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diff --git a/doc/paper/taler.tex b/doc/paper/taler.tex index e9c795857..5ad93ec32 100644 --- a/doc/paper/taler.tex +++ b/doc/paper/taler.tex @@ -41,17 +41,17 @@ % 'transaction' already when we talk about taxable % transfers of Taler coins and database 'transactions'. % - wallet = coins at customer -% - reserve = currency entrusted to mint waiting for withdrawal -% - deposit = SEPA to mint -% - withdrawal = mint to customer +% - reserve = currency entrusted to exchange waiting for withdrawal +% - deposit = SEPA to exchange +% - withdrawal = exchange to customer % - spending = customer to merchant -% - redeeming = merchant to mint (and then mint SEPA to merchant) -% - refreshing = customer-mint-customer +% - redeeming = merchant to exchange (and then exchange SEPA to merchant) +% - refreshing = customer-exchange-customer % - dirty coin = coin with exposed public key % - fresh coin = coin that was refreshed or is new -% - coin signing key = mint's online key used to (blindly) sign coin -% - message signing key = mint's online key to sign mint messages -% - mint master key = mint's key used to sign other mint keys +% - coin signing key = exchange's online key used to (blindly) sign coin +% - message signing key = exchange's online key to sign exchange messages +% - exchange master key = exchange's key used to sign other exchange keys % - owner = entity that knows coin private key % - transaction = coin ownership transfer that should be taxed % - sharing = coin copying that should not be taxed @@ -74,7 +74,7 @@ blind signatures that enables anonymous payments while ensuring that entities that receive payments are auditable and thus taxable. Taler differs from Chaum's original proposal in that customers can never defraud anyone, merchants can only fail to deliver the merchandise to -the customer, and mints can be fully audited. Consequently, +the customer, and exchanges can be fully audited. Consequently, enforcement of honest behavior is better and more timely than with Chaum, and is at least as strict as with legacy credit card payment systems that do not provide for privacy. Furthermore, Taler allows @@ -112,11 +112,11 @@ anarchistic economies. The Taler protocol is heavily based on ideas from Chaum~\cite{chaum1983blind} and also follows Chaum's basic architecture of -customer, merchant and mint (Figure~\ref{fig:cmm}). The two designs +customer, merchant and exchange (Figure~\ref{fig:cmm}). The two designs share the key first step where the {\em customer} withdraws digital -{\em coins} from the {\em mint} with unlinkability provided via blind +{\em coins} from the {\em exchange} with unlinkability provided via blind signatures. The coins can then be spent at a {\em merchant} who {\em - deposits} them at the mint. Taler uses online detection of + deposits} them at the exchange. Taler uses online detection of double-spending, thus assuring the merchant instantly that a transaction is valid. @@ -125,17 +125,17 @@ transaction is valid. \begin{tikzpicture} \tikzstyle{def} = [node distance= 5em and 7em, inner sep=1em, outer sep=.3em]; \node (origin) at (0,0) {}; - \node (mint) [def,above=of origin,draw]{Mint}; + \node (exchange) [def,above=of origin,draw]{Exchange}; \node (customer) [def, draw, below left=of origin] {Customer}; \node (merchant) [def, draw, below right=of origin] {Merchant}; \node (auditor) [def, draw, above right=of origin]{Auditor}; \tikzstyle{C} = [color=black, line width=1pt] - \draw [<-, C] (customer) -- (mint) node [midway, above, sloped] (TextNode) {withdraw coins}; - \draw [<-, C] (mint) -- (merchant) node [midway, above, sloped] (TextNode) {deposit coins}; + \draw [<-, C] (customer) -- (exchange) node [midway, above, sloped] (TextNode) {withdraw coins}; + \draw [<-, C] (exchange) -- (merchant) node [midway, above, sloped] (TextNode) {deposit coins}; \draw [<-, C] (merchant) -- (customer) node [midway, above, sloped] (TextNode) {spend coins}; - \draw [<-, C] (mint) -- (auditor) node [midway, above, sloped] (TextNode) {verify}; + \draw [<-, C] (exchange) -- (auditor) node [midway, above, sloped] (TextNode) {verify}; \end{tikzpicture} \caption{Taler's system model for the payment system is based on Chaum~\cite{chaum1983blind}.} @@ -147,7 +147,7 @@ believe needs a payment system with the following properties: \begin{description} \item[Customer Anonymity] - It must be impossible for mints, merchants and even a global active + It must be impossible for exchanges, merchants and even a global active adversary, to trace the spending behavior of a customer. \item[Unlinkability] As a strong form of customer anonymity, it must be infeasible to @@ -170,11 +170,11 @@ believe needs a payment system with the following properties: Nevertheless, customers must never be able to defraud anyone, and merchants must at best be able to defraud their customers by not 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 exchange. Additionally, both customers and merchants must receive cryptographic proofs of - bad behavior in case of protocol violations by the mint. - 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. + bad behavior in case of protocol violations by the exchange. + In this way, only the exchange will need to be tightly audited and regulated. + The design must make it easy to audit the finances of the exchange. \item[Ease of Deployment] %The system should be easy to deploy for % real-world applications. In order to lower the entry barrier and % acceptance of the system, a gateway to the existing financial @@ -206,8 +206,8 @@ say a \EUR{0,01} coin and a \EUR{50,00} coin. A merchant cannot simply give the customer their coins in another transaction; however, as this reverses the role of merchant and customer, and 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 +also cannot withdraw exact change from his account from the exchange, as this +would allow the exchange 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 @@ -226,9 +226,9 @@ A key contribution of Taler is the {\em refresh} protocol, which enables a customer to exchange the residual value of the exchanged coin for unlinkable freshly anonymized change. -Taler mints ensure that all transactions involving the same coin +Taler exchanges ensure that all transactions involving the same coin do not exceed the total value of the coin simply by -requiring that merchants clear transactions immediately with the mint. +requiring that merchants clear transactions immediately with the exchange. This improves dramatically on systems that support offline merchants with cryptographic threats to deanonymizing customers who double-spend, like restrictive blind signatures~\cite{brands1993efficient}. @@ -308,17 +308,17 @@ Taler avoids include: \item The use of patents to protect the technology; a payment system should be free software (libre) to have a chance for widespread adoption. \item The use of off-line payments and thus deferred detection of - double-spending, which could require the mint to attempt to recover + double-spending, which could require the exchange to attempt to recover funds from customers via the legal system. This creates a - significant business risk for the mint, as the system is not - self-enforcing from the perspective of the mint. In 1983 off-line + significant business risk for the exchange, as the system is not + self-enforcing from the perspective of the exchange. In 1983 off-line payments might have been a necessary feature. However, today requiring network connectivity is feasible and avoids the business risks associated with deferred fraud detection. \item % In addition to the risk of legal disputes with fraudulent % merchants and customers, Chaum's published design does not clearly - limit the financial damage a mint might suffer from the + limit the financial damage a exchange might suffer from the disclosure of its private online signing key. \item Chaum did not support fractional payments or refunds without breaking customer anonymity. @@ -359,16 +359,16 @@ description of the Opencoin protocol is available to date. Peppercoin~\cite{rivest2004peppercoin} is a microdonation protocol. The main idea of the protocol is to reduce transaction costs by minimizing the number of transactions that are processed directly by -the mint. Instead of always paying, the customer ``gambles'' with the +the exchange. Instead of always paying, the customer ``gambles'' with the merchant for each microdonation. Only if the merchant wins, the microdonation is upgraded to a macropayment to be deposited at the -mint. Peppercoin does not provide customer-anonymity. The proposed -statistical method by which mints detect fraudulent cooperation between -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 +exchange. Peppercoin does not provide customer-anonymity. The proposed +statistical method by which exchanges detect fraudulent cooperation between +customers and merchants at the expense of the exchange not only creates +legal risks for the exchange, but would also require that the exchange learns about microdonations where the merchant did not get upgraded to a macropayment. It is therefore unclear how Peppercoin would actually -reduce the computational burden on the mint. +reduce the computational burden on the exchange. \section{Design} @@ -382,32 +382,32 @@ As with Chaum, the Taler system comprises three principal types of actors (Figure~\ref{fig:cmm}): The \emph{customer} is interested in receiving goods or services from the \emph{merchant} in exchange for payment. When making a transaction, both the customer and the -merchant must agree on the same \emph{mint}, which serves as an -intermediary for the financial transaction between the two. The mint +merchant must agree on the same \emph{exchange}, which serves as an +intermediary for the financial transaction between the two. The exchange is responsible for allowing the customer to obtain the anonymous digital currency and for enabling the merchant to convert the digital coins back to some traditional currency. The \emph{auditor} -assures customers and merchants that the mint operates correctly. +assures customers and merchants that the exchange operates correctly. \subsection{Security model} Taler's security model assumes that cryptographic primitives are secure and that each participant is under full control of his system. -The contact information of the mint is known to both customer and +The contact information of the exchange is known to both customer and merchant from the start. Furthermore, the merchant communication's authenticity is assured to the customer, such as by using X.509 certificates~\cite{rfc5280}, and we assume that an anonymous, reliable bi-directional communication channel can be established by the -customer to both the mint and the merchant, such as by using Tor. +customer to both the exchange and the merchant, such as by using Tor. -The mint is trusted to hold funds of its customers and to forward them +The exchange is trusted to hold funds of its customers and to forward them when receiving the respective deposit instructions from the merchants. -Customer and merchant can have some assurances about the mint's -liquidity and operation, as the mint has proven reserves, is subject +Customer and merchant can have some assurances about the exchange's +liquidity and operation, as the exchange has proven reserves, is subject to the law, and can have its business regularly audited by a government or third party. -Regular audits of the mint's accounts should reveal any possible fraud -before the mint is allowed to destroy the corresponding accumulated +Regular audits of the exchange's accounts should reveal any possible fraud +before the exchange is allowed to destroy the corresponding accumulated cryptographic proofs and book its fees as profits. % The merchant is trusted to deliver the service or goods to the @@ -416,7 +416,7 @@ to achieve this, as he must get cryptographic proofs of the contract and that he paid his obligations. % Neither the merchant nor the customer may have any ability to {\em - effectively} defraud the mint or the state collecting taxes. Here, + effectively} defraud the exchange or the state collecting taxes. Here, ``effectively'' means that the expected return for fraud is negative. Note that customers do not need to be trusted in any way, and that in particular it is never necessary for anyone to try to recover funds @@ -455,10 +455,10 @@ thus {\bf not} recorded for taxation. Taler ensures taxability only when some entity acquires exclusive control over the value of digital coins, which requires an interaction -with the mint. For such transactions, the state can obtain information -from the mint, or a bank, that identifies the entity that received the +with the exchange. For such transactions, the state can obtain information +from the exchange, or a bank, that identifies the entity that received the 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 exchange, and hence the state, to learn the value of digital coins withdrawn by a customer---but not how, where, or when they were spent. @@ -468,7 +468,7 @@ An anonymous communication channel such as Tor~\cite{tor-design} is used for all communication between the customer and the merchant. Ideally, the customer's anonymity is limited only by this channel; 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 exchange. There are naturally risks that the customer-merchant business operation may leak identifying information about the customer. At least, customers have some options to defend their privacy against nosey merchants however, @@ -477,7 +477,7 @@ We consider information leakage specific to the business logic to be outside of the scope of the design of Taler. Ideally, customer should use an anonymous communication channel with -the mint to avoid leaking IP address information; however, the mint +the exchange to avoid leaking IP address information; however, the exchange would typically learn the customer's identity from the wire transfer that funds the customer's withdraw anonymous digital coins. In fact, this is desirable as there might be rules and regulations @@ -485,33 +485,33 @@ designed to limit the amount of anonymous digital cash that an individual customer can withdraw in a given time period, similar to how states today sometimes impose limits on cash 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 exchange will be unable to link the known identity of the customer that withdrew anonymous digital currency to the {\em purchase} performed later at the merchant. In this respect, Taler provides exactly the same scheme for unconditional anonymous payments as was proposed by Chaum~\cite{chaum1983blind,chaum1990untraceable} over 30 years ago. -While the customer thus has anonymity for purchases, the mint 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 account. This is simply necessary for taxation, as Taler is supposed to make information about funds received by any entity transparent to the state. % Technically, the merchant could still -%use an anonymous communication channel to communicate with the mint. -%However, in order to receive the traditional currency the mint will +%use an anonymous communication channel to communicate with the exchange. +%However, in order to receive the traditional currency the exchange will %require (SEPA) account details for the deposit. -%As both the initial transaction between the customer and the mint as -%well as the transactions