Allow the introduction_node to be set for the BlindedPaymentPath

[dangeross]

This PR introduces introduction_node as an optional parameter when creating a BlindedPaymentPath. The reason for this change is to allow the introduction node to be configured as a DirectedShortChannelId.

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[jkczyz]

Could you expand upon why this is necessary? Invoices don't have a human-readable format, so QR size shouldn't be an issue.

[dangeross]

Could you expand upon why this is necessary?

Sure. We should be able to reference a short channel id as the channel leading to the introduction node, as in the spec.

Invoices don't have a human-readable format, so QR size shouldn't be an issue.

I'm sure how this is related?

[jkczyz]

Could you expand upon why this is necessary?

Sure. We should be able to reference a short channel id as the channel leading to the introduction node, as in the spec.

Invoices don't have a human-readable format, so QR size shouldn't be an issue.

I'm sure how this is related?

The reason for allowing an SCID instead of a pubkey is to reduce the size of offer QR codes. Since BlindedPaymentPath is used in Bolt12Invoice but not in Offer, there is no need to create the compact representation.

If we are to allow it, we should use the same approach as BlindedMessagePath unless there is a good reason not to.

rust-lightning/lightning/src/blinded_path/message.rs

Line 106 in 16307f6

pub fn use_compact_introduction_node(&mut self, network_graph: &ReadOnlyNetworkGraph) {

[TheBlueMatt]

Wait, are we sure the spec allows for it? I thought the spec similarly only allowed it in message paths precisely for the reasons @jkczyz lays out.

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[jkczyz]

Wait, are we sure the spec allows for it? I thought the spec similarly only allowed it in message paths precisely for the reasons @jkczyz lays out.

The spec really doesn't differentiate the two. We support sending a payment to one with a compact introduction node. We just don't create them.

[dangeross]

Thanks for the feedback @jkczyz @TheBlueMatt

The motivation is similar to how Boltz adds a specific scid as a routing hop hint to a BOLT11 invoice to denote that there is a direct payment alternative (Magic Routing Hint). Similarly for the BOLT12 implementation we would also like to include this specific scid as the introduction node scid in the BOLT12 invoice.

Would an alternative option be to set the DirectedShortChannelId as part of the BlindedMessagePath in the offer and then this path then be copied to the invoice? Can you propose any other options?

[jkczyz]

The messages are extensible with TLVs, so a custom TLV setting along with a feature bit in the invoice would be the typical way of achieving this.

[dangeross]

We've gone a different route. Thanks for your feedback