Blinded route should hold alias instead of actual scid?

[orbitalturtle]

Issue and Steps to Reproduce

Ahoy there, I'm helping @carlaKC with interop of her lovely route blinding PR (lightningnetwork/lnd#7267) with CLN.

If I'm understanding right, one issue seems to be that when CLN creates a blinded route (returned by fetchinvoice), the encrypted data for the final hop (reached by a private channel) should include a scid alias as the short_channel_id field. But instead it adds the actual scid.

Walking through this, I use the setup described at the bottom of @carlaKC's PR. So we have nodes A (LND) - B (CLN) - C (CLN) - D (CLN), where C and D are connected by a private channel. Then I follow the rest of Carla's instructions to send a blinded payment from A to D.

The HTLC gets stuck at C because C cannot find a route to D, according to logs in C. This seems to happen here:

lightning/lightningd/channel.c

Line 687 in fa58cd9

if (!privacy_leak_ok

Using a debugger here, the problem seems to be that the short_channel_id provided to this function (which originally comes from the encrypted data of the final hop) is the short_channel_id, instead of the alias, which the comment on 681 explains is required here.

When I comment out #686-688, and recompile, it successfully finds the channel, and forwards the HTLC to D! Not a good solution of course, but helps to verify that this is the problem area...

[orbitalturtle]

Closing as it seems like this has been fixed :)