Rendez vous mechanism on top of Sphinx

Problem Statement

We are currently using sphinx based onion routing to route a payment from one endpoint, the sender, to another endpoint, the recipient. This requires that the sender constructs the onion packet that contains the entire route of the payment, i.e., the sender has to compute the entire route and has to know who the recipient is.

This has a number of downsides, most importantly that the sender will learn the identity of the recipient and its surroundings. While it is possible to partially hide the recipient behind private channels and use aliases that hide the last part of the route using the route hints in the bolt11 payment requests, this approach is rather limited, as it still leaks the general location and route length.

A common feature in onion routed networks is the ability to rendez-vous on the public network and have the recipient generate a route onion from that public location. The sender then just takes this trailing part of the route and generates a route to the public location, concatenating the two routes. This results in a route that is only partially known to either endpoint, providing anonymity to both sender and recipient.