\ref{fig:withdraw:fees}
 shows a Taler wallet showing the fees for the withdraw operation.
 
 \begin{figure}[h!]
 \begin{center}
 \includegraphics[width=0.3\textwidth]{Screenshots/Screenshot_20221121-210653.png}
 \end{center}
 \caption{The Taler wallet will show applicable exchange fees before starting the
   actual process. Note that these fees do not include the on-chain transaction fees of the
   Bitcoin blockchain.}
 \label{fig:withdraw:fees}
 \end{figure}
 
 After the user agreed to the terms of service and the fees, the wallet
 internally sets up the withdraw process and shows instructions for how to
 transfer Bitcoins to the Taler exchange operator to top-up the Taler wallet
 (Figure~\ref{fig:withdraw:instructions}).  The Bitcoin transfer is a bit
 unusual, as the Deploymerization project encodes the access key of the Taler
 wallet in two fake Bitcoin addresses, while the actual transfer to the Bitcoin
 Taler exchange operator happens in a third address.  Thus, users need to copy
 the three lines with amounts and Bitcoin target addresses into their clipboard
 and from there import them into their Bitcoin wallet (see
 Figure~\ref{fig:bitcoin:import}).
 
 \begin{figure}[h!]
 \begin{center}
 \includegraphics[width=0.3\textwidth]{Screenshots/Screenshot_20221121-210709.png}
 \end{center}
 \caption{This is is the information needed for the Bitcoin wallet to make the
   on-chain wire transfer to send Bitcoins to your Taler wallet. You should copy
   this information into your Bitcoin wallet. If the Bitcoin wallet runs on the
   same device ``Open in banking app'' may work (if the Bitcoin wallet supports it).}
 \label{fig:withdraw:instructions}
 \end{figure}
 
 We will now show how to complete the operation using the Electrum wallet.
 The first step is to open the Electrum wallet and open the screen to
 ``Send'' funds (Figure~\ref{fig:electrum:send}).
 
 \begin{figure}[h!]
 \begin{center}
 \includegraphics[width=0.6\textwidth]{Screenshots/ScreenShot-2022-11-22_14.17.37.png}
 \end{center}
 \caption{Using your Bitcoin wallet (here: Electrum), you now need to send BTC to
   the Taler exchange to obtain BITCOINBTC in your Taler wallet.}
 \label{fig:electrum:send}
 \end{figure}
 
 Next, one needs to tell Electrum that the payment will go to multiple
 recipients. This requires using the ``Pay to many'' option in the
 menu (Figure~\ref{fig:electrum:many}).
 
 \begin{figure}[h!]
 \begin{center}
 \includegraphics[width=0.3\textwidth]{Screenshots/ScreenShot-2022-11-22_14.17.44.png}
 \end{center}
 \caption{When sending the funds, use the ``Pay to many'' option in the menu to be
   allowed to enter multiple recipients.}
 \label{fig:electrum:many}
 \end{figure}
 
 Once the ``Pay to many'' option has been activated, the three lines from the
 GNU Taler wallet can be pasted into the address field
 (Figure~\ref{fig:bitcoin:import}).  Electrum expects amounts in mBTC, while
 the lines generated by GNU Taler used BTC.  Hence, all amounts must be
 multiplied by 1000 and the ``BTC'' unit should be removed. Also, Electrum
 expects the lines in CSV format, thus commas need to be added between the
 addresses and the amounts.
 
 \begin{figure}[h!]
 \begin{center}
 \includegraphics[width=0.6\textwidth]{Screenshots/ScreenShot-2022-11-22_14.19.45.png}
 \end{center}
 \caption{Now you can import the three destinations that were generated by the GNU Taler
   wallet.  Note that Electrum expects the amounts to be specified in mBTC, while
   GNU Taler generated the amounts in BTC.
   Future versions of the Taler wallet may support the Electrum syntax directly.}
 \label{fig:bitcoin:import}
 \end{figure}
 
 The last step is then to confirm the transaction to Electrum
 and to provide Electrum with the passphrase (Figure~\ref{fig:electrum:pass}).
 
 \begin{figure}[h!]
 \begin{center}
 \includegraphics[width=0.3\textwidth]{Screenshots/ScreenShot-2022-11-22_14.20.32.png}
 \end{center}
 \caption{As usual, you need to unlock your Electrum wallet with your passphrase and
   specify how much you are willing to pay in terms of transaction fees.}
 \label{fig:electrum:pass}
 \end{figure}
 
 Afterwards, Electrum will show a final confirmation screen. Here, the two 234
 Satoshi transactions are used to encode which Taler wallet should receive the
 funds, while the main output goes to the Taler exchange to hold the funds in
 escrow. The fourth output is your remaining balance.  It will then take a few
 hours (or possibly days, if the Bitcoin transaction fee offered was low) for
 the Bitcoins to arrive at the Taler exchange. During that time, the GNU Taler
 wallet will show the transaction as ``pending''.  Once the Bitcoins have
 arrived at the Taler exchange operator, the wallet will withdraw the coins and
 show that the ``withdraw'' operation has completed.
 
