This walkthrough is based on this Medium post by Manuel Araoz.
Proof of Existence is a service that verifies the existence of a document or file at a specific time via timestamped transactions. PoE utilizes the one-way nature of cryptographic hash functions to reduce the amount of information that needs to be stored on the blockchain.
In this walkthrough, we are going to introduce some basic Ethereum smart contract development best practices by creating a simple proof of existence contract and interacting with it on the blockchain.
To develop Ethereum applications, you will need a client to connect to an Ethereum blockchain. You can use Geth, Parity or a development blockchain such as Ganache.
In this walkthrough we will be using the Ganache command line interface, ganache-cli. You can find the documentation on ganache-cli here.
You can install ganache-cli with the following command:
$ sudo npm install -g ganache-cli
And start ganache-cli with
$ ganache-cli
When ganache-cli starts, you can see that it starts with 10 accounts. Each of those accounts comes prefunded with Ether so we do not need to mine or acquire funds from a faucet.
Ganache-cli starts a development blockchain that needs to be running while develop the application so leave it running while we continue working on the application.
Solidity is the most popular programming language for writing smart contracts in Ethereum. We will be using Solidity throughout this course.
The Truffle development framework is one of the most popular development tools for writing Solidity smart contracts for Ethereum. Truffle will help us compile, deploy and test our smart contracts once they are written.
To install truffle
$ sudo npm install -g truffle
Create a project directory and set up truffle
$ mkdir proof-of-existence
$ cd proof-of-existence/
$ truffle init
Truffle sets up a contracts directory where we will write our contracts, a migrations directory where we will write scripts to deploy our contracts and a test directory where we will write tests to make sure that our contracts work as expected.
The project also comes with truffle-config.js and truffle.js configuration files. These files serve the same purpose, but the project initializes with both because of naming conflicts on Windows machines. We will use truffle-config.js, so you can delete truffle.js. In truffle-config.js we need to specify the network that we will be using.
In the module.exports object, add the following information:
module.exports = {
networks: {
development: {
host: “localhost”,
port: 8545,
network_id: “*”
}
}
};
We use this information because ganache-cli is running our development blockchain on localhost:8545, and this is what we want to connect to. Specifying network_id as * means that any truffle will deploy to any network running at localhost:8545.
Truffle comes with a Migrations.sol contract that keeps track our migrations as well as a 1_initial_migrations.js script to deploy the Migrations.sol contract.
Run the following command in the terminal in your project directory:
$ truffle migrate
The terminal should print “Using network ‘development’. Network up to date.”
This means that you environment is set up correctly and we can move on to writing our smart contracts.
Run the following command in the terminal in your project directory:
$ truffle create contract ProofOfExistence1
This command will create a Solidity file in the contracts directory called “ProofOfExistence1.sol” and set up the boilerplate code for the contract, a contract definition and a contract constructor. Open ProofOfExistence.sol in your text editor.
Update your ProofOfExistence1.sol file so that it looks like this.
We are starting with something simple, but incorrect and we are going to work towards a better contract.
Our contract has a state and two functions. This contract actually has two different kinds of functions, a transactional function (notarize) and a read-only, or view, function (proofFor). Transactional functions can modify state whereas constant functions can only read the state and return values.
Let’s deploy this contract to our test network.
Create a new migrations file in the migrations directory called 2_deploy_contracts.js. In this file add the following:
var ProofOfExistence1 = artifacts.require("./ProofOfExistence1.sol");
module.exports = function(deployer) {
deployer.deploy(ProofOfExistence1);
};This script tells Truffle to get the contract information from ProofOfExitence.sol and deploy it to the specified network. Now we just need to tell Truffle to run the deployment.
Run the following command in the terminal in your project directory:
$ truffle migrate
In the terminal output you should see that Truffle compiled ProofOfExistence.sol and printed some warning regarding the contract. Then it runs the migrations using the network ‘development’ that we specified in truffle-config.js.
Truffle remembers which contracts it has migrated to the network, so if we want to run the migration again on the same network, we need to use the --reset option like so:
$ truffle migrate --reset
You can find more information about truffle migrations here.
Our contract is now on the development blockchain, so we can interact with it. We can read the contract state from the blockchain and update the state by calling the notarize function. We can do this using the Truffle console.
Bring up the truffle console with the command:
$truffle console
You should see
truffle(development)>
On the first line enter:
var poe = ProofOfExistence1.at(ProofOfExistence1.address)
This line says that the variable “poe” is an instance of ProofOfExistence1.sol found at the address that we just deployed.
You can see the address by entering
truffle(development)> poe.address
'0xc490df1850010ea8146c1dd3e961fedbf6b85bef'
To call the notarize function, we call it like any other javascript function.
truffle(development)> poe.notarize(‘Hello World!’)
{ tx: '0x60ae...2643cbea65',
receipt: …
}
This function causes a state change, so it is a transactional function. Transactional functions return a Promise that resolves to a transaction object.
We can get the proof for the string with
truffle(development)> poe.proofFor(‘Hello World!’)
‘0x7f83b...126d9069’
And check that the contract’s state was correctly changed
truffle(development)> poe.proof()
‘0x7f83b...126d9069’
The hashes match!
Our contract works! But it can only store one proof at a time. Let’s change that.
