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[taler-docs] branch master updated: DD 39

From: gnunet
Subject: [taler-docs] branch master updated: DD 39
Date: Wed, 22 Mar 2023 19:46:46 +0100

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dold pushed a commit to branch master
in repository docs.

The following commit(s) were added to refs/heads/master by this push:
     new cb35966  DD 39
cb35966 is described below

commit cb359669eb3a8d11c4a4dbfd7fe0b9a1779aed5a
Author: Florian Dold <>
AuthorDate: Wed Mar 22 19:46:38 2023 +0100

    DD 39
 design-documents/039-taler-browser-integration.rst | 138 +++++++++++++++++++++
 design-documents/index.rst                         |   1 +
 2 files changed, 139 insertions(+)

diff --git a/design-documents/039-taler-browser-integration.rst 
new file mode 100644
index 0000000..a7044e2
--- /dev/null
+++ b/design-documents/039-taler-browser-integration.rst
@@ -0,0 +1,138 @@
+DD 39: Taler Wallet Browser Integration Considerations
+This design document discusses considerations for integrating the GNU Taler
+wallet with browsers and highlights difficulties with the implementation of a
+GNU Taler wallet as a cross-browser WebExtension.
+GNU Taler is a payment system based on open standards with a free and open
+source reference implementation.  The GNU Taler wallet is the main component
+used by end users to manage their electronic cash balance and payments.
+Payments with GNU Taler are typically initiated via a QR code or link that
+contains a ``taler://pay/{merchant}/{order_id}`` URI.  Navigating to such a
+link should result in a context switch to the wallet, where the payment can can
+be approved/declined, and the user is subsequently redirected to the merchant's
+website again.
+Other ``taler://`` URIs (for withdrawals, refunds, etc.) are also commonly
+used, but not explicitly discussed in this document, as very similar
+considerations apply.
+There are multiple reference implementations available for multiple
+platforms (command line, Android, iOS, WebExtension).
+While native applications can register themselves as a handler for the
+``taler`` URI scheme, the story is different for WebExtensions: There is
+currently no reasonable, cross-platform mechanism that allows a WebExtension to
+register itself as the handler for the ``taler`` URI scheme.
+This is unfortunate, as a WebExtension could otherwise easily provide a Taler
+wallet implementation without requiring the user to install a native App,
+providing a better and safer user experience.
+The problems with individual browsers are:
+* Firefox allows specifying ``protocol_handlers`` in the extension manifest.
+  However, this list only allows schemes with the prefix ``ext+`` and
+  schemes that are included in an allowlist.  The ``taler`` URI scheme
+  is not part of this list yet.
+* Chromium / Google Chrome allows extensions to use the
+  ``registerProtocolHandler`` API.  However, the same allowlist restrictions
+  apply.  Furthermore, the registered protocol scheme is not listed as the
+  extension's required/optional permissions.  Instead, a different permission
+  prompt is dynamically shown to the user.
+* Safari currently neither supports ``registerProtocolHandler`` nor the
+  ``protocol_handlers`` mechanism.
+* Opera does not seem to have any support for WebExtension protocol handlers
+  either.
+Another issue is that Websites can't easily find out whether a browser
+extension handling the ``taler://`` protocol is installed.  To avoid using this
+information for fingerprinting, anchors could provide an alternative ``href``
+in case the main one is not handled, such as:
+.. code:: html
+  <a href="taler://pay/..." 
+* No vendor lock-in: The integration should not require merchant
+  Websites to rely on a particular list of extension IDs but instead
+  any WebExtension to potentially handle ``taler://`` URIs.
+* Security: The integration mechanism should require as few
+  permissions as possible.
+* Ergonomic user experience: As few clicks and permission
+  prompts as possible should be shown to the user.
+* Ergonomic developer experience: The code size and
+  effort to trigger a Taler payment on a merchant's Website
+  should be minimized.
+* Forward compatibility: The integration mechanism
+  should work smoothly with future browsers that
+  have native, built-in support for Taler payments.
+* JavaScript API: The WebExtension could inject a JavaScript API into Websites
+  that allow interacting with the Taler wallet.  This is the approach taken by
+  the MetaMask crypto wallet.  It requires excessive permissions, may break
+  some Websites ( and
+  requires merchants to include extra JavaScript.
+  * This type of interaction is useful for Single Page Apps and
+    might be provided by the GNU Taler wallet reference implementation,
+    at least when the user grants additional permissions.
+  * Unfortunately, browsers currently do not provide a safe way
+    for the communication between a WebExtension and the page
+    without excessive permissions.  This especially applies
+    if the Website does not know the extension's ID. Hard-coding
+    the extension IDs would violate the "no vendor lock-in requirement".
+* Handling ``taler://`` URIs by overriding the onclick handler of ``a`` HTML 
+  This requires excessive permissions but would be a viable work-around,
+  at least on pages that opt in with a special ``<meta name="taler-support" 
content="uri">`` tag.
+  It does not work in all use-cases, for example when a navigation
+  to a ``taler://`` URI is initiated programmatically or by pasting
+  the URI in the browser's address bar.
+* Triggering interactions with the ``taler://`` URI in a ``Taler:`` HTTP
+  header.  This approach would allow browsers with native Taler support
+  (or a WebExtension) to handle payment/withdrawal initiations directly,
+  without rendering a page that shows the QR code or link.
+  However, the WebExtension APIs do not allow extensions to
+  read particular headers without excessive permissions.  Furthermore,
+  more recent versions of Chrome/Chromium do not allow blocking
+  processing of headers, leading to flickering when the extension
+  redirects based on the presence of the ``Taler:`` header.
+* Browser and wallet presence detection. Merchants' Websites could include 
+  code to detect the browser and/or presence of a Taler WebExtension and show
+  different instructions to guide the user towards processing the payment or to
+  show ``ext+taler`` URIs instead of ``taler`` URIs. This is not a viable
+  solution, as it requires a lot of extra and brittle logic on merchants'
+  Websites.
+* Always use ``ext+taler`` URIs.  This would help with Firefox.
+* Web Payments API: Using the Web Payments API is not possible, because current
+  browsers do not allow specifying a WebExtension as a handler. Furthermore,
+  the Web Payments API would not support the withdrawal flow
+  (``taler://withdraw`` URIs).
+Related Work and References
+* **[1]**
+* **[2]**
+* **[3]**
diff --git a/design-documents/index.rst b/design-documents/index.rst
index eef16ce..098bca1 100644
--- a/design-documents/index.rst
+++ b/design-documents/index.rst
@@ -47,4 +47,5 @@ and protocol.
+  039-taler-browser-integration.rst

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