between the merchant and the mint do not have +%As both the initial transaction between the customer and the exchange as +%well as the transactions between the merchant and the exchange do not have %to be done anonymously, there might be a formal business contract -%between the customer and the mint and the merchant and the mint. Such +%between the customer and the exchange and the merchant and the exchange. Such %a contract may provide customers and merchants some assurance that -%they will actually receive the traditional currency from the mint +%they will actually receive the traditional currency from the exchange %given cryptographic proof about the validity of the transaction(s). %However, given the business overheads for establishing such contracts -%and the natural goal for the mint to establish a reputation and to -%minimize cost, it is more likely that the mint will advertise its +%and the natural goal for the exchange to establish a reputation and to +%minimize cost, it is more likely that the exchange will advertise its %external auditors and proven reserves and thereby try to convince %customers and merchants to trust it without a formal contract. @@ -519,57 +519,57 @@ to the state. \subsection{Coins} A \emph{coin} in Taler is a public-private key pair which derives its -financial value from a signature over the coin's public key by a mint. -The mint is expected to have multiple {\em coin signing key} pairs +financial value from a signature over the coin's public key by a exchange. +The exchange is expected to have multiple {\em coin signing key} pairs available for signing, each representing a different coin denomination. These coin signing keys have an expiration date, before which any coins -signed with it must be spent or refreshed. This allows the mint to +signed with it must be spent or refreshed. This allows the exchange to eventually discard records of old transactions, thus limiting the -records that the mint must retain and search to detect double-spending -attempts. Furthermore, the mint is expected to use each coin signing +records that the exchange must retain and search to detect double-spending +attempts. Furthermore, the exchange is expected to use each coin signing key only for a limited number of coins. % 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, -the mint would detect this once more coins were redeemed than the total +the exchange would detect this once more coins were redeemed than the total 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 exchange could allow authentic customers to exchange their unspent coins that were signed with the compromised private key, while refusing further anonymous transactions involving those coins. 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. -To ensure that the mint does not enable deanonymization of users by -signing each coin with a fresh coin signing key, the mint must publicly +To ensure that the exchange does not enable deanonymization of users by +signing each coin with a fresh coin signing key, the exchange must publicly announce the coin signing keys in advance. Those announcements are 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 exchange and the auditor. -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. +Before a customer can withdraw a coin from the exchange, +he has to pay the exchange the value of the coin, as well as processing fees. 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 exchange, which is equivalent to a bank account with a positive balance. Taler assumes that the customer has a {\em withdrawal authorization key} to identify himself as authorized to withdraw funds from the reserve. 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 exchange that he is the individual authorized to withdraw anonymous digital coins from his reserve. -The mint will record the withdrawal messages with the reserve record as +The exchange will record the withdrawal messages with the reserve record as proof that the anonymous digital coin was created for the correct 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 exchange are orthogonal to the rest of the system, and multiple methods can be supported. %To put it differently, unlike %modern cryptocurrencies like BitCoin, Taler's design simply %acknowledges that primitive accumulation~\cite{engels1844} predates %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 exchanged, the customer is the only entity that knows the 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 -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 +The coin can be identified by the exchange by its public key; however, due +to the use of blind signatures, the exchange does not learn the public key +during the exchange process. Knowledge of the private key of the coin enables the owner to spent the coin. If the private key is shared with others, they also become owners of the coin. @@ -579,10 +579,10 @@ To spend a coin, the coin's owner needs to sign a {\em deposit request} specifying the amount, the merchant's account information and a {\em business transaction-specific hash} using the coin's private key. A merchant can then transfer this permission of the -coin's owner to the mint to obtain the amount in traditional currency. -If the customer is cheating and the coin was already spent, the mint +coin's owner to the exchange to obtain the amount in traditional currency. +If the customer is cheating and the coin was already spent, the exchange provides cryptographic proof of the fraud to the merchant, who will -then refuse the transaction. The mint is typically expected to +then refuse the transaction. The exchange is typically expected to transfer the funds to the merchant using a wire transfer or by crediting the merchant's individual account, depending on what is appropriate to the domain of the traditional currency. @@ -591,7 +591,7 @@ To allow exact payments without requiring the customer to keep a large amount of ``change'' in stock and possibly perform thousands of signatures for larger transactions, the payment systems allows partial spending where just a fraction of a coin's total value is transferred. -Consequently, the mint the must not only store the identifiers of +Consequently, the exchange the must not only store the identifiers of spent coins, but also the fraction of the coin that has been spent. @@ -601,7 +601,7 @@ In this and other scenarios it is thus possible that a customer has revealed the public key of a coin to a merchant, but not ultimately signed over the full value of the coin. If the customer then continues to directly use the coin in other transactions, merchants -and the mint could link the various transactions as they all share the +and the exchange could link the various transactions as they all share the same public key for the coin. The owner of such a {\em dirty} coin might therefore want to exchange it @@ -620,21 +620,21 @@ must assure that owner stays the same. % Meh, this paragraph sucks : 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 -the public key of the dirty coin. Second, the mint can ensure that the +First, the exchange must not be able to link the fresh coin's public key to +the public key of the dirty coin. Second, the exchange can ensure that the owner of the dirty coin can determine the private key of the fresh coin, thereby preventing the refresh protocol from being used to construct a transaction. %As with other operations, the refreshing protocol must also protect -%the mint from double-spending; similarly, the customer has to have -%cryptographic evidence if there is any misbehavior by the mint. -%Finally, the mint may choose to charge a transaction fee for +%the exchange from double-spending; similarly, the customer