offer: make the merkle tree signature public #3892

vincenzopalazzo · 2025-06-26T10:30:20Z

This is helpful for users who want to use the Merkle tree signature in their own code, for example, to verify the signature of bolt12 invoices or recreate it.

Very useful for people who are building command line tools for the bolt12 offers.

I am opening this, but I do not know if it is something that you want to do, but at the same time, IMHO, this is very useful to expose because it allows to use LDK in command line tools and in learning tools.

However, this is not strictly necessary because Bolt12Invoice::try_from already verifies the signature, but I would open this to know your point of view.

ldk-reviews-bot · 2025-06-26T10:30:22Z

👋 Thanks for assigning @jkczyz as a reviewer!
I'll wait for their review and will help manage the review process.
Once they submit their review, I'll check if a second reviewer would be helpful.

dunxen · 2025-06-26T11:51:14Z

I'm not personally opposed to this and it may be nice to expose it for the use cases you mention.
However, maybe the doc comments for these should be updated to indicate that these don't need to be used directly and they're really for internal use, but made public for use in applications like you specified. We should redirect them to the appropriate methods for production use.

I'm also not sure about any API guarantees for these methods. I don't think it's too much of a hassle to treat them the same way we treat any other public API.

codecov · 2025-06-26T15:08:45Z

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 88.99%. Comparing base (1d507f2) to head (f5fbd6c).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3892      +/-   ##
==========================================
- Coverage   89.00%   88.99%   -0.01%     
==========================================
  Files         166      166              
  Lines      119990   120093     +103     
  Branches   119990   120093     +103     
==========================================
+ Hits       106798   106878      +80     
- Misses      10770    10788      +18     
- Partials     2422     2427       +5

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lightning/src/offers/merkle.rs

ldk-reviews-bot · 2025-06-27T16:20:29Z

👋 The first review has been submitted!

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lightning/src/offers/merkle.rs

ldk-reviews-bot · 2025-07-07T10:48:01Z

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Hey @TheBlueMatt @valentinewallace! This PR has been waiting for your review.
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ldk-reviews-bot · 2025-07-07T10:48:02Z

🔔 1st Reminder

Hey @TheBlueMatt @valentinewallace! This PR has been waiting for your review.
Please take a look when you have a chance. If you're unable to review, please let us know so we can find another reviewer.

vincenzopalazzo · 2025-07-07T17:18:52Z

cc @jkczyz

lightning/src/offers/invoice.rs

lightning/src/offers/merkle.rs

TheBlueMatt · 2025-07-09T01:06:27Z

Will take a look at this after @jkczyz's comments are addressed, assuming he thinks it needs a second reviewer at that point.

ldk-reviews-bot · 2025-07-09T10:49:03Z

🔔 2nd Reminder

Hey @valentinewallace! This PR has been waiting for your review.
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vincenzopalazzo · 2025-07-09T11:19:29Z

Ok, I pushed the change that @jkczyz requested, and now the PR is organised in two commits as follows:

bed3812 where expose the sign and verify method to verify manually an invoice signature, this is nice for people that are building command line tools and want to show how the signature verification is described inside the spec
-e60b87c38c043114642eda5499864c3c6898aa64 expose the offer_id inside the Bolt12Invoice that allows to compare it with the Offer to make sure that the invoice is linked with the Bolt12 one. This is useful for a sanity check, but requires trusting that the decoding of ldk will always verify the signature of the invoice

Let me know if you are fine with the commit divided in this way, or you prefer e60b87c

lightning/src/offers/invoice.rs

lightning/src/offers/offer.rs

lightning/src/offers/merkle.rs

lightning/src/offers/invoice.rs

lightning/src/offers/offer.rs

jkczyz · 2025-07-10T13:33:08Z

BTW, could you make sure the commit messages are wrapped at 72 characters?

vincenzopalazzo · 2025-07-10T15:48:58Z

Done thanks, should be ready for another pass!

jkczyz · 2025-07-11T02:53:43Z

lightning/src/offers/invoice.rs

+	/// Returns the [`OfferId`] if this invoice corresponds to an [`Offer`].
+	///
+	/// [`Offer`]: crate::offers::offer::Offer
+	pub fn offer_id(&self) -> Option<OfferId> {
+		self.offer_id
+	}


Ah, sorry... we should put this in invoice_accessors_common such that UnsignedBolt12Invoice, UnsignedStaticInvoice, and StaticInvoice also get this method.

