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Christian Grothoff 2016-05-29 21:25:08 +02:00
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@ -83,30 +83,30 @@ payments while maintaining unlinkability of transactions. We argue
that Taler provides a secure digital currency for modern liberal that Taler provides a secure digital currency for modern liberal
societies as it is a flexible, libre and efficient protocol and societies as it is a flexible, libre and efficient protocol and
adequately balances the state's need for monetary control with the adequately balances the state's need for monetary control with the
citizen's needs for private economic activity. citizen's needs for private economic activity.
\end{abstract} \end{abstract}
\section{Introduction} \section{Introduction}
The design of payment systems shapes economies and societies. The design of payment systems shapes economies and societies.
Strong, developed nation states are evolving towards transparent Strong, developed nation states are evolving towards transparent
payment systems, such as the MasterCard and VisaCard credit card payment systems, such as the MasterCard and VisaCard credit card
schemes and computerized bank transactions such as SWIFT. schemes and computerized bank transactions such as SWIFT.
These systems enable mass surveillance by both governments and These systems enable mass surveillance by both governments and
private companies, chilling customer activity~\cite{???}. private companies, chilling customer activity~\cite{???}.
Aspects of this government control benifit the economy, by enabling Aspects of this government control benifit the economy, by enabling
taxation. Also, bribery and corruption are limited to elites who taxation. Also, bribery and corruption are limited to elites who
can afford to escape the dragnet. can afford to escape the dragnet.
At the other extreme, weaker developing nation states have economic At the other extreme, weaker developing nation states have economic
activity based largely on coins, paper money or even barter. activity based largely on coins, paper money or even barter.
Here, the state is often unable to effectively monitor or tax economic Here, the state is often unable to effectively monitor or tax economic
activity, and this limits the ability of the state to shape the society. activity, and this limits the ability of the state to shape the society.
As bribery is virtually impossible to detect, corruption is widespread As bribery is virtually impossible to detect, corruption is widespread
and not limited to social elites. and not limited to social elites.
% %
ZeroCoin~\cite{miers2013zerocoin} is an example for translating an ZeroCoin~\cite{miers2013zerocoin} is an example for translating an
anarchistic economy into the digital realm. anarchistic economy into the digital realm.
% FIXME: Unclear referee comment : % FIXME: Unclear referee comment :
% I didn’t understand why ZeroCoin is particularly suited for % I didn’t understand why ZeroCoin is particularly suited for
% developing nations? % developing nations?
% => clarified: suited to model anarchistic economy. % => clarified: suited to model anarchistic economy.
@ -123,7 +123,7 @@ customer, merchant and exchange (Figure~\ref{fig:cmm}).
The two designs share the key first step where the {\em customer} The two designs share the key first step where the {\em customer}
withdraws digital {\em coins} from the {\em exchange} with unlinkability withdraws digital {\em coins} from the {\em exchange} with unlinkability
provided via blind signatures. The coins can then be spent at a provided via blind signatures. The coins can then be spent at a
{\em merchant} who {\em deposits} them at the exchange. {\em merchant} who {\em deposits} them at the exchange.
Taler uses online detection of double-spending, thus assuring the merchant Taler uses online detection of double-spending, thus assuring the merchant
instantly that a transaction is valid. instantly that a transaction is valid.
@ -158,7 +158,7 @@ be too inefficient, even for modern systems. The customer should not
withdraw exact change from her account, as doing so reduces anonymity withdraw exact change from her account, as doing so reduces anonymity
due to the obvious corrolation. A practical payment system must thus due to the obvious corrolation. A practical payment system must thus
support giving change in the form of spendable coins, say a \EUR{0,01} support giving change in the form of spendable coins, say a \EUR{0,01}
coin and a \EUR{50,00} coin. coin and a \EUR{50,00} coin.
Taler solves the problem of giving change by introducing a new {\em Taler solves the problem of giving change by introducing a new {\em
refresh} protocol. Using this protocol, a customer can obtain refresh} protocol. Using this protocol, a customer can obtain
@ -233,9 +233,9 @@ include:
should be free software (libre) to have a chance for widespread adoption. should be free software (libre) to have a chance for widespread adoption.
\item Support for payments to off-line merchants, and thus deferred \item Support for payments to off-line merchants, and thus deferred
detection of double-spending, requires the exchange to attempt to detection of double-spending, requires the exchange to attempt to
recover funds from delinquent customers via the legal system. recover funds from delinquent customers via the legal system.
Any system that fails to be self-enforcing creates a major Any system that fails to be self-enforcing creates a major
business risk for the exchange and merchants. business risk for the exchange and merchants.