has to have +%cryptographic evidence if there is any misbehavior by the exchange. +%Finally, the exchange may choose to charge a transaction fee for %refreshing by reducing the value of the generated fresh coins %in relation to the value of the melted coins. % %Naturally, all such transaction fees should be clearly stated as part -%of the business contract offered by the mint to customers and +%of the business contract offered by the exchange to customers and %merchants. @@ -642,7 +642,7 @@ construct a transaction. % In this section, we describe the protocols for Taler in detail. -We shall assume for the sake of brevity that a recipient of a signed +We shall assume for the sake of brevity that a recipient of a signed message always first checks that the signature is valid, aborting the operation if not. Additionally, we assume that the receiver of a signed message is either told the public key, or knows it from the @@ -650,10 +650,10 @@ context, and that the signature contains additional identification as to the purpose of the signature, making it impossible to use a signature in a different context. -The mint has an {\em online message signing key} used for signing -messages, as opposed to coins. The mint's long-term offline key is used +The exchange has an {\em online message signing key} used for signing +messages, as opposed to coins. The exchange's long-term offline key is used to certify both the coin signing keys and the online message signing key -of the mint. The mint's long-term offline key is assumed to be known to +of the exchange. The exchange's long-term offline key is assumed to be known to both customers and merchants and is certified by the auditors. As we are dealing with financial transactions, we explicitly describe @@ -665,19 +665,19 @@ information. Keys and thus coins always have a well-known expiration date; information committed to disk can be discarded after the expiration date of the respective public key. Customers can also discard information once the respective coins have been fully spent, -and merchants may discard information once payments from the mint have +and merchants may discard information once payments from the exchange have been received, assuming the records are also no longer needed for tax -purposes. The mint's bank transfers dealing in traditional currency +purposes. The exchange's bank transfers dealing in traditional currency are expected to be recorded for tax authorities to ensure taxability. \subsection{Withdrawal} Let $G$ be the generator of an elliptic curve. To withdraw anonymous digital coins, the customer performs the following interaction with -the mint: +the exchange: \begin{enumerate} - \item The customer identifies a mint with an auditor-approved + \item The customer identifies a exchange with an auditor-approved coin signing public-private key pair $K := (K_s, K_p)$ and randomly generates: \begin{itemize} @@ -685,19 +685,19 @@ the mint: \item coin key $C := (c_s,C_p)$ with private key $c_s$ and public key $C_p := c_s G$, \item blinding factor $b$, and commits $\langle W, C, b \rangle$ to disk. \end{itemize} - \item The customer transfers an amount of money corresponding to at least $K_p$ to the mint, with $W_p$ in the subject line of the transaction. - \item The mint receives the transaction and credits the $W_p$ reserve with the respective amount in its database. - \item The customer sends $S_W(B_b(C_p))$ to the mint to request withdrawal of $C$; here, $B_b$ denotes Chaum-style blinding with blinding factor $b$. - \item The mint checks if the same withdrawal request was issued before; in this case, it sends $S_{K}(B_b(C_p))$ to the customer.\footnote{Here $S_K$ + \item The customer transfers an amount of money corresponding to at least $K_p$ to the exchange, with $W_p$ in the subject line of the transaction. + \item The exchange receives the transaction and credits the $W_p$ reserve with the respective amount in its database. + \item The customer sends $S_W(B_b(C_p))$ to the exchange to request withdrawal of $C$; here, $B_b$ denotes Chaum-style blinding with blinding factor $b$. + \item The exchange checks if the same withdrawal request was issued before; in this case, it sends $S_{K}(B_b(C_p))$ to the customer.\footnote{Here $S_K$ denotes a Chaum-style blind signature with private key $K_s$.} - If this is a fresh withdrawal request, the mint performs the following transaction: + If this is a fresh withdrawal request, the exchange performs the following transaction: \begin{enumerate} \item checks if the reserve $W_p$ has sufficient funds for a coin of value corresponding to $K_p$ \item stores the withdrawal request and response $\langle S_W(B_b(C_p)), S_K(B_b(C_p)) \rangle$ in its database for future reference, \item deducts the amount corresponding to $K_p$ from the reserve, \end{enumerate} and then sends $S_{K}(B_b(C_p))$ to the customer. - If the guards for the transaction fail, the mint sends a descriptive error back to the customer, + If the guards for the transaction fail, the exchange sends a descriptive error back to the customer, with proof that it operated correctly. Assuming the signature was valid, this would involve showing the transaction history for the reserve. \item The customer computes and verifies the unblinded signature $S_K(C_p) = U_b(S_K(B_b(C_p)))$. @@ -705,7 +705,7 @@ the mint: \end{enumerate} We note that the authorization to create and access a reserve using a withdrawal key $W$ is just one way to establish that the customer is -authorized to withdraw funds. If a mint has other ways to securely +authorized to withdraw funds. If a exchange has other ways to securely authenticate customers and establish that they are authorized to withdraw funds, those can also be used with Taler. @@ -713,19 +713,19 @@ withdraw funds, those can also be used with Taler. \subsection{Exact and partial spending} A customer can spend coins at a merchant, under the condition that the -merchant trusts the specific mint that minted the coin. Merchants are +merchant trusts the specific exchange that exchanged the coin. Merchants are identified by their key $M := (m_s, M_p)$ where the public key $M_p$ must be known to the customer a priori. -We now describe the protocol between the customer, merchant, and mint +We now describe the protocol between the customer, merchant, and exchange for a transaction in which the customer spends a coin $C := (c_s, C_p)$ with signature $\widetilde{C} := S_K(C_p)$ - where $K$ is the mint's demonination key. + where $K$ is the exchange's demonination key. \begin{enumerate} \item\label{contract} - Let $\vec{D} := D_1, \ldots, D_n$ be the list of mints accepted by - the merchant where each $D_j$ is a mint's public key. + Let $\vec{D} := D_1, \ldots, D_n$ be the list of exchanges accepted by + the merchant where each $D_j$ is a exchange's public key. The merchant creates a digitally signed contract $\mathcal{A} := S_M(m, f, a, H(p, r), \vec{D})$ where $m$ is an identifier for this transaction, $a$ is data relevant @@ -734,40 +734,40 @@ with signature $\widetilde{C} := S_K(C_p)$ $p$ is the merchant's payment information (e.g. his IBAN number), and $r$ is a random nonce. The merchant commits $\langle \mathcal{A} \rangle$ to disk and sends $\mathcal{A}$ to the customer. -\item\label{deposit} - The customer should already possess a coin minted by a mint that is +\item\label{deposit} + The customer should already possess a coin exchanged by a exchange that is accepted by the merchant, meaning $K$ should be publicly signed by - some $D_j \in \{D_1, D_2, \ldots, D_n\}$, and has a value $\geq f$. + some $D_j \in \{D_1, D_2, \ldots, D_n\}$, and has a value $\geq f$. \item The customer generates a \emph{deposit-permission} $\mathcal{D} := S_c(\widetilde{C}, m, f, H(a), H(p,r), M_p)$ and sends $\langle \mathcal{D}, D_j\rangle$ to the merchant, - where $D_j$ is the mint which signed $K$. -\item The merchant gives $(\mathcal{D}, p, r)$ to the mint, revealing $p$ - only to the mint. -\item The mint validates $\mathcal{D}$ and checks for double spending. + where $D_j$ is the exchange which signed $K$. +\item The merchant gives $(\mathcal{D}, p, r)$ to the exchange, revealing $p$ + only to the exchange. +\item The exchange validates $\mathcal{D}$ and checks for double spending. If the coin has been involved in previous transactions and the new one would exceed its remaining value, it sends an error with the records from the previous transactions back to the merchant. % - If double spending is not found, the mint commits $\langle \mathcal{D} \rangle$ to disk + If double spending is not found, the exchange commits $\langle \mathcal{D} \rangle$ to disk and notifies the merchant that the deposit operation was successful. -\item The merchant commits and forwards the notification from the mint to the +\item The merchant commits and forwards the notification from the exchange to the customer, confirming the success or failure of the operation. \end{enumerate} We have simplified the exposition by assuming that one coin suffices, but -in practice a customer can use multiple coins from the same mint where +in practice a customer can use multiple coins from the same exchange where the total value adds up to $f$ by running the following steps for each of the coins. There is a risk of metadata leakage if a customer acquires a coin in responce to the merchant, or if a customer uses -coings issued by multiple mints together. +coings issued by multiple exchanges together. If a transaction is aborted after Step~\ref{deposit}, subsequent transactions with the same coin could be linked to the coin, but not directly to the coin's owner. The same applies to partially spent coins where $f$ is smaller than the actual value of the coin. To unlink subsequent transactions from a coin, the customer has to -execute the coin refreshing protocol with the mint. +execute the coin refreshing protocol with the exchange. %\begin{figure}[h] %\centering @@ -777,12 +777,12 @@ execute the coin refreshing protocol with the mint. %\node (origin) at (0,0) {}; %\node (offer) [def,below=of origin]{make offer (merchant $\rightarrow$ customer)}; %\node (A) [def,below=of offer]{permit lock (customer $\rightarrow$ merchant)}; -%\node (B) [def,below=of A]{apply lock (merchant $\rightarrow$ mint)}; -%\node (C) [def,below=of B]{confirm (or refuse) lock (mint $\rightarrow$ merchant)}; +%\node (B) [def,below=of A]{apply lock (merchant $\rightarrow$ exchange)}; +%\node (C) [def,below=of B]{confirm (or refuse) lock (exchange $\rightarrow$ merchant)}; %\node (D) [def,below=of C]{sign contract (merchant $\rightarrow$ customer)}; %\node (E) [def,below=of D]{permit deposit (customer $\rightarrow$ merchant)}; -%\node (F) [def,below=of E]{make deposit (merchant $\rightarrow$ mint)}; -%\node (G) [def,below=of F]{transfer confirmation (mint $\rightarrow$ merchant)}; +%\node (F) [def,below=of E]{make deposit (merchant $\rightarrow$ exchange)}; +%\node (G) [def,below=of F]{transfer confirmation (exchange $\rightarrow$ merchant)}; % %\tikzstyle{C} = [color=black, line width=1pt] %\draw [->,C](offer) -- (A); @@ -796,7 +796,7 @@ execute the coin refreshing protocol with the mint. %\draw [->,C, bend right, shorten <=2mm] (E.east) % to[out=-135,in=-45,distance=3.8cm] node[left] {aggregate} (D.east); %\end{tikzpicture} -%\caption{Interactions between a customer, merchant and mint in the coin spending +%\caption{Interactions between a customer, merchant and exchange in the coin spending % protocol} %\label{fig:spending_protocol_interactions} %\end{figure} @@ -804,11 +804,11 @@ execute the coin refreshing protocol with the mint. \subsection{Refreshing} \label{sec:refreshing} -We now describe the refresh protocol whereby a dirty coin $C'$ of +We now describe the refresh protocol whereby a dirty coin $C'$ of denomination $K$ is melted to obtain a fresh coin $\widetilde{C}$ with the same denomination. In practice, Taler uses a natural extension where multiple fresh coins are generated a the same time to -enable giving precise change matching any amount. +enable giving precise change matching any amount. In the protocol, $\kappa \ge 3$ is a security parameter and $G$ is the generator of the elliptic curve. @@ -830,18 +830,18 @@ generator of the elliptic curve. between the private key $c'_s$ of the original coin with the public transfer key $T_p^{(i)}$. \item The customer computes $B^{(i)} := B_{b^{(i)}}(C^{(i)}_p)$ for $i \in \{1,\ldots,\kappa\}$ and sends a commitment - $S_{C'}(\vec{E}, \vec{B}, \vec{T_p})$ to the mint. - \item The mint generates a random $\gamma$ with $1 \le \gamma \le \kappa$ and + $S_{C'}(\vec{E}, \vec{B}, \vec{T_p})$ to the exchange. + \item The exchange generates a random $\gamma$ with $1 \le \gamma \le \kappa$ and marks $C'_p$ as spent by committing $\langle C', \gamma, S_{C'}(\vec{E}, \vec{B}, \vec{T_p}) \rangle$ to disk. Auditing processes should assure that $\gamma$ is unpredictable until - this time to prevent the mint from assisting tax evasion. - \item The mint sends $S_{K'}(C'_p, \gamma)$ to the customer where - $K'$ is the mint's message signing key. + this time to prevent the exchange from assisting tax evasion. + \item The exchange sends $S_{K'}(C'_p, \gamma)$ to the customer where + $K'$ is the exchange's message signing key. \item The customer commits $\langle C', S_K(C'_p, \gamma) \rangle$ to disk. \item The customer computes $\mathfrak{R} := \left(t_s^{(i)}\right)_{i \ne \gamma}$ - and sends $S_{C'}(\mathfrak{R})$ to the mint. - \item \label{step:refresh-ccheck} The mint checks whether $\mathfrak{R}$ is consistent with the commitments; + and sends $S_{C'}(\mathfrak{R})$ to the exchange. + \item \label{step:refresh-ccheck} The exchange checks whether $\mathfrak{R}$ is consistent with the commitments; specifically, it computes for $i \not= \gamma$: \vspace{-2ex} @@ -864,8 +864,8 @@ generator of the elliptic curve. \item \label{step:refresh-done} If the commitments were consistent, - the mint sends the blind signature $\widetilde{C} := - S_{K}(B^{(\gamma)})$ to the customer. Otherwise, the mint responds + the exchange sends the blind signature $\widetilde{C} := + S_{K}(B^{(\gamma)})$ to the customer. Otherwise, the exchange responds with an error indicating the location of the failure. \end{enumerate} @@ -875,7 +875,7 @@ generator of the elliptic curve. \subsection{Linking} -For a coin that was successfully refreshed, the mint responds to a +For a coin that was successfully refreshed, the exchange responds to a request $S_{C'}(\mathtt{link})$ with $(T^{(\gamma)}_p$, $E^{(\gamma)}, \widetilde{C})$. % @@ -883,7 +883,7 @@ This allows the owner of the melted coin to also obtain the private key of the new coin, even if the refreshing protocol was illicitly executed with the help of another party who generated $\vec{c_s}$ and only provided $\vec{C_p}$ and other required information to the old owner. -As a result, linking ensures that access to the new coins minted by +As a result, linking ensures that access to the new coins exchanged by the refresh protocol is always {\em shared} with the owner of the melted coins. This makes it impossible to abuse the refresh protocol for {\em transactions}. @@ -892,7 +892,7 @@ The linking request is not expected to be used at all during ordinary operation of Taler. If the refresh protocol is used by Alice to obtain change as designed, she already knows all of the information and thus has little reason to request it via the linking protocol. -The fundamental reason why the mint must