chain: implement Bitcoin Merkle root computation

zebra-chain/src/block/header.rs

@@ -30,11 +30,14 @@ pub struct Header {
     /// block’s header.
     pub previous_block_hash: Hash,
 
-    /// The root of the transaction Merkle tree.
+    /// The root of the Bitcoin-inherited transaction Merkle tree, binding the
+    /// block header to the transactions in the block.
     ///
-    /// The Merkle root is derived from the SHA256d hashes of all transactions
-    /// included in this block as assembled in a binary tree, ensuring that none
-    /// of those transactions can be modied without modifying the header.
+    /// Note that because of a flaw in Bitcoin's design, the `merkle_root` does
+    /// not always precisely bind the contents of the block (CVE-2012-2459). It
+    /// is sometimes possible for an attacker to create multiple distinct sets of
+    /// transactions with the same Merkle root, although only one set will be
+    /// valid.
     pub merkle_root: merkle::Root,
 
     /// Some kind of root hash.

zebra-chain/src/block/merkle.rs

@@ -1,51 +1,132 @@
 //! The Bitcoin-inherited Merkle tree of transactions.
 #![allow(clippy::unit_arg)]
 
-use std::{fmt, io};
+use std::{fmt, io::Write};
 
 #[cfg(any(any(test, feature = "proptest-impl"), feature = "proptest-impl"))]
 use proptest_derive::Arbitrary;
 
-use crate::serialization::{sha256d, SerializationError, ZcashDeserialize, ZcashSerialize};
-use crate::transaction::Transaction;
+use crate::serialization::sha256d;
+use crate::transaction::{self, Transaction};
 
-/// A binary hash tree of SHA256d (two rounds of SHA256) hashes for
-/// node values.
-#[derive(Default)]
-pub struct Tree<T> {
-    _leaves: Vec<T>,
-}
+/// The root of the Bitcoin-inherited transaction Merkle tree, binding the
+/// block header to the transactions in the block.
+///
+/// Note that because of a flaw in Bitcoin's design, the `merkle_root` does
+/// not always precisely bind the contents of the block (CVE-2012-2459). It
+/// is sometimes possible for an attacker to create multiple distinct sets of
+/// transactions with the same Merkle root, although only one set will be
+/// valid.
+///
+/// # Malleability
+///
+/// The Bitcoin source code contains the following note:
+///
+/// > WARNING! If you're reading this because you're learning about crypto
+/// > and/or designing a new system that will use merkle trees, keep in mind
+/// > that the following merkle tree algorithm has a serious flaw related to
+/// > duplicate txids, resulting in a vulnerability (CVE-2012-2459).
+/// > The reason is that if the number of hashes in the list at a given time
+/// > is odd, the last one is duplicated before computing the next level (which
+/// > is unusual in Merkle trees). This results in certain sequences of
+/// > transactions leading to the same merkle root. For example, these two
+/// > trees:
+/// >
+/// > ```ascii
+/// >              A               A
+/// >            /  \            /   \
+/// >          B     C         B       C
+/// >         / \    |        / \     / \
+/// >        D   E   F       D   E   F   F
+/// >       / \ / \ / \     / \ / \ / \ / \
+/// >       1 2 3 4 5 6     1 2 3 4 5 6 5 6
+/// > ```
+/// >
+/// > for transaction lists [1,2,3,4,5,6] and [1,2,3,4,5,6,5,6] (where 5 and
+/// > 6 are repeated) result in the same root hash A (because the hash of both
+/// > of (F) and (F,F) is C).
+/// >
+/// > The vulnerability results from being able to send a block with such a
+/// > transaction list, with the same merkle root, and the same block hash as
+/// > the original without duplication, resulting in failed validation. If the
+/// > receiving node proceeds to mark that block as permanently invalid
+/// > however, it will fail to accept further unmodified (and thus potentially
+/// > valid) versions of the same block. We defend against this by detecting
+/// > the case where we would hash two identical hashes at the end of the list
+/// > together, and treating that identically to the block having an invalid
+/// > merkle root. Assuming no double-SHA256 collisions, this will detect all
+/// > known ways of changing the transactions without affecting the merkle
+/// > root.
+///
+/// This vulnerability does not apply to Zebra, because it does not store invalid
+/// data on disk, and because it does not permanently fail blocks or use an
+/// aggressive anti-DoS mechanism.
+#[derive(Clone, Copy, Eq, PartialEq, Serialize, Deserialize)]
+#[cfg_attr(any(test, feature = "proptest-impl"), derive(Arbitrary))]
+pub struct Root(pub [u8; 32]);
 
