MerkleTree.js

Construct Merkle Trees and verify proofs in JavaScript.

Contents

• Install
• Getting started
• Diagrams
• Documentation
• Test
• FAQ
• Notes
• Resources
• Contributing
• License

Install

From NPM:

npm install merkletreejs

CDN

Available on sDelivr CDN:

<script src="https://cdn.jsdelivr.net/npm/merkletreejs@latest/merkletree.js"></script>

Getting started

Construct tree, generate proof, and verify proof:

const { MerkleTree } = require('merkletreejs')
const SHA256 = require('crypto-js/sha256')

const leaves = ['a', 'b', 'c'].map(x => SHA256(x))
const tree = new MerkleTree(leaves, SHA256)
const root = tree.getRoot().toString('hex')
const leaf = SHA256('a')
const proof = tree.getProof(leaf)
console.log(tree.verify(proof, leaf, root)) // true


const badLeaves = ['a', 'x', 'c'].map(x => SHA256(x))
const badTree = new MerkleTree(badLeaves, SHA256)
const badLeaf = SHA256('x')
const badProof = tree.getProof(badLeaf)
console.log(tree.verify(badProof, leaf, root)) // false

Print tree to console:

MerkleTree.print(tree)

Output

└─ 311d2e46f49b15fff8b746b74ad57f2cc9e0d9939fda94387141a2d3fdf187ae
   ├─ 176f0f307632fdd5831875eb709e2f68d770b102262998b214ddeb3f04164ae1
   │  ├─ 3ac225168df54212a25c1c01fd35bebfea408fdac2e31ddd6f80a4bbf9a5f1cb
   │  └─ b5553de315e0edf504d9150af82dafa5c4667fa618ed0a6f19c69b41166c5510
   └─ 0b42b6393c1f53060fe3ddbfcd7aadcca894465a5a438f69c87d790b2299b9b2
      └─ 0b42b6393c1f53060fe3ddbfcd7aadcca894465a5a438f69c87d790b2299b9b2

Diagrams

▾ Visualization of Merkle Tree

▾ Visualization of Merkle Tree Proof

▾ Visualization of Invalid Merkle Tree Proofs

▾ Visualization of Bitcoin Merkle Tree

Documentation

Class: MerkleTree

Class reprensenting a Merkle Tree

namespace MerkleTree

Hierarchy

• MerkleTree

Index

Constructors

• constructor

Methods

• getDepth
• getHexLayers
• getHexLayersFlat
• getHexLeaves
• getHexMultiProof
• getHexProof
• getHexRoot
• getLayers
• getLayersAsObject
• getLayersFlat
• getLeaves
• getMultiProof
• getProof
• getProofFlags
• getProofIndices
• getRoot
• print
• toString
• verify
• verifyMultiProof
• bufferify
• getMultiProof
• isHexString
• print

Constructors

constructor

+ new MerkleTree(leaves: any[], hashAlgorithm: any, options: Options): MerkleTree

desc 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.

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 => keccak(x))

const tree = new MerkleTree(leaves, sha256)

Parameters:

Name	Type	Default	Description
leaves	any[]	-	Array of hashed leaves. Each leaf must be a Buffer.
hashAlgorithm	any	SHA256	Algorithm used for hashing leaves and nodes
options	Options	{}	Additional options

Returns: MerkleTree

Methods

getDepth

▸ getDepth(): number

getDepth

desc Returns the tree depth (number of layers)

example

const depth = tree.getDepth()

Returns: number

---

getHexLayers

▸ getHexLayers(): string[]

getHexLayers

desc Returns multi-dimensional array of all layers of Merkle Tree, including leaves and root as hex strings.

example

const layers = tree.getHexLayers()

Returns: string[]

---

getHexLayersFlat

▸ getHexLayersFlat(): string[]

getHexLayersFlat

desc Returns single flat array of all layers of Merkle Tree, including leaves and root as hex string.

example

const layers = tree.getHexLayersFlat()