 %\begin{figure}
 %\begin{center}
 %\includegraphics[width=0.6\textwidth]{Screenshots/ScreenShot-2022-11-22_14.22.44.png}
 %\end{center}
 %\caption{The final transaction should look like this. Here, the two 234 Satoshi transactions
 %  are used to encode which Taler wallet should receive the funds, while the main output goes
 %  to the Taler exchange to hold the funds in escrow. The fourth output is your remaining
 %  balance.}
 %\end{figure}
 
 \section{Making Peer-to-Peer payments}
 
 This section describes how to transact directly between Taler wallets.  It is
 possible to either create a QR code to send money to another wallet, or to
 create a QR code that allows the recipient to pay the sender.
 
 \subsection{Sending money}
 
 To send money, select the ``send funds'' button just next to the current
 balance (Figure~\ref{fig:p2p:menu}). If a wallet contains multiple currencies,
 one needs to first select the desired currency.
 
 \begin{figure}[h!]
 \begin{center}
 \includegraphics[width=0.6\textwidth]{Screenshot_2023-06-23-13-35-34-228_net.taler.wallet.jpg}
 \end{center}
 \caption{The buttons for peer-to-peer payments are next to the balance on the
   screen with the transaction history.}
 \label{fig:p2p:menu}
 \end{figure}
 
 Next, the wallet will ask for the amount to send, and ask for the destination
 (Figure~\ref{fig:p2p:amount}).  Sending back ``to a bank account'' will be
 discussed in Section~\ref{sec:to_chain}, for peer-to-peer payments select ``to
 another wallet''.
 
 \begin{figure}[h!]
 \begin{center}
 \includegraphics[width=0.6\textwidth]{Screenshot_2023-06-23-13-36-07-426_net.taler.wallet.jpg}
 \end{center}
 \caption{Next you will need to specify where to send the funds, and to whom. FIXME:
     this screen is changing as it will be split into two in the next version!}
 \label{fig:p2p:amount}
 \end{figure}
 
 % The wallet will now show the applicable fees.  FIXME: not implemented!
 
 Finally, a purpose for the transfer must be specified
 (Figure~\ref{fig:p2p:subject}). This is basically the summary message that
 will be shown in the transaction history (of both parties) for this
 transaction.
 
 \begin{figure}[h!]
   \begin{center}
 % FIXME: fees not shown!
 %\includegraphics[width=0.6\textwidth]{}
 \end{center}
 \caption{The buttons for peer-to-peer payments are next to the balance on the
   screen with the transaction history.}
 \label{fig:p2p:subject}
 \end{figure}
 
 Once all of the information has been specified, the wallet will show a QR code
 that the recipient must scan to receive the funds. It is also possible to copy
 the URL encoded in the QR code to the clipboard to exchange it via some secure
 messaging application.  Note that the first person to scan the code or enter
 the URL will receive the funds.
 
 To scan the QR code, the recipient must use the QR code button in the bottom
 right corner of the Taler wallet app. To enter a URL or paste one from a
 clipboard, hold the QR code button to get to the ``Enter Taler URI'' dialog
 (Figure~\ref{fig:qr:entry}).
 
 \begin{figure}[h!]
   \begin{center}
 %\includegraphics[width=0.6\textwidth]{}
 \end{center}
 \caption{Entering a Taler payment URL into the Android wallet requires briefly
   holding the QR code button.}
 \label{fig:qr:entry}
 \end{figure}
 
 After scanning the QR code or pasting the URL the wallet will show the
 purpose and amount of the payment and allow the user to confirm the action
 before proceeding.
 
 % FIXME: fees not shown!
 \begin{figure}[h!]
 \begin{center}
 %\includegraphics[width=0.6\textwidth]{}
 \end{center}
 \caption{Entering a Taler payment URL into the Android wallet requires briefly
   holding the QR code button.}
 \label{fig:qr:accept}
 \end{figure}
 
 
 \subsection{Sending invoices}
 
 To send money, select the ``receive funds'' button just next to the current
 balance (Figure~\ref{fig:p2p:menu}).  The remaining dialogs mirror the
 process for sending money, except that this time the direction of the
 transfer is reversed.
 
