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The GNU Taler tutorial for PHP Web shops

This tutorial is about implementing a merchant frontend to run against a GNU Taler merchant backend (version 0.2.0, 11 November 2016),

Copyright © 2016 INRIA

Permission is granted to copy, distribute and/or modify this document under the terms of the GNU Free Documentation License, Version 1.3 or any later version published by the Free Software Foundation; with no Invariant Sections, with no Front-Cover Texts, and with no Back-Cover Texts. A copy of the license is included in the section entitled “GNU Free Documentation License”.

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1. Introduction

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1.1 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).

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1.2 About this tutorial

This tutorial is for Web developers and addresses how to integrate GNU Taler with Web shops. It describes how to create a Web shop that processes payments with the help of a GNU Taler merchant backend. In the second chapter, you will learn how to trigger the payment process from the Web site, how to communicate with the backend, how to generate a order and process the payment. The third chapter covers the integration of a back office with the backend, which includes tracking payments for orders, matching payments to orders, and persisting and retrieving contracts.

You can download all of the code examples given in this tutorial from https://git.taler.net/merchant-frontend-examples.git/tree/php/.

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1.3 Architecture overview

The Taler software stack for a merchant consists of the following main components:

The following image illustrates the various interactions of these key components:


Basically, 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.

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2. Setting up a simple donation page

This section describes how to setup a simple shop, which exposes a button to get donations via Taler. The expected behaviour is that once the “donate” button is clicked, the customer will receive a Taler *proposal* offering him the opportunity to make a fixed donation, for example to donate 1 KUDOS to the charity operating the shop.

All the code samples shown below in the tutorial can be found at https://git.taler.net/merchant-frontend-examples.git/tree/php.

Note that if the customer does not have the Taler wallet installed, they should instead be prompted to proceed with a classic dialog for credit card payments.

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2.1 Specifying the backend

For many critical operations, the frontend needs to communicate with a Taler backend. Assuming that you do not yet have a backend configured, you can use the public backend provided by the Taler project for testing. This public backend has been set-up at http://backend.demo.taler.net/ specifically for testing frontends. It uses the currency “KUDOS” and all payments will go into the “Tutorial” account at the Taler “bank” running at https://bank.demo.taler.net/public-accounts.

To point the frontend being developed in this tutorial to some backend, it suffices to set the variable $BACKEND in php/config.php to the desired backend’s base URL. You also need to specify the currency used by the backend. For example:

// php/config.php
  // This file is in the public domain.

  // Which backend should we use? Must end in "/".
  $BACKEND = "http://backend.demo.taler.net/";

  // The currency must match the one used by the backend.

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2.2 Talking to the backend

Given the above configuration, we can now implement two simple functions get_to_backend and post_to_backend to send requests to the backend. The function get_to_backend is in charge of performing HTTP GET requests to the backend, while post_to_backend will send HTTP POST requests.

// php/backend.php
  // This file is in the public domain.

  include_once 'config.php';
  include_once 'helpers.php';

   * 'body' is an object, representing the JSON to POST. NOTE: we do NOT
   * rely on a more structured way of doing HTTP, like the one offered by
   * pecl_http, as its installation was NOT always straightforward.
  function post_to_backend($backend_uri, $body){
    $json = json_encode($body);
    $c = curl_init(url_join ($GLOBALS['BACKEND'], $backend_uri));
    $options = array(CURLOPT_RETURNTRANSFER => true,
                     CURLOPT_CUSTOMREQUEST => "POST",
                     CURLOPT_POSTFIELDS => $json,
                     CURLOPT_HTTPHEADER =>
                       array('Content-Type: application/json'));
    curl_setopt_array($c, $options);
    $r = curl_exec($c);
    return array("status_code" => curl_getinfo($c, CURLINFO_HTTP_CODE),
                 "body" => $r);

   * Sends a GET request to the backend.
  function get_to_backend($backend_url, $args){
    $path = sprintf("%s?%s", $backend_url, http_build_query($args));
    $c = curl_init(url_join($GLOBALS['BACKEND'], $path));

    $options = array(CURLOPT_RETURNTRANSFER => true,
                     CURLOPT_CUSTOMREQUEST => "GET");
    curl_setopt_array($c, $options);
    $r = curl_exec($c);
    return array("status_code" => curl_getinfo($c, CURLINFO_HTTP_CODE),
                 "body" => $r);

The given backend.php code uses a few helper functions from php/helpers.php, which should be self-explanatory.

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2.3 Prompting for payment

Our goal is to trigger a Taler payment once the customer has clicked on a donation button. We will use a button that issues an HTTP GET to the frontend /donate.php URL. For this, the HTML would be as follows:

// php/index.html
<!DOCTYPE html>
<html lang="en">
  <!-- This file is in the public domain -->
    <title>A donation button</title>
    <form action='/donate.php' method='GET'>
      <input type='submit' value='Donate!'></input>

When the server-side handler for /donate.php receives the form submission, it will return a HTML page that will take care of:

A minimalistic donate.php implementation is shown below (in PHP):

// php/donate.php
  // This file is in the public domain.

  // Next two lines offer Taler payment option for Taler wallets:
  http_response_code(402); // 402: Payment required
  header ('X-Taler-Contract-Url: /generate-order.php');
<!DOCTYPE html>
<html lang="en">
    <title>Select payment method</title>
    Here you should put the HTML for the non-Taler (credit card) payment.

Given this response, the Taler wallet will fetch the proposal from /generate-order.php and display it to the user.

If the wallet is not present, the HTML body will be shown and the Taler headers and the 402 status code ought to be ignored by the browser.

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2.4 A helper function to generate the order

We make distinction between three different stages of what it informally called "contract".

