ngi-pointer-ap3/m4/ngi-ap3-m4-report.tex

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\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}{<m.ross@digital-worx.de>}\\[\medskipamount]
}

\date{October 29, 2022}

\maketitle

\section*{Management summary}
\begin{abstract}
\noindent 
	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 results of our usability study, the state of the implementation of
	the features and projected future work.

\end{abstract}

\vfill
\hfill {\footnotesize Version: 1.0}

\thispagestyle{empty}

\newpage
\tableofcontents
\newpage


\section{Age Restriction}

We designed and implemented a scheme for age restriction in GNU Taler based on
the following basic ideas:

Parents/warden can choose to \textbf{commit} a certain maximum age out of a
predefined list of age groups and bind that commitment to a particular coin.
The minors receive those coins and can now \textbf{attest} a required minimum
age (provided that age is less or equal to the committed age of the coins) to
merchants, who can \textbf{verify} the minimum age.  For the rest values
(change) after an transaction, the minor/ward can \textbf{derive} new
age-restricted coins. The exchange can \textbf{compare} the equality of the
age-restriction of the old coin with the new coin (in a zero-knowledge
protocol $\DeriveCompare$, that gives the minor a 1/$\kappa$ chance to raise
the minimum age for the new coin).

Figure~\ref{scheme} gives an overview of the scheme for age restriction
detached from the payment flow.

\begin{figure}[h]
	\begin{center}\footnotesize
\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{center}
	\caption{Scheme of the age restriction performed between a guardian
	$\Guardian$, a child $\Child$, a merchant $\Merchant$ and an exchange
	$\Exchange$, using the functions $\Commit$, $\Attest$, $\Verify$ and
	the zero-knowledge protocol $\DeriveCompare$ which is based on
	functions $\Derive$ and $\Compare$.  $\commitment$ is the age
	commitment for a maximum age $\age \in \{1,...,\Age\}$ and
	$\pruf_{\age}$ is the corresponding proof.  $\attest_{\minage}$ is an
	attestation of a required age $\minage \leq \age$.}
	\label{scheme}
\end{figure}

We have implemented all parts of this scheme in GNU Taler's exchange, wallet
and merchant code bases for the most common protocols: withdraw, purchase,
deposit and refresh.

\subsection{Technical details}

Our implementation of the five functions $\Commit$, $\Attest$, $\Verify$,
$\Derive$ and $\Compare$ is based on the following main building blocks:

\begin{itemize}
	\item The exchange $\Exchange$ defines and publishes M+1 different
		\textit{age groups} of increasing order: $0 < a_1 < \ldots <
		a_M$ with $a_i \in \mathbb{N}$. The zeroth age group is
		$\{0,\ldots,a_1-1\}$.

	\item An \textit{unrestricted} age commitment is defined as a vector of
		length $\Age$ of pairs of 
		\href{https://docs.taler.net/design-documents/024-age-restriction.html#edx25519}{Edx25519}
		public and private keys on Curve25519. In other words: one key
		pair for each age group after the zeroth: $\bigl\langle (p_1,
		q_1), \ldots, (p_M, q_M) \bigr\rangle$.  (Here, $p_i$ are
		\textit{private} and $q_i$ are public keys).

	\item A \textit{restricted} age commitment to age group m (or m-th age
		group) is derived from an unrestricted age commitment by
		removing all private keys for indices larger than m:
		\[
			\bigl\langle (p_1, q_1), \ldots, (p_m, q_m), 
			\, (\perp, q_{m+1}), 
			\ldots, (\perp, q_M)\bigr\rangle
		\] F.e. if none of the private keys is provided, the age
		commitment would be restricted to the zeroth age group.

		Note that the action of dropping private keys is performed by
		the guardian $\Guardian$.

	\item An \textit{age commitment} (without prefix) is just the vector of
		public keys: $\commitment := \langle q_1, \ldots, q_M \rangle$.
		Note that from just the age commitment one can not deduce if it
		was originated from an unrestricted or restricted age
		commitment (and what age).

	\item A child $\Child$ receives the commitment $\commitment$ along with
		the proof, the restricted vector\\
		$\pruf_\age := (p_1,\ldots,p_\age,\perp,\ldots,\perp)$.
		$\Child$ can now create an \textit{attestation}
		$\attest_\minage$ for age group $\minage \leq \age$, which is
		simply a signature to some message with the private key
		$p_\minage$.

