Expose more granular data in TaggedHash struct

lightning/src/offers/invoice.rs

@@ -439,6 +439,7 @@ impl UnsignedBolt12Invoice {
 			bytes: self.bytes,
 			contents: self.contents,
 			signature,
+			tagged_hash: self.tagged_hash,
 		})
 	}
 }
@@ -463,6 +464,7 @@ pub struct Bolt12Invoice {
 	bytes: Vec<u8>,
 	contents: InvoiceContents,
 	signature: Signature,
+	tagged_hash: TaggedHash,
 }
 
 /// The contents of an [`Bolt12Invoice`] for responding to either an [`Offer`] or a [`Refund`].
@@ -707,7 +709,7 @@ impl Bolt12Invoice {
 
 	/// Hash that was used for signing the invoice.
 	pub fn signable_hash(&self) -> [u8; 32] {
-		merkle::message_digest(SIGNATURE_TAG, &self.bytes).as_ref().clone()
+		self.tagged_hash.as_digest().as_ref().clone()
 	}
 
 	/// Verifies that the invoice was for a request or refund created using the given key. Returns
@@ -1212,11 +1214,11 @@ impl TryFrom<ParsedMessage<FullInvoiceTlvStream>> for Bolt12Invoice {
 			None => return Err(Bolt12ParseError::InvalidSemantics(Bolt12SemanticError::MissingSignature)),
 			Some(signature) => signature,
 		};
-		let message = TaggedHash::new(SIGNATURE_TAG, &bytes);
+		let tagged_hash = TaggedHash::new(SIGNATURE_TAG, &bytes);
 		let pubkey = contents.fields().signing_pubkey;
-		merkle::verify_signature(&signature, message, pubkey)?;
+		merkle::verify_signature(&signature, &tagged_hash, pubkey)?;
 
-		Ok(Bolt12Invoice { bytes, contents, signature })
+		Ok(Bolt12Invoice { bytes, contents, signature, tagged_hash })
 	}
 }
 
@@ -1431,7 +1433,7 @@ mod tests {
 		assert_eq!(invoice.signing_pubkey(), recipient_pubkey());
 
 		let message = TaggedHash::new(SIGNATURE_TAG, &invoice.bytes);
-		assert!(merkle::verify_signature(&invoice.signature, message, recipient_pubkey()).is_ok());
+		assert!(merkle::verify_signature(&invoice.signature, &message, recipient_pubkey()).is_ok());
 
 		let digest = Message::from_slice(&invoice.signable_hash()).unwrap();
 		let pubkey = recipient_pubkey().into();
@@ -1528,7 +1530,7 @@ mod tests {
 		assert_eq!(invoice.signing_pubkey(), recipient_pubkey());
 
 		let message = TaggedHash::new(SIGNATURE_TAG, &invoice.bytes);
-		assert!(merkle::verify_signature(&invoice.signature, message, recipient_pubkey()).is_ok());
+		assert!(merkle::verify_signature(&invoice.signature, &message, recipient_pubkey()).is_ok());
 
 		assert_eq!(
 			invoice.as_tlv_stream(),

lightning/src/offers/invoice_request.rs

@@ -876,7 +876,7 @@ impl TryFrom<Vec<u8>> for InvoiceRequest {
 			Some(signature) => signature,
 		};
 		let message = TaggedHash::new(SIGNATURE_TAG, &bytes);
-		merkle::verify_signature(&signature, message, contents.payer_id)?;
+		merkle::verify_signature(&signature, &message, contents.payer_id)?;
 
 		Ok(InvoiceRequest { bytes, contents, signature })
 	}
@@ -1013,7 +1013,7 @@ mod tests {
 		assert_eq!(invoice_request.payer_note(), None);
 
 		let message = TaggedHash::new(SIGNATURE_TAG, &invoice_request.bytes);
-		assert!(merkle::verify_signature(&invoice_request.signature, message, payer_pubkey()).is_ok());
+		assert!(merkle::verify_signature(&invoice_request.signature, &message, payer_pubkey()).is_ok());
 
 		assert_eq!(
 			invoice_request.as_tlv_stream(),

lightning/src/offers/merkle.rs

@@ -30,20 +30,41 @@ tlv_stream!(SignatureTlvStream, SignatureTlvStreamRef, SIGNATURE_TYPES, {
 ///
 /// [BIP 340]: https://github.com/bitcoin/bips/blob/master/bip-0340.mediawiki
 /// [BOLT 12]: https://github.com/rustyrussell/lightning-rfc/blob/guilt/offers/12-offer-encoding.md#signature-calculation
-#[derive(Debug, PartialEq)]
-pub struct TaggedHash(Message);
+#[derive(Clone, Debug, PartialEq)]
+pub struct TaggedHash {
+	tag: String,
+	merkle_root: sha256::Hash,
+	digest: Message,
+}
 