 % FIXME: expand once we have invoice attestation!
 
 \section{Paying Taler-enabled merchants}
 
 The GNU Taler merchant backend makes it easy to accept GNU Taler payments as
 part of an e-commerce site or at a point-of-sale.  In contrast to the
 peer-to-peer payments described above, accepting payments with the merchant
 backend always results in the money being transfer to the merchant using the
 real-time gross settlement (RTGS) layer underpinning transactions in the
 currency. In case of Bitcoin, this means that the merchant will always receive
 the money on-chain in their Bitcoin wallet as a UTXO.
 
 \subsection{Payment aggregation}
 
 As on-chain (or even traditional banking RTGS) transactions are expensive,
 the GNU Taler exchange does not trigger such a transfer for each GNU Taler
 transaction, but only at a frequency determined by the merchant backend.
 Three settings primarily affect when such a transfer will happen:
 \begin{itemize}
 \item The wire transfer fee set by the exchange. As the transfer is expensive,
   the exchange charges a fee, but will not execute transfers if the fee is
   larger than the amount to be transferred.
 \item The refund deadline. GNU Taler allows a merchant to give a customer a
   refund until the refund deadline, which is part of the contract between
   customer and merchant.  As refunds are not possible after the exchange
   has send the money to the merchant, any transfers to the merchant happen
   {\bf after the refund deadline} set by the merchant in the contract.
 \item The wire deadline. The wire deadline must be after the refund deadline
   and specifies the latest time by which the merchant wants to receive the
   funds. Thus, the exchange will execute the transfer {\bf before the wire deadline}.
 \end{itemize}
 
 When any outstanding payment hits the wire deadline, the exchange will
 sum up all other payments to the same merchant that are past the refund
 deadline and wire the total amount (minus the wire fee) to the merchant.
 
 
 \subsection{Configuring the merchant backend}
 
 To setup a merchant backend, you first need to install the {\tt
   taler-merchant-httpd} backend. This is described for various platforms in
 the GNU Taler merchant
 manual.\footnote{\url{https://docs.taler.net/taler-merchant-manual.html}}
 To configure the system for Bitcoin, you need to set the currency:
 \begin{verbatim}
 $ taler-config -s TALER -o CURRENCY -V BITCOINBTC
 \end{verbatim}
 Furthermore, you need to tell the backend to trust the respective
 exchange:
 \begin{verbatim}
 $ taler-config -s merchant-exchange-benoist \
   -o CURRENCY -V BITCOINBTC
 $ taler-config -s merchant-exchange-benoist \
   -o EXCHANGE_BASE_URL -V https://bitcoin.ice.bfh.ch/
 $ taler-config -s merchant-exchange-benoist \
   -o MASTER_KEY -V YCWD4QXP607YDZ47NF40MZ6BNMKNFKPECD6JYMTTMTAANZ6C7W00
 \end{verbatim}
 
 It is possible to support multiple exchanges in the same currency by varying
 the section name, as long as the section prefix is ``merchant-exchange-''.
 
 \begin{figure}[h!]
 \begin{center}
 %\includegraphics[width=0.6\textwidth]{}
 \end{center}
 \caption{The Taler Backoffice application with the page to configure
   the key settings for an e-commerce site or point-of-sale.}
 \label{fig:spa:settings}
 \end{figure}
 
 Assuming everything else was setup correctly, the rest of the configuration
 can be done with the browser (Figure~\ref{fig:spa:settings}).  Configuring the
 instance under ``Settings'' should be largely straightforward.  The only
 special setting is that under ``Bank account'' you need to specify ``bitcoin''
 as the ``Target type'' and enter your Bitcoin wallet address under
 ``Address''. The ``Name'' should be set to a human-readable identifier of the
 owner of the wallet; however, it is not checked or used in the on-chain
 transfer.
 
 The ``Default max deposit fee'' refers to the exchange's per-coin deposit
 fees.  The merchant backend will agree to pay up to this amount in fees for
 any transaction. This is, however, merely a default: the frontend can override
 the deposit fee the merchant is willing to cover each time it creates an
 order. If the actual deposit fees are higher, the customer will be asked to
 cover the excess deposit fees.
 
 The ``Default max wire fee'' is not a hard cap on the wire fee to be paid to
 the exchange. Instead, if the wire fee charged by the exchange is higher, the
 merchant backend will ask the customer to cover a fraction of the excess fee,
 as determined by the ``Default wire fee amortization'' setting.
 
 The ``Default wire transfer delay'' corresponds to the ``Wire deadline''
 mentioned above. If a specific contract from the frontend does not include a
 ``Wire deadline'' the ``Wire deadline'' will be automatically set to the
 current time plus the ``Default wire transfer delay'' by the merchant backend.
 