	\item An age commitment $\commitment$ is bound to a particular coin
		$C_p$ by incorporating the SHA256 hash value of $\commitment$
		into the signature of the coin. So, instead of signing the
		full-domain-hash $\text{FDH}(C_p)$ with the RSA private key of
		a denomination, the exchange signs $\text{FDH}(C_p,
		\orange{H(\commitment)})$.

	\item $\DeriveCompare$ is a zero-knowledge, cut-and-choose protocol,
		using the functions $\Derive$ and $\Compare$, which allows
		$\Child$ to derive new commitments from existing ones in an
		unlinkable way and $\Exchange$ to statistically trust them to
		be equivalent.  The protocol is defined roughly as follows,
		with $\kappa \in \N$ the security parameter (currently $\kappa
		= 3$):
	\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}

\end{itemize}

The integration of both schemes for age restriction and the scheme for payment
in GNU Taler (protocols \textsf{withdraw}, \textsf{purchase}, \textsf{deposit}
and \textsf{refresh}) is sketched in Figure~\ref{bothschemes}.
\begin{figure}[ht]
\begin{center}
\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]
		{\textsf{withdraw}\orange{, using}\\ $\FDH(C_p\orange{, H(\commitment)})$} (Exchange);
	\draw[<->] (Client)   to node[sloped,below,align=center]
		{\textsf{refresh} \orange{ + }\\ \orange{$\DeriveCompare$}} (Exchange);
	\draw[<->] (Client)   to node[sloped, below]
		{\textsf{purchase} \blue{+ $(\attest_\minage, \commitment)$}} (Merchant);
	\draw[<->] (Merchant) to node[sloped, above]
		{\textsf{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{center}
	\caption{Sketch of the integration of the schemes for age restriction
	and payment in GNU Taler.}
	\label{bothschemes}
\end{figure}

The proposed solution maintains the guarantees of GNU Taler with respect to
anonymity and unlinkability.  Precise formulations of the functions, protocols,
requirements and security guarantees---together with proofs---can be found in
our paper
\href{https://taler.net/papers/esorics2022-age-restriction.pdf}
{\textit{Zero-Knowledge Age Restriction for GNU Taler}},
published in the 
\href{https://link.springer.com/chapter/10.1007/978-3-031-17140-6\_6}
{proceedings to ESORICS 2022}.  



\subsection{Legal evaluation}

We are in correspondence with the \href{https://www.kjm-online.de/en/}{German
Commission for the Protections of Minors in the Media (KJM)}.  The commission
acts as the central supervisory body for the protection of minors in private
broadcasting and media in Germany and runs a Berlin-based joint office of state
authorities for media and journalism within the Federal republic of Germany.
Its plenary congregation meets in Berlin on December 7, 2022, for which GNU
Taler applies with a systematic description and the whitepaper on Taler tokens
with age restrictions in German language. The application is assisted and
coordinated by Mr. Henning Mellage, Legal \& Supervision Officer, at the
Landesanstalt für Medien NRW in Düsseldorf which is part of the KJM.

The KJM confirmed its intention to evaluate GNU Taler as recommendable concept
for the protection of minors and to add GNU Taler to its list of positively
evaluated, so-called ``cross-channel concepts'' at
\href{https://www.kjm-online.de/en/supervision/technical-protection-of-minors-in-the-media/cross-channel-concepts}%
{\texttt{https://www.kjm-online.de/en/supervision/{\small \keys{\return}}\\
technical-protection-of-minors-in-the-media/cross-channel-concepts}}.  This
includes the presentation of GNU Taler in the document
\href{https://www.kjm-online.de/fileadmin/user_upload/KJM/Aufsicht/Technischer_Jugendmedienschutz/Uebergreifende_Jugendschutzkonzepte.pdf}{\textit{Übersicht
über positiv bewertete übergreifende Jugendschutzkonzepte}}, which mirrors the
content of the mentioned website with recommended concepts of youth protection.

GNU Taler is going to be evaluated for the category ``holistic youth protection
concept''. The KJM uses this category only for concepts that are widely
accessible to the public and \textit{do not} serve a closed user group.  Here,
\textit{serving a closed user group} means granting access to content or goods
only after ID-based verification of the membership in a particular group, such
as: proof of belonging to a certain age-group by presenting an official ID
card, driving licenses or the like---a type of verification which is
endangering privacy, data security, and informational self-determination.