 impl TaggedHash {
 	/// Creates a tagged hash with the given parameters.
 	///
 	/// Panics if `tlv_stream` is not a well-formed TLV stream containing at least one TLV record.
 	pub(super) fn new(tag: &str, tlv_stream: &[u8]) -> Self {
-		Self(message_digest(tag, tlv_stream))
+		let tag_hash = sha256::Hash::hash(tag.as_bytes());
+		let merkle_root = root_hash(tlv_stream);
+		let digest = Message::from_slice(&tagged_hash(tag_hash, merkle_root)).unwrap();
+		Self {
+			tag: tag.to_owned(),
+			merkle_root,
+			digest,
+		}
 	}
 
 	/// Returns the digest to sign.
 	pub fn as_digest(&self) -> &Message {
-		&self.0
+		&self.digest
+	}
+
+	/// Returns the tag used in the tagged hash.
+	pub fn tag(&self) -> &str {
+		&self.tag
+	}
+
+	/// Returns the merkle root used in the tagged hash.
+	pub fn merkle_root(&self) -> sha256::Hash {
+		self.merkle_root
 	}
 }
 
@@ -91,20 +112,14 @@ where
 /// Verifies the signature with a pubkey over the given message using a tagged hash as the message
 /// digest.
 pub(super) fn verify_signature(
-	signature: &Signature, message: TaggedHash, pubkey: PublicKey,
+	signature: &Signature, message: &TaggedHash, pubkey: PublicKey,
 ) -> Result<(), secp256k1::Error> {
 	let digest = message.as_digest();
 	let pubkey = pubkey.into();
 	let secp_ctx = Secp256k1::verification_only();
 	secp_ctx.verify_schnorr(signature, digest, &pubkey)
 }
 
-pub(super) fn message_digest(tag: &str, bytes: &[u8]) -> Message {
-	let tag = sha256::Hash::hash(tag.as_bytes());
-	let merkle_root = root_hash(bytes);
-	Message::from_slice(&tagged_hash(tag, merkle_root)).unwrap()
-}
-
 /// Computes a merkle root hash for the given data, which must be a well-formed TLV stream
 /// containing at least one TLV record.
 fn root_hash(data: &[u8]) -> sha256::Hash {
@@ -258,12 +273,13 @@ mod tests {
 	use super::{SIGNATURE_TYPES, TlvStream, WithoutSignatures};
 
 	use bitcoin::hashes::{Hash, sha256};
-	use bitcoin::secp256k1::{KeyPair, Secp256k1, SecretKey};
+	use bitcoin::secp256k1::{KeyPair, Message, Secp256k1, SecretKey};
 	use bitcoin::secp256k1::schnorr::Signature;
 	use core::convert::Infallible;
 	use crate::offers::offer::{Amount, OfferBuilder};
 	use crate::offers::invoice_request::InvoiceRequest;
 	use crate::offers::parse::Bech32Encode;
+	use crate::offers::test_utils::{payer_pubkey, recipient_pubkey};
 	use crate::util::ser::Writeable;
 
 	#[test]
@@ -322,6 +338,25 @@ mod tests {
 		);
 	}
 
+        #[test]
+        fn compute_tagged_hash() {
+                let unsigned_invoice_request = OfferBuilder::new("foo".into(), recipient_pubkey())
+                        .amount_msats(1000)
+                        .build().unwrap()
+                        .request_invoice(vec![1; 32], payer_pubkey()).unwrap()
+                        .payer_note("bar".into())
+                        .build().unwrap();
+
+                // Simply test that we can grab the tag and merkle root exposed by the accessor
+                // functions, then use them to succesfully compute a tagged hash.
+                let tagged_hash = unsigned_invoice_request.as_ref();
+                let expected_digest = unsigned_invoice_request.as_ref().as_digest();
+                let tag = sha256::Hash::hash(tagged_hash.tag().as_bytes());
+                let actual_digest = Message::from_slice(&super::tagged_hash(tag, tagged_hash.merkle_root()))
+                        .unwrap();
+                assert_eq!(*expected_digest, actual_digest);
+        }
+
 	#[test]
 	fn skips_encoding_signature_tlv_records() {
 		let secp_ctx = Secp256k1::new();