-impl<Transaction> ZcashSerialize for Tree<Transaction> {
-    fn zcash_serialize<W: io::Write>(&self, _writer: W) -> Result<(), io::Error> {
-        unimplemented!();
+impl fmt::Debug for Root {
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        f.debug_tuple("Root").field(&hex::encode(&self.0)).finish()
     }
 }
 
-impl<Transaction> ZcashDeserialize for Tree<Transaction> {
-    fn zcash_deserialize<R: io::Read>(_reader: R) -> Result<Self, SerializationError> {
-        unimplemented!();
+fn hash(h1: &[u8; 32], h2: &[u8; 32]) -> [u8; 32] {
+    let mut w = sha256d::Writer::default();
+    w.write_all(h1).unwrap();
+    w.write_all(h2).unwrap();
+    w.finish()
+}
+
+impl<T> std::iter::FromIterator<T> for Root
+where
+    T: std::convert::AsRef<Transaction>,
+{
+    fn from_iter<I>(transactions: I) -> Self
+    where
+        I: IntoIterator<Item = T>,
+    {
+        transactions
+            .into_iter()
+            .map(|tx| tx.as_ref().hash())
+            .collect()
     }
 }
 
-/// A SHA-256d hash of the root node of a merkle tree of SHA256-d
-/// hashed transactions in a block.
-#[derive(Clone, Copy, Eq, PartialEq, Serialize, Deserialize)]
-#[cfg_attr(any(test, feature = "proptest-impl"), derive(Arbitrary))]
-pub struct Root(pub [u8; 32]);
+impl std::iter::FromIterator<transaction::Hash> for Root {
+    fn from_iter<I>(hashes: I) -> Self
+    where
+        I: IntoIterator<Item = transaction::Hash>,
+    {
+        let mut hashes = hashes.into_iter().map(|hash| hash.0).collect::<Vec<_>>();
 
-impl From<Tree<Transaction>> for Root {
-    fn from(merkle_tree: Tree<Transaction>) -> Self {
-        let mut hash_writer = sha256d::Writer::default();
-        merkle_tree
-            .zcash_serialize(&mut hash_writer)
-            .expect("Sha256dWriter is infallible");
-        Self(hash_writer.finish())
+        while hashes.len() > 1 {
+            hashes = hashes
+                .chunks(2)
+                .map(|chunk| match chunk {
+                    [h1, h2] => hash(h1, h2),
+                    [h1] => hash(h1, h1),
+                    _ => unreachable!("chunks(2)"),
+                })
+                .collect();
+        }
+
+        Self(hashes[0])
     }
 }
 
-impl fmt::Debug for Root {
-    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
-        f.debug_tuple("Root").field(&hex::encode(&self.0)).finish()
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    use crate::{block::Block, serialization::ZcashDeserialize};
+
+    #[test]
+    fn block_test_vectors() {
+        for block_bytes in zebra_test::vectors::BLOCKS.iter() {
+            let block = Block::zcash_deserialize(&**block_bytes).unwrap();
+            let merkle_root = block.transactions.iter().collect::<Root>();
+            assert_eq!(merkle_root, block.header.merkle_root);
+        }
     }
 }