Messenger Implementation for Webxdc Apps

This section describes requirements and guidance for messenger implementors to add support for their users to run and interact with webxdc apps. Webxdc apps are attached to a chat message using the container file format, and they can be "started" on all receiving devices of the chat message. Each such chat message constitutes a separate "webxdc app". The same webxdc container file shared in two separate chat messages will be regarded as two separate "webxdc apps" and the two apps can not communicate with, or even know about, each other.

Webview Constraints

When starting a web view for a webxdc app to run, messenger implementors

• MUST serve all resources from the webxdc container file to the webxdc application.

• on initial opening, MUST open root URL for the webview with index.html appended

• MUST implement the Webxdc Javascript API and serve a webxdc.js file accordingly.

• MUST deny all forms of Internet access; if you don't restrict internet access unsuspecting users may run 3rd party apps that leak data of private interactions to outside third parties. See "Bringing E2E privacy to the web" which contains a deep discussion of the unique privacy guarantees of webxdc.

  Users may click on internet links provided by an app (e.g., http:// or https:// URL schemes). If a messenger implementation chooses to allow accessing the internet through such links, it MUST show the full URL to the user and ask for explicit confirmation before opening the link. This confirmation dialog MUST include a clear warning that the link leads to an external website and may compromise the user's privacy.

• MUST support localStorage, sessionStorage and indexedDB

• MUST isolate all storage and state of one webxdc app from any other

• MUST support visibilitychange events

• MUST support window.navigator.language

• MUST support window.location.href but you can not specify or assume anything about the protocol scheme or domain part of the url.

• MUST support HTML links such as <a href="localfile.html">

• MUST support mailto links, such as <a href="mailto:addr@example.org?body=...">

• MUST support <meta name="viewport" ...> is useful especially as webviews from different platforms have different defaults

• MUST support <input type="file"> to allow importing of files; see sendToChat() for a way to export files.

UI Interactions in Chats

• Text from update.info SHOULD be shown in the chats and tapping them SHOULD open the webxdc app directly. If update.href was set then the webxdc app MUST be started with the root URL for the webview with the value of update.href appended.

• The most recent text from update.document and update.summary SHOULD be shown inside the webxdc message, together with name and icon. Only one line of text SHOULD be shown and truncation is fine as webxdc application developers SHOULD NOT be encouraged to send long texts here.

• A "Start" button MUST be offered that runs the webxdc app. Note that there is no need to ask for "privacy" or "cookie" consent because there is no way a webxdc app can implicitly transfer user data to the internet.

Example Messenger Implementations

• Delta Chat Android using Java

• Delta Chat iOS using Swift

• Delta Chat Desktop using TypeScript