Thanks, I was worried about this accessor in another invoice. Because this change is a little bit disruptive, I made a fixup commit 2da0ac8 to have a first iteration of review on the changes

Yeah... come to think of it we probably don't need this for the unsigned invoices. Sorry about the hassle. Instead, we can drop the fixup and simply implement it directly for StaticInvoice. That should never come from a Refund, so its offer_id method won't need to use Option. Thus, we shouldn't use the macro.

vincenzopalazzo · 2025-07-11T08:40:44Z

lightning/src/offers/invoice.rs

+		let offer_id = match &contents {
+			InvoiceContents::ForOffer { .. } => Some(OfferId::from_valid_bolt12_tlv_stream(&bytes)),
+			InvoiceContents::ForRefund { .. } => None,
+		};
+		Self { bytes, experimental_bytes, contents, tagged_hash, offer_id }


@jkczyz do you think that we should add in OfferId a maybe_from_valid_bolt12_tlv_stream(&bytes) -> Option<OfferId> to include the refund case without duplicate the match here?

We'd need to take in the InvoiceContents then, too. Leaning towards not doing this for unsigned invoices after all. See next comment.

lightning/src/offers/static_invoice.rs

jkczyz · 2025-07-11T14:12:21Z

lightning/src/offers/invoice.rs

+		let offer_id = match &contents {
+			InvoiceContents::ForOffer { .. } => Some(OfferId::from_valid_bolt12_tlv_stream(&bytes)),
+			InvoiceContents::ForRefund { .. } => None,
+		};
+		Self { bytes, experimental_bytes, contents, tagged_hash, offer_id }


We'd need to take in the InvoiceContents then, too. Leaning towards not doing this for unsigned invoices after all. See next comment.

jkczyz · 2025-07-11T14:14:51Z

lightning/src/offers/invoice.rs

+	/// Returns the [`OfferId`] if this invoice corresponds to an [`Offer`].
+	///
+	/// [`Offer`]: crate::offers::offer::Offer
+	pub fn offer_id(&self) -> Option<OfferId> {
+		self.offer_id
+	}


Yeah... come to think of it we probably don't need this for the unsigned invoices. Sorry about the hassle. Instead, we can drop the fixup and simply implement it directly for StaticInvoice. That should never come from a Refund, so its offer_id method won't need to use Option. Thus, we shouldn't use the macro.

lightning/src/offers/invoice.rs

vincenzopalazzo · 2025-07-15T08:39:12Z

Ok pushed the changes, not sure that I have got 100% this review, but let me know if there is something that is still pending #3892 (comment)

vincenzopalazzo · 2025-07-15T08:57:33Z

I noted that for some reason, I was not making public the tagged_hash inside the Bolt12Invoice which was the way to execute manual verification. I pushed the code in ea5685c and added a test to show a user how to use the API that we are now making public

jkczyz · 2025-07-15T19:33:49Z

lightning/src/offers/invoice.rs

+		let issuer_sign_pubkey = offer.issuer_signing_pubkey().unwrap();
+		let tagged_hash = invoice.tagged_hash();
+		let signature = invoice.signature();
+		assert!(merkle::verify_signature(&signature, tagged_hash, issuer_sign_pubkey).is_ok());


Seems like exposing verify_signature doesn't provide much value as it's simply a wrapper around the secp256k1 crate. Maybe it is fine to expose as a convenience function, but it isn't strictly necessary to support the use case (i.e., all we need to do is make tagged_hash accessible).

I see it, from an API point of view I think it is nice to have a verify_signature, but I am not oppose to use secp256k1

lightning/src/offers/invoice.rs

This is helpfull for the users that want to use the merkle tree signature in their own code, for example to verify the signature of bolt12 invoices or recreate it. Very useful for people that are building command line tools for the bolt12 offers. Signed-off-by: Vincenzo Palazzo <[email protected]>

- Add an Option<OfferId> field to Bolt12Invoice to track the originating offer. - Compute the offer_id for invoices created from offers by extracting the offer TLV records and hashing them with the correct tag. - Expose a public offer_id() accessor on invoice. - Add tests to ensure the offer_id in the invoice matches the originating Offer, and that refund invoices have no offer_id. - All existing and new tests pass. This enables linking invoices to their originating offers in a robust and spec-compliant way. Co-authored-by: graphite-app[bot] <96075541+graphite-app[bot]@users.noreply.github.com> Signed-off-by: Vincenzo Palazzo <[email protected]>

vincenzopalazzo · 2025-07-16T09:58:00Z

I had to rebase to make the CI happy because there was a problem fixed by some PR that is now merged.

So I think the only missing thing to resolve is the following one #3892 (comment)

Would love to have the API to hide the secp256k1, but I also understand that the code is simple enough that we do not need an API for skilled users. Let me know what you preferer

jkczyz · 2025-07-16T22:34:47Z

lightning/src/offers/invoice.rs

+	}
+
+	#[test]
+	fn verifies_invoice_signature_with_tagged_hash() {


But this third test belongs in the first commit. :P

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offer: make the merkle tree signature public #3892

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offer: make the merkle tree signature public #3892

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