In a very first stage, we call it the order: that occurs when the frontend generates the first JSON that misses some information that the backend is supposed to add. When the backend completes the order and signs it, we have a proposal. The proposal is what the user is prompted with, and allows them to confirm the purchase. Once the user confirms the purchase, the wallet makes a signature over the proposal, turning it into a contract.

We first define a helper function make_order that will generate a complete Taler order as a nested PHP array. The function takes only the order ID and the timestamp as arguments; all of the other details of the order are hardcoded in this simple example.

The following code generate a order about donating 1 KUDOS to the ’Taler charity program’:

// php/order.php
  // This file is in the public domain.

  include_once 'config.php';
  include_once 'helpers.php';

  function make_order($nonce,
      = array(
        'nonce' => $nonce,
        'amount' =>
          array('value' => 0,
    	        'fraction' => 10000000,
                'currency' => $GLOBALS['CURRENCY']),
    	'max_fee' =>
  	  array('value' => 0,
    	        'fraction' => 5000000,
    	        'currency' => $GLOBALS['CURRENCY']),
        'products' =>
          array(array('description' =>
                         "Donation to charity program",
    		      'quantity' => 1,
    		      'price' =>
  		         array ('value' => 0,
    			        'fraction' => 10000000,
                                'currency' => $GLOBALS['CURRENCY']),
    		      'product_id' => 0,
    		      'taxes' =>
    		      'delivery_date' =>
                         "/Date(" . $now->getTimestamp() . ")/",
    		      'delivery_location' =>
        'summary' =>
          "Personal donation to charity program",
        'order_id' => $order_id,
    	'timestamp' =>
          "/Date(" . $now->getTimestamp() . ")/",
  	'fulfillment_url' =>
  	'pay_url' =>
    	'refund_deadline' =>
  	  "/Date(" . $now->getTimestamp() . ")/",
    	'pay_deadline' =>
          "/Date(" . $now->add(new DateInterval('P2W'))->getTimestamp() . ")/",
    	'merchant' =>
  	  array('address' =>
                'instance' => "tutorial",
    		'name' =>
                    "Charity donation shop",
    		'jurisdiction' =>
        'locations' =>
  	  array ('LNAME1' =>
  		    array ('country' => 'Test Country 1',
    			   'city' => 'Test City 1',
    			   'state' => 'Test State 1',
    			   'region' => 'Test Region 1',
    			   'province' => 'Test Province 1',
    			   'ZIP code' => 49081,
    			   'street' => 'test street 1',
    			   'street number' => 201),
    		 'LNAME2' =>
  		    array ('country' => 'Test Country 2',
    		           'city' => 'Test City 2',
    		           'state' => 'Test State 2',
    		           'region' => 'Test Region 2',
    		           'province' => 'Test Province 2',
    		           'ZIP code' => 49082,
    		           'street' => 'test street 2',
    		           'street number' => 202)
    return array ('order' => $order);

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2.5 Signing and returning the proposal

The server-side handler for /generate-order.php has to call make_order and then POST the result to the backend at /proposal. By POSTing the order to the backend we get a cryptographic signature over its contents. The result is then returned to the wallet.

A simple /generate-order.php handler may thus look like this:

// php/generate-order.php
  // This file is in the public domain.

  include 'order.php';
  include 'backend.php';
  include 'error.php';

  $order_id = "tutorial-" . dechex(rand(0,99999999)) . date("-H_i_s");
  $_SESSION["order_id"] = $order_id;
    return build_error(array("body" => null),
                       "no nonce given",
  $order = make_order($_GET["nonce"],
                      new DateTime('now'));
  // Here the frontend POSTs the proposal to the backend
  $response = post_to_backend("/proposal", $order);
  // We always return verbatim what the backend returned
  if (200 != $response["status_code"]) {
    echo build_error($response,
                     "Failed to generate proposal",
  echo $response["body"];

Note that in practice the frontend might want to generate a monotonically increasing series of order IDs to avoid a chance of collisions. Order IDs must be in the range of [0:2^51).

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2.6 Handling errors

In the above example, the helper function build_error is used to generate an error response in the case that the backend somehow failed to respond properly to our request.

The function build_error is shown below, it returns JSON data matching a particular format for reporting errors, see http://api.taler.net/api-common.html#errordetail:

// php/error.php
  // This file is in the public domain.

  function build_error($response, $hint, $http_code){
    return json_encode(array(
                       'error' => "internal error",
                       'hint' => $hint,
                       'detail' => $response["body"]),

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2.7 Initiating the payment process

After the browser has fetched the proposal, the user will be given the opportunity to affirm the payment. Assuming the user affirms, the browser will navigate to the “fulfillment_url” that was specified in the proposal.

The fulfillment page can be called by users that have already paid for the item, as well as by users that have not yet paid at all. The fulfillment page must thus use the HTTP session state to detect if the payment has been performed already, and if not request payment from the wallet.

For our example, the fulfillment URL will contain the order id of the donation, like in the following example:


The fulfillment handler at /fulfillment.php will use this information to check if the user has already paid, and if so confirm the donation. If the user has not yet paid, it will instead return another “402 Payment Required” header, requesting the wallet to pay:

// php/fulfillment.php
  // This file is in the public domain.

  include 'helpers.php';


  if(pull($_SESSION, 'paid', false)){
    echo sprintf("<p>Thanks for your donation!</p>
                  <br><p>The order ID is: <b>%s</b>; use it to
                  <a href=\"backoffice.html\">track</a> your money,
                  or make <a href=\"/\">another donation!</a></p>",

  // The user needs to pay, instruct the wallet to send the payment.
  header('X-Taler-Contract-Url: ' . url_rel('/generate-order.php'));
  header('X-Taler-Contract-Query: ' . "fulfillment_url");
  header('X-Taler-Offer-Url: ' . url_rel('/donate.php'));

Here, this second 402 response contains the following Taler-specific headers:


The URL that generated the proposal that led to this payment. The wallet may need to reconstruct the proposal.