\subsection{Future Works}

\begin{description}
	\item[Completion of support for all GNU Taler protocols:] So far, age restriction is
		only implemented for the GNU Taler protocols \textsf{withdraw},
		\textsf{purchase}, \textsf{deposit} and \textsf{refresh}.  We
		will extend the support for age restriction in GNU Taler to
		include the protocols for P2P payments, tipping and refund.

	\item[Support for minors with bank accounts:]  The current design
		of age restriction is based on the assumption that only
		adults can have bank accounts. That is: wire transfers to the
		exchange are assumed to be originated by adults.

		However, in some countries, like Germany, it is possible for
		minors to have bank accounts, too, starting from a certain age.
		In those cases, the wire transfer record will indicate that the
		originating account is owned by a minor.

		We plan to extend the current design and implementation of age
		restriction to handle those situations as well:  After the
		exchange receives a wire transfer from a bank account of a
		minor, it will require in a zero-knowledge-proof for a) the
		presence of age restriction and b) the appropriate
		\textit{maximum} age for the age commitment during the
		\textsf{withdraw} protocol.

\end{description}


\subsection{Links}

Our scheme for age restriction in GNU Taler has been 
\href{https://link.springer.com/chapter/10.1007/978-3-031-17140-6\_6}{published
in the proceedings to ESORICS 2022}.  
In addition,
\href{https://docs.taler.net/design-documents/024-age-restriction.html}%
{document 24} at \url{https://docs.taler.net/design-documents} also lays out
the design.  The implementation is distributed across multiple repositories:

{ \small
\begin{description}
	\item[Exchange:] The following REST endpoint handlers and their
		accompanying helper functions in
		\url{https://git.taler.net/exchange.git/tree/src}:
		\begin{itemize}
			\item \href{https://git.taler.net/exchange.git/tree/src/exchange/taler-exchange-httpd_deposit.c}{\texttt{TEH\_handler\_deposit}}
			\item \href{https://git.taler.net/exchange.git/tree/src/exchange/taler-exchange-httpd_melt.c}{\texttt{TEH\_handler\_melt}}
			\item \href{https://git.taler.net/exchange.git/tree/src/exchange/taler-exchange-httpd_refreshes_reveal.c}{\texttt{TEH\_handler\_reveal}}
			\item \href{https://git.taler.net/exchange.git/tree/src/exchange/taler-exchange-httpd_recoup.c}{\texttt{TEH\_handler\_recoup}}
			\item \href{https://git.taler.net/exchange.git/tree/src/exchange/taler-exchange-httpd_recoup-refresh.c}{\texttt{TEH\_handler\_recoup\_refresh}}
		\end{itemize}

	Under \url{https://git.taler.net/exchange.git/tree/src/exchangedb}:\\
		\href{https://git.taler.net/exchange.git/tree/src/exchangedb/common.sql}{common.sql}, 
		\href{https://git.taler.net/exchange.git/tree/src/exchangedb/exchange-0001.sql}{exchange-0001.sql},
		\href{https://git.taler.net/exchange.git/tree/src/exchangedb/plugin\_exchangedb\_postgres.c}{plugin\_exchangedb\_postgres.c}.
	\item[Merchant:]
		Under \url{https://git.taler.net/merchant.git/tree/src/},
		\begin{itemize}
			\item schema changes in
				\href{https://git.taler.net/merchant.git/tree/src/backenddb/merchant-0001.sql}{backenddb/merchant-0001.sql} and\\
				\href{https://git.taler.net/merchant.git/tree/src/backenddb/plugin_merchantdb_postgres.c}{backenddb/plugin\_merchantdb\_postgres.c}
			\item functions \verb|process_pay_with_exchange| and \verb|parse_pay| in\\
				\href{https://git.taler.net/merchant.git/tree/src/backend/taler-merchant-httpd_post-orders-ID-pay.c}{backend/taler-merchant-httpd\_post-orders-ID-pay.c}
		\end{itemize}

