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+The GNU Taler Merchant API Tutorial
+About GNU Taler
+GNU Taler is an open protocol for an electronic payment system with a
+free software reference implementation. GNU Taler offers secure, fast
+and easy payment processing using well understood cryptographic
+techniques. GNU Taler allows customers to remain anonymous, while
+ensuring that merchants can be held accountable by governments. Hence,
+GNU Taler is compatible with anti-money-laundering (AML) and
+know-your-customer (KYC) regulation, as well as data protection
+regulation (such as GDPR).
+About this tutorial
+This tutorial addresses how to process payments using the GNU Taler
+merchant Backend. This chapter explains some basic concepts. In the
+second chapter, you will learn how to do basic payments.
+This version of the tutorial has examples for Python3. It uses the
+requests library for HTTP requests. Versions for other
+languages/environments are available as well.
+If you want to look at some simple, running examples, check out these:
+- The `essay
+ merchant <>`__
+ that sells single chapters of a book.
+- The `donation
+ page <>`__
+ that accepts donations for software projects and gives donation
+ receipts.
+- The
+ `survey <>`__
+ that gives users who answer a question a small reward.
+Architecture overview
+The Taler software stack for a merchant consists of the following main
+- frontend
+ A frontend which interacts with the customer’s browser. The frontend
+ enables the customer to build a shopping cart and place an order.
+ Upon payment, it triggers the respective business logic to satisfy
+ the order. This component is not included with Taler, but rather
+ assumed to exist at the merchant. This tutorial describes how to
+ develop a Taler frontend.
+- backend
+ A Taler-specific payment backend which makes it easy for the frontend
+ to process financial transactions with Taler. For this tutorial, you
+ will use a public sandbox backend. For production use, you must
+ either set up your own backend or ask another person to do so for
+ you.
+The following image illustrates the various interactions of these key
+The backend provides the cryptographic protocol support, stores
+Taler-specific financial information and communicates with the GNU Taler
+exchange over the Internet. The frontend accesses the backend via a
+RESTful API. As a result, the frontend never has to directly communicate
+with the exchange, and also does not deal with sensitive data. In
+particular, the merchant’s signing keys and bank account information are
+encapsulated within the Taler backend.
+Some functionality of the backend (the “public interface“) is also
+exposed to the customer’s browser directly. In the HTTP API, all public
+endpoints are prefixed with ``/public/``.
+Public Sandbox Backend and Authentication
+How the frontend authenticates to the Taler backend depends on the
+configuration. See Taler Merchant Operating Manual.
+The public sandbox backend uses an API
+key in the ``Authorization`` header. The value of this header must be
+``ApiKey sandbox`` for the public sandbox backend.
+ >>> import requests
+ >>> requests.get("",
+ ... headers={"Authorization": "ApiKey sandbox"})
+ <Response [200]>
+If an HTTP status code other than 200 is returned, something went wrong.
+You should figure out what the problem is before continuing with this
+The sandbox backend uses ``KUDOS`` as an
+imaginary currency. Coins denominated in ``KUDOS`` can be withdrawn from
+Merchant Instances
+The same Taler merchant backend server can be used by multiple separate
+merchants that are separate business entities. Each of these separate
+business entities is called a *merchant instance*, and is identified by
+an alphanumeric *instance id*. If the instance is omitted, the instance
+id ``default`` is assumed.
+The following merchant instances are configured on
+- ``GNUnet`` (The GNUnet project)
+- ``FSF`` (The Free Software Foundation)
+- ``Tor`` (The Tor Project)
+- ``default`` (Kudos Inc.)
+Note that these are fictional merchants used for our demonstrators and
+not affiliated with or officially approved by the respective projects.
+.. _Accepting-a-Simple-Payment:
+Accepting a Simple Payment
+Creating an Order for a Payment
+Payments in Taler revolve around an *order*, which is a machine-readable
+description of the business transaction for which the payment is to be
+made. Before accepting a Taler payment as a merchant you must create
+such an order.
+This is done by posting a JSON object to the backend’s ``/order`` API
+endpoint. At least the following fields must be given:
+- amount: The amount to be paid, as a string in the format
+ ``CURRENCY:DECIMAL_VALUE``, for example ``EUR:10`` for 10 Euros or
+ ``KUDOS:1.5`` for 1.5 KUDOS.