Returns: string[]

---

getHexLeaves

▸ getHexLeaves(): string[]

getHexLeaves

desc Returns array of leaves of Merkle Tree as hex strings.

example

const leaves = tree.getHexLeaves()

Returns: string[]

---

getHexMultiProof

▸ getHexMultiProof(tree: Buffer[], indices: number[]): string[]

getHexMultiProof

desc Returns the multiproof for given tree indices as hex strings.

example

const indices = [2, 5, 6]
const proof = tree.getHexMultiProof(indices)

Parameters:

Name	Type	Description
tree	Buffer[]	-
indices	number[]	Tree indices.

Returns: string[]

• Multiproofs as hex strings.

---

getHexProof

▸ getHexProof(leaf: Buffer, index?: number): string[]

getHexProof

desc Returns the proof for a target leaf as hex strings.

example

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

Parameters:

Name	Type	Description
leaf	Buffer	Target leaf
index?	number	-

Returns: string[]

• Proof array as hex strings.

---

getHexRoot

▸ getHexRoot(): string

getHexRoot

desc Returns the Merkle root hash as a hex string.

example

const root = tree.getHexRoot()

Returns: string

---

getLayers

▸ getLayers(): Buffer[]

getLayers

desc Returns multi-dimensional array of all layers of Merkle Tree, including leaves and root.

example

const layers = tree.getLayers()

Returns: Buffer[]

---

getLayersAsObject

▸ getLayersAsObject(): any

getLayersAsObject

desc Returns the layers as nested objects instead of an array.

example

const layersObj = tree.getLayersAsObject()

Returns: any

---

getLayersFlat

▸ getLayersFlat(): Buffer[]

getLayersFlat

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

example

const layers = tree.getLayersFlat()

Returns: Buffer[]

---

getLeaves

▸ getLeaves(values?: any[]): Buffer[]

getLeaves

desc Returns array of leaves of Merkle Tree.

example

const leaves = tree.getLeaves()

Parameters:

Name	Type
values?	any[]

Returns: Buffer[]

---

getMultiProof

▸ getMultiProof(tree?: any[], indices?: any[]): Buffer[]

getMultiProof

desc Returns the multiproof for given tree indices.

example

const indices = [2, 5, 6]
const proof = tree.getMultiProof(indices)

Parameters:

Name	Type	Description
tree?	any[]	-
indices?	any[]	Tree indices.

Returns: Buffer[]

• Multiproofs

---

getProof

▸ getProof(leaf: Buffer, index?: number): any[]

getProof

desc Returns the proof for a target leaf.

example

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

example

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

Parameters:

Name	Type	Description
leaf	Buffer	Target leaf
index?	number	-

Returns: any[]

• Array of objects containing a position property of type string with values of 'left' or 'right' and a data property of type Buffer.

---

getProofFlags

▸ getProofFlags(leaves: Buffer[], proofs: Buffer[]): boolean[]

getProofFlags

desc Returns list of booleans where proofs should be used instead of hashing. Proof flags are used in the Solidity multiproof verifiers.

example

const indices = [2, 5, 6]
const proof = tree.getMultiProof(indices)
const proofFlags = tree.getProofFlags(leaves, proof)

Parameters:

Name	Type
leaves	Buffer[]
proofs	Buffer[]

Returns: boolean[]

• Boolean flags

---

getProofIndices

▸ getProofIndices(treeIndices: number[], depth: number): number[]

getProofIndices

desc Returns the proof indices for given tree indices.

example

const proofIndices = tree.getProofIndices([2,5,6], 4)
console.log(proofIndices) // [ 23, 20, 19, 8, 3 ]

Parameters:

Name	Type	Description
treeIndices	number[]	Tree indices
depth	number	Tree depth; number of layers.

