Fixing offers when offer_paths are present and offer_issuer_id is not needed

[21M4TW]

Fixing offers when offer_paths are present and offer_issuer_id is no longer needed:
-When offer_paths are present and offer_issuer_id is no longer needed, set offer_issuer_id to NULL in found_best_peer
-In listoffers_done, when there is no offer_issuer_id, set invoice_node_id to the final blinded_node_id from the used offer path (as described in the Bolt12 specs)
-handle_sign_bolt12 was only used to sign invoices, but was not able to sign using the blinded node id. I modified the function to support using a path_pubkey (I created the function node_blinded_privkey as well).

• [ x] The changelog has been updated in the relevant commit(s) according to the guidelines.
• [ x] Tests have been added or modified to reflect the changes.
• [ x] Documentation has been reviewed and updated as needed.
• [ x] Related issues have been listed and linked, including any that this PR closes.

[21M4TW]

As requested by @vincenzopalazzo

[rustyrussell]

This invoice is invalid: let's break it down by type, length, value (hex)

• 00 10 fc84667539e41bd6564fbbdb87d3375e
• 02 20 43497fd7f826957108f4a30fd9cec3aeba79972084e90ead01ea330900000000
• 0a 07 6e657774657374
• 10 ee 03e5c9e190e93af5f757e533ad12ca2da9daef99f337a7af0320e70feae9a7890003b224fa465507bf714641c64a1018ccc05bbcda60d3aa15212e577dfd8e0fb9ea020202a9628f99168a0b09c8ddc71bd9191e31f871075cd8854968dbb72d47c3912100338139ad02276007ea918fa7208765a306c200c7af814c0284deca7ced13b36863c84d03393572ef44d44d78cce9b0bf58a6ff8c02c1a165fe61b4fc6c2097bee34d538fbc84902df62e0bf8eac81495971d491b3600324a46184f1c237f594b5cd4797f794c0b3f2267b8ff51e55376aa7053a5b708ad7c7056dd0b537494580af8f872b9972cebba
• 50 20 43497fd7f826957108f4a30fd9cec3aeba79972084e90ead01ea330900000000
• 52 01 0b
• 54 00
• 58 21 037d942d1f385d68bf600ed3ce14d1e0b84f99eaa594196e60dedc4a6eae3064fe
• a0 fd00fd 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
• a2 1c 000000010000000a0010000000000000000000000000002380000000
• a4 04 68ac799e
• a8 20 d6e635a2de2108f755ac860e2a6e80ab9798dc73ab55e6b6c54eda1025e6dadf
• aa 01 0b
• ae 03 020000
• f0 40 9558b15edc9a8b402eb1a54512caae3b82d31f61c04e849a5491d075d5d0ebf7b3f0ae39e8541496ec374a38ec8ea947de2a74ba4b22efb34a859e55506726cc

This has no "b0" field (i.e. type: 176 (invoice_node_id)).

To quote bolt12:

A writer of an invoice:
...
  - if `offer_issuer_id` is present:
    - MUST set `invoice_node_id` to the `offer_issuer_id`
  - otherwise, if `offer_paths` is present:
    - MUST set `invoice_node_id` to the final `blinded_node_id` on the path it received the invoice request

And:

A reader of an invoice:
  - MUST reject the invoice if `invoice_amount` is not present.
  - MUST reject the invoice if `invoice_created_at` is not present.
  - MUST reject the invoice if `invoice_payment_hash` is not present.
  - MUST reject the invoice if `invoice_node_id` is not present.

So, who is producing this invalid invoice?

[21M4TW]

Thank you Rusty, and sorry for the late reply.

I just updated this PR with what I think fixes the issue. It now does the following:
-When offer_paths are present and offer_issuer_id is no longer needed, set offer_issuer_id to NULL in found_best_peer
-In listoffers_done, when there is no offer_issuer_id, set invoice_node_id to the final blinded_node_id from the used offer path (as described in the Bolt12 specs)
-handle_sign_bolt12 was only used to sign invoices, but was not able to sign using the blinded node id. I modified the function to support using a path_pubkey (I created the function node_blinded_privkey as well).

So as far as I know, this code is now properly working and addresses the issues I had identified. The invoice signature is recognized as valid by fetchinvoice, and it works whether or not an offer contains an issuer_id. Thank you!

[21M4TW]

This solves the issuer_id issue identified in #4689

[madelinevibes]

It seems this one slipped through the 25.12 release crack because the issue it was linked to had been closed. will request and prompt for review ahead of early 2026 release. @21M4TW can you please rebase?

[21M4TW]

It seems this one slipped through the 25.12 release crack because the issue it was linked to had been closed. will request and prompt for review ahead of early 2026 release. @21M4TW can you please rebase?

Thanks. I just rebased it to the latest commit from master.

[madelinevibes]

thanks! now over to @endothermicdev for review please.