taler/exchange
1
0
Fork 0
Code Issues Pull Requests Projects Releases Wiki Activity

-minor corrections to the corrections

Browse Source
This commit is contained in:
Christian Grothoff 2015-10-08 20:47:17 +02:00
parent 2ce2c1dac7
commit 99865ad6d4
1 changed files with 61 additions and 55 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

116
doc/paper/taler.tex
View File

@ -146,33 +146,33 @@ Taler was designed for use in a modern social-liberal society, which we
believe needs a payment system with the following properties: believe needs a payment system with the following properties:
\begin{description} \begin{description}
\item[Customer Anonymity] \item[Customer Anonymity]
It must be impossible for mints, merchants and even a global active It must be impossible for mints, merchants and even a global active
adversary, to trace the spending behavior of a customer. adversary, to trace the spending behavior of a customer.
\item[Unlinkability] \item[Unlinkability]
As a strong form of customer anonymity, it must be infeasible to As a strong form of customer anonymity, it must be infeasible to
link a set of transactions to the same (anonymous) customer. link a set of transactions to the same (anonymous) customer.
%, even when taking aborted transactions into account. %, even when taking aborted transactions into account.
\item[Taxability] \item[Taxability]
In many current legal systems, it is the responsibility of the merchant In many current legal systems, it is the responsibility of the merchant
to deduct (sales) taxes from purchases made by customers, or to to deduct (sales) taxes from purchases made by customers, or to
pay (income) taxes for payments received for work. pay (income) taxes for payments received for work.
%Taxation is necessary for the state to %Taxation is necessary for the state to
%provide legitimate social functions, such as education. Thus, a payment %provide legitimate social functions, such as education. Thus, a payment
%system must facilitate sales, income and transaction taxes. %system must facilitate sales, income and transaction taxes.
This requires that merchants, or anybody receiving electronic transfers, This requires that merchants are easily identifiable and that
is easily identifiable, so that the state can ensure that its an audit trail is always generated, so that the state can ensure that its
taxation regime is obeyed. taxation regime is obeyed.
\item[Verifiability] \item[Verifiability]
The payment system should try to minimize the trust necessary between The payment system should try to minimize the trust necessary between
the participants. In particular, digital signatures should be used, the participants. In particular, digital signatures should be used,
and retained, whenever they would play a role in resolving disputes. % between the involved parties. and retained, whenever they would play a role in resolving disputes. % between the involved parties.
Nevertheless, customers must never be able to defraud anyone, and Nevertheless, customers must never be able to defraud anyone, and
merchants must at best be able to defraud their customers by not merchants must at best be able to defraud their customers by not
delivering on the agreed contract. Neither merchants nor customers delivering on the agreed contract. Neither merchants nor customers
should ever be able to commit fraud against the mint. Additionally, should ever be able to commit fraud against the mint. Additionally,
both customers and merchants must receive cryptographic proofs of both customers and merchants must receive cryptographic proofs of
bad behavior in case of protocol violations by the mint. bad behavior in case of protocol violations by the mint.
In this way, only the mint will need to be tightly audited and regulated. In this way, only the mint will need to be tightly audited and regulated.
The design must make it easy to audit the finances of the mint. The design must make it easy to audit the finances of the mint.
\item[Ease of Deployment] %The system should be easy to deploy for \item[Ease of Deployment] %The system should be easy to deploy for
@ -187,11 +187,11 @@ believe needs a payment system with the following properties:
a free software reference implementation. a free software reference implementation.
% The protocol should % The protocol should
% be able to run easily over HTTP(S). % be able to run easily over HTTP(S).
\item[Low resource consumption] \item[Low resource consumption]
In order to minimize the operating costs and environmental impact of In order to minimize the operating costs and environmental impact of
the payment system, it should avoid the reliance on expensive or the payment system, it should avoid the reliance on expensive or
``wasteful'' computations, such as proof-of-work. ``wasteful'' computations, such as proof-of-work.
\item[Fractional payments] \item[Fractional payments]
The payment system needs to handle both small and large payments in The payment system needs to handle both small and large payments in
an efficient and reliable manner. It is inefficient to simply issue an efficient and reliable manner. It is inefficient to simply issue
coins in the smallest unit of currency, so instead a mechanism to coins in the smallest unit of currency, so instead a mechanism to
@ -200,34 +200,40 @@ believe needs a payment system with the following properties:
\end{description} \end{description}
% %
We give a concise example of how these properties interact: We give a concise example of how these properties interact:
A customer may want to pay \EUR{49,99} using a \EUR{100,00} coin. A customer may want to pay \EUR{49,99} using a \EUR{100,00} coin.
the system must thus support giving change in the form of spendable coins, the system must thus support giving change in the form of spendable coins,
say a \EUR{0,01} coin and a \EUR{50,00} coin. say a \EUR{0,01} coin and a \EUR{50,00} coin.
