From c9e4192335b67d867a6c0afa956c03cfaf4f91a4 Mon Sep 17 00:00:00 2001
From: "Rodrigo Q. Saramago" <rodrigoqsaramago@gmail.com>
Date: Mon, 22 Aug 2022 22:26:44 +0200
Subject: [PATCH] Add method to verify merkle proof

---
 src/BaseTree.ts              | 31 +++++++++++++++++++++++++++++++
 test/fixedMerkleTree.spec.ts | 31 ++++++++++++++++++++++++++++++-
 2 files changed, 61 insertions(+), 1 deletion(-)

diff --git a/src/BaseTree.ts b/src/BaseTree.ts
index c8819ee..732c0cd 100644
--- a/src/BaseTree.ts
+++ b/src/BaseTree.ts
@@ -179,6 +179,37 @@ export class BaseTree {
     }
   }
 
+  /**
+   * Verifies a merkle proof
+   * @param {Element} root the root of the merkle tree
+   * @param {number} levels the number of levels of the tree
+   * @param {HashFunction<Element>} hashFn hash function
+   * @param {Element} leaf the leaf to be verified
+   * @param {Element[]} pathElements adjacent path elements
+   * @param {number[]} pathIndices left-right indices
+   * @returns {Boolean} whether the proof is valid for the given root
+   */
+  static verifyProof(
+    root: Element,
+    levels: number,
+    hashFn: HashFunction<Element>,
+    leaf: Element,
+    pathElements: Element[],
+    pathIndices: number[],
+  ): boolean {
+    const layerProofs: Element[] = []
+
+    for (let level = 0; level < levels; level++) {
+      let elem = level == 0 ? leaf : layerProofs[level - 1]
+      if (pathIndices[level] == 0) {
+        layerProofs[level] = hashFn(elem, pathElements[level])
+      } else {
+        layerProofs[level] = hashFn(pathElements[level], elem)
+      }
+    }
+    return root === layerProofs[levels - 1]
+  }
+
   /**
    * Verifies a merkle multiproof
    * @param {Element} root the root of the merkle tree
diff --git a/test/fixedMerkleTree.spec.ts b/test/fixedMerkleTree.spec.ts
index 150e7db..63706fa 100644
--- a/test/fixedMerkleTree.spec.ts
+++ b/test/fixedMerkleTree.spec.ts
@@ -1,4 +1,4 @@
-import { MerkleTree, MultiProofPath, PartialMerkleTree, TreeEdge } from '../src'
+import { MerkleTree, ProofPath, MultiProofPath, PartialMerkleTree, TreeEdge } from '../src'
 import { assert, should, expect } from 'chai'
 import { createHash } from 'crypto'
 import { it } from 'mocha'
@@ -262,6 +262,35 @@ describe('MerkleTree', () => {
     })
   })
 
+  describe('#verifyProof', () => {
+    it('should verify a merkle-proof', () => {
+      const leaves = [...Array(16)].map((_, i) => i + 1)
+      const tree = new MerkleTree(4, leaves)
+      const proof: ProofPath = {
+        pathElements: [
+          6,
+          '1177811302158128621769756786551727063040',
+          '81822854828781486047086122479545722339328',
+          '3473994785217814971484840545214368055296'
+        ],
+        pathIndices: [ 0, 0, 1, 0 ],
+        pathPositions: [ 5, 3, 0, 1 ],
+        pathRoot: '4813607112316126252402222488335589310464'
+      }
+
+      expect(
+        BaseTree.verifyProof(
+          tree.root,
+          tree.levels,
+          defaultHash,
+          5,
+          proof.pathElements,
+          proof.pathIndices,
+        ),
+      ).to.be.true
+    })
+  })
+
   describe('#multiproof', () => {
     it('should return a merkle-multiproof for a range of leaves in a full tree', () => {
       const leaves = [...Array(8)].map((_, i) => i + 1)