 \subsection{Creating an order}
 
 You can setup a new order using the backend Web interface or using the GNU
 Taler merchant REST
 API.\footnote{\url{https://docs.taler.net/core/api-merchant.html}} The GNU
 Taler Merchant API
 tutorial\footnote{\url{https://docs.taler.net/taler-merchant-api-tutorial.html}}
 provides a tutorial for using the REST API. Here we will just use the backend
 Web interface. Select the ``Orders'' tab to get a list of the current set of
 orders for this merchant. Clicking on the blue ``+'' icon launches the dialog
 for setting up a new order (Figure~\ref{fig:spa:order}).
 
 \begin{figure}[h!]
 \begin{center}
 %\includegraphics[width=0.6\textwidth]{}
 \end{center}
 \caption{The Taler Backoffice application with the page to configure
   the key settings for an e-commerce site or point-of-sale.}
 \label{fig:spa:order}
 \end{figure}
 
 Only the order price and a human-readable summary to be shown to the buyer are
 required. Once an order has been created it will appear in the order
 list. Clicking on it will show the order details. To get a customer to pay for
 it, you can either redirect the user to the ``Order status URL'' given on that
 page, or pass the ``Payment URL'' to the wallet (e.g. via clipboard, NFC or QR
 code).  Naturally, in practice, the e-commerce frontend or point-of-sale
 system would use the REST API to automatically setup an order and hand the
 resulting URL to the customer's wallet.
 
 
 \subsection{Making the payment}
 
 From the user's perspective, paying a Taler-enabled merchant is very similar
 to paying a peer-to-peer invoice, in that the user needs to scan the QR code,
 check the amount and purpose of the payment, and hit the confirm button.  With
 the WebExtension wallet, the process can be even further simplified: after
 granting the wallet the permission to ``Automatically open wallet based on
 page content'' in the settings, Web pages can directly transition into the
 wallet payment dialog without ever showing a QR code or asking the user to
 explicitly open the Taler wallet.  Note that it is also possible to make
 payments on Web sites opened in a browser on a desktop with a mobile wallet
 running on a different device.
 
 \section{Going back onto the chain} \label{sec:to_chain}
 
 Last but not least, it is possible to transfer the BITCOINBTC from the Taler
 wallet back to a Bitcoin wallet, effectively moving the funds out of the Taler
 payment system and back onto the chain.
 
 The process is started via the ``send funds'' button next to the current
 balance (Figure~\ref{fig:p2p:menu}).  When the wallet asks for the amount to
 send and the destination (Figure~\ref{fig:p2p:amount}, sending the money to a
 Bitcoin wallet corresponds to the ``to a bank account'' action, as in Bitcoin,
 the equivalent of a bank account is an actual native Bitcoin wallet.
 
 % FIXME: maybe change text shown for bitcoin?
 % FIXME: current Android wallet dialog for sending funds back onto chain is broken!
 
 
 \section{Future work}
 
 In the future, GNU Taler could be combined with technologies like the
 Lightning~\cite{lightening} network to create a federation of GNU Taler
 exchanges.  This would likely address open challenges for Lightening in terms
 of scalability and availability.  The main work required here is to add
 support for federation to the GNU Taler payment system.
 
 \section{Future work}
 
 As described, the current system requires trust in the Taler exchange
 operator. In banking, this trust is usually established by the operator being
 a regulated entity.  However, the existing system is NOT supervised by any
 regulator and there is no depost insurance.\footnote{More specifically, the
 deployment is a research prototype and all funds sent to the exchange are
 considered to be a donation to the research group. The research group may use
 the funds to make payments to third parties as specified above, or terminate
 the system at any time for any reason. Specifically, holders of the respective
 coins do not have a legal rights to any funds held by the research group.}
 
 While possible with GNU Taler, the existing system does not perform any kind
 of KYC or AML checks. As pointed out in our paper~\cite{depo2022}, the
 non-fungible nature of Bitcoin creates challenges for any regulated operator.
 Future business and legal work is required to overcome these challenges if
 this type of system were to be operated commercially.
 
 \section{Conclusion}
 
 We have described how GNU Taler can be used as a layer-2 solution on top of
 Bitcoin to facilitate inexpensive and virtually instant transactions, both
 between GNU Taler wallets and between customer wallets and merchants. The
 system supports the use of cryptographically secured microtransactions
 off-chain with large transactions on-chain when withdrawing or paying out to
 merchants.
 
 
 \bibliographystyle{alpha}
 \bibliography{taler,ui}
 
 \end{document}