provide the link protocol is +The fundamental reason why the exchange must provide the link protocol is simply to provide a threat: if Bob were to use the refresh protocol for a transaction of funds from Alice to him, Alice may use a link request to gain shared access to Bob's coins. Thus, this threat @@ -903,8 +903,8 @@ exchanging the original private coin keys. This is permitted in our taxation model as with such trust they are assumed to be the same entity. -The auditor can anonymously check if the mint correctly implements the -link request, thus preventing the mint operator from legally disabling +The auditor can anonymously check if the exchange correctly implements the +link request, thus preventing the exchange operator from legally disabling this protocol component. Without the link operation, Taler would devolve into a payment system where both sides can be anonymous, and thus no longer provide taxability. @@ -926,7 +926,7 @@ server indicates that the client is violating the protocol, the client should record the interaction and enable the user to file a bug report. -The second case is a faulty mint service provider. Such faults will +The second case is a faulty exchange service provider. Such faults will be detected because of protocol violations, such as providing a faulty proof or no proof. In this case, the client is expected to notify the auditor, providing a transcript of the interaction. The @@ -935,11 +935,11 @@ provide the now correct response to the client or take appropriate legal action against the faulty provider. The third case are transient failures, such as network failures or -temporary hardware failures at the mint service provider. Here, the +temporary hardware failures at the exchange service provider. Here, the client may receive an explicit protocol indication, such as an HTTP response code 500 ``internal server error'' or simply no response. The appropriate behavior for the client is to automatically retry -after 1s, and twice more at randomized times within 1 minute. +after 1s, and twice more at randomized times within 1 minute. If those three attempts fail, the user should be informed about the delay. The client should then retry another three times within the next 24h, and after that time the auditor be informed about the outage. @@ -947,7 +947,7 @@ next 24h, and after that time the auditor be informed about the outage. Using this process, short term failures should be effectively obscured from the user, while malicious behavior is reported to the auditor who can then presumably rectify the situation, such as by shutting down -the operator and helping customers to regain refunds for coins in +the operator and helping customers to regain refunds for coins in their wallets. To ensure that such refunds are possible, the operator is expected to always provide adequate securities for the amount of coins in circulation as part of the certification process. @@ -961,14 +961,14 @@ details, and having the customer keep the private key of the spent coins on file. Given this, the merchant can simply sign a {\em refund confirmation} -and share it with the mint and the customer. Assuming the mint has +and share it with the exchange and the customer. Assuming the exchange has a way to recover the funds from the merchant, or has not yet performed -the wire transfer, the mint can simply add the value of the refunded -transaction back to the original coin, re-enabling those funds to be +the wire transfer, the exchange can simply add the value of the refunded +transaction back to the original coin, re-enabling those funds to be spent again by the original customer. This anonymous customer can then use the refresh protocol to melt the -refunded coin and create a fresh coin that is unlinkable to the +refunded coin and create a fresh coin that is unlinkable to the refunded transaction. @@ -976,11 +976,11 @@ refunded transaction. Taler's security is largely equivalent to that of Chaum's original design without online checks or the cut-and-choose revelation of -double-spending customers for offline spending. -We specifically note that the digital equivalent of the ``Columbian -Black Market Exchange''~\cite{fatf1997} is a theoretical problem for +double-spending customers for offline spending. +We specifically note that the digital equivalent of the ``Columbian +Black Market Exchange''~\cite{fatf1997} is a theoretical problem for both Chaum and Taler, as individuals with a strong mutual trust -foundation can simply copy electronic coins and thereby establish a +foundation can simply copy electronic coins and thereby establish a limited form of black transfers. However, unlike the situation with physical checks with blank recipients in the Columbian black market, the transitivity is limited as each participant can deposit the electronic @@ -990,11 +990,11 @@ check and not also all previous owners of the physical check. As with any unconditionally anonymous payment system, the ``Perfect Crime'' attack~\cite{solms1992perfect} where blackmail is used to -force the mint to issue anonymous coins also continues to apply in +force the exchange to issue anonymous coins also continues to apply in principle. However, as mentioned Taler does facilitate limits on withdrawals, which we believe is a better trade-off than the problematic escrow systems where the necessary intransparency -actually facilitates voluntary cooperation between the mint and +actually facilitates voluntary cooperation between the exchange and criminals~\cite{sander1999escrow} and where state can selectively deanonymize activists to support the deep state's quest for absolute security. @@ -1002,12 +1002,12 @@ security. \subsection{Offline Payments} Chaum's original proposals for anonymous digital cash avoided the need -for online interactions with the mint to detect double spending by +for online interactions with the exchange to detect double spending by providing a means to deanonymize customers involved in -double-spending. We believe that this is problematic as the mint or +double-spending. We believe that this is problematic as the exchange or the merchant will then still need out-of-band means to recover funds from the customer, which may be impossible in practice. In contrast, -in our design only the mint may try to defraud the other participants +in our design only the exchange may try to defraud the other participants and disappear. While this is still a risk, and regular financial audits are required to protect against it, this is more manageable and significantly cheaper compared to recovering funds via the court @@ -1017,12 +1017,12 @@ Chaum's method for offline payments would also be incompatible with the refreshing protocol (Section~\ref{sec:refreshing}) which enables the crucial feature of giving unlinkable change. The reason is that if the owner's identity were embedded in coins, it would be leaked to -the mint as part of the reveal operation in the cut-and-choose +the exchange as part of the reveal operation in the cut-and-choose operation of the refreshing protocol. %\subsection{Merchant Tax Audits} % -%For a tax audit on the merchant, the mint includes the business +%For a tax audit on the merchant, the exchange includes the business %transaction-specific hash in the transfer of the traditional %currency. A tax auditor can then request the merchant to reveal %(meaningful) details about the business transaction ($\mathcal{D}$, @@ -1048,13 +1048,13 @@ computing base (TCB) is public and free software. %\subsection{System Performance} % %We performed some initial performance measurements for the various -%operations on our mint implementation. The main conclusion was that +%operations on our exchange implementation. The main conclusion was