	\item[Wallet:]
		Under \url{https://git.taler.net/wallet-core.git/tree/packages/taler-util}
		\begin{itemize}
			\item low-level cryptographic primitives in 
				\href{https://git.taler.net/wallet-core.git/tree/packages/taler-util/src/nacl-fast.ts}{\texttt{crypto\_edx25519}} and
				\href{https://git.taler.net/wallet-core.git/tree/packages/taler-util/src/talerCrypto.ts#n851}{namespace \texttt{Edx25519}}
			\item  high-level cryptographic primitives in
				\href{https://git.taler.net/wallet-core.git/tree/packages/taler-util/src/talerCrypto.ts#n966}{namespace \texttt{AgeRestrictions}}
			\item API changes to wallet-core RPC API in
				\href{https://git.taler.net/wallet-core.git/tree/packages/taler-util/src/walletTypes.ts}{\texttt{restrictAge}}
		\end{itemize}
		Under \url{https://git.taler.net/wallet-core.git/tree/packages/taler-wallet-core/}
		\begin{itemize}
			\item withdrawal and refresh primitives in \href{https://git.taler.net/wallet-core.git/tree/packages/taler-wallet-core/src/crypto/cryptoImplementation.ts}{\texttt{crypto/cryptoImplementation.ts}}
			\item wallet database requests and HTTP requests in \\
				\href{https://git.taler.net/wallet-core.git/tree/packages/taler-wallet-core/src/db.ts}{\texttt{db.ts}},
				\href{https://git.taler.net/wallet-core.git/tree/packages/taler-wallet-core/src/operations/withdraw.ts}{\texttt{withdraw.ts}} and
				\href{https://git.taler.net/wallet-core.git/tree/packages/taler-wallet-core/src/operations/refresh.ts}{\texttt{refresh.ts}}
			\item coin/denomination selection in
				\href{https://git.taler.net/wallet-core.git/tree/packages/taler-wallet-core/src/util/coinSelection.ts}{\texttt{util/coinSelection.ts}}
		\end{itemize}


	\item[Tests:] Under \url{https://git.taler.net/exchange.git/tree/src}:
		\begin{itemize}
			\item \href{https://git.taler.net/exchange.git/tree/src/util/test\_crypto.c}{util/test\_crypto.c}
			\item \href{https://git.taler.net/exchange.git/tree/src/util/test\_age\_restriction.c}{util/test\_age\_restriction.c}
			\item \href{https://git.taler.net/exchange.git/tree/src/util/tv\_age\_restriction.c}{util/tv\_age\_restriction.c}
			\item \href{https://git.taler.net/exchange.git/tree/src/testing/test\_exchange\_api.c}{testing/test\_exchange\_api.c}
		\end{itemize}
\end{description}


The definition of Edx25519, a variant of EdDSA that we designed for usage in
our age restriction scheme in GNU Taler, and its implementation is located at
\url{https://git.gnunet.org/gnunet.git/} in file
\href{https://git.gnunet.org/gnunet.git/tree/src/util/crypto\_edx25519.c}{util/crypto\_edx25519.c}.

}



\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 KYC%
\footnote{KYC: Know-Your-Customer, a legally required procedure for customers
to identify themselves with the bank.}ed
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}

\href{https://pub.dss.in.tum.de/brandt-research/ijis2006.pdf}{In 2006, Felix
Brandt} designed cryptographic Vickrey-style auction protocols that only yield
the winners’ identities and the selling price. Losing bidders learn no
information at all, except that they lost.  This approach does not rely on
trusted third parties, e.g., auctioneers or---in our case---the payment service
provider.
\href{https://grothoff.org/christian/teich2017ms.pdf}{In 2017, Marcus Teich}
updated the cryptographic primitives in the Brandt-Vickrey protocols to use
elliptic curves and provided an implementation in
\url{https://git.gnunet.org/libbrandt.git/}.

Based on that work, we implemented a Proof-of-Concept for \textsc{NGI Pointer}
of conditional payments in GNU Taler in the form of sealed-bid auctions of type
Brandt-Vickrey:

Bidders put coins in escrow with the exchange in order to participate in a
particular auction.  The coins are locked and can only be unlocked by timeout
or by winning the auction.  After successfully running the Brandt-Vickrey
auction, the seller has to provide the transcript of all (signed) messages
received from all bidders.  The exchange then replays the transcript and
determines the winning bidder(s) and price(s) and pays the seller accordingly.
Non-winning coins and rest amounts can be claimed by the bidders.

\subsection{Technical details}

We designed and implemented a general framework for conditional payments with
GNU Taler---so-called \textit{policies}.  Our proof-of-concept of sealed-bid
auction support is implemented as an policy extension to GNU Taler.  In
particular, we implemented the following parts:  

\begin{description}
	\item[Transcript generation:]  \texttt{libbrandt} has been extended to
		generate a transcript of the auction including all
		cryptographically signed messages from all bidders in
		JSON-encoding.

