[Lightning-dev] A Mobile Lightning User Goes to Pay a Mobile Lightning User...

[ZmnSCPxj]

Good morning Matt,

> On 10/13/21 02:58, ZmnSCPxj wrote:
>
> > Good morning Matt,
> >
> > >      The Obvious (tm) solution here is PTLCs - just have the sender always add some random nonce * G to
> > >      the PTLC they're paying and send the recipient a random nonce in the onion. I'd generally suggest we
> > >      just go ahead and do this for every PTLC payment, cause why not? Now the sender and the lnurl
> > >      endpoint have to collude to steal the funds, but, like, the sender could always just give the lnurl
> > >      endpoint the money. I'd love suggestions for fixing this short of PTLCs, but its not immediately
> > >      obvious to me that this is possible.
> > >
> >
> > Use two hashes in an HTLC instead of one, where the second hash is from a preimage the sender generates, and which the sender sends (encrypted via onion) to the receiver.
> > You might want to do this anyway in HTLC-land, consider that we have a `payment_secret` in invoices, the second hash could replace that, and provide similar protection to what `payment_secret` provides (i.e. resistance against forwarding nodes probing; the information in both cases is private to the ultimate sender and ultimate reeceiver).
>
> Yes, you could create a construction which does this, sure, but I'm not sure how you'd do this
> without informing every hop along the path that this is going on, and adapting each hop to handle
> this as well. I suppose I should have been more clear with the requirements, or can you clarify
> somewhat what your proposed construction is?

Just that: two hashes instead of one.
Make *every* HTLC on LN use two hashes, even for current "online RPi user pays online RPi user" --- just use the `payment_secret` for the preimage of the second hash, the sender needs to send it anyway.

>
> If you're gonna adapt every node in the path, you might as well just use PTLC.

Correct, we should just do PTLCs now.
(Basically, my proposal was just a strawman to say "we should just do PTLCs now")


Regards,
ZmnSCPxj