Merkle root hash computation

Offers uses a merkle root hash construction for signature calculation
and verification. Add a submodule implementing this so that it can be
used when parsing and signing invoice_request and invoice messages.

Author: jkczyz

lightning/src/offers/merkle.rs

@@ -0,0 +1,139 @@
+// This file is Copyright its original authors, visible in version control
+// history.
+//
+// This file is licensed under the Apache License, Version 2.0 <LICENSE-APACHE
+// or http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your option.
+// You may not use this file except in accordance with one or both of these
+// licenses.
+
+//! Tagged hashes for use in signature calculation and verification.
+
+use bitcoin::hashes::{Hash, HashEngine, sha256};
+use crate::io;
+use crate::util::ser::{BigSize, Readable};
+
+use crate::prelude::*;
+
+/// Valid type range for signature TLV records.
+const SIGNATURE_TYPES: core::ops::RangeInclusive<u64> = 240..=1000;
+
+/// 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 {
+	let mut tlv_stream = TlvStream::new(&data[..]).peekable();
+	let nonce_tag = tagged_hash_engine(sha256::Hash::from_engine({
+		let mut engine = sha256::Hash::engine();
+		engine.input("LnNonce".as_bytes());
+		engine.input(tlv_stream.peek().unwrap().type_bytes);
+		engine
+	}));
+	let leaf_tag = tagged_hash_engine(sha256::Hash::hash("LnLeaf".as_bytes()));
+	let branch_tag = tagged_hash_engine(sha256::Hash::hash("LnBranch".as_bytes()));
+
+	let mut leaves = Vec::new();
+	for record in tlv_stream {
+		if !SIGNATURE_TYPES.contains(&record.r#type.0) {
+			leaves.push(tagged_hash_from_engine(leaf_tag.clone(), &record));
+			leaves.push(tagged_hash_from_engine(nonce_tag.clone(), &record.type_bytes));
+		}
+	}
+
+	// Calculate the merkle root hash in place.
+	let num_leaves = leaves.len();
+	for level in 0.. {
+		let step = 2 << level;
+		let offset = step / 2;
+		if offset >= num_leaves {
+			break;
+		}
+
+		for (i, j) in (0..num_leaves).step_by(step).zip((offset..num_leaves).step_by(step)) {
+			leaves[i] = tagged_branch_hash_from_engine(branch_tag.clone(), leaves[i], leaves[j]);
+		}
+	}
+
+	*leaves.first().unwrap()
+}
+
+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)
+}
+
+fn tagged_hash_engine(tag: sha256::Hash) -> sha256::HashEngine {
+	let mut engine = sha256::Hash::engine();
+	engine.input(tag.as_ref());
+	engine.input(tag.as_ref());
+	engine
+}
+
+fn tagged_hash_from_engine<T: AsRef<[u8]>>(mut engine: sha256::HashEngine, msg: T) -> sha256::Hash {
+	engine.input(msg.as_ref());
+	sha256::Hash::from_engine(engine)
+}
+
+fn tagged_branch_hash_from_engine(
+	mut engine: sha256::HashEngine, leaf1: sha256::Hash, leaf2: sha256::Hash,
+) -> sha256::Hash {
+	if leaf1 < leaf2 {
+		engine.input(leaf1.as_ref());
+		engine.input(leaf2.as_ref());
+	} else {
+		engine.input(leaf2.as_ref());
+		engine.input(leaf1.as_ref());
+	};
+	sha256::Hash::from_engine(engine)
+}
+
+/// [`Iterator`] over a sequence of bytes yielding [`TlvRecord`]s. The input is assumed to be a
+/// well-formed TLV stream.
+struct TlvStream<'a> {
+	data: io::Cursor<&'a [u8]>,
+}
+
+impl<'a> TlvStream<'a> {
+	fn new(data: &'a [u8]) -> Self {
+		Self {
+			data: io::Cursor::new(data),
+		}
+	}
+}
+
+/// A slice into a [`TlvStream`] for a record.
+struct TlvRecord<'a> {
+	r#type: BigSize,
+	type_bytes: &'a [u8],
+	data: &'a [u8],
+}
+
+impl AsRef<[u8]> for TlvRecord<'_> {
+	fn as_ref(&self) -> &[u8] { &self.data }
+}
+
+impl<'a> Iterator for TlvStream<'a> {
+	type Item = TlvRecord<'a>;
+
+	fn next(&mut self) -> Option<Self::Item> {
+		if self.data.position() < self.data.get_ref().len() as u64 {
+			let start = self.data.position();
+
+			let r#type: BigSize = Readable::read(&mut self.data).unwrap();
+			let offset = self.data.position();
+			let type_bytes = &self.data.get_ref()[start as usize..offset as usize];
+
+			let length: BigSize = Readable::read(&mut self.data).unwrap();
+			let offset = self.data.position();
+			let end = offset + length.0;
+
+			let _value = &self.data.get_ref()[offset as usize..end as usize];
+			let data = &self.data.get_ref()[start as usize..end as usize];
+
+			self.data.set_position(end);
+
+			Some(TlvRecord { r#type, type_bytes, data })
+		} else {
+			None
+		}
+	}
+}

lightning/src/offers/mod.rs

@@ -13,6 +13,7 @@
 //! Offers are a flexible protocol for Lightning payments.
 
 pub mod invoice_request;
+mod merkle;
 pub mod offer;
 pub mod parse;
 mod payer;