The way the wallet should lookup for replayable payments. NOTE that for each payment done, the wallet stores the coins it spent for it in an internal database. And each set of used coins is associated to the fulfillment page where they have been spent. So whenever an already known fulfillment page requests a payment, the wallet will pick those coins it spent on that fulfillment page and resend them (therefore replaying the payment). In other words, new coins are used only on unknown fulfillment pages. This header is supposed to be removed in future versions of the wallet though, as it only works with the value "fulfillment_url".


In case that the wallet does not know about this payment already, i.e. because a user shared the URL with another user, this tells the wallet where to go to retrieve a fresh offer.

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2.8 Receiving payments via Taler

The final next step for the frontend is to accept the payment from the wallet. For this, the frontend must implement a payment handler at the URI specified in the pay_url proposal field, as explained above.

The role of the /pay.php handler is to receive the payment from the wallet and forward it to the backend. If the backend reports that the payment was successful, the handler needs to update the session state with the browser to remember that the user paid.

The following code implements this in PHP:

// php/pay.php
  // This file is in the public domain.

  include "backend.php";
  include "error.php";

    echo "<p>No session active. Aborting.</p>";
  // Get coins.
  $body = json_decode(file_get_contents("php://input"));

  $response = post_to_backend("/pay", $body);
  $proposal_data = json_decode($response["body"])->proposal_data;
   * NOTE: the order id is fetched from the data returned by the
   * backend. This information is then shown in the final page from
   * the fulfillment URL. This way, if a malicious wallet sends a
   * old deposit permission for a new donation, then the user can
   * still detect that, since the old order id would be shown on
   * the fulfillment page.
  $_SESSION["order_id"] = $proposal_data->order_id;

  if (200 != $response["status_code"]){
    echo build_error($response,
                     "Could not send payment to backend",
  // Payment went through!
  $_SESSION["paid"] = true;

Do not be confused by the isset test for the session state. In our simple example, it will be set to “false” by the fulfillment URL which the browser actually always visits first.

After the pay.php handler has affirmed that the payment was successful, the wallet will refresh the fulfillment page, this time receiving the message that the donation was successful. If anything goes wrong, the wallet will handle the respective error.

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3. Integration with the back office

This chapter shows how to implement the back office Web interface.

We will use the term transaction to refer to the business transaction that a merchant has with a customer (and the related communication with the Taler exchange for payment), and the term wire transfer to refer to the exchange settling its accounts with the merchant.

However, from the frontend’s perspective, any transaction is denoted by the order id contained in the proposal that led to the transaction.

Given that Taler deals with microtransactions, not every customer payment processed with Taler will necessarily correspond to a wire transfer. The Taler exchange may aggregate multiple payments from transactions into one larger wire transfer. The refund_deadline and the pay_deadline values in the contract specify the timeframes within which the exchange is permitted to perform such aggregations, see @ref{Taler contracts}.

In this chapter, we will see how a merchant can obtain the mapping from transactions to wire transfers and vice versa. Additionally, we will describe how to obtain a list of all transactions known to the backend.

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3.1 Entry page

Given this charge, the back office’s main tasks are:

We implement these with a simple HTML form. For simplicity, we have one single page for gathering input data for both tracking directions:

// php/backoffice.html
<!DOCTYPE html>
<html lang="en">
  <!-- This file is in the public domain -->
    <title>Minimal merchant back office</title>
    <script src="/history.js" type="application/javascript"></script>
    <form action='/track-transaction.php' method='GET'>
      <input type='text' name='order_id' placeholder='Order ID'></input>
      <input type='text' name='instance' value='tutorial' hidden></input>
      <input type='submit' value='Track transaction'></input>
    <form action='/track-transfer.php' method='GET'>
      <input type='text' name='wtid' placeholder='Wire transfer ID'></input>
      <input type='text' name='exchange' placeholder='Exchange URL'></input>
      <input type='text' name='instance' value="tutorial" hidden></input>
      <input type='submit' value='Track transfer'></input>
    <form id="history_form" action="" method="GET">
      <a href="#" onclick="submit_history()">Get transactions list</a>
      <small>Works only if JavaScript enabled</small>
    <table id="history" style="visibility: hidden;">
        <th>Order ID</th>

The imported script history.js is responsible for dynamically get the list of known transactions. See below.

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3.2 Tracking a transaction

The track-transaction.php script is now responsible for taking all the URL query parameter and use them on the /track/transaction request to the backend, see http://api.taler.net/api-merchant.html#get--track-transaction. The parameters are the order_id and the instance (see Instances) of this merchant. Note that the backend may then request this information from the exchange, or retrieve it from its own cache if it has already obtained it. The backend will also check signatures from the exchange, persist the information obtained, and complain if the exchange ever changes its facts in an inconsistent manner.

