Understanding and Implementing a Merkle Tree in JavaScript

In this tutorial, we’ll explore the concept of a Merkle Tree and implement it in JavaScript. A Merkle Tree is a fundamental data structure used in cryptography and distributed systems to efficiently verify the integrity and consistency of data.

Prerequisites

Before we begin, ensure you have the following:

• Basic knowledge of JavaScript
• Node.js installed on your machine
• A text editor or IDE of your choice

Step 1: Setting Up the Project

1. Create a New JavaScript File: Open your text editor and create a new file named merkleTree.js.

2. Copy the Initial Code: Copy the following JavaScript code, which defines the Node and MerkleTree classes along with utility functions for hashing:

const crypto = require('crypto');<br /> class Node {<br />    constructor(left, right, value) {<br />        this.left = left;<br />        this.right = right;<br />        this.value = value;<br />    }<br />     static hash(val) {<br />        return crypto.createHash('sha256').update(val).digest('hex');<br />    }<br />     static doubleHash(val) {<br />        return Node.hash(Node.hash(val));<br />    }<br />}<br /> class MerkleTree {<br />    constructor(values) {<br />        this.buildTree(values);<br />    }<br />     buildTree(values) {<br />        let leaves = values.map(e => new Node(null, null, Node.doubleHash(e)));<br />         if (leaves.length % 2 === 1) {<br />            leaves.push(leaves[leaves.length - 1]); // duplicate last element if odd number of elements<br />        }<br />         this.root = this.buildTreeRec(leaves);<br />    }<br />     buildTreeRec(nodes) {<br />        let half = Math.floor(nodes.length / 2);<br />         if (nodes.length === 2) {<br />            return new Node(nodes[0], nodes[1], Node.doubleHash(nodes[0].value + nodes[1].value));<br />        }<br />         let left = this.buildTreeRec(nodes.slice(0, half));<br />        let right = this.buildTreeRec(nodes.slice(half));<br />         let value = Node.doubleHash(left.value + right.value);<br />        return new Node(left, right, value);<br />    }<br />     printTree() {<br />        this.printTreeRec(this.root);<br />    }<br />     printTreeRec(node) {<br />        if (node !== null) {<br />            console.log(node.value);<br />            this.printTreeRec(node.left);<br />            this.printTreeRec(node.right);<br />        }<br />    }<br />     getRootHash() {<br />        return this.root.value;<br />    }<br />}<br /> function test() {<br />    let elems = ["Hello", "mister", "Merkle", "tree"];<br />    let mtree = new MerkleTree(elems);<br />    console.log(mtree.getRootHash());<br />}<br /> test();

Step 2: Explaining the Code
Node Class

• Constructor: The Node class represents a node in the Merkle Tree. Each node has left and right children (left and right) and a value.

• Static Methods:

  • hash(val): Computes the SHA-256 hash of a given value using Node.js’s crypto module.
  • doubleHash(val): Computes the double SHA-256 hash of a given value by calling hash twice.

MerkleTree Class

• Constructor:

  • Initializes a new Merkle Tree instance by calling buildTree with an array of values.

• buildTree(values):

  • Constructs the Merkle Tree structure from an array of values.
  • Generates leaf nodes from each value and ensures the number of leaves is even.
  • Calls buildTreeRec to recursively build the tree structure.

• buildTreeRec(nodes):

  • Recursively builds the Merkle Tree from a list of Node objects.
  • Splits the list into halves until base cases (length of 2) are reached, then creates parent nodes with double-hashed values.

• printTree() and printTreeRec(node):

  • Helper methods to print the structure of the Merkle Tree for debugging purposes.

• getRootHash():

  • Returns the root hash of the Merkle Tree, which is the hash of all concatenated values in the tree.

Test Function

• test():
  • Demonstrates how to create a Merkle Tree instance with example elements "Hello", "mister", "Merkle", "tree".
  • Prints the root hash of the Merkle Tree instance.

Step 3: Running the Code

1. Save Your File: Save merkleTree.js after pasting the code.

2. Open Terminal/Command Prompt: Open a terminal or command prompt.

3. Navigate to Directory: Use the cd command to navigate to the directory where merkleTree.js is saved.

    
   <span class="hljs-built_in">cd</span> path/to/your/directory


4. Execute the Code: Run the following command to execute the JavaScript file:

   node merkleTree.js


5. Expected Output: You should see the root hash of the Merkle Tree printed in the terminal, which verifies the integrity of the data elements.

Conclusion

In this tutorial, we’ve explored the Merkle Tree data structure and implemented it in JavaScript. We learned about the Node and MerkleTree classes, static hashing methods, and how to construct and verify a Merkle Tree. You can now use this implementation for cryptographic proofs, data verification in distributed systems, and more.

Feel free to experiment further with different values and understand how the Merkle Tree adapts to changes in data elements.