[Lightning-dev] lnurl to enable payer/payee interactions

[Anton Kumaigorodskiy]

LN as it exists today is not too convinient for end users, I believe some kind of standard is needed for various payer/payee interactions which builds on Lightning invoices and a fact that Bitcoin wallets contain keychains which can be creatively repurposed, my attempt to get this ball rolling is https://github.com/btcontract/lnurl-rfc/blob/master/spec.md


So far this spec defines 4 use cases:
1. Incoming payment channel request  
2. Withdrawing funds from a service  
3. Linkable payments  
4. Log in with Bitcoin Wallet  


I'd like to give some rationale in particular for use cases 3. (Linkable payments) and 4. (Log in with Bitcoin Wallet) since those are related and define a cryptographic protocol (I think) so I'd be greatful if someone more skilled than me checks if those has any obvious flaws.


In both of these cases some kind of stable user identifier is needed, a popular candidate for that would be user's LN node key. But that approach seems flawed because it allows to easily link user actions across different services so what's proposed is to derive a special `LinkingKey` which would be unique for each `(user, service)` pair where `user` is basically wallet seed and `service` is a domain name.


Anton