// php/track-transaction.php
  // This file is in the public domain.

  include 'error.php';
  include 'backend.php';

  $response = get_to_backend("/track/transaction", $_GET);

  if (!in_array($response["status_code"], array(200, 202, 424))){
    echo build_error($response,
                     "Backend error",

  // Report conflict
  if (424 == $response["status_code"]){
    $body = json_decode($response["body"]);
    echo sprintf("<p>Exchange provided conflicting information about
                  transaction '%s': what claimed by the exchange does
                  not match what stored in our DB.</p>",

  // Render HTML
  $decoded = json_decode($response["body"]);
  if (202 == $response["status_code"]){
    $pretty_date = get_pretty_date($decoded->details->execution_time);
    echo "<p>The exchange accepted the transaction.
          The estimated time for when the related wire transfer
          is to be performed is: $pretty_date</p>";

  echo "<ul>";
  foreach ($decoded as $entry){
    $pretty_date = get_pretty_date($entry->execution_time);
    echo sprintf("<li>Wire transfer ID: %s, date: %s</li>",
  echo "</ul>";

If the backend returned an HTTP status code 202 (Accepted), this means that the exchange simply did not yet perform the wire transfer. This is usually the case before pay_deadline, as the exchange is waiting for additional opportunities to aggregate transactions. In this case, we tell the user when to retry this operation later.

In the foreach loop, we construct the list of all the wire transfers which paid back transaction order_id. For simplicity, the list will report only two values: the wire transfer identifier and the date when the transfer occurred. Nonetheless, the data returned by the backend contains more information that can be shown to the user.

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3.3 Tracking a wire transfer

To track a wire transfer, the frontend just needs to forward the request it got from the Web form, to the backend. Again, the backend may request missing information from the exchange, verify signatures, persist the result and complain if there are inconsistencies.

In the case that the backend detects inconsistencies, an HTTP status code of 402 is returned. In this case, we report this situation to the user, who should now report this situation to the exchange’s auditors as the exchange is misbehaving.

In the usual case where everything went fine, we first output the amount that was supposed to have been transfered under the given wtid, and when it was performed (according to the exchange). Finally, in the foreach loop, we construct the list of the order ids paid by the wire transfer:

  // This file is in the public domain.

  include 'error.php';
  include 'backend.php';

  $response = get_to_backend("/track/transfer", $_GET);

  if (!in_array($response["status_code"], array(200, 424))){
    echo build_error($response,
                     "Backend error",

  // Render HTML
  if (424 == $response["status_code"]){
    $body = json_decode($response["body"]);
    echo sprintf("<p>The backend detected that the amount wire
                 transferred by the exchange for coin '%s', differs
                 from the coin's original amount.</p>",

  $json_response = json_decode($response["body"]);
  $pretty_date = get_pretty_date($json_response->execution_time);
  $amount = get_amount($json_response->total);
  echo "<p>$amount transferred on $pretty_date. The list of involved
       transactions is shown below:</p>";
  echo "<ul>";

  foreach ($json_response->deposits_sums as $entry){
    echo sprintf("<li>Order id: %s", $entry->order_id);
  echo "</ul>";

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3.4 Listing all transactions

In order to track transactions, order ids are needed as input. To that purpose, the frontend needs to make a HTTP GET request to /history, which is offered by the backend.

The returned data lists the transactions from the youngest back to the oldest.

The /history API is actually more rich, as it offers the way to skim results based on time or on index, but that goes beyond the scope of this tutorial.

Our example frontend implements this feature by orchestrating two parts:

See below both parts:

// ../history.js
var FRACTION = 100000000;

// Stringify amounts. Take a {value: x, fraction: y, currency: "Z"}
// and return a "a.b Z" form.
function parse_amount(amount){
  var v = amount.value + (amount.fraction/FRACTION);
  return v + " " + amount.currency;

// Parse Taler date ("/Date(TIMESTAMP)/") string and
// return a JavaScript Date object.
function get_date(date){
  var split = date.match(/Date\((.*)\)/);
  var seconds;
  if(isNaN(seconds = Number(split[1]))){
    console.error("Malformed date gotten from backend");
  console.log("Extracting timestamp", split[1]);
  var d = new Date(seconds * 1000);
  return d;

// Perform the call to /history.php?instance=tutorial.
// It also takes care of cleaning/filling the table showing
// the results.
function submit_history(){

  // Clean the table showing old results
  var table = document.getElementById("history");
  /* We don't want to kill the first child */
  for (var i = 2; i < table.childNodes.length; i++)
  var req = new XMLHttpRequest();
  var get_column = function(value){
    var column = document.createElement("td");
    column.textContent = value;
    return column;
  var on_error = function(){
    table.innerHTML = "<p>Could not get transactions list from server</p>"
  req.open("GET", "/history.php?instance=tutorial", true);
  req.onload = function(){
    if(req.readyState == 4 && req.status == 200){
      console.log("Got history:", req.responseText);
      var history = JSON.parse(req.responseText); 
        console.log("Got invalid JSON");
      if(0 == history.length){
        table.innerHTML = "<p>No transaction was that young!</p>"; 
      // Fill the table with fresh results
      for (var i=0; i<history.length; i++){
	var entry = history[i];
        var tr = document.createElement("tr");
	var date = get_date(entry.timestamp);
      table.style.visibility = "";
// ../history.php
  include "helpers.php";
  include "backend.php";
  include "error.php";

  // Just relay the request we got from the JavaScript
  $response = get_to_backend("/history", $_GET);

  if (200 != $response["status_code"]){
    echo build_error($response,
                     "Backend error",

  // Give the response "verbatim" back.
  echo $response["body"];

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4. Advanced topics

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4.1 Reacting to the presence of a Taler wallet

Taler offers the way to the frontend developer to detect whether a user has the wallet installed in their browser, and take actions accordingly.

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4.1.1 The no-JavaScript way

The follwing example shows all that is needed to perform the detection without using JavaScript:

<!DOCTYPE html>
<html lang="en" data-taler-nojs="true">
    <link rel="stylesheet"
          id="taler-presence-stylesheet" />
    <p class="taler-installed-hide">
      No wallet found.
    <p class="taler-installed-show">
      Wallet found!

The taler-fallback.css is part of the Taler’s web-common repository, available at https://git.taler.net/web-common.git. Please adjust the href attribute in order to make it work with your Web site.

