First draft of the Telegram strategy

[vheathen]

Resolves #153

Telegram offers a convenient way of authenticating users by allowing them to enter their Telegram phone number and confirm the request directly in the Telegram messenger app without a password.

However, they don’t support the traditional method of redirecting to their OAuth server to enter credentials. Instead, it is necessary to include their JS library on the page and either use an embedded iframe with a standard button (with limited styles) or - via an undocumented method - directly call the auth(callback) function with a callback to get the results. The official login widget also supports the redirect mode to a provided callback URL. This means that the authorize_url/1 call is useless for this implementation (more on this later).

Additionally, Telegram has published the Web Mini App API. This API allows direct interaction with many internal Telegram features. The Mini Web App is intended to be opened directly from the messenger app and to work inside the app.

To authenticate a user of a web app, Telegram sends a URL-encoded query string (protected by a hash) that contains all the initial data when opening the web app. This string doesn’t pass through a WebView instance and should be collected inside the frontend and sent to the backend for authentication. The backend needs to check the data’s authenticity by calculating the hash of the string and comparing it with the provided one, as well as validating the authentication date.

Returning to the authorization_url/1: it is possible to use it to create a script to embed in the HTML page. The generator, depending on config parameters, can then generate different embedding options. However, this contradicts the meaning of authorization_url.

But current functionality is full and works.

WDYT?

[danschultzer]

Thanks! I've refactored the strategy to conform with how other strategies are set up. There actually is a URL that can be used so I think we should just use that and have the script in the docs. This is in line with other strategies like Facebook and Apple where a JS widget can be used as well.

I have tested with this the Login Widget which works well, but haven't tested with the mini app.