How to Construct and Verify Merkle Trees with MerkleTree.js and Solidity

In the world of blockchain technology, Merkle trees (also known as binary hash trees) play a crucial role in securely organizing and verifying data. This blog will guide you through creating Merkle trees using MerkleTree.js and verifying them in Solidity. Whether you’re looking to enhance your project security or simply expand your knowledge, this comprehensive guide is for you!

Prerequisites

• Basic understanding of JavaScript and Solidity.
• Node.js installed on your machine.
• Access to Ganache or another Ethereum development environment.

Creating the Merkle Tree in JavaScript

First, we will create a Merkle tree using the MerkleTree.js library.

const MerkleProof = artifacts.require("MerkleProof");
const MerkleTree = require("merkletreejs");
const keccak256 = require("keccak256");

const contract = await MerkleProof.new();
const leaves = [a, b, c, d].map(v => keccak256(v));
const tree = new MerkleTree(leaves, keccak256, { sort: true });
const root = tree.getHexRoot();
const leaf = keccak256(a);
const proof = tree.getHexProof(leaf);
console.log(await contract.verify.call(root, leaf, proof)); // true

Understanding the Code: An Analogy

Think of a Merkle tree as a multi-layered cake. Each layer of the cake is represented by a hash, which is created by combining the ingredients (or data points, like ‘a’, ‘b’, ‘c’, ‘d’). Just as a baker needs to ensure all layers are properly stacked to form a perfect cake, we need to ensure all hashes are correctly constructed to validate the entire structure of our Merkle tree.

In the code above:

• We start with individual ingredients (the leaves) and hash them using `keccak256` to form the base layer.
• These hashes are then combined and hashed again to form the higher layers until we get to the root hash (the top layer of our cake).
• To verify if a piece of cake (leaf) belongs to this multi-layer cake, we prepare a proof that allows us to check if it matches the root hash.

Verifying the Merkle Proof in Solidity

Next, let’s see how we can verify the Merkle proof in Solidity with the following contract code:

pragma solidity ^0.5.2;

contract MerkleProof {
  function verify(
    bytes32 root,
    bytes32 leaf,
    bytes32[] memory proof
  ) public pure returns (bool) {
    bytes32 computedHash = leaf;
    for (uint256 i = 0; i < proof.length; i++) {
      bytes32 proofElement = proof[i];
      if (computedHash == proofElement) {
        computedHash = keccak256(abi.encodePacked(computedHash, proofElement));
      } else {
        computedHash = keccak256(abi.encodePacked(proofElement, computedHash));
      }
    }
    return computedHash == root;
  }
}

Running the Test

Now that you have crafted and verified your Merkle tree, it’s time to run the test to ensure everything is functioning as expected.

bash make test

Troubleshooting Common Issues

As with any programming endeavor, you may encounter a few bumps along the road. Here are some common problems and their solutions:

• Error: Merkle proof validation failed. Ensure that the leaf being verified matches the one that was hashed.
• Error: Contract deployment failed. Check your Solidity version; this example uses version ^0.5.2.
• Unexpected output. Verify your leaves are correctly hashed using `keccak256` before creating the tree.

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Conclusion

Congratulations! You have successfully created a Merkle tree, learned how to verify it using Solidity, and understood the analogy that connects the code to the real world. At fxis.ai, we believe that such advancements are crucial for the future of AI, as they enable more comprehensive and effective solutions. Our team is continually exploring new methodologies to push the envelope in artificial intelligence, ensuring that our clients benefit from the latest technological innovations.