+- summary: A human-readable summary for what the payment is about. The
+ summary should be short enough to fit into titles, though no hard
+ limit is enforced.
+- fulfillment_url: A URL that will be displayed once the payment is
+ completed. For digital goods, this should be a page that displays the
+ product that was purchased. On successful payment, the wallet
+ automatically appends the ``order_id`` as a query parameter, as well
+ as the ``session_sig`` for session-bound payments (discussed later).
+Orders can have many more fields, see `The Taler Order
+Format <#The-Taler-Order-Format>`__.
+After successfully ``POST``\ ing to ``/order``, an ``order_id`` will be
+returned. Together with the merchant ``instance``, the order id uniquely
+identifies the order within a merchant backend.
+ >>> import requests
+ >>> order = dict(order=dict(amount="KUDOS:10",
+ ... summary="Donation",
+ ... fulfillment_url=""))
+ >>> order_resp ="", json=order,
+ ... headers={"Authorization": "ApiKey sandbox"})
+ <Response [200]>
+The backend will fill in some details missing in the order, such as the
+address of the merchant instance. The full details are called the
+*contract terms*. contract terms
+Checking Payment Status and Prompting for Payment
+The status of a payment can be checked with the ``/check-payment``
+endpoint. If the payment is yet to be completed by the customer,
+``/check-payment`` will give the frontend a URL (the
+payment_redirect_url) that will trigger the customer’s wallet to execute
+the payment.
+Note that the only way to obtain the payment_redirect_url is to check
+the status of the payment, even if you know that the user did not pay
+ >>> import requests
+ >>> r = requests.get("",
+ ... params=dict(order_id=order_resp.json()["order_id"]),
+ ... headers={"Authorization": "ApiKey sandbox"})
+ >>> print(r.json())
+If the paid field in the response is ``true``, the other fields in the
+response will be different. Once the payment was completed by the user,
+the response will contain the following fields:
+- paid: Set to true.
+- contract_terms: The full contract terms of the order.
+- refunded: ``true`` if a (possibly partial) refund was granted for
+ this purchase.
+- refunded_amount: Amount that was refunded
+- last_session_id: Last session ID used by the customer’s wallet. See
+ `Session-Bound Payments <#Session_002dBound-Payments>`__.
+Once the frontend has confirmed that the payment was successful, it
+usually needs to trigger the business logic for the merchant to fulfill
+the merchant’s obligations under the contract.
+.. _Giving-Refunds:
+Giving Refunds
+A refund in GNU Taler is a way to “undo” a payment. It needs to be
+authorized by the merchant. Refunds can be for any fraction of the
+original amount paid, but they cannot exceed the original payment.
+Refunds are time-limited and can only happen while the exchange holds
+funds for a particular payment in escrow. The time during which a refund
+is possible can be controlled by setting the ``refund_deadline`` in an
+order. The default value for this refund deadline is specified in the
+configuration of the merchant’s backend.
+The frontend can instruct the merchant backend to authorize a refund by
+``POST``\ ing to the ``/refund`` endpoint.
+The refund request JSON object has the following fields:
+- order_id: Identifies for which order a customer should be refunded.
+- instance: Merchant instance to use.
+- refund: Amount to be refunded. If a previous refund was authorized
+ for the same order, the new amount must be higher, otherwise the
+ operation has no effect. The value indicates the total amount to be
+ refunded, *not* an increase in the refund.
+- reason: Human-readable justification for the refund. The reason is
+ only used by the Back Office and is not exposed to the customer.
+If the request is successful (indicated by HTTP status code 200), the
+response includes a ``refund_redirect_url``. The frontend must redirect
+the customer’s browser to that URL to allow the refund to be processed
+by the wallet.
+This code snipped illustrates giving a refund:
+ >>> import requests
+ >>> refund_req = dict(order_id="2018.",
+ ... refund="KUDOS:10",
+ ... instance="default",
+ ... reason="Customer did not like the product")
+ >>>"", json=refund_req,
+ ... headers={"Authorization": "ApiKey sandbox"})
+ <Response [200]>
+.. _Giving-Customers-Tips:
+Giving Customers Tips
+GNU Taler allows Web sites to grant small amounts directly to the
+visitor. The idea is that some sites may want incentivize actions such
+as filling out a survey or trying a new feature. It is important to note
+that tips are not enforceable for the visitor, as there is no contract.