that %the computational and bandwidth cost for transactions described in %this paper is smaller than $10^{-3}$ cent/transaction, and thus -%dwarfed by the other business costs for the mint. However, this +%dwarfed by the other business costs for the exchange. However, this %figure excludes the cost of currency transfers using traditional -%banking, which a mint operator would ultimately have to interact with. -%Here, mint operators should be able to reduce their expenses by +%banking, which a exchange operator would ultimately have to interact with. +%Here, exchange operators should be able to reduce their expenses by %aggregating multiple transfers to the same merchant. @@ -1065,8 +1065,8 @@ computing base (TCB) is public and free software. %citizen's need for privacy and the state's need for taxation. The %coin refreshing protocol makes the design flexible and enables a %variety of payment methods. The current balance and profits of the -%mint are also easily determined, thus audits can be used to ensure -%that the mint operates correctly. The libre implementation and open +%exchange are also easily determined, thus audits can be used to ensure +%that the exchange operates correctly. The libre implementation and open %protocol may finally enable modern society to upgrade to proper %electronic wallets with efficient, secure and privacy-preserving %transactions. @@ -1097,17 +1097,17 @@ certain interactions. However, we note that Taler's transaction costs are expected to be so low that these features are likely not particularly useful in practice: When we performed some initial performance measurements for -the various operations on our mint implementation, the main conclusion +the various operations on our exchange implementation, the main conclusion was that the computational and bandwidth cost for transactions described in this paper is smaller than $10^{-3}$ cent/transaction, -and thus dwarfed by the other business costs for the mint. We note +and thus dwarfed by the other business costs for the exchange. We note that the $10^{-3}$ cent/transaction estimate excludes the cost of wire -transfers using traditional banking, which a mint operator would -ultimately have to interact with. Here, mint operators should be able +transfers using traditional banking, which a exchange operator would +ultimately have to interact with. Here, exchange operators should be able to reduce their expenses by aggregating multiple transfers to the same merchant. -As a result of the low cost of the interaction with the mint in Taler +As a result of the low cost of the interaction with the exchange in Taler today, we expect that transactions with amounts below Taler's internal transaction costs to be economically meaningless. Nevertheless, we document various ways how such transactions could be achieved within @@ -1120,7 +1120,7 @@ Taler. For services that include pay-as-you-go billing, customers can over time sign deposit permissions for an increasing fraction of the value of a coin to be paid to a particular merchant. As checking with the -mint for each increment might be expensive, the coin's owner can +exchange for each increment might be expensive, the coin's owner can instead sign a {\em lock permission}, which allows the merchant to get an exclusive right to redeem deposit permissions for the coin for a limited duration. The merchant uses the lock permission to determine @@ -1128,41 +1128,41 @@ if the coin has already been spent and to ensure that it cannot be spent by another merchant for the {\em duration} of the lock as specified in the lock permission. If the coin has insufficient funds because too much has been spent or is -already locked, the mint provides the owner's deposit or locking +already locked, the exchange provides the owner's deposit or locking request and signature to prove the attempted fraud by the customer. -Otherwise, the mint locks the coin for the expected duration of the +Otherwise, the exchange locks the coin for the expected duration of the transaction (and remembers the lock permission). The merchant and the customer can then finalize the business transaction, possibly exchanging a series of incremental payment permissions for services. -Finally, the merchant then redeems the coin at the mint before the +Finally, the merchant then redeems the coin at the exchange before the lock permission expires to ensure that no other merchant redeems the coin first. \begin{enumerate} \item\label{offer2} The merchant sends an \emph{offer:} $\langle S_M(m, f), \vec{D} \rangle$ containing the price of the offer $f$, a transaction - ID $m$ and the list of mints $D_1, \ldots, D_n$ accepted by the merchant - where each $D_j$ is a mint's public key. + ID $m$ and the list of exchanges $D_1, \ldots, D_n$ accepted by the merchant + where each $D_j$ is a exchange's public key. \item\label{lock2} The customer must possess or acquire a coin $\widetilde{C}$ - signed by a mint that is + signed by a exchange that is accepted by the merchant, i.e. $K$ should be signed by some $D_j \in \{D_1, D_2, \ldots, D_n\}$, and has a value $\geq f$. Customer then generates a \emph{lock-permission} $\mathcal{L} := S_c(\widetilde{C}, t, m, f, M_p)$ where $t$ specifies the time until which the lock is valid and sends $\langle \mathcal{L}, D_j\rangle$ to the merchant, - where $D_j$ is the mint which signed $K$. -\item The merchant asks the mint to apply the lock by sending $\langle - \mathcal{L} \rangle$ to the mint. -\item The mint validates $\widetilde{C}$ and detects double spending + where $D_j$ is the exchange which signed $K$. +\item The merchant asks the exchange to apply the lock by sending $\langle + \mathcal{L} \rangle$ to the exchange. +\item The exchange validates $\widetilde{C}$ and detects double spending in the form of existing \emph{deposit-permission} or lock-permission records for $\widetilde{C}$. If such records exist - and indicate that insufficient funds are left, the mint sends those + and indicate that insufficient funds are left, the exchange sends those records to the merchant, who can then use the records to prove the double spending to the customer. If double spending is not found, - the mint commits $\langle \mathcal{L} \rangle$ to disk + the exchange commits $\langle \mathcal{L} \rangle$ to disk and notifies the merchant that locking was successful. \item\label{contract2} The merchant creates a digitally signed contract $\mathcal{A} := S_M(m, f, a, H(p, r))$ where $a$ is data relevant to the contract @@ -1173,23 +1173,23 @@ coin first. \emph{deposit-permission} $\mathcal{D} := S_c(\widetilde{C}, \widetilde{L}, f, m, M_p, H(a), H(p, r))$, commits $\langle \mathcal{A}, \mathcal{D} \rangle$ to disk and sends $\mathcal{D}$ to the merchant. \item\label{invoice_paid2} The merchant commits the received $\langle \mathcal{D} \rangle$ to disk. -\item The merchant gives $(\mathcal{D}, p, r)$ to the mint, revealing his +\item The merchant gives $(\mathcal{D}, p, r)$ to the exchange, revealing his payment information. -\item The mint verifies $(\mathcal{D}, p, r)$ for its validity and +\item The exchange verifies $(\mathcal{D}, p, r)$ for its validity and checks against double spending, while of course permitting the merchant to withdraw funds from the amount that had been locked for this merchant. \item If $\widetilde{C}$ is valid and no equivalent \emph{deposit-permission} for $\widetilde{C}$ and $\widetilde{L}$ exists on disk, the - mint performs the following transaction: + exchange performs the following transaction: \begin{enumerate} \item $\langle \mathcal{D}, p, r \rangle$ is committed to disk. \item\label{transfer2} transfers an amount of $f$ to the merchant's bank account given in $p$. The subject line of the transaction to $p$ must contain $H(\mathcal{D})$. \end{enumerate} - Finally, the mint sends a confirmation to the merchant. + Finally, the exchange sends a confirmation to the merchant. \item If the deposit record $\langle \mathcal{D}, p, r \rangle$ already exists, - the mint sends the confirmation to the merchant, + the exchange sends the confirmation to the merchant, but does not transfer money to $p$ again. \end{enumerate} @@ -1223,7 +1223,7 @@ incremental amount up to $f_{max}$: For transactions with multiple coins, the steps of the protocol are executed in parallel for each coin. During the time a coin is locked, the locked fraction may not be spent at a different merchant or via a -deposit permission that does not contain $\mathcal{L}$. The mint will +deposit permission that does not contain $\mathcal{L}$. The exchange will release the locks when they expire or are used in a deposit operation. Thus the coins can be used with other merchants once their locks expire, even if the original merchant never executed any deposit for @@ -1234,7 +1234,7 @@ Similarly, if a transaction is aborted after Step 2, subsequent transactions with the same coin can be linked to the coin, but not directly to the coin's owner. The same applies to partially spent coins. Thus, to unlink subsequent transactions from a coin, the -customer has to execute the coin refreshing protocol with the mint. +customer has to execute the coin refreshing protocol with the exchange. %\begin{figure}[h] %\centering @@ -1244,12 +1244,12 @@ customer has to execute the coin refreshing protocol with the mint. %\node (origin) at (0,0) {}; %\node (offer) [def,below=of origin]{make offer (merchant $\rightarrow$ customer)}; %\node (A) [def,below=of offer]{permit lock (customer $\rightarrow$ merchant)}; -%\node (B) [def,below=of A]{apply lock (merchant $\rightarrow$ mint)}; -%\node (C) [def,below=of B]{confirm (or refuse) lock (mint $\rightarrow$ merchant)}; +%\node (B) [def,below=of A]{apply lock (merchant $\rightarrow$ exchange)}; +%\node (C) [def,below=of B]{confirm (or refuse) lock (exchange $\rightarrow$ merchant)}; %\node (D) [def,below=of C]{sign contract (merchant $\rightarrow$ customer)}; %\node (E) [def,below=of D]{permit deposit (customer $\rightarrow$ merchant)}; -%\node (F) [def,below=of E]{make deposit (merchant $\rightarrow$ mint)}; -%\node (G) [def,below=of F]{transfer confirmation (mint $\rightarrow$ merchant)}; +%\node (F) [def,below=of E]{make deposit (merchant $\rightarrow$ exchange)}; +%\node (G) [def,below=of F]{transfer confirmation (exchange $\rightarrow$ merchant)}; % %\tikzstyle{C} = [color=black, line width=1pt] %\draw [->,C](offer) -- (A); @@ -1263,7 +1263,7 @@ customer has to execute the coin refreshing protocol with the mint. %\draw [->,C, bend right, shorten <=2mm] (E.east) % to[out=-135,in=-45,distance=3.8cm] node[left] {aggregate} (D.east); %\end{tikzpicture} -%\caption{Interactions between a customer, merchant and mint in the coin spending +%\caption{Interactions between a customer, merchant and exchange in the coin spending % protocol} %\label{fig:spending_protocol_interactions} %\end{figure} @@ -1274,7 +1274,7 @@ customer has to execute the coin refreshing protocol with the mint. Similar to Peppercoin, Taler supports probabilistic {\em micro}donations of coins to support cost-effective transactions for small amounts. We consider amounts to be ``micro'' if the value of the transaction is close or -even below the business cost of an individual transaction to the mint. +even below the business cost of an individual transaction to the exchange. To support microdonations, an ordinary transaction is performed based on the result of a biased coin flip with a probability related to the @@ -1282,7 +1282,7 @@ desired transaction amount in relation to the value of the coin. More specifically, a microdonation of value $\epsilon$ is upgraded to a macropayment of value $m$ with a probability of $\frac{\epsilon}{m}$. Here, $m$ is chosen such that the business transaction cost at the -mint is small in relation to $m$. The mint is only involved in the +exchange is small in relation to $m$. The exchange is only involved in the tiny fraction of transactions that are upgraded. On average both customers and merchants end up paying (or receiving) the expected amount $\epsilon$ per microdonation. @@ -1290,12 +1290,12 @@ amount $\epsilon$ per microdonation. Unlike Peppercoin, in Taler either the merchant wins and the customer looses the coin, or the merchant looses and the customer keeps the coin. Thus, there is no opportunity for the merchant and the customer -to conspire against the mint. To determine if the coin is to be +to conspire against the exchange. To determine if the coin is to be transferred, merchant and customer execute a secure coin flipping protocol~\cite{blum1981}. The commit values are included in the business contract and are revealed after the contract has been signed using the private key of the coin. If the coin flip is decided in -favor of the merchant, the merchant can redeem the coin at the mint. +favor of the merchant, the merchant can redeem the coin at the exchange. One issue in this protocol is that the customer may use a worthless coin by offering a coin that has already been spent. This kind of @@ -1306,9 +1306,9 @@ already spent coins until the coin flip is in his favor. As with incremental spending, lock permissions could be used to ensure that the customer cannot defraud the merchant by offering a coin that -has already been spent. However, as this means involving the mint +has already been spent. However, as this means involving the exchange even if the merchant looses the coin flip, such a scheme is unsuitable -for microdonations as the transaction costs from involving the mint +for microdonations as the transaction costs from involving the exchange might be disproportionate to the value of the transaction, and thus with locking the probabilistic scheme has no advantage over simply using fractional payments. @@ -1366,7 +1366,7 @@ indicate the application of a function $f$ to one or more arguments. Records of data being committed to disk are represented in between $\langle\rangle$. \begin{description} - \item[$K_s$]{Private (RSA) key of the mint used for coin signing} + \item[$K_s$]{Private (RSA) key of the exchange used for coin signing} \item[$K_p$]{Public (RSA) key corresponding to $K_s$} \item[$K$]{Public-priate (RSA) coin signing key pair $K := (K_s, K_p)$} \item[$b$]{RSA blinding factor for RSA-style blind signatures} @@ -1389,11 +1389,11 @@ data being committed to disk are represented in between $\langle\rangle$. \item[$c_s'$]{Private key of a ``dirty'' coin (otherwise like $c_s$)} \item[$C_p'$]{Public key of a ``dirty'' coin (otherwise like $C_p$)} \item[$C'$]{Dirty coin (otherwise like $C$)} - \item[$\widetilde{C}$]{Mint signature $S_K(C_p)$ indicating validity of a fresh coin (with key $C$)} - \item[$n$]{Number of mints accepted by a merchant} - \item[$j$]{Index into a set of accepted mints, $i \in \{1,\ldots,n\}$} - \item[$D_j$]{Public key of a mint (not used to sign coins)} - \item[$\vec{D}$]{Vector of $D_j$ signifying mints accepted by a merchant} + \item[$\widetilde{C}$]{Exchange signature $S_K(C_p)$ indicating validity of a fresh coin (with key $C$)} + \item[$n$]{Number of exchanges accepted by a merchant} + \item[$j$]{Index into a set of accepted exchanges, $i \in \{1,\ldots,n\}$} + \item[$D_j$]{Public key of a exchange (not used to sign coins)} + \item[$\vec{D}$]{Vector of $D_j$ signifying exchanges accepted by a merchant} \item[$a$]{Complete text of a contract between customer and merchant} \item[$f$]{Amount a customer agrees to pay to a merchant for a contract} \item[$m$]{Unique transaction identifier chosen by the merchant} |