Returns: number[]

• Proof indices

---

getRoot

▸ getRoot(): Buffer

getRoot

desc Returns the Merkle root hash as a Buffer.

example

const root = tree.getRoot()

Returns: Buffer

---

print

▸ print(): void

print

desc Prints out a visual representation of the merkle tree.

example

tree.print()

Returns: void

---

toString

▸ toString(): string

toString

desc Returns a visual representation of the merkle tree as a string.

example

console.log(tree.toString())

Returns: string

---

verify

▸ verify(proof: any[], targetNode: Buffer, root: Buffer): boolean

verify

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

example

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

Parameters:

Name	Type	Description
proof	any[]	Array of proof objects that should connect target node to Merkle root.
targetNode	Buffer	Target node Buffer
root	Buffer	Merkle root Buffer

Returns: boolean

---

verifyMultiProof

▸ verifyMultiProof(root: Buffer, indices: number[], leaves: Buffer[], depth: number, proof: Buffer[]): boolean

verifyMultiProof

desc Returns true if the multiproofs can connect the leaves to the Merkle root.

example

const root = tree.getRoot()
const treeFlat = tree.getLayersFlat()
const depth = tree.getDepth()
const indices = [2, 5, 6]
const proofLeaves = indices.map(i => leaves[i])
const proof = tree.getMultiProof(treeFlat, indices)
const verified = tree.verifyMultiProof(root, indices, proofLeaves, depth, proof)

Parameters:

Name	Type	Description
root	Buffer	Merkle tree root
indices	number[]	Leave indices
leaves	Buffer[]	Leaf values at indices.
depth	number	Tree depth
proof	Buffer[]	Multiproofs given indices

Returns: boolean

---

Static bufferify

▸ bufferify(value: any): Buffer

bufferify

desc Returns a buffer type for the given value.

example

const buf = MerkleTree.bufferify('0x1234')

Parameters:

Name	Type
value	any

Returns: Buffer

---

Static getMultiProof

▸ getMultiProof(tree: Buffer[], indices: number[]): Buffer[]

getMultiProof

desc Returns the multiproof for given tree indices.

example

const flatTree = tree.getLayersFlat()
const indices = [2, 5, 6]
const proof = MerkleTree.getMultiProof(flatTree, indices)

Parameters:

Name	Type	Description
tree	Buffer[]	Tree as a flat array.
indices	number[]	Tree indices.

Returns: Buffer[]

• Multiproofs

---

Static isHexString

▸ isHexString(v: string): boolean

isHexString

desc Returns true if value is a hex string.

example

console.log(MerkleTree.isHexString('0x1234'))

Parameters:

Name	Type
v	string

Returns: boolean

---

Static print

▸ print(tree: any): void

print

desc Prints out a visual representation of the given merkle tree.

example

MerkleTree.print(tree)

Parameters:

Name	Type	Description
tree	any	Merkle tree instance.

Returns: void

Interface: Options

Hierarchy

• Options

Index

Properties

• duplicateOdd
• hashLeaves
• isBitcoinTree
• sort
• sortLeaves
• sortPairs

Properties

Optional duplicateOdd

• duplicateOdd? : boolean

If set to true, an odd node will be duplicated and combined to make a pair to generate the layer hash.

---

Optional hashLeaves

• hashLeaves? : boolean

If set to true, the leaves will hashed using the set hashing algorithms.

---

Optional isBitcoinTree

• 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.

---

Optional sort

• sort? : boolean

If set to true, the leaves and hashing pairs will be sorted.

---

Optional sortLeaves

• sortLeaves? : boolean

If set to true, the leaves will be sorted.

---

Optional sortPairs

• sortPairs? : boolean

If set to true, the hashing pairs will be sorted.

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.

• Q: How do you verify merkle multiproofs in Solidity?

  • A: Check out the example repo merkletreejs-multiproof-solidity on how to generate merkle multiproofs 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?

• Compact Merkle Multiproofs

• Eth 2.0 specs - Merkle Multiproofs

Contributing

Pull requests are welcome!

For contributions please create a new branch and submit a pull request for review.

License

MIT