+It is simply a voluntary gesture of appreciation of the site to its
+visitor. However, once a tip has been granted, the visitor obtains full
+control over the funds provided by the site.
+The “merchant” backend of the site must be properly configured for
+tipping, and sufficient funds must be made available for tipping See
+Taler Merchant Operating Manual.
+To check if tipping is configured properly and if there are sufficient
+funds available for tipping, query the ``/tip-query`` endpoint:
+ >>> import requests
+ >>> requests.get("",
+ ... headers={"Authorization": "ApiKey sandbox"})
+ <Response [200]>
+authorize tip
+To authorize a tip, ``POST`` to ``/tip-authorize``. The following fields
+are recognized in the JSON request object:
+- amount: Amount that should be given to the visitor as a tip.
+- instance: Merchant instance that grants the tip (each instance may
+ have its own independend tipping funds configured).
+- justification: Description of why the tip was granted. Human-readable
+ text not exposed to the customer, but used by the Back Office.
+- next_url: The URL that the user’s browser should be redirected to by
+ the wallet, once the tip has been processed.
+The response from the backend contains a ``tip_redirect_url``. The
+customer’s browser must be redirected to this URL for the wallet to pick
+up the tip. pick up tip
+This code snipped illustrates giving a tip:
+ >>> import requests
+ >>> tip_req = dict(amount="KUDOS:0.5",
+ ... instance="default",
+ ... justification="User filled out survey",
+ ... next_url="")
+ >>>"", json=tip_req,
+ ... headers={"Authorization": "ApiKey sandbox"})
+ <Response [200]>
+.. _Advanced-topics:
+Advanced topics
+.. _Detecting-the-Presence-of-the-Taler-Wallet:
+Detecting the Presence of the Taler Wallet
+Taler offers ways to detect whether a user has the wallet installed in
+their browser. This allows Web sites to adapt accordingly. Note that not
+all platforms can do presence detection reliably. Some platforms might
+have a Taler wallet installed as a separate App instead of using a Web
+extension. In these cases, presence detection will fail. Thus, sites may
+want to allow users to request Taler payments even if a wallet could not
+be detected, especially for visitors using mobiles.
+Presence detection without JavaScript
+Presence detection without JavaScript is based on CSS classes. You can
+hide or show elements selectively depending on whether the wallet is
+detected or not.
+In order to work correctly, a special fallback stylesheet must be
+included that will be used when the wallet is not present. The
+stylesheet can be put into any file, but must be included via a ``link``
+tag with the ``id`` attribute set to ``taler-presence-stylesheet``. If a
+wallet is present, it will “hijack” this stylesheet to change how
+elements with the following classes are rendered:
+The following CSS classes can be used:
+ A CSS rule will set the ``display`` property for this class to
+ ``none`` once the Taler wallet is installed and enabled. If the
+ wallet is not installed, ``display`` will be ``inherit``.
+ A CSS rule will set the ``display`` property for this class to
+ ``inherit`` once the Taler wallet is installed and enabled. If the
+ wallet is not installed, ``display`` will be ``none``.
+The following is a complete example:
+ <!DOCTYPE html>
+ <html data-taler-nojs="true">
+ <head>
+ <title>Tutorial</title>
+ <link rel="stylesheet"
+ type="text/css"
+ href="/web-common/taler-fallback.css"
+ id="taler-presence-stylesheet" />
+ </head>
+ <body>
+ <p class="taler-installed-hide">
+ No wallet found.
+ </p>
+ <p class="taler-installed-show">
+ Wallet found!
+ </p>
+ </body>
+ </html>
+The ``taler-fallback.css`` is part of the Taler’s *web-common*
+repository, available at
+ You may
+have to adjust the ``href`` attribute in the HTML code above to point to
+the correct location of the ``taler-fallback.css`` file on your Web
+Detection with JavaScript
+The following functions are defined in the ``taler`` namespace of the
+``taler-wallet-lib`` helper library available at
+``onPresent(callback: () => void)``
+ Adds a callback to be called when support for Taler payments is
+ detected.
+``onAbsent(callback: () => void)``
+ Adds a callback to be called when support for Taler payments is
+ disabled.
