expose more granular data in TaggedHash struct

lightning/src/offers/invoice_request.rs

@@ -922,8 +922,9 @@ mod tests {
 	use super::{InvoiceRequest, InvoiceRequestTlvStreamRef, SIGNATURE_TAG, UnsignedInvoiceRequest};
 
 	use bitcoin::blockdata::constants::ChainHash;
+	use bitcoin::hashes::{sha256, Hash};
 	use bitcoin::network::constants::Network;
-	use bitcoin::secp256k1::{KeyPair, Secp256k1, SecretKey, self};
+	use bitcoin::secp256k1::{KeyPair, Message, Secp256k1, SecretKey, self};
 	use core::convert::{Infallible, TryFrom};
 	use core::num::NonZeroU64;
 	#[cfg(feature = "std")]
@@ -934,7 +935,7 @@ mod tests {
 	use crate::ln::inbound_payment::ExpandedKey;
 	use crate::ln::msgs::{DecodeError, MAX_VALUE_MSAT};
 	use crate::offers::invoice::{Bolt12Invoice, SIGNATURE_TAG as INVOICE_SIGNATURE_TAG};
-	use crate::offers::merkle::{SignError, SignatureTlvStreamRef, TaggedHash, self};
+	use crate::offers::merkle::{tagged_hash, SignError, SignatureTlvStreamRef, TaggedHash, self};
 	use crate::offers::offer::{Amount, OfferBuilder, OfferTlvStreamRef, Quantity};
 	use crate::offers::parse::{Bolt12ParseError, Bolt12SemanticError};
 	use crate::offers::payer::PayerTlvStreamRef;
@@ -1537,6 +1538,23 @@ mod tests {
 		assert_eq!(tlv_stream.payer_note, Some(&String::from("baz")));
 	}
 
+	#[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 tosuccesfully compute a tagged hash.
+		let taggedhash = unsigned_invoice_request.as_ref();
+		let tag = sha256::Hash::hash(taggedhash.tag().as_bytes());
+		let _ = Message::from_slice(&tagged_hash(tag, taggedhash.merkle_root_hash()))
+			.unwrap();
+        }
+
 	#[test]
 	fn fails_signing_invoice_request() {
 		match OfferBuilder::new("foo".into(), recipient_pubkey())

lightning/src/offers/merkle.rs

@@ -31,19 +31,37 @@ 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);
+pub struct TaggedHash {
+	tag: String,
+	merkle_root_hash: 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))
+		Self{
+			tag: tag.to_owned(),
+			merkle_root_hash: root_hash(tlv_stream),
+			digest: message_digest(tag, tlv_stream),
+		}
 	}
 
 	/// Returns the digest to sign.
 	pub fn as_digest(&self) -> &Message {
-		&self.0
+		&self.digest
+	}
+
+	/// Returns the tag used in the TaggedHash.
+	pub fn tag(&self) -> &str {
+		&self.tag
+	}
+
+	/// Returns the merkle root hash used in the TaggedHash.
+	pub fn merkle_root_hash(&self) -> sha256::Hash {
+		self.merkle_root_hash
 	}
 }
 
@@ -144,7 +162,7 @@ fn root_hash(data: &[u8]) -> sha256::Hash {
 	*leaves.first().unwrap()
 }
 
-fn tagged_hash<T: AsRef<[u8]>>(tag: sha256::Hash, msg: T) -> sha256::Hash {
+pub(crate) fn tagged_hash<T: AsRef<[u8]>>(tag: sha256::Hash, msg: T) -> sha256::Hash {
 	let engine = tagged_hash_engine(tag);
 	tagged_hash_from_engine(engine, msg)
 }