Exit the Truffle console and create a new file called ProofOfExistence2.sol:
truffle(development)> .exit
$ truffle create contract ProofOfExistence2
Update the ProofOfExistence2 contract to match this contract.
The main changes between the first version and this version are that we changed the “proof” variable to a bytes32 array called “proofs” and made it private. We also added a function called “hasProof” to check if a proof has already been stored in the array.
Update the migration script 2_deploy_contracts.js to deploy the new contract and deploy it to the development blockchain.
$ truffle migrate --reset
Launch the console to interact with the new contract.
$ truffle console
Save the deployed contract
truffle(development)> var poe = ProofOfExistence2.at(ProofOfExistence2.address)
We can check for a proof.
truffle(development)> poe.checkDocument(“Hello World!”)
false
It returns false because we haven’t added anything yet. Let’s do that now.
truffle(development)> poe.notarize(“Hello World!”)
{ tx: '0xd6f72...10a6e',
receipt:
{ transactionHash: ...
logs: [] }
truffle(development)> poe.checkDocument(“Hello World!”)
true
We can check to make sure that our contract will store multiple proofs.
truffle(development)> poe.notarize(“Hello Consensys!”)
{ tx: '0x8b566...091ace',
receipt:
{ transactionHash: ...
logs: [] }
truffle(development)> poe.checkDocument(“Hello Consensys!”)
True
Looping over arrays in smart contracts can get expensive as arrays get longer. Using a mapping is a better solution.
Let’s create a final version in ProofOfExistence3.sol using mappings. You can use this code for the contract.
Modify the deployment script to deploy the new contract and test it in the console to make sure that it behaves just like ProofOfExistence2.sol.
From here, we are going to deviate from the Zeppelin Solutions walkthrough. If you want to learn how to deploy contracts using the geth console (which requires syncing with the testnet), you can consult the Zeppelin Solutions walkthrough.
The first step in our alternate deployment method is to get an Ethereum account on Metamask. On the landing page, click “Get Chrome Extension.”
Once the extension is installed, accept the terms of use and enter a password for your account.
You will be shown 12 words that can be used to restore your wallet. A word of caution, do NOT publish these words anywhere public. Anyone that has these 12 words has access to your wallet. Save these 12 words in a variable called mnemonic in your truffle-config.js file.
// truffle-config.js
var mnemonic = “amateur fan ... insect police”Truffle will use these words to access your wallet and deploy the contract.
Deploying the contract requires us to make a transaction on the testnet, so we need some ether to pay for the transaction. You can get free Rinkeby ether by going to this website and following the instructions. Make sure you enter the Ethereum address for your 1st Metamask account.
Now that we have a testnet account with Ether, we need to configure Truffle to be able to deploy the contract.
To deploy contracts to the testnet using Truffle without having to sync a local node, you can use Infura. Infura allows you to access a fully synced Ethereum node via their API. We will use their API to deploy our contracts to the Rinkeby testnet.
Go to the Infura website and sign up for a free account. Save the Rinkeby test network URL that Infrua provides in a variable called infura in truffle-config.js.
// truffle-config.js
var infura = "https://rinkeby.infura.io/Umc...8z"For Truffle to derive our ethereum address from the mnemonic, we need to install the Truffle HD wallet provider. In the terminal located in the proof-of-existence project root run:
$ npm install truffle-hdwallet-provider
And import the HD wallet provider into your truffle-config.js file.
var HDWalletProvider = require("truffle-hdwallet-provider")Now we just need to add the rinkeby network configuration to the networks object in truffle-config.js module.exports.
Change the module.exports object to resemble this:
networks: {
development: {
host: "localhost",
port: 8545,
network_id: "*"
},
rinkeby: {
provider: new HDWalletProvider(mnemonic, infura),
network_id: "4",
gas: 4500000
}
}Where the “mnemonic” variable is the 12 word seed phrase that you saved from metamask and the “infura” variable is your infura API endpoint URL.
You just have to run “truffle migrate” for the correct network and your contract will be deployed!
$ truffle migrate --network rinkeby
Using network 'rinkeby'.
Running migration: 1_initial_migration.js
Replacing Migrations...
... 0xb2d008d20b247444705ef40bbcb325a6fbb1f21c333753408cf1d04f77f10818
Migrations: 0x75d1d15f6e4be82683c162b26471c84ffa1b890e
Saving successful migration to network...
... 0x9f16180ffe6eb9125bd4c3dbbe6c9336553760e93317c866de2b498a586bdcf0
Saving artifacts...
Running migration: 2_deploy_contracts.js
Replacing ProofOfExistence2...
... 0x337bdcce47003c4426d5eea0c86df86559c3520d1788fb1af0ec1662ca227546
ProofOfExistence2: 0xdc5921ba88697df7198b73d0a40fc27fa80d8e5d
Saving successful migration to network...
... 0xfeff42682a4d9e66ca362a539b32cc433b1093b94997f75ab46eec914a508b86
Saving artifacts...
The terminal prints the addresses of the deployed contracts as well as the transaction hashes of the deployment transactions. This information can also be referenced in the contract artifacts, which are stored in proof-of-existence/build/contracts/. Deployment information is found at the bottom of each JSON file.