\documentclass{scrartcl} \usepackage[a4paper]{geometry} \usepackage{hyperref} \usepackage[dvipsnames]{xcolor} \hypersetup{ colorlinks = true, allcolors = {black}, linkcolor = DarkOrchid, urlcolor = DarkOrchid, } \usepackage{url} \usepackage[font=footnotesize]{caption} \usepackage{amssymb} \usepackage{amsmath} \usepackage{pdfpages} \usepackage{graphicx} \usepackage{listings} \usepackage{fontspec} \setmonofont[Path = ../fonts/, Extension = .ttf, UprightFont = *-Regular, ItalicFont = *-Italic, BoldFont = *-Bold, Scale=0.85]{RobotoMono} \lstdefinelanguage{typescript}{ keywords={typeof, new, true, false, catch, function, return, null, catch, switch, var, if, in, while, do, else, case, break, interface}, keywordstyle=\bfseries, ndkeywords={class, export, boolean, number, Amount, string, Timestamp, RelativeTime, EddsaPublicKey, BrandtVickreyAuction, BrandtVickreyAuctionMessage, BrandtVickreyAuctionWinner, EddsaSignature, HashCode, throw, implements, import, this, BrandtVickreyReplayOutcome}, ndkeywordstyle=\bfseries, identifierstyle=\color{black}, sensitive=false, comment=[l]{//}, morecomment=[s]{/*}{*/}, commentstyle=\itshape, %stringstyle=\color{red}, morestring=[b]', morestring=[b]" } \lstset{ language=typescript, %backgroundcolor=\color{lightgray}, extendedchars=true, basicstyle=\footnotesize\color{NavyBlue}\ttfamily, showstringspaces=false, showspaces=false, %numbers=left, %numberstyle=\footnotesize, %numbersep=9pt, tabsize=2, breaklines=true, showtabs=false, captionpos=b, emphstyle=\bfseries } \newcommand{\TODO}[1]{{\color{orange}#1}\marginpar{{\color{orange}TODO}}} \begin{document} \title{AP³\\ Report for Milestone IV\\ NGI Pointer} \author{Özgür Kesim\\ Christan Grothoff\\ Florian Dold\\ Stefan Kügel\\ Emmanuel Benoist\\[\bigskipamount] \normalsize Mentor: Mirko Ross \href{mailto:m.ross@digital-worx.de}{}\\[\medskipamount] } \date{October 29, 2022} \maketitle \section*{Management summary} \begin{abstract} For the \textsc{NGI Pointer} programme, the AP³ project team extended GNU Taler with \begin{itemize} \item age-restricted payments, \item peer-to-peer (P2P) payments and \item a proof-of-concept escrow functionality for privacy-preserving auctions. \end{itemize} This document provides the report for the final milestone IV with details on the state of the implementation, the results of user experience studies and projected future work. \end{abstract} \vfill \hfill {\footnotesize Version: 1.0} \thispagestyle{empty} \newpage \tableofcontents \newpage \section{Age Restriction} \TODO{} \subsection{Technical details} \TODO{} \subsection{Future Works} \TODO{} \subsection{Links} \TODO{} \newpage \section{P2P Payments} We implemented two styles of P2P payments: \textbf{push payments} where a user sends money to another Taler wallet (which can then accept the payment), and \textbf{pull payments} where a user sends an invoice to another Taler wallet (which can then pay the invoice). Both styles of payment only require one asynchronous uni-directional secure communication between the wallets to make the payment. The information that is exchanged is in both cases a short \texttt{taler://}-URI which allows the wallet to obtain further details from the payment service provider. These further details include the contract/invoice that the payment is for. The contract/invoice is stored encrypted at the exchange and thus cannot be learned by the exchange. The actual P2P payment works by having the initiator wallet first communicate with the Taler exchange to setup the payment. Then the Taler URI is communicated to the other wallet, for now usually via QR code or NFC transmission; however, users could also copy the URI to the clipboard and exchange it via some (secure) messenger. Upon receiving the URI, the other wallet then again interacts with the exchange to obtain more details and complete the payment. If the recipient of a push payment does not accept the payment (say because the message was lost in transmission, or they do not like the attached contract terms) the money is automatically refunded to the payer after some expiration date is reached. \subsection{Technical details} P2P payments always work by establishing a \textbf{purse} at the exchange. A purse has an expiration date, target amount, minimum age, associated business contract (or invoice), an actual balance and two public-private key pairs representing permissions to operate on the purse. For a push payment, the wallet of the sending user creates a purse (with contract and expiration time) and immediately ensures that the balance of the purse is the target amount. The payer then sends the merge-capability key of the purse to the payee. The payee can then use the merge-capability key to merge (move) the balance of the purse into a KYCed (i.e. know-your-customer ready) reserve of the receiving wallet. A KYCed reserve basically is a long-term public-private key pair that identifies a wallet and a user at an exchange, ensuring income transparency. Once the money is in the reserve, the wallet can then use the reserve private key to withdraw fresh coins. When invoicing, the initiator again creates a purse, but this time does not put any money into it and keeps the merge capability key private. The initiator wallet instead immediately specifies its own KYCed reserve as the one the purse should be merged into. The initiator then shares a contract-download capability key with the payer. The payer downloads the contract (including the purse public key) and can then decide to put money into the purse. Once the purse balance reaches the target amount, it is then automatically merged with the initiator's reserve. The initiator's wallet (long) polls the reserve and withdraws the funds as soon as they become available. The protocol and implementation includes a few refinements, like fees to be paid to the exchange for managing a purse and age-restrictions on the coins used to pay an invoice. \subsection{Future Work} The following features (from tiny to major) have been discussed in the team and ought to be implemented as part of future work. \begin{itemize} \item \textbf{Wallets:} The wallet UIs currently do not allow the user to actually specify any age-restrictions on the payer when sending P2P pull payment requests (invoicing). This mostly is about adding one more drop-down widget. \item \textbf{Wallets:} The wallet UIs currently do not allow the user to actually specify the expiration date for the purse. Instead, the duration is hard-coded to a few hours (likely too short). This mostly is about adding one more input field. \item \textbf{Auditor:} The Taler auditor was extended to support P2P transactions in its audits, but the code has not been adequately tested (no fault injection). \item \textbf{Exchange:} P2P payments currently only work if both wallets are using the same exchange. If multiple exchanges operate in the same currency domain and the recipient has made their (expensive) KYC process at one exchange and the payer has withdrawn from the other, a direct payment between the wallets is not possible right now. We envision federation protocol using periodic aggregated (``wad'') payments between exchanges should be added to support P2P payments involing multiple exchanges in the future. \item \textbf{Mailbox (NEW):} Currently, sending the payment URI is largely left to the user. We would like to implement a ``mailbox'' service for Taler wallets that would enable wallets to asynchronously exchange URIs over the Internet. Once a wallet knows the mailbox address of another wallet (which would include a public key to encrypt messages to), the user would no longer be required to manually exchange the QR code. \item \textbf{TalDir (NEW):} To lookup the mailbox address of another wallet, we would like to implement a directory service that maps existing addresses like e-mail addresses, phone numbers or accounts in messengers or social media platforms to a mailbox. When establishing the directory service entry, the directory service would validate that the user registering the wallet has control over the respective address. Naturally, the directory service would need to be trusted to return the correct mapping. \item Both the mailbox and the directory service operators could themselves be paid via Taler for their service. That should help ensure a high quality of service from those operators. Naturally, using the mailbox or the directory service would be optional. \item \textbf{Messaging:} In the long term, we would like to see more direct integration of Taler payment functionality with messaging applications, especially for spam prevention (``The recipient has configured its software to demand a payment of 50 cents before displaying a message from an unknown sender to the user. If they like your message, they promise they would refund this attention fee. Do you want to pay to have your message shown? (Y/N)''). \end{itemize} \subsection{Links} \href{https://docs.taler.net/design-documents/013-peer-to-peer-payments.html}{Document 013} at \url{https://docs.taler.net/design-documents/} presents the design. The main