+Note that the registered callbacks may be called more than once. This
+may happen if a user disables or enables the wallet in the browser’s
+extension settings while a shop’s frontend page is open.
+.. _Integration-with-the-Back-Office:
+Integration with the Back Office
+Taler ships a Back Office application as a stand-alone Web application.
+The Back Office has its own documentation at
+Developers wishing to tightly integrate back office support for
+Taler-based payments into an existing back office application should
+focus on the wire transfer tracking and transaction history sections of
+the Taler Backend API specification at
+.. _Session_002dBound-Payments:
+Session-Bound Payments
+Sometimes checking if an order has been paid for is not enough. For
+example, when selling access to online media, the publisher may want to
+be paid for exactly the same product by each customer. Taler supports
+this model by allowing the mechant to check whether the “payment
+receipt” is available on the user’s current device. This prevents users
+from easily sharing media access by transmitting a link to the
+fulfillment page. Of course sophisticated users could share payment
+receipts as well, but this is not as easy as sharing a link, and in this
+case they are more likely to just share the media directly.
+To use this feature, the merchant must first assign the user’s current
+browser an ephemeral ``session_id``, usually via a session cookie. When
+executing or re-playing a payment, the wallet will receive an additional
+signature (``session_sig``). This signature certifies that the wallet
+showed a payment receipt for the respective order in the current
+session. cookie
+Session-bound payments are triggerd by passing the ``session_id``
+parameter to the ``/check-payment`` endpoint. The wallet will then
+redirect to the fulfillment page, but include an additional
+``session_sig`` parameter. The frontend can query ``/check-payment``
+with both the ``session_id`` and the ``session_sig`` to verify that the
+signature is correct.
+The last session ID that was successfuly used to prove that the payment
+receipt is in the user’s wallet is also available as ``last_session_id``
+in the response to ``/check-payment``.
+.. _Product-Identification:
+Product Identification
+resource url
+In some situations the user may have paid for some digital good, but the
+frontend does not know the exact order ID, and thus cannot instruct the
+wallet to reveil the existing payment receipt. This is common for simple
+shops without a login system. In this case, the user would be prompted
+for payment again, even though they already purchased the product.
+To allow the wallet to instead find the existing payment receipt, the
+shop must use a unique fulfillment URL for each product. Then, the
+frontend must provide an additional ``resource_url`` parameter to to
+``/check-payment``. It should identify this unique fulfillment URL for
+the product. The wallet will then check whether it has paid for a
+contract with the same ``resource_url`` before, and if so replay the
+previous payment.
+.. _The-Taler-Order-Format:
+The Taler Order Format
+A Taler order can specify many details about the payment. This section
+describes each of the fields in depth.
+Financial amounts are always specified as a string in the format
+ amount
+ Specifies the total amount to be paid to the merchant by the
+ customer.
+ fees
+ maximum deposit fee
+ This is the maximum total amount of deposit fees that the merchant is
+ willing to pay. If the deposit fees for the coins exceed this amount,
+ the customer has to include it in the payment total. The fee is
+ specified using the same triplet used for amount.
+ fees
+ maximum wire fee
+ Maximum wire fee accepted by the merchant (customer share to be
+ divided by the ’wire_fee_amortization’ factor, and further reduced if
+ deposit fees are below ’max_fee’). Default if missing is zero.
+ fees
+ maximum fee amortization
+ Over how many customer transactions does the merchant expect to
+ amortize wire fees on average? If the exchange’s wire fee is above
+ ’max_wire_fee’, the difference is divided by this number to compute
+ the expected customer’s contribution to the wire fee. The customer’s
+ contribution may further be reduced by the difference between the
+ ’max_fee’ and the sum of the actual deposit fees. Optional, default
+ value if missing is 1. 0 and negative values are invalid and also
+ interpreted as 1.
+ pay_url
+ Which URL accepts payments. This is the URL where the wallet will
+ POST coins.
+ fulfillment URL
+ Which URL should the wallet go to for obtaining the fulfillment, for
+ example the HTML or PDF of an article that was bought, or an order
+ tracking system for shipments, or a simple human-readable Web page
+ indicating the status of the contract.
+ order ID
+ Alphanumeric identifier, freely definable by the merchant. Used by
+ the merchant to uniquely identify the transaction.