A merchant cannot simply give the customer their coins in another transaction A merchant cannot simply give the customer their coins in another transaction,
however, as this reverses the role of merchant and customer, and however, as this reverses the role of merchant and customer, and
our taxability requirement would deanonymize the customer. our taxability requirement would deanonymize the customer. The customer
also cannot withdraw exact change from his account from the mint, as this
would allow the mint to link the identity of the customer that is revealed
during withdrawal to the subsequent deposit operation that follows shortly
afterwards.
Instead, the customer should obtain new freshly anonymized coins that cannot be Instead, the customer should obtain new freshly anonymized coins that cannot be
linked with this transaction, the original \EUR{100,00} coin, or each other. linked with this transaction, the original \EUR{100,00} coin, or each other.
Instead of using cryptographic methods like $k$-show signatures Instead of using cryptographic methods like $k$-show
\cite{brands1993efficient} to achieve divisibility, signatures~\cite{brands1993efficient} to achieve divisibility,
Taler's fractional payments use a simpler, more powerful mechanism. Taler's fractional payments use a simpler, more powerful mechanism.
In Taler, a coin is not simply a unique random token, but a private key. In Taler, a coin is not simply a signed unique random token, but signature
over the hash of a public key where the private key is only known to the owner
of the coin.
A customer transfers currency from a coin to a merchant by cryptographically A customer transfers currency from a coin to a merchant by cryptographically
signing a deposit message with this private key. This deposit message signing a deposit message with this private key. This deposit message
specifies the fraction of the coin's value to be paid to the merchant. specifies the fraction of the coin's value to be paid to the merchant.
A key contribution of Taler is the {\em refresh} protocol, which enables A key contribution of Taler is the {\em refresh} protocol, which enables
a customer to exchange the residual value of the exchanged coin for a customer to exchange the residual value of the exchanged coin for
unlinkable fleshy anonymized change. unlinkable fleshy anonymized change.
Taler mints ensure that all transactions involving the same coin Taler mints ensure that all transactions involving the same coin
do not exceed the total value of the coin, thereby do not exceed the total value of the coin simply by
requiring that merchants process transactions online. requiring that merchants clear transactions immediately with the mint.
This improves dramatically on systems that support offline merchants with This improves dramatically on systems that support offline merchants with
cryptographic threats to deanonymizing customers who double-spend, like cryptographic threats to deanonymizing customers who double-spend, like
restrictive blind signatures \cite{brands1993efficient}. restrictive blind signatures~\cite{brands1993efficient}.
In such schemes, if a transaction is interrupted, then any coins sent to In such schemes, if a transaction is interrupted, then any coins sent to
the merchant become tainted, but may never arrive or be spent. the merchant become tainted, but may never arrive or be spent.
It becomes tricky to extract the value of the tainted coins without linking It becomes tricky to extract the value of the tainted coins without linking
to the aborted transaction and risking deanonymization. to the aborted transaction and risking deanonymization.
As with support for fractional payments, Taler addresses these problems by As with support for fractional payments, Taler addresses these problems by
@ -262,8 +268,8 @@ Yet, there are several major irredeemable problems inherent in their designs:
of securing the block chain consume a considerable amount of computational of securing the block chain consume a considerable amount of computational
resources and energy. So Bitcoin does not an environmentally responsible resources and energy. So Bitcoin does not an environmentally responsible
design. design.
\item Bitcoin transactions are not easily taxable, leading to legal hurdles. \item Bitcoin transactions are not easily taxable, leading to legal hurdles.
% The legality and legitimacy of this aspect is questionable. % The legality and legitimacy of this aspect is questionable.
The Zerocoin extension would only make this worse. The Zerocoin extension would only make this worse.
\item Bitcoins can not easily be bound to any fiat currency, leading to \item Bitcoins can not easily be bound to any fiat currency, leading to
significant fluctuations in value. These fluctuations may be desirable in significant fluctuations in value. These fluctuations may be desirable in
@ -360,7 +366,7 @@ mint. Peppercoin does not provide customer-anonymity. The proposed
statistical method by which mints detect fraudulent cooperation between statistical method by which mints detect fraudulent cooperation between
customers and merchants at the expense of the mint not only creates customers and merchants at the expense of the mint not only creates
legal risks for the mint, but would also require that the mint learns legal risks for the mint, but would also require that the mint learns
about microdonations where the merchant did not get upgraded to a about microdonations where the merchant did not get upgraded to a
macropayment. It is therefore unclear how Peppercoin would actually macropayment. It is therefore unclear how Peppercoin would actually
reduce the computational burden on the mint. reduce the computational burden on the mint.
