Support async payments in BOLT 12 #1149

valentinewallace · 2024-03-19T17:59:01Z

This builds on #989 by adding the ability to fetch an invoice from an
always-online node on behalf of an often-offline recipient, e.g. a mobile node.

The idea is that often-offline recipients will supply some always-online node
such as their wallet vendor with a static keysend (i.e. payment_hash-less)
invoice to return on its behalf. The recipient will then publish an offer
containing blinded paths that terminate at this always-online node, who payers
can request the invoice from if the recipient is offline at the time. After
receiving the keysend invoice, payers will commence the protocol outlined in
1 to send the HTLC asynchronously.

Some context on the top commit where we include the invoice request in the payment onion:
This definitely warrants discussion, but the idea is that this field may be useful for often-offline recipients who did not receive the invoice request when it was originally sent. Recipients may want to verify the invreq or be provided some other relevant data about the payment, while keeping the payment stateless until an HTLC is actually received. For example, future extensions have been proposed 2 that require the recipient to know a unique token for a payment, and this field would provide that to them.

Seeking conceptual feedback! I'm also working on the implementation in LDK.

Based on ~~#798~~ and #989.

valentinewallace · 2024-06-03T17:25:55Z

Changed up the feature bits, required single-chain offers, and disallowed setting invoice_amount_msat for static invoices. Also moved away from the "keysend" language for clarity. These changes were based on implementing this in LDK in lightningdevkit/rust-lightning#3082, will take this out of draft once the design solidifies a bit more in that PR.

valentinewallace · 2024-06-24T20:26:29Z

Added a fixup requiring the invreq to be included in the payment onion per spec meeting discussion a week ago!

Per <lightning/bolts#1149>, when paying a static invoice we need to include our original invoice request in the HTLC onion since the recipient wouldn't have received it previously. We use an experimental TLV type for this new onion payload field, since the spec is still not merged in the BOLTs.

Add a new invoice request parameter to onion_utils::build_onion_payloads. As of this commit it will always be passed in as None, to be updated in future commits. Per <lightning/bolts#1149>, when paying a static invoice we need to include our original invoice request in the HTLC onion since the recipient wouldn't have received it previously.

Add a new invoice request parameter to onion_utils::create_payment_onion. As of this commit it will always be passed in as None, to be updated in future commits. Per <lightning/bolts#1149>, when paying a static invoice we need to include our original invoice request in the HTLC onion since the recipient wouldn't have received it previously.

Add a new invoice request parameter to outbound_payments and channelmanager send-to-route internal utils. As of this commit the invreq will always be passed in as None, to be updated in future commits. Per <lightning/bolts#1149>, when paying a static invoice we need to include our original invoice request in the HTLC onion since the recipient wouldn't have received it previously.

When transitioning outbound payments from AwaitingInvoice to StaticInvoiceReceived, include the invreq in the new state's outbound payment storage for future inclusion in an async payment onion. Per <lightning/bolts#1149>, when paying a static invoice we need to include our original invoice request in the HTLC onion since the recipient wouldn't have received it previously.

Past commits have set us up to include invoice requests in outbound async payment onions. Here we actually pull the invoice request from where it's stored in outbound_payments and pass it into the correct utility for inclusion in the onion on initial send. Per <lightning/bolts#1149>, when paying a static invoice we need to include our original invoice request in the HTLC onion since the recipient wouldn't have received it previously.

While in the last commit we began including invoice requests in async payment onions on initial send, further work is needed to include them on retry. Here we begin storing invreqs in our retry data, and pass them along for inclusion in the onion on payment retry. Per <lightning/bolts#1149>, when paying a static invoice we need to include our original invoice request in the HTLC onion since the recipient wouldn't have received it previously.