The detection works by taler-fallback.css hiding any tag from the taler-installed-show class, in case no wallet is installed. If otherwise the wallet is installed, the wallet takes action by hiding any tag from the taler-installed-hide class and overriding taler-fallback.css logic by showing any tag from the taler-installed-show class.

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4.1.2 The JavaScript way

taler-wallet-lib.js helps the frontend, by providing the way to register two callbacks: one to be executed if a wallet is present, the other if it is not. See the example below:

<html lang="en">
    <script src="/web-common/taler-wallet-lib.js" type="application/javascript">
    <div id="content">
    <script type="application/javascript">

      content = document.getElementById("content");
      p = document.createElement("p");

      function walletInstalled(){
        p.textContent = "Wallet installed!";
      function walletNotInstalled(){
        p.textContent = "Wallet not found.";

taler-wallet-lib.js exports the taler object that exposes the onPresent and the onAbsent functions needed to register the frontend’s callbacks. Thus the function walletInstalled will be executed whenever a wallet is installed, and walletNotInstalled if not. Note that since now we can use JavaScript we can register callbacks that do more than just showing and hiding elements.

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4.2 Building Taler proposals

A Taler proposal can specify many details about the transaction. This section describes each of the fields in depth.


Specifies the total amount to be paid to the merchant by the customer. The amount is broken up into a value, a fraction (100.000.000 fraction units correspond to one value) and the currency. For example, EUR 1.50 would be represented as the tuple value = 1, fraction = 50000000, currency = "EUR".


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.


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.


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.


Which URL accepts payments. This is the URL where the wallet will POST coins.


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.


Alphanumeric identifier, freely definable by the merchant. Used by the merchant to uniquely identify the transaction.


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.


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.


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.


Array of products that are being sold to the customer. Each entry contains a tuple with the following values:


Description of the product.


Quantity of the items to be shipped. May specify a unit (1 kg) or just the count.


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.


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 [0,2^51).


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.


Time by which the product is to be delivered to the 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.


This should give a label in the locations map, specifying where the merchant is located.


This should give a human-readable name for the merchant’s business.


This should give a label in the locations map, specifying the jurisdiction under which this contract is to be arbitrated.


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:


Name of the country for delivery, as found on a postal package, i.e. “France”.


Name of the state for delivery, as found on a postal package, i.e. “NY”.


Name of the region for delivery, as found on a postal package.


Name of the province for delivery, as found on a postal package.


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 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 it 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.

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4.3 Inlining proposals in HTTP headers

In the example in Prompting for payment, we told the wallet the URL from where it should fetch the proposal. Instead of specifying the proposal via an URL, it is also possible to inline short proposals directly.

That may be accomplished by the frontend including the X-Taler-Proposal HTTP header, whenever the requested page is meant to trigger a payment. The example shown below may be an alternative version of /donate.php, which implements inline contracts.

// ../inline.php
  // This file is in the public domain.

  include "order.php";

  $order_id = rand(1,90000); // simplified, do not do this!
  $order = make_order($order_id, new DateTime("now"));

  $response = post_to_backend("/proposal", $order);
  $ret = $response["body"];

  if (200 != $response["status_code"]) {
    $ret =  build_error($response,
                        "Failed to generate proposal",

  http_response_code(402); // Payment required
  header ("X-Taler-Proposal: $ret"); // Inline proposal
<!DOCTYPE html>
<html lang="en">
    <title>Select payment method</title>
    Here you should put the HTML for the non-Taler (credit card) payment.

The wallet will then look for the X-Taler-Proposal header, and fetch the inlined contract if this header is found. Please note that this feature is not yet implemented in the wallet.

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4.4 Design considerations for the fulfillment page

The fulfillment page mechanism is designed to provide the following two properties:

  1. Taler payments can be implemented in DB-less frontends.
  2. Taler payments are replayable, meaning that each purchase is associated with a URL (the fulfillment URL) that shows the product each time it gets visited (and of course, only the first time takes the user’s money).

Both properties are gotten "for free" by the way replayable payments are implemented. Since pay.php simply relays payments to the backend, if the latter returns "200 OK", then the frontend delivers what is mentioned in the backend’s response. Note that along with the "200 OK" response, the backend returns the whole proposal associated with the fulfillment URL that triggered the payment, so the frontend has all the information useful to pick the right product to deliver. The "payment" relayed to the backend contains the order id, that allows the backend to perform all the integrity checks on the payment. This way, the frontend doesn’t need any database to replay payments.

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4.5 Instances

Taler’s design allows a single backend to manage multiple frontends. In other words, we might have multiple shops relying on the same backend. As of terminology, we call instance any of the frontends accounted by the same backend.

The backend’s RESTful API allows frontends to specify which instance they are. However, this specification is optional, and a “default” instance will be used whenever the frontend does not specify one.

Please note that in this stage of development, the backend’s REST call /history returns records for any instance. The rationale behind is to allow grouping “public” business entities under the same backend.

This way, a single frontend can expose multiple donation buttons for multiple receivers, and still operate against one backend. So in this scenario, there is no harm if the operator of instance ‘a’ sees history entries related to instance ‘b’.

See https://donations.demo.taler.net/, which uses this functionality.

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4.6 Normalized base URLs

Exchanges and merchants have a base URL for their service. This URL must be in a canonical form when it is stored (e.g. in the wallet’s database) or transmitted (e.g. to a bank page).

When a user enters a URL that is, technically, relative (such as "alice.example.com/exchange"), wallets *may* transform it into a canonical base URL ("http://alice.example.com/exchange/"). Other components *should not* accept URLs that are not canonical.

