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Part One: DIY Blockchain

What better way to learn about blockchains than to build your own? In this section you will construct your own simple blockchain in the vein of Bitcoin and other cryptocurrencies. Using cryptographic private/public key pairs, users will be able to create transactions sending funds from their public key to that of another user. Like any other blockchain, these transactions will become a permanent immutable part of the ledger, protected by the hashes that link each block to the one that came before it in the chain.

Contents

Getting Started and Running Tests

This section uses Node and npm to install dependencies and run tests. To begin, first install Node 8 or higher. Then run these commands from your terminal:

cd code/part-one/
npm install
npm test

You should see a number of tests run, most of which are failing.

(╯︵╰,)

Your job, in short, is to make these tests pass. As you work your way through the sections below, keep running npm test to check your progress and get clues to what you should do next.

The Curriculum

Blockchain Lecture

The first part of the Cryptomoji lecture is a brief overview of the blockchain data structure and the technologies that power it: hashing, signatures, and consensus. The lecture itself is 15 minutes long with an additional 10 minutes of Q&A. They are included in this repo as two MP4 files:

In addition to the video, the slides are available in a variety of formats. The general blockchain section is the first 8 slides:

Zulfikar Ramzan: Bitcoin

For a deeper dive, follow up the lecture with Zulfikar Ramzan's excellent Khan Academy course about the inner workings of the Bitcoin protocol. While your blockchain will have some differences from Bitcoin (in particular the way "inputs" and "outputs" are handled), there will be a lot of overlap:

You don't necessarily need to take the whole course, but makes sure you watch (and absorb!) the videos on hashing, signatures, and transaction blockchains. If you are going to tackle the extra credit, you will also want to watch the videos on proof of work and the money supply.

The Project

The intro blockchain project is broken into four modules, one of which is extra credit.

01 Signing

Module: signing.js

Useful APIs:

Signatures form the basis for verifiable identity and correctness on most, if not all, blockchains. First, a private key is generated; it's basically just a random set of bytes. This key is kept secret. Next, a cryptographic algorithm uses these bytes to derive a public key which can be widely shared. Finally, a signature is generated by combining the private key with some message. This signature, the message, and the public key are then all distributed together.

private key             message    private key
    |                         \     /
    v                          v   v
public key                   signature

While others won't ever be able to deduce the original private key, they will be able to confirm that the public key and signature came from the same private key and that the message was not altered. Not even a single byte. This powerful cryptographic tool is fundamental to how blockchains work.

public key -
             \
 signature - - - > ???
             /
   message -

In this section, you will build a simple signing API using Secp256k1, a common cryptographic algorithm used by Bitcoin, Ethereum, and Hyperledger Sawtooth. The underlying math is rather complex, so we will be relying on the library secp256k1-node to do the heavy lifting for us. Make sure you familiarize yourself with its API.

Note that this library uses Node Buffers (basically raw bytes) as the format of choice for keys and signatures. One of your jobs will be to convert these bytes to and from hex strings, which are slightly more convenient for our purposes. Make sure you are familiar with Buffer's from and toString methods.

You will be implementing four stub methods:

  • createPrivateKey
  • getPublicKey
  • sign
  • verify

02 Blockchain

Module: blockchain.js

Useful APIs:

Before you can understand the blockchain data structure itself, you need to be familiar with the concept of hashing: creating a deterministic digest of some arbitrary data. Importantly, while the same data will always produce the same hash, even a small change in the underlying data will create a completely different digest:

SHA-512: 'Hello, World!' -> '374d794a95cdcfd8b35993185fef9ba368f160d8daf432d08ba9f1ed1e5abe6cc69291e0fa2fe0006a52570ef18c19def4e617c33ce52ef0a6e5fbe318cb0387'

SHA-512: 'Hello, World?' -> '04d176b6977a4ee37d66e6c5b4a6cb9df46f73b453441af997b27f5f82c36bb18308b6ff5d29e4189fa41553e7ae7246db0482c9b78e42cbedc727f2ad639d9f'

You are free to use any hashing algorithm you like for this project, but your best bet is to familiarize yourself with Node's crypto module. In particular, use it to create SHA-512 hashes. This will come up later when you start working with Hyperledger Sawtooth.

Now that you understand hashing, a blockchain should actually be rather straightforward. It's just bundles of data linked sequentially by hashes of that data. Start with a "genesis" block. This is the only block which won't be linked to a previous hash. Then gather some data into a new block, combine it with the genesis hash, and create a new hash. When more data comes in, repeat the process: bundle the data into a new block, combine it with the previous hash, and generate a new hash.

[
  {
    data: '',
    hash: 'cf83e1357eefb8bd...'  // <- SHA-512: ''
  },
  {
    data: 'foo',
    previousHash: 'cf83e1357eefb8bd...',
    hash: '0bfc4817f6e1e5f3...'  // <- SHA-512: data + previousHash
  },
  {
    data: 'bar',
    previousHash: '0bfc4817f6e1e5f3...',
    hash: 'cef9981655e46b59...'  // <- SHA-512: data + previousHash
  }
]

Now all of your data is linked all the way back to your original genesis block. If anyone attempts to tamper with the data in a block, the hashes will also have to change. Anyone checking the chain of hashes would immediately see that one does not match. