Construct Merkle Trees and verify proofs in JavaScript.

Contents

• Diagrams
• Install
• Documentation
• Test
• FAQ
• Notes
• Resources
• License

Diagrams

Diagram of Merkle Tree

Diagram of Merkle Tree Proof

Diagram of Invalid Merkle Tree Proofs

Diagram of Bitcoin Merkle Tree

Install

npm install merkletreejs

Documentation

Classes

MerkleTree

Objects

MerkleTree : object

Class reprensenting a Merkle Tree

MerkleTree

Kind: global class

• MerkleTree
  • new MerkleTree(leaves, hashAlgorithm, options)
  • .getLeaves() ⇒ Array.<Buffer>
  • .getLayers() ⇒ Array.<Buffer>
  • .getRoot() ⇒ Buffer
  • .getProof(leaf, [index]) ⇒ Array.<Object>
  • .verify(proof, targetNode, root) ⇒ Boolean

---

new MerkleTree(leaves, hashAlgorithm, options)

Constructs a Merkle Tree. All nodes and leaves are stored as Buffers. Lonely leaf nodes are promoted to the next level up without being hashed again.

Param	Type	Description
leaves	Array.<Buffer>	Array of hashed leaves. Each leaf must be a Buffer.
hashAlgorithm	function	Algorithm used for hashing leaves and nodes
options	Object	Additional options
options.isBitcoinTree	Boolean	If set to true, constructs the Merkle Tree using the Bitcoin Merkle Tree implementation. Enable it when you need to replicate Bitcoin constructed Merkle Trees. In Bitcoin Merkle Trees, single nodes are combined with themselves, and each output hash is hashed again.

Example

const MerkleTree = require('merkletreejs')
const crypto = require('crypto')

function sha256(data) {
  // returns Buffer
  return crypto.createHash('sha256').update(data).digest()
}

const leaves = ['a', 'b', 'c'].map(x => sha256(x))

const tree = new MerkleTree(leaves, sha256)

---

merkleTree.getLeaves() ⇒ Array.<Buffer>

Returns array of leaves of Merkle Tree.

Kind: instance method of MerkleTree Example

const leaves = tree.getLeaves()

---

merkleTree.getLayers() ⇒ Array.<Buffer>

Returns array of all layers of Merkle Tree, including leaves and root.

Kind: instance method of MerkleTree Example

const layers = tree.getLayers()

---

merkleTree.getRoot() ⇒ Buffer

Returns the Merkle root hash as a Buffer.

Kind: instance method of MerkleTree Example

const root = tree.getRoot()

---

merkleTree.getProof(leaf, [index]) ⇒ Array.<Object>

Returns the proof for a target leaf.

Kind: instance method of MerkleTree Returns: Array.<Object> - - Array of objects containing a position property of type string with values of 'left' or 'right' and a data property of type Buffer.

Param	Type	Description
leaf	Buffer	Target leaf
[index]	Number	Target leaf index in leaves array. Use if there are leaves containing duplicate data in order to distinguish it.

Example

const proof = tree.getProof(leaves[2])

Example

const leaves = ['a', 'b', 'a'].map(x => sha256(x))
const tree = new MerkleTree(leaves, sha256)
const proof = tree.getProof(leaves[2], 2)

---

merkleTree.verify(proof, targetNode, root) ⇒ Boolean

Returns true if the proof path (array of hashes) can connect the target node to the Merkle root.

Kind: instance method of MerkleTree

Param	Type	Description
proof	Array.<Object>	Array of proof objects that should connect target node to Merkle root.
targetNode	Buffer	Target node Buffer
root	Buffer	Merkle root Buffer

Example

const root = tree.getRoot()
const proof = tree.getProof(leaves[2])
const verified = tree.verify(proof, leaves[2], root)

---

MerkleTree : object

Class reprensenting a Merkle Tree

Kind: global namespace

• MerkleTree : object
  • new MerkleTree(leaves, hashAlgorithm, options)
  • .getLeaves() ⇒ Array.<Buffer>
  • .getLayers() ⇒ Array.<Buffer>
  • .getRoot() ⇒ Buffer
  • .getProof(leaf, [index]) ⇒ Array.<Object>
  • .verify(proof, targetNode, root) ⇒ Boolean

---

new MerkleTree(leaves, hashAlgorithm, options)

Constructs a Merkle Tree. All nodes and leaves are stored as Buffers. Lonely leaf nodes are promoted to the next level up without being hashed again.

Param	Type	Description
leaves	Array.<Buffer>	Array of hashed leaves. Each leaf must be a Buffer.
hashAlgorithm	function	Algorithm used for hashing leaves and nodes
options	Object	Additional options
options.isBitcoinTree	Boolean	If set to true, constructs the Merkle Tree using the Bitcoin Merkle Tree implementation. Enable it when you need to replicate Bitcoin constructed Merkle Trees. In Bitcoin Merkle Trees, single nodes are combined with themselves, and each output hash is hashed again.

Example

const MerkleTree = require('merkletreejs')
const crypto = require('crypto')

function sha256(data) {
  // returns Buffer
  return crypto.createHash('sha256').update(data).digest()
}

const leaves = ['a', 'b', 'c'].map(x => sha256(x))

const tree = new MerkleTree(leaves, sha256)

---

merkleTree.getLeaves() ⇒ Array.<Buffer>

Returns array of leaves of Merkle Tree.

Kind: instance method of MerkleTree Example

const leaves = tree.getLeaves()

---

merkleTree.getLayers() ⇒ Array.<Buffer>

Returns array of all layers of Merkle Tree, including leaves and root.

Kind: instance method of MerkleTree Example

const layers = tree.getLayers()

---

merkleTree.getRoot() ⇒ Buffer

Returns the Merkle root hash as a Buffer.

Kind: instance method of MerkleTree Example

const root = tree.getRoot()

---

merkleTree.getProof(leaf, [index]) ⇒ Array.<Object>

Returns the proof for a target leaf.

Kind: instance method of MerkleTree Returns: Array.<Buffer> - - Array of objects containing a position property of type string with values of 'left' or 'right' and a data property of type Buffer.

Param	Type	Description
leaf	Buffer	Target leaf
[index]	Number	Target leaf index in leaves array. Use if there are leaves containing duplicate data in order to distinguish it.

Example

const proof = tree.getProof(leaves[2])

Example

const leaves = ['a', 'b', 'a'].map(x => sha256(x))
const tree = new MerkleTree(leaves, sha256)
const proof = tree.getProof(leaves[2], 2)

---

merkleTree.verify(proof, targetNode, root) ⇒ Boolean

Returns true if the proof path (array of hashes) can connect the target node to the Merkle root.

Kind: instance method of MerkleTree

Param	Type	Description
proof	Array.<Buffer>	Array of proof Buffer hashes that should connect target node to Merkle root.
targetNode	Buffer	Target node Buffer
root	Buffer	Merkle root Buffer

Example

const root = tree.getRoot()
const proof = tree.getProof(leaves[2])
const verified = tree.verify(proof, leaves[2], root)

---

Test

npm test

FAQ

• Q: How do you verify merkle proofs in Solidity?
  • A: Check out the example repo merkletreejs-solidity on how to generate merkle proofs with this library and verify them in Solidity.

Notes

As is, this implemenation is vulnerable to a second pre-image attack. Use a difference hashing algorithm function for leaves and nodes, so that H(x) != H'(x).

Also, as is, this implementation is vulnerable to a forgery attack for an unbalanced tree, where the last leaf node can be duplicated to create an artificial balanced tree, resulting in the same Merkle root hash. Do not accept unbalanced tree to prevent this.

More info here.

Resources

• Bitcoin mining the hard way: the algorithms, protocols, and bytes

• Bitcoin Talk - Merkle Trees

• How Log Proofs Work

• Raiden Merkle Tree Implemenation

• Why aren't Solidity sha3 hashes not matching what other sha3 libraries produce?

• What is the purpose of using different hash functions for the leaves and internals of a hash tree?

• Why is the full Merkle path needed to verify a transaction?

• Where is Double hashing performed in Bitcoin?

License

MIT