Rationale: Joining non-canonical URLs with relative URLs (e.g. "exchange.example.com" with "reserve/status") results in different and slightly unexpected behavior in some URL handling libraries. Canonical URLs give more predictable results with standard URL joining.

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5. Reference

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5.1 Headers for HTTP 402

The HTTP status code 402 Payment Required can be used by the merchant frontend to trigger operations related to payments in the user agent. The user agent associates at most one proposal with every URL via the proposal’s fulfillment_url field. The associated proposal is either missing (in case it doesn’t exist), paid (in case the payment for it was successfully sent to the merchant) or unpaid. If the associated proposal is unpaid, 402 Payment Required will cause the user agent to pay for the associated proposal.

The following headers for 402 Payment Required are recognized by Taler and further influence the processing:


If this header present, the value of this header must be a URL that the user agent can use to request and process refunds.


If there is no associated proposal, the user agent will fetch a proposal from this URL and process it. This typically prompts the user to agree to pay.


If there is no associated proposal and X-Taler-Contract-Url is not specified, the browser will navigate to this URL.

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5.2 JavaScript API

The following functions are defined in the taler namespace of the taler-wallet-lib helper library available at https://git.taler.net/web-common.git/tree/taler-wallet-lib.js.

onPresent(callback: () => void)

Add a callback to be called when support for Taler payments is detected.

onAbsent(callback: () => void)

Add a callback to be called when support for Taler payments is disabled.

pay({contract_url: string, offer_url: string})

Results in the same action as a 402 Payment Required with contract_url in the X-Taler-Contract-Url header and offer_url in the X-Taler-Payment-Url header.

refund(refund_url: string)

Results in the same action as a 402 Payment Required with refund_url in the X-Taler-Refund-Url header.

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5.3 Stylesheet-based presence detection

Stylesheet-based presence detection will be applied on all pages that have the data-taler-nojs attribute of the html element set true. The default/fallback stylesheet, that will be taken over by the wallet once installed, must be included with the id taler-presence-stylesheet, like this:

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.

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  15. If you wish to incorporate parts of the Library into other free programs whose distribution conditions are incompatible with these, write to the author to ask for permission. For software which is copyrighted by the Free Software Foundation, write to the Free Software Foundation; we sometimes make exceptions for this. Our decision will be guided by the two goals of preserving the free status of all derivatives of our free software and of promoting the sharing and reuse of software generally.




How to Apply These Terms to Your New Libraries

If you develop a new library, and you want it to be of the greatest possible use to the public, we recommend making it free software that everyone can redistribute and change. You can do so by permitting redistribution under these terms (or, alternatively, under the terms of the ordinary General Public License).

To apply these terms, attach the following notices to the library. It is safest to attach them to the start of each source file to most effectively convey the exclusion of warranty; and each file should have at least the “copyright” line and a pointer to where the full notice is found.

one line to give the library's name and an idea of what it does.
Copyright (C) year  name of author

This library is free software; you can redistribute it and/or modify it
under the terms of the GNU Lesser General Public License as published by
the Free Software Foundation; either version 2.1 of the License, or (at
your option) any later version.

This library is distributed in the hope that it will be useful, but
WITHOUT ANY WARRANTY; without even the implied warranty of
Lesser General Public License for more details.

You should have received a copy of the GNU Lesser General Public
License along with this library; if not, write to the Free Software
Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301,

Also add information on how to contact you by electronic and paper mail.

You should also get your employer (if you work as a programmer) or your school, if any, to sign a “copyright disclaimer” for the library, if necessary. Here is a sample; alter the names:

Yoyodyne, Inc., hereby disclaims all copyright interest in the library
`Frob' (a library for tweaking knobs) written by James Random Hacker.

signature of Ty Coon, 1 April 1990
Ty Coon, President of Vice

That’s all there is to it!

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Version 1.3, 3 November 2008

Copyright © 2000, 2001, 2002, 2007, 2008 Free Software Foundation, Inc.

Everyone is permitted to copy and distribute verbatim copies
of this license document, but changing it is not allowed.

    The purpose of this License is to make a manual, textbook, or other functional and useful document free in the sense of freedom: to assure everyone the effective freedom to copy and redistribute it, with or without modifying it, either commercially or noncommercially. Secondarily, this License preserves for the author and publisher a way to get credit for their work, while not being considered responsible for modifications made by others.

    This License is a kind of “copyleft”, which means that derivative works of the document must themselves be free in the same sense. It complements the GNU General Public License, which is a copyleft license designed for free software.

    We have designed this License in order to use it for manuals for free software, because free software needs free documentation: a free program should come with manuals providing the same freedoms that the software does. But this License is not limited to software manuals; it can be used for any textual work, regardless of subject matter or whether it is published as a printed book. We recommend this License principally for works whose purpose is instruction or reference.


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    If you publish printed copies (or copies in media that commonly have printed covers) of the Document, numbering more than 100, and the Document’s license notice requires Cover Texts, you must enclose the copies in covers that carry, clearly and legibly, all these Cover Texts: Front-Cover Texts on the front cover, and Back-Cover Texts on the back cover. Both covers must also clearly and legibly identify you as the publisher of these copies. The front cover must present the full title with all words of the title equally prominent and visible. You may add other material on the covers in addition. Copying with changes limited to the covers, as long as they preserve the title of the Document and satisfy these conditions, can be treated as verbatim copying in other respects.

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    It is requested, but not required, that you contact the authors of the Document well before redistributing any large number of copies, to give them a chance to provide you with an updated version of the Document.