@ -442,17 +448,17 @@ the payment system has effective double-spending detection, this means
that either entity has to constantly fear that the funds might no that either entity has to constantly fear that the funds might no
longer be available to it. It follows that sharing coins by copying longer be available to it. It follows that sharing coins by copying
a private key implies mutual trust between the two parties, in which a private key implies mutual trust between the two parties, in which
case Taler will treat them as the same entity. case Taler will treat them as the same entity.
In Taler, making funds available by copying a private key and thus In Taler, making funds available by copying a private key and thus
sharing control is {\bf not} considered a {\em transaction} and sharing control is {\bf not} considered a {\em transaction} and
thus {\bf not} recorded for taxation. thus {\bf not} recorded for taxation.
Taler ensures taxability only when some entity acquires exclusive Taler ensures taxability only when some entity acquires exclusive
control over the value of digital coins, which requires an interaction control over the value of digital coins, which requires an interaction
with the mint. For such transactions, the state can obtain information with the mint. For such transactions, the state can obtain information
from the mint, or a bank, that identifies the entity that received the from the mint, or a bank, that identifies the entity that received the
digital coins as well as the exact value of those coins. digital coins as well as the exact value of those coins.
Taler also allows the mint, and hence the state, to learn the value of Taler also allows the mint, and hence the state, to learn the value of
digital coins withdrawn by a customer---but not how, where, or when digital coins withdrawn by a customer---but not how, where, or when
they were spent. they were spent.
@ -463,22 +469,22 @@ used for all communication between the customer and the merchant.
Ideally, the customer's anonymity is limited only by this channel; 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 mint. is one of the patrons of the mint.
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. At least, customers may leak identifying information about the customer. At least, customers
have some options to defend their privacy against nosey merchants however, have some options to defend their privacy against nosey merchants however,
possibly even when dealing with physical goods \cite{apod}. possibly even when dealing with physical goods \cite{apod}.
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.
Ideally, customer should use an anonymous communication channel with Ideally, customer should use an anonymous communication channel with
the mint to avoid leaking IP address information; however, the mint the mint to avoid leaking IP address information; however, the mint
would typically learn the customer's identity from the wire transfer would typically learn the customer's identity from the wire transfer
that funds the customer's withdraw anonymous digital coins. that funds the customer's withdraw anonymous digital coins.
In fact, this is desirable as there might be rules and regulations In fact, this is desirable as there might be rules and regulations
designed to limit the amount of anonymous digital cash that an designed to limit the amount of anonymous digital cash that an
individual customer can withdraw in a given time period, similar to individual customer can withdraw in a given time period, similar to
how states today sometimes impose limits on cash how states today sometimes impose limits on cash
withdrawals~\cite{france2015cash,greece2015cash}. withdrawals~\cite{france2015cash,greece2015cash}.
Taler is only anonymous with respect to {\em payments}, as the mint Taler is only anonymous with respect to {\em payments}, as the mint
will be unable to link the known identity of the customer that withdrew will be unable to link the known identity of the customer that withdrew
anonymous digital currency to the {\em purchase} performed later at the anonymous digital currency to the {\em purchase} performed later at the
@ -519,18 +525,18 @@ available for signing, each representing a different coin
denomination. denomination.
These coin signing keys have an expiration date, before which any coins These coin signing keys have an expiration date, before which any coins
signed with it must be spent or refreshed. It follows the mint can signed with it must be spent or refreshed. This allows the mint to
eventually discard records of old transactions, thus limiting the eventually discard records of old transactions, thus limiting the
records that the mint must retain and search to detect double-spending records that the mint must retain and search to detect double-spending
attempts. Furthermore, the mint is expected to use each coin signing attempts. Furthermore, the mint is expected to use each coin signing
key only for a limited number of coins. key only for a limited number of coins.
% for example by limiting its use to sign coins to a week or a month. % for example by limiting its use to sign coins to a week or a month.
In this way, if a private coin signing key were to be compromised, In this way, if a private coin signing key were to be compromised,
the mint would detect this once more coins were redeemed than the total the mint would detect this once more coins were redeemed than the total
that was signed into existence using that coin signing key. that was signed into existence using that coin signing key.
In this case, the mint could allow authentic customers to exchange their In this case, the mint could allow authentic customers to exchange their
unspent coins that were signed with the compromised private key, unspent coins that were signed with the compromised private key,
while refusing further anonymous transactions involving those coins. while refusing further anonymous transactions involving those coins.
