Implement library for verifying Merkle proofs.

Cargo.toml

@@ -9,6 +9,7 @@ members = [
 	"eth2/utils/boolean-bitfield",
 	"eth2/utils/hashing",
 	"eth2/utils/honey-badger-split",
+	"eth2/utils/merkle_proof",
 	"eth2/utils/int_to_bytes",
 	"eth2/utils/slot_clock",
 	"eth2/utils/ssz",

eth2/utils/merkle_proof/Cargo.toml

@@ -0,0 +1,9 @@
+[package]
+name = "merkle_proof"
+version = "0.1.0"
+authors = ["Michael Sproul <michael@sigmaprime.io>"]
+edition = "2018"
+
+[dependencies]
+ethereum-types = "0.5"
+hashing = { path = "../hashing" }

eth2/utils/merkle_proof/src/lib.rs

@@ -0,0 +1,148 @@
+use ethereum_types::H256;
+use hashing::hash;
+
+/// Verify a proof that `leaf` exists at `index` in a Merkle tree rooted at `root`.
+///
+/// The `branch` argument is the main component of the proof: it should be a list of internal
+/// node hashes such that the root can be reconstructed (in bottom-up order).
+pub fn verify_merkle_proof(
+    leaf: H256,
+    branch: &[H256],
+    depth: usize,
+    index: usize,
+    root: H256,
+) -> bool {
+    if branch.len() == depth {
+        merkle_root_from_branch(leaf, branch, depth, index) == root
+    } else {
+        false
+    }
+}
+
+/// Compute a root hash from a leaf and a Merkle proof.
+fn merkle_root_from_branch(leaf: H256, branch: &[H256], depth: usize, index: usize) -> H256 {
+    assert_eq!(branch.len(), depth, "proof length should equal depth");
+
+    let mut merkle_root = leaf.as_bytes().to_vec();
+
+    for i in 0..depth {
+        let ith_bit = (index >> i) & 0x01;
+        if ith_bit == 1 {
+            let input = concat(branch[i].as_bytes().to_vec(), merkle_root);
+            merkle_root = hash(&input);
+        } else {
+            let mut input = merkle_root;
+            input.extend_from_slice(branch[i].as_bytes());
+            merkle_root = hash(&input);
+        }
+    }
+
+    H256::from_slice(&merkle_root)
+}
+
+/// Concatenate two vectors.
+fn concat(mut vec1: Vec<u8>, mut vec2: Vec<u8>) -> Vec<u8> {
+    vec1.append(&mut vec2);
+    vec1
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    fn hash_concat(h1: H256, h2: H256) -> H256 {
+        H256::from_slice(&hash(&concat(
+            h1.as_bytes().to_vec(),
+            h2.as_bytes().to_vec(),
+        )))
+    }
+
+    #[test]
+    fn verify_small_example() {
+        // Construct a small merkle tree manually
+        let leaf_b00 = H256::from([0xAA; 32]);
+        let leaf_b01 = H256::from([0xBB; 32]);
+        let leaf_b10 = H256::from([0xCC; 32]);
+        let leaf_b11 = H256::from([0xDD; 32]);
+
+        let node_b0x = hash_concat(leaf_b00, leaf_b01);
+        let node_b1x = hash_concat(leaf_b10, leaf_b11);
+
+        let root = hash_concat(node_b0x, node_b1x);
+
+        // Run some proofs
+        assert!(verify_merkle_proof(
+            leaf_b00,
+            &[leaf_b01, node_b1x],
+            2,
+            0b00,
+            root
+        ));
+        assert!(verify_merkle_proof(
+            leaf_b01,
+            &[leaf_b00, node_b1x],
+            2,
+            0b01,
+            root
+        ));
+        assert!(verify_merkle_proof(
+            leaf_b10,
+            &[leaf_b11, node_b0x],
+            2,
+            0b10,
+            root
+        ));
+        assert!(verify_merkle_proof(
+            leaf_b11,
+            &[leaf_b10, node_b0x],
+            2,
+            0b11,
+            root
+        ));
+        assert!(verify_merkle_proof(
+            leaf_b11,
+            &[leaf_b10],
+            1,
+            0b11,
+            node_b1x
+        ));
+
+        // Ensure that incorrect proofs fail
+        // Zero-length proof
+        assert!(!verify_merkle_proof(leaf_b01, &[], 2, 0b01, root));
+        // Proof in reverse order
+        assert!(!verify_merkle_proof(
+            leaf_b01,
+            &[node_b1x, leaf_b00],
+            2,
+            0b01,
+            root
+        ));
+        // Proof too short
+        assert!(!verify_merkle_proof(leaf_b01, &[leaf_b00], 2, 0b01, root));
+        // Wrong index
+        assert!(!verify_merkle_proof(
+            leaf_b01,
+            &[leaf_b00, node_b1x],
+            2,
+            0b10,
+            root
+        ));
+        // Wrong root
+        assert!(!verify_merkle_proof(
+            leaf_b01,
+            &[leaf_b00, node_b1x],
+            2,
+            0b01,
+            node_b1x
+        ));
+    }
+
+    #[test]
+    fn verify_zero_depth() {
+        let leaf = H256::from([0xD6; 32]);
+        let junk = H256::from([0xD7; 32]);
+        assert!(verify_merkle_proof(leaf, &[], 0, 0, leaf));
+        assert!(!verify_merkle_proof(leaf, &[], 0, 7, junk));
+    }
+}