    You may copy and distribute a Modified Version of the Document under the conditions of sections 2 and 3 above, provided that you release the Modified Version under precisely this License, with the Modified Version filling the role of the Document, thus licensing distribution and modification of the Modified Version to whoever possesses a copy of it. In addition, you must do these things in the Modified Version:

    1. Use in the Title Page (and on the covers, if any) a title distinct from that of the Document, and from those of previous versions (which should, if there were any, be listed in the History section of the Document). You may use the same title as a previous version if the original publisher of that version gives permission.
    2. List on the Title Page, as authors, one or more persons or entities responsible for authorship of the modifications in the Modified Version, together with at least five of the principal authors of the Document (all of its principal authors, if it has fewer than five), unless they release you from this requirement.
    3. State on the Title page the name of the publisher of the Modified Version, as the publisher.
    4. Preserve all the copyright notices of the Document.
    5. Add an appropriate copyright notice for your modifications adjacent to the other copyright notices.
    6. Include, immediately after the copyright notices, a license notice giving the public permission to use the Modified Version under the terms of this License, in the form shown in the Addendum below.
    7. Preserve in that license notice the full lists of Invariant Sections and required Cover Texts given in the Document’s license notice.
    8. Include an unaltered copy of this License.
    9. Preserve the section Entitled “History”, Preserve its Title, and add to it an item stating at least the title, year, new authors, and publisher of the Modified Version as given on the Title Page. If there is no section Entitled “History” in the Document, create one stating the title, year, authors, and publisher of the Document as given on its Title Page, then add an item describing the Modified Version as stated in the previous sentence.
    10. Preserve the network location, if any, given in the Document for public access to a Transparent copy of the Document, and likewise the network locations given in the Document for previous versions it was based on. These may be placed in the “History” section. You may omit a network location for a work that was published at least four years before the Document itself, or if the original publisher of the version it refers to gives permission.
    11. For any section Entitled “Acknowledgements” or “Dedications”, Preserve the Title of the section, and preserve in the section all the substance and tone of each of the contributor acknowledgements and/or dedications given therein.
    12. Preserve all the Invariant Sections of the Document, unaltered in their text and in their titles. Section numbers or the equivalent are not considered part of the section titles.
    13. Delete any section Entitled “Endorsements”. Such a section may not be included in the Modified Version.
    14. Do not retitle any existing section to be Entitled “Endorsements” or to conflict in title with any Invariant Section.
    15. Preserve any Warranty Disclaimers.

    If the Modified Version includes new front-matter sections or appendices that qualify as Secondary Sections and contain no material copied from the Document, you may at your option designate some or all of these sections as invariant. To do this, add their titles to the list of Invariant Sections in the Modified Version’s license notice. These titles must be distinct from any other section titles.

    You may add a section Entitled “Endorsements”, provided it contains nothing but endorsements of your Modified Version by various parties—for example, statements of peer review or that the text has been approved by an organization as the authoritative definition of a standard.

    You may add a passage of up to five words as a Front-Cover Text, and a passage of up to 25 words as a Back-Cover Text, to the end of the list of Cover Texts in the Modified Version. Only one passage of Front-Cover Text and one of Back-Cover Text may be added by (or through arrangements made by) any one entity. If the Document already includes a cover text for the same cover, previously added by you or by arrangement made by the same entity you are acting on behalf of, you may not add another; but you may replace the old one, on explicit permission from the previous publisher that added the old one.

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    You may combine the Document with other documents released under this License, under the terms defined in section 4 above for modified versions, provided that you include in the combination all of the Invariant Sections of all of the original documents, unmodified, and list them all as Invariant Sections of your combined work in its license notice, and that you preserve all their Warranty Disclaimers.

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    In the combination, you must combine any sections Entitled “History” in the various original documents, forming one section Entitled “History”; likewise combine any sections Entitled “Acknowledgements”, and any sections Entitled “Dedications”. You must delete all sections Entitled “Endorsements.”


    You may make a collection consisting of the Document and other documents released under this License, and replace the individual copies of this License in the various documents with a single copy that is included in the collection, provided that you follow the rules of this License for verbatim copying of each of the documents in all other respects.

    You may extract a single document from such a collection, and distribute it individually under this License, provided you insert a copy of this License into the extracted document, and follow this License in all other respects regarding verbatim copying of that document.


    A compilation of the Document or its derivatives with other separate and independent documents or works, in or on a volume of a storage or distribution medium, is called an “aggregate” if the copyright resulting from the compilation is not used to limit the legal rights of the compilation’s users beyond what the individual works permit. When the Document is included in an aggregate, this License does not apply to the other works in the aggregate which are not themselves derivative works of the Document.

    If the Cover Text requirement of section 3 is applicable to these copies of the Document, then if the Document is less than one half of the entire aggregate, the Document’s Cover Texts may be placed on covers that bracket the Document within the aggregate, or the electronic equivalent of covers if the Document is in electronic form. Otherwise they must appear on printed covers that bracket the whole aggregate.


    Translation is considered a kind of modification, so you may distribute translations of the Document under the terms of section 4. Replacing Invariant Sections with translations requires special permission from their copyright holders, but you may include translations of some or all Invariant Sections in addition to the original versions of these Invariant Sections. You may include a translation of this License, and all the license notices in the Document, and any Warranty Disclaimers, provided that you also include the original English version of this License and the original versions of those notices and disclaimers. In case of a disagreement between the translation and the original version of this License or a notice or disclaimer, the original version will prevail.

    If a section in the Document is Entitled “Acknowledgements”, “Dedications”, or “History”, the requirement (section 4) to Preserve its Title (section 1) will typically require changing the actual title.


    You may not copy, modify, sublicense, or distribute the Document except as expressly provided under this License. Any attempt otherwise to copy, modify, sublicense, or distribute it is void, and will automatically terminate your rights under this License.