	\item[Transcript replay:]  An external program to replay a transcript
		has been implemented as part of \texttt{libbrandt}.  It
		consumes transcript of an auction in JSON-encoding, generates
		an Brandt-Vickrey-auction and replays all provided messages.
		On success, it returns the winning bidder(s) and price(s)
		according the transcript.

	\item[Policy extension for auctions:]  A policy extension for GNU Taler
		has been implemented that handles deposit requests with set
		policy of type \texttt{\small
		policy\_brandt\_vickrey\_auction}.  It locks the coins and
		persists the policy details under a hash-code, that it
		generates by calculating the hash $H(h_a || p_b)$, where $h_a$
		is the hash of the auction meta data and $p_b$ is the public
		key of the bidder.  Both parameters are part of the policy
		structure provided during a deposit.

	\item[POST handler for auction transcripts:]  A POST handler for the
		policy extension of that type has been implemented that expects
		a valid transcript of a Brandt-Vickrey-auction.  It uses the
		external program to perform a replay of the auction and
		determine the winning bidder(s) and price(s).  Based on the
		outcome information, the exchange unlocks the corresponding
		winning coins' value for transfer to the seller---minus an
		auction fee---and the rest value of all coins for refresh.

\end{description}


\subsection{Future Works}

For a fully functional and maintainable solution, further development is needed
in the following areas:

\begin{description}

\item[libbrandt upgrade:] The code base of this library is from 2017 and doesn't
	compile with the latest version of
		\href{https://git.gnunet.org/gnunet.git}{GNUnet}.  It also
		uses \href{https://gnupg.org/software/libgcrypt/index.html}%
		{\ttfamily libgcrypt} as cryptographic library, which is quite
		slow compared to its modern alternative 
		\href{https://doc.libsodium.org/}{\ttfamily libsodium}, at
		least with respect to the primitives used in libbrandt.
		For future work we plan to refactor libbrandt to use libsodium
		and the current version of GNUnet. 

\item[Brandt-Vickrey-auction continuation:] The current code is a
	proof-of-concept.  The following known problem exist:
	\begin{itemize}
		\item Signature verification of the transcript and its messages
			is missing
		\item Unit- and integration-tests with the exchange are missing
		\item A fully featured auction(er) system, including billboard
			and client software, is missing.
	\end{itemize}

		We will address at least the first two issues and plan to
		address the lack of an auction system once we find volunteers
		and funding for it.

\item[Policy framework continuation:]   Based on the current design of policy
	extensions for deposit---aka conditional payments---we plan to add
		following policy extensions to the GNU Taler exchange:
	\begin{itemize}
	\item{\itshape Merchant refunds:} Merchant can grant customers refundable
		payments.  Right now, this policy is implicit and optional in
			the usual deposit-flow.  Future work on Taler will lift
			this into a policy-extension.

	\item{\itshape Escrowed payments:} A trustor puts coins into escrow
		with the exchange. It can be claimed by a beneficiary until a
				certain deadline, when the claim is signed by
				both, the beneficiary’s and the trustor’s keys.
	\end{itemize}

\end{description}

\subsection{Links}

\begin{description}
	\item[Changes to libbrandt:]
		\href{https://git.kesim.org/oec/libbrandt/src/branch/transcript/test\_brandt.c}{\ttfamily test\_brandt.c}
		has been extended to generate and print a transcript in JSON
		encoding for each auction.  The new file
		\href{https://git.kesim.org/oec/libbrandt/src/branch/transcript/replay.c}{\ttfamily
		replay.c} implements a standalone program to replay an auction.
	\item[Changes to the Exchange:]
		The changes to the exchange can be found at\\
		\url{https://git.kesim.org/taler/exchange/src/branch/auction\_brandt/},\\
		in particular:
		\begin{description}
	\item[\href{https://git.kesim.org/taler/exchange/src/branch/auction\_brandt/src/extensions/policy\_brandt\_vickrey\_auction/policy\_brandt\_vickrey\_auction.c\#L767-L821}{\ttfamily auction\_create\_policy\_details}]
		This implements the {\ttfamily create\_policy\_details}
		function of the policy extension interface. 

	\item[\href{https://git.kesim.org/taler/exchange/src/branch/auction\_brandt/src/extensions/policy\_brandt\_vickrey\_auction/policy\_brandt\_vickrey\_auction.c\#L694-L764}{\ttfamily auction\_policy\_post\_handler}]
		This implements the {\ttfamily policy\_post\_handler} function
				of the policy extension interface.
		\end{description}
\end{description}


\newpage
\section{Usability Study}

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/b.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/a.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}