Udacity Blockchain Capstone

The capstone builds upon the knowledge gained in the blockchain course in order to build a decentralized housing product. Each house is represented by an ERC721 non-fungible token (NFT). Before a token can be minted, the owner needs to be verified using zkSnarks.

Install

Download or pull the code from from this repo above. Then, open it locally within Visual Studio Code (VS Code) as its own workspace. Next, open Terminal mode within VS Code and run the following to install all the proper node.js libraries. Importantly, make sure Truffle and Truffle-HDWallet libraries are installed:

npm install

1. Install and run Ganache (either from CLI or the user interface)

2. Inside the Terminal, cd to the "eth-contracts" directory then run:

truffle compile

This will create all of the smart contract artifacts in the proper directories.

3. Next, deploy to your local blockchain in Ganache. Make sure the proper localhost URL and port number to Ganache are specified in the truffle-config.js file. Also, make sure to copy the latest Ganache mnemonic to this same file:

truffle migrate --network development

Create a zkSnarks Proof Using Zokrates

zkSnarks is used for enhancing privacy and scalability in the blockchain space. It will be used to enhance the privacy of the real estate contracts in this project.

1. Install Docker to easily deploy Zokrates via a Docker container

2. Run Zokrates in the container using the following command line code:

docker run -v <path to your project folder>:/home/zokrates/code -ti zokrates/zokrates /bin/bash

3. There is a 5-step process to creating the correct Zokrates files:

a. Compile Program

b. Trusted Setup

c. Compute-Witness

d. Generate-Proof

e. Export-Verifier

4. Compile the program written in Zokrates DSL:

cd code/square

~/zokrates compile -i square.code

5. Generate the trusted setup:

~/zokrates setup

6. Compute the witness:

~/zokrates compute-witness -a 3 9

5. Generated proof:

~/zokrates generate-proof

5. Export verifier:

~/zokrates export-verifier

This should end up creating a file within the zokrates\code\square directory called "verifier.sol". This is the solidity file that is used to secure the contract via zkSnarks.

Testing

There are 3 separate unit test files to run. Start by testing the TestERC721Mintable class:

truffle test test/TestERC721Mintable.js

This should return 6 successful test results:

Next, test the TestSolSquareVerifier class:

truffle test test/TestSolSquareVerifier.js

This should return 2 successful test results:

Finally, test the TestSquareVerifier class:

truffle test test/TestSquareVerifier.js

This should return 2 successful test results:

Deployment to the Rinkeby Test Network

1. Goto Infura.io and create an account if you already have not done so. The Infura web service is a bridge between your application and the Rinkeby test blockchain network. Create a project in Infura and copy the Rinkeby network URL into the truffle-config.js file (or the hidden .env environment file).

2. Make sure Metamask is installed as a plug-in in the Chrome browser. Copy the Metamask mnemonic into the hidden environment file, too. Connect the Metamask account to Rinkeby.

3. Run the following command to create a new contract(s) on the Rinkeby network:

truffle migrate --network rinkeby --reset

Mint a Token Using Remix

The easiest way to interact with the smart contracts after deploying them to the Rinkeby network is by using Remix, the Solidity IDE. Go to https://remix.ethereum.org/ and create a new workspace. Then, copy the main ".sol" contract files located in the "eth-contracts/contracts" directory and also several supporting files. See the image below for all of the required files. You will also need to change some of the import statement relative directory locations in Remix:

Next, select the SolnSquareVerifier.sol file from the list and select the Solidity Compiler option on the far left then press the Compile button to compile this file. You may get some warnings but not errors if you properly updated the import statement relative file locations.

Next, select the Deploy and Run Transactions option. Make sure to select "Injected Web3", then select your account address from the dropdown. Select the SolnSquareVerifier contract in the "Contract" dropdown. Then, click the Deploy button to deploy it to Rinkeby. Under the list of Deployed Contracts, select the first option, then you should see a list of all of the exposed functions from this smart contract.

Under the Mint function option, enter the your account address and a token id (from 1 to 5). Then, click transact. This will create a new transaction that mints a token associated with an image (NFT) located at the URI specified in the code.

Here is what it looks like in Etherscan. You can see that 5 tokens have been minted:

Sell it on OpenSea

Now that the tokens have been minted and are assigned to real estate images, they can be offered for sale on OpenSea. Since we are using the test networks, the same can be done on the OpenSea network.

On the item detail page on OpenSea, click "Sell". This will walk you through the steps for selling an item. Note that the first time you auction an item, you will need to complete several MetaMask transactions in order to give the exchange contracts access to your items. After you complete these initial steps, creating an auction will only require signing a MetaMask message. This means that you can auction items without paying gas.

Once an account has been created on OpenSea, then the easiest way to assign tokenized objects to it is by including the contract address and token id in the proper url. Here's an example of one:

https://testnets.opensea.io/assets/0x0EB9a81A0b0b540beEFc7a41F7c4Fe40488769Bd/1

The first part of the URL includes the Rinkeby contract address. The second part includes the token id. Below is a screenshot of the tokenized images in Opensea:

Below is an example of one of the tokenized images for sale:

I sold 5 of the tokenized images to one test account (0xe77f929f093415d2d7221ec961394e8b33ddb066):

Below is the Etherscan list of the 5 tokenized images transferred to this account (0xe77f929f093415d2d7221ec961394e8b33ddb066):

Other Information

Here is some additional information about this project:

Contract address on Rinkeby for CustomERC721Token: 0x32f2330b649A9bFCe5C90B2EaaF10dc8998cc03B

Contract address on Rinkeby for SolnSquareVerifier: 0x0EB9a81A0b0b540beEFc7a41F7c4Fe40488769Bd

Link to OpenSea repository: https://testnets.opensea.io/collection/unidentified-contract-eu35jzqksd

All ABI files are in the /eth-contracts/build/contracts subdirectory

Project Resources

• Remix - Solidity IDE
• Visual Studio Code
• Truffle Framework
• Ganache - One Click Blockchain
• Open Zeppelin
• Interactive zero knowledge 3-colorability demonstration
• Docker
• ZoKrates