implementation parts for P2P are distributed over various code locations under \url{https://git.taler.net/wallet-core.git/tree/packages/} (wallet) and \url{https://git.taler.net/exchange.git/tree/src/exchange/} (exchange): \begin{description} \item[Payment operations:] \href{https://git.taler.net/wallet-core.git/tree/packages/taler-wallet-core/src/operations/pay-peer.ts}{taler-wallet-core/src/operations/pay-peer.ts} \item[Database schema related:] \href{https://git.taler.net/wallet-core.git/tree/packages/taler-wallet-core/src/db.ts#n2065}{taler-wallet-core/src/db.ts} \item[Transaction list type declarations:] \href{https://git.taler.net/wallet-core.git/tree/packages/taler-util/src/transactions-types.ts#n251}{taler-util/src/transactions-types.ts} \item[WebExtension UI:] \href{https://git.taler.net/wallet-core.git/tree/packages/taler-wallet-webextension/src/cta/InvoiceCreate}{taler-wallet-webextension/src/cta/InvoiceCreate} and \href{https://git.taler.net/wallet-core.git/tree/packages/taler-wallet-webextension/src/cta/InvoicePay}{InvoicePay} \item[Exchange] \url{https://git.taler.net/exchange.git/tree/src/exchange/}, specifically:\\ \texttt{taler-exchange-httpd\_purses\_*.c}, \texttt{taler-exchange-httpd\_reserves\_purse\_*.c}, \texttt{taler-exchange-httpd\_contract.*}, \texttt{taler-exchange-expire.c} (plus related changes in the database, client libraries, history, auditor), and the main test case at \href{https://git.taler.net/exchange.git/tree/src/testing/test\_exchange\_p2p.c}{testing/test\_exchange\_p2p.c} \end{description} \newpage \section{Brandt-Vickrey Auctions} \TODO{} \subsection{Technical details} \TODO{} \subsection{Future Works} \TODO{} \subsection{Links} \TODO{} \newpage \section{User Experience Studies} For the \textsc{NGI Pointer} programme, the AP³ project team performed a usability study to gather feedback and inform further development of the age-restricted and P2P payment functionalities. The BFH ``Digitaltag'' is an annual day-long event where the university presents itself to the public. It is held right next to the central train station of Biel/Bienne, and is open to the general public. It was attended by a mixture of prospective students, normal adults, Swiss executives and retirees. We used the opportunity to both present GNU Taler to the public, and to conduct usability studies with interested volunteers. \begin{figure}[h!] \includegraphics[width=\textwidth]{pics/digitaltag-ux-setup.jpg} \caption{Our booth, with GNU Taler publications (including on age-restrictions), NGI stickers, and a Taler-enabled coffee machine.} \end{figure} \subsection{Preparation} We prepared several notebooks with a browser running a Taler wallet as well as several Android phones with the Taler Android wallet. We set up the coffee machine and three Taler backends, one for CHF (used by the coffee machine), one for KUDOS (used with age-restrictions in the browser-based setup) and one for Bitcoin (used for P2P payments). We also prepared a rough write-up describing what we would like users to do. These intended user stories are included in the appendix. We note that during the day, we permitted participants to deviate from the script if they desired to do so, sometimes leading them to explore other GNU Taler features (and us learning interesting lessons about those). For the UX study, we prepared four tables: two tables with the coffee machine and information materials, and two tables with additional chairs for guests for the actual UX experiment. \begin{figure}[h!] \includegraphics[width=\textwidth]{pics/digitaltag-ux-chairs.jpg} \caption{Tables for the UX study with Prof. Benoist.} \end{figure} \subsection{Data collection} We did not collect any PII on the participants.\footnote{Except for one executive who had come just for our booth from Zug and who gave us his business card as he hopes to collaborate with us in the future.} Instead, each team member wrote down their observations. We afterwards de-duplicated the observations and turned those that could lead to improvements into over 20 new issues on the GNU Taler bug-tracker (\#7334--\#7354). \subsection{Key conclusions} The day revealed the existence of several previously unknown bugs (like refresh not working properly with the new features) as well as quite a few surprising difficulties of users (not finding the QR code button, not finding the account balance, not understanding that the \texttt{shop.demo.taler.net} page is the shop where they should buy things). We will try to rectify those as soon as possible. \end{document}