    However, if you cease all violation of this License, then your license from a particular copyright holder is reinstated (a) provisionally, unless and until the copyright holder explicitly and finally terminates your license, and (b) permanently, if the copyright holder fails to notify you of the violation by some reasonable means prior to 60 days after the cessation.

    Moreover, your license from a particular copyright holder is reinstated permanently if the copyright holder notifies you of the violation by some reasonable means, this is the first time you have received notice of violation of this License (for any work) from that copyright holder, and you cure the violation prior to 30 days after your receipt of the notice.

    Termination of your rights under this section does not terminate the licenses of parties who have received copies or rights from you under this License. If your rights have been terminated and not permanently reinstated, receipt of a copy of some or all of the same material does not give you any rights to use it.


    The Free Software Foundation may publish new, revised versions of the GNU Free Documentation License from time to time. Such new versions will be similar in spirit to the present version, but may differ in detail to address new problems or concerns. See http://www.gnu.org/copyleft/.

    Each version of the License is given a distinguishing version number. If the Document specifies that a particular numbered version of this License “or any later version” applies to it, you have the option of following the terms and conditions either of that specified version or of any later version that has been published (not as a draft) by the Free Software Foundation. If the Document does not specify a version number of this License, you may choose any version ever published (not as a draft) by the Free Software Foundation. If the Document specifies that a proxy can decide which future versions of this License can be used, that proxy’s public statement of acceptance of a version permanently authorizes you to choose that version for the Document.


    “Massive Multiauthor Collaboration Site” (or “MMC Site”) means any World Wide Web server that publishes copyrightable works and also provides prominent facilities for anybody to edit those works. A public wiki that anybody can edit is an example of such a server. A “Massive Multiauthor Collaboration” (or “MMC”) contained in the site means any set of copyrightable works thus published on the MMC site.

    “CC-BY-SA” means the Creative Commons Attribution-Share Alike 3.0 license published by Creative Commons Corporation, a not-for-profit corporation with a principal place of business in San Francisco, California, as well as future copyleft versions of that license published by that same organization.

    “Incorporate” means to publish or republish a Document, in whole or in part, as part of another Document.

    An MMC is “eligible for relicensing” if it is licensed under this License, and if all works that were first published under this License somewhere other than this MMC, and subsequently incorporated in whole or in part into the MMC, (1) had no cover texts or invariant sections, and (2) were thus incorporated prior to November 1, 2008.

    The operator of an MMC Site may republish an MMC contained in the site under CC-BY-SA on the same site at any time before August 1, 2009, provided the MMC is eligible for relicensing.

ADDENDUM: How to use this License for your documents

To use this License in a document you have written, include a copy of the License in the document and put the following copyright and license notices just after the title page:

  Copyright (C)  year  your name.
  Permission is granted to copy, distribute and/or modify this document
  under the terms of the GNU Free Documentation License, Version 1.3
  or any later version published by the Free Software Foundation;
  with no Invariant Sections, no Front-Cover Texts, and no Back-Cover
  Texts.  A copy of the license is included in the section entitled ``GNU
  Free Documentation License''.

If you have Invariant Sections, Front-Cover Texts and Back-Cover Texts, replace the “with…Texts.” line with this:

    with the Invariant Sections being list their titles, with
    the Front-Cover Texts being list, and with the Back-Cover Texts
    being list.

If you have Invariant Sections without Cover Texts, or some other combination of the three, merge those two alternatives to suit the situation.

If your document contains nontrivial examples of program code, we recommend releasing these examples in parallel under your choice of free software license, such as the GNU General Public License, to permit their use in free software.

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Concept Index

Jump to:   4  
A   B   C   E   F   G   I   L   M   O   P   R   S   W   X  
Index Entry Section

4022.3 Prompting for payment
402 payment required2.7 Initiating the payment process

amount4.2 Building Taler proposals

back office1.3 Architecture overview
backend1.3 Architecture overview
backend2.1 Specifying the backend
backend2.2 Talking to the backend
backend2.5 Signing and returning the proposal
button2.3 Prompting for payment

configuration2.1 Specifying the backend
contract2.4 A helper function to generate the order
contract4.2 Building Taler proposals
contract4.3 Inlining proposals in HTTP headers
currency2.1 Specifying the backend

examples1.2 About this tutorial

fees4.2 Building Taler proposals
fees4.2 Building Taler proposals
fees4.2 Building Taler proposals
frontend1.3 Architecture overview
fulfillment page4.4 Design considerations for the fulfillment page
fulfillment URL2.7 Initiating the payment process
fulfillment URL4.2 Building Taler proposals

git1.2 About this tutorial
GNU Free Documentation LicenseGNU-FDL

instances4.5 Instances

location4.2 Building Taler proposals

maximum deposit fee4.2 Building Taler proposals
maximum fee amortization4.2 Building Taler proposals
maximum wire fee4.2 Building Taler proposals

optimization4.3 Inlining proposals in HTTP headers
order2.5 Signing and returning the proposal
order ID4.2 Building Taler proposals

pay handler2.3 Prompting for payment
payment deadline4.2 Building Taler proposals
pay_url4.2 Building Taler proposals
product description4.2 Building Taler proposals

refund deadline4.2 Building Taler proposals

signature2.5 Signing and returning the proposal
summary4.2 Building Taler proposals

wallet4.1 Reacting to the presence of a Taler wallet

X-Taler-Contract-Query2.7 Initiating the payment process
X-Taler-Contract-Url2.3 Prompting for payment
X-Taler-Contract-Url2.7 Initiating the payment process
X-Taler-Offer-Url2.7 Initiating the payment process

Jump to:   4  
A   B   C   E   F   G   I   L   M   O   P   R   S   W   X  

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Table of Contents

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Short Table of Contents

[Top] [Contents] [Index] [ ? ]

About This Document

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