+ summary
+ Short, human-readable summary of the contract. To be used when
+ displaying the contract in just one line, for example in the
+ transaction history of the customer.
+ Time at which the offer was generated.
+ payment deadline
+ Timestamp of the time by which the merchant wants the exchange to
+ definitively wire the money due from this contract. Once this
+ deadline expires, the exchange will aggregate all deposits where the
+ contracts are past the refund_deadline and execute one large wire
+ payment for them. Amounts will be rounded down to the wire transfer
+ unit; if the total amount is still below the wire transfer unit, it
+ will not be disbursed.
+ refund deadline
+ Timestamp until which the merchant willing (and able) to give refunds
+ for the contract using Taler. Note that the Taler exchange will hold
+ the payment in escrow at least until this deadline. Until this time,
+ the merchant will be able to sign a message to trigger a refund to
+ the customer. After this time, it will no longer be possible to
+ refund the customer. Must be smaller than the pay_deadline.
+ product description
+ Array of products that are being sold to the customer. Each entry
+ contains a tuple with the following values:
+ description
+ Description of the product.
+ quantity
+ Quantity of the items to be shipped. May specify a unit (``1 kg``)
+ or just the count.
+ price
+ Price for quantity units of this product shipped to the given
+ delivery_location. Note that usually the sum of all of the prices
+ should add up to the total amount of the contract, but it may be
+ different due to discounts or because individual prices are
+ unavailable.
+ product_id
+ Unique ID of the product in the merchant’s catalog. Can generally
+ be chosen freely as it only has meaning for the merchant, but
+ should be a number in the range :math:`[0,2^{51})`.
+ taxes
+ Map of applicable taxes to be paid by the merchant. The label is
+ the name of the tax, i.e. VAT, sales tax or income tax, and the
+ value is the applicable tax amount. Note that arbitrary labels are
+ permitted, as long as they are used to identify the applicable tax
+ regime. Details may be specified by the regulator. This is used to
+ declare to the customer which taxes the merchant intends to pay,
+ and can be used by the customer as a receipt. The information is
+ also likely to be used by tax audits of the merchant.
+ delivery_date
+ Time by which the product is to be delivered to the
+ delivery_location.
+ delivery_location
+ This should give a label in the locations map, specifying where
+ the item is to be delivered.
+ Values can be omitted if they are not applicable. For example, if a
+ purchase is about a bundle of products that have no individual prices
+ or product IDs, the product_id or price may not be specified in the
+ contract. Similarly, for virtual products delivered directly via the
+ fulfillment URI, there is no delivery location.
+ address
+ This should give a label in the locations map, specifying where
+ the merchant is located.
+ name
+ This should give a human-readable name for the merchant’s
+ business.
+ jurisdiction
+ This should give a label in the locations map, specifying the
+ jurisdiction under which this contract is to be arbitrated.
+ location
+ Associative map of locations used in the contract. Labels for
+ locations in this map can be freely chosen and used whenever a
+ location is required in other parts of the contract. This way, if the
+ same location is required many times (such as the business address of
+ the customer or the merchant), it only needs to be listed (and
+ transmitted) once, and can otherwise be referred to via the label. A
+ non-exhaustive list of location attributes is the following:
+ country
+ Name of the country for delivery, as found on a postal package,
+ i.e. “France”.
+ state
+ Name of the state for delivery, as found on a postal package, i.e.
+ “NY”.
+ region
+ Name of the region for delivery, as found on a postal package.
+ province
+ Name of the province for delivery, as found on a postal package.
+ city
+ Name of the city for delivery, as found on a postal package.
+ ZIP code
+ ZIP code for delivery, as found on a postal package.
+ street
+ Street name for delivery, as found on a postal package.
+ street number
+ Street number (number of the house) for delivery, as found on a
+ postal package.
+ name receiver name for delivery, either business or person name.
+ Note that locations are not required to specify all of these fields,
+ and they is also allowed to have additional fields. Contract
+ renderers must render at least the fields listed above, and should
+ render fields that they do not understand as a key-value list.
+.. _GNU_002dLGPL:
+Version 2.1, February 1999
+ Copyright © 1991, 1999 Free Software Foundation, Inc.
+ 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301, USA
+ Everyone is permitted to copy and distribute verbatim copies
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+.. _Concept-Index:
+Concept Index
+.. |image0| image:: arch-api.png