In 1983, there were merchants without network connectivity, making that In 1983, there were merchants without network connectivity, making that
feature relevant, but today network connectivity is feasible for most feature relevant, but today network connectivity is feasible for most
merchants, and saves both the exchange and merchants the business risks merchants, and saves both the exchange and merchants the business risks
@ -416,13 +416,13 @@ Ideally, the customer's anonymity is limited only by this channel;
however, the payment system does additionally reveal that the customer however, the payment system does additionally reveal that the customer
is one of the patrons of the exchange. is one of the patrons of the exchange.
There are naturally risks that the customer-merchant business operation There are naturally risks that the customer-merchant business operation
may leak identifying information about the customer. may leak identifying information about the customer.
We consider information leakage specific to the business logic to be We consider information leakage specific to the business logic to be
outside of the scope of the design of Taler. outside of the scope of the design of Taler.
Aside from refreshing and obtaining denomination key, the customer Aside from refreshing and obtaining denomination key, the customer
should ideally use an anonymous communication channel with the exchange should ideally use an anonymous communication channel with the exchange
to obscure their IP address for location privacy, but naturally to obscure their IP address for location privacy, but naturally
the exchange would typically learn the customer's identity from the wire the exchange would typically learn the customer's identity from the wire
transfer that funds the customer's withdrawal of anonymous digital coins. transfer that funds the customer's withdrawal of anonymous digital coins.
We believe this may even be desirable as there are laws, or bank policies, We believe this may even be desirable as there are laws, or bank policies,
@ -432,7 +432,7 @@ Taler is thus only anonymous with respect to {\em payments}.
In particular, the exchange In particular, the exchange
is unable to link the known identity of the customer that withdrew is unable to link the known identity of the customer that withdrew
anonymous digital coins to the {\em purchase} performed later at the anonymous digital coins to the {\em purchase} performed later at the
merchant. merchant.
While the customer thus has anonymity for purchases, the exchange will While the customer thus has anonymity for purchases, the exchange will
always learn the merchant's identity in order to credit the merchant's always learn the merchant's identity in order to credit the merchant's
@ -590,9 +590,9 @@ resumed at any step. Commitments to disk are cumulative, that is an
additional commitment does not erase the previously committed additional commitment does not erase the previously committed
information. Keys and thus coins always have a well-known expiration information. Keys and thus coins always have a well-known expiration
date; information committed to disk can be discarded after the date; information committed to disk can be discarded after the
expiration date of the respective public key. expiration date of the respective public key.
Customers may discard information once the respective coins have been Customers may discard information once the respective coins have been
fully spent, so long as refunds are not required. fully spent, so long as refunds are not required.
Merchants may discard information once payments from the exchange have Merchants may discard information once payments from the exchange have
been received, assuming the records are also no longer needed for tax been received, assuming the records are also no longer needed for tax
purposes. The exchange's bank transfers dealing in traditional currency purposes. The exchange's bank transfers dealing in traditional currency
@ -610,7 +610,7 @@ performs the following interaction with the exchange:
% others, so probably withdrawal key should be renamed to reserve key. % others, so probably withdrawal key should be renamed to reserve key.
% FIXME: These steps occur at very different points in time, so probably % FIXME: These steps occur at very different points in time, so probably
% they should be restructured into more of a protocol discription. % they should be restructured into more of a protocol discription.
% It does create some confusion, like is a withdrawal key semi-ephemeral % It does create some confusion, like is a withdrawal key semi-ephemeral
% like a linking key? % like a linking key?
@ -946,7 +946,7 @@ provides a payment system with the following key properties:
Additionally, customers cannot defraud anyone, and Additionally, customers cannot defraud anyone, and
merchants can only defraud their customers by not merchants can only defraud their customers by not
delivering on the agreed contract. Neither merchants nor customers delivering on the agreed contract. Neither merchants nor customers
are able to commit fraud against the exchange. are able to commit fraud against the exchange.
Only the exchange needs be tightly audited and regulated. Only the exchange needs be tightly audited and regulated.
\item[No speculation] % It's actually "Specualtion not required" \item[No speculation] % It's actually "Specualtion not required"
The digital coins are denominated in existing currencies, The digital coins are denominated in existing currencies,
@ -984,7 +984,7 @@ principle. However, as mentioned Taler does facilitate limits on
withdrawals, which we believe is a better trade-off than the withdrawals, which we believe is a better trade-off than the
problematic escrow systems where the necessary intransparency problematic escrow systems where the necessary intransparency
actually facilitates voluntary cooperation between the exchange and actually facilitates voluntary cooperation between the exchange and
criminals~\cite{sander1999escrow} and where the state could criminals~\cite{sander1999escrow} and where the state could
deanonymize citizens. deanonymize citizens.
\subsection{Offline Payments} \subsection{Offline Payments}