As a result, the financial damage of losing a private signing key can be As a result, the financial damage of losing a private signing key can be
limited to at most twice the amount originally signed with that key. limited to at most twice the amount originally signed with that key.
To ensure that the mint does not enable deanonymization of users by To ensure that the mint does not enable deanonymization of users by
@ -539,19 +545,19 @@ announce the coin signing keys in advance. Those announcements are
expected to be signed with an off-line long-term private {\em master expected to be signed with an off-line long-term private {\em master
signing key} of the mint and the auditor. signing key} of the mint and the auditor.
Before a customer can withdraw a coin from the mint, Before a customer can withdraw a coin from the mint,
he has to pay the mint the value of the coin, as well as processing fees. he has to pay the mint the value of the coin, as well as processing fees.
This is done using other means of payments, such as wire transfers or This is done using other means of payments, such as wire transfers or
by having a personal {\em reserve} at the mint, by having a personal {\em reserve} at the mint,
which is equivalent to a bank account with a positive balance. which is equivalent to a bank account with a positive balance.
Taler assumes that the customer has a {\em withdrawal authorization key} Taler assumes that the customer has a {\em withdrawal authorization key}
to identify himself as authorized to withdraw funds from the reserve. to identify himself as authorized to withdraw funds from the reserve.
By signing the withdrawal request messages using this withdrawal By signing the withdrawal request messages using this withdrawal
authorization key, the customer can prove to the mint that he is the authorization key, the customer can prove to the mint that he is the
individual authorized to withdraw anonymous digital coins from his reserve. individual authorized to withdraw anonymous digital coins from his reserve.
The mint will record the withdrawal messages with the reserve record as The mint will record the withdrawal messages with the reserve record as
proof that the anonymous digital coin was created for the correct proof that the anonymous digital coin was created for the correct
customer. We note that the specifics of how the customer authenticates customer. We note that the specifics of how the customer authenticates
to the mint are orthogonal to the rest of the system, and to the mint are orthogonal to the rest of the system, and
multiple methods can be supported. multiple methods can be supported.
%To put it differently, unlike %To put it differently, unlike
@ -560,7 +566,7 @@ to the mint are orthogonal to the rest of the system, and
%the system and that a secure method to authenticate owners exists. %the system and that a secure method to authenticate owners exists.
After a coin is minted, the customer is the only entity that knows the After a coin is minted, the customer is the only entity that knows the
private key of the coin, making him the \emph{owner} of the coin. private key of the coin, making him the \emph{owner} of the coin.
The coin can be identified by the mint by its public key; however, due The coin can be identified by the mint by its public key; however, due
to the use of blind signatures, the mint does not learn the public key to the use of blind signatures, the mint does not learn the public key
during the minting process. Knowledge of the private key of the coin during the minting process. Knowledge of the private key of the coin
@ -600,7 +606,7 @@ same public key for the coin.
The owner of such a {\em dirty} coin might therefore want to exchange it The owner of such a {\em dirty} coin might therefore want to exchange it
for a {\em fresh} coin to ensure unlinkability with future transactions. for a {\em fresh} coin to ensure unlinkability with future transactions.
% with the previous operation. % with the previous operation.
Even if a coin is not dirty, the owner of a coin may want to exchange it Even if a coin is not dirty, the owner of a coin may want to exchange it
if the respective coin signing key is about to expire. All of these if the respective coin signing key is about to expire. All of these
operations are supported with the {\em coin refreshing protocol}, which operations are supported with the {\em coin refreshing protocol}, which
@ -608,11 +614,11 @@ allows the owner of a coin to {\em melt} it for fresh coins of the same
value with a new public-private key pairs. Refreshing does not use the value with a new public-private key pairs. Refreshing does not use the
ordinary spending operation as the owner of a coin should not have to ordinary spending operation as the owner of a coin should not have to
pay taxes on this operation. Because of this, the refreshing protocol pay taxes on this operation. Because of this, the refreshing protocol
must assure that owner stays the same. must assure that owner stays the same.
% After all, the coin refreshing protocol must not be usable for transactions, as % After all, the coin refreshing protocol must not be usable for transactions, as
% transactions in Taler must be taxable. % transactions in Taler must be taxable.
% Meh, this paragraph sucks : % Meh, this paragraph sucks :
We therefore demand two main properties from the refresh protocol: We therefore demand two main properties from the refresh protocol:
First, the mint must not be able to link the fresh coin's public key to First, the mint must not be able to link the fresh coin's public key to
the public key of the dirty coin. Second, the mint can ensure that the the public key of the dirty coin. Second, the mint can ensure that the