Registration via indirect URL request

The web server on the facilitator should be able to answer requests for URLs like

http⁠://facilitator.example.com/reg/GGX4n12cGCjOIQ45...

Where GGX4n12cGCjOIQ45... is a base64-encoded encrypted client registration (as produced by flashproxy-reg-email internally) of about 350 bytes. When such a URL is requested, the facilitator should decrypt it and do the registration.

The idea behind this is that a censored client can use any third-party URL retrieval service to register on their behalf, if the facilitator is blocked directly. For example,

1. Client: Dear Mr. W3C Validator, please validate the page at http⁠://facilitator.example.com/reg/GGX4n12cGCjOIQ45....
2. The validator requests the URL.
3. The facilitator returns a 204 or empty HTML page, and in the background makes the registration.
4. Validator: Your page checks out!

We will want a helper program that prints out a facilitator URL which the client can paste into their favorite URL retrieval service.

This could also become how flashproxy-reg-http works, rather than a POST request.