An Efficient Unidirectional Micropayment Channel on Ethereum

This is the accompanying code for the article "An Efficient Unidirectional Micropayment Channel on Ethereum". It consists of:

• Four Solidity contracts: EthWord, Pay50, PayMerkle, PayMerkleExtension.
• Helper scripts in order to test PayMerkle contract.

Test Case

Senario

Assume that Alice runs an online service for which she accepts (Ether) payments on a micro-level (i.e., a very low fraction of Ether that costs less than one dollar). Bob is interested in that service and he wants to utilize it.

requirements

The following programs are required to test this senario:

1. Nodejs
2. Web3
3. ethereumjs-util

Channel Setup (Running by Bob)

Suppose that Bob estimates the number of maximum payments he is willing to make is 4096 with a minimum unit of payments of 1 Wei. He create a Merkle Tree with number of leaves equal to channelCapacity = 4096 by executing the following code on his Nodejs console.

const { MerkleTree } = require('../helpers/merkletree.js');
const Web3 = require('web3');
const { keccak256, bufferToHex } = require('ethereumjs-util');
const crypto = require('crypto');   
const web3 = new Web3(new Web3.providers.HttpProvider('http://127.0.0.1:8545'));
//Create MerkleTree
const channelCapacity = 4096; 
const leaves =[];
const seed = crypto.randomBytes(256);
let rn = keccak256(seed);
for(i = channelCapacity; i > 0; i--){
     let a = web3.eth.abi.encodeParameter('uint256',web3.utils.toWei(`${i}`, "wei"));
     let b = bufferToHex(rn);
     let ab = a + b.slice(2);
     leaves.unshift(ab);//push at the beginning of the array 
     rn = keccak256(rn);
}
const merkleTree = new MerkleTree(leaves);
const treeRoot = merkleTree.getHexRoot();
console.log("seed : " + bufferToHex(seed));
console.log("Tree Root : " + treeRoot);

Channel Deployment (Running by Bob)

Bob deploys PayMerkle smart contract on Ethereum to act as a trusted third party. The mart contract constructor parameters are:

1. Alice's address on Ethereum.
2. A timeout value before the channel is closed.
3. The root of the Merkle tree treeRoot.

Also, Bob has to send the amount of 4096 Wei to the smart contract to be held in escrow and pay Alice when she submits a valid Merkle proof.

Off-chain Payments (Running by Bob and Alice)

Alice and Bob are performing transactions off-chain. Every time Bob wants to utilize Alice's service, he sends her a new Merkle proof proof for an amount of i of Wei. To obtain the Merkle proof, he executes the following code on his previously opened Nodejs console:

const claimedLeaf = leaves[i - 1];
const claimedValue = claimedLeaf.slice(0,66);
const randomValue = '0x'+claimedLeaf.slice(66);
const proof = merkleTree.getHexProof(claimedLeaf);

Then, Bob sends the values of claimedValue, randomValue, proof to Alice. From her side, she verifies that claimedValue||randomValue is a part of the Merkle tree depolyed in the smart contract by calls VerifyMerkleProof function on her Nodejs console, and based on the output she decides to accept or reject.

const { keccak256, bufferToHex } = require('ethereumjs-util');
function VerifyMerkleProof(_claimedValue, _randomValue, _proof, _treeRoot) {
	let node = bufferToHex(keccak256(_claimedValue + _randomValue.slice(2)));
        for (i = 0; i < _proof.length; i++) {
          let proofElement = _proof[i];
          if (node < proofElement)
            node = bufferToHex(keccak256(node + proofElement.slice(2)));
          else
            node = bufferToHex(keccak256(proofElement + node.slice(2)));
          }
         if (node == treeRoot)
	    return true;
	 else
	    return false;
     }  

console.log(VerifyMerkleProof(claimedValue, randomValue, proof, treeRoot))  

Channel Close (Runing by Alice)

Once, Alice and Bob agree that there is no more transactions going between them, Alice invokes the CloseChannel function on PayMerkle smart contract by sending the latest values of claimedValue, randomValue, proof (the proof for largest amount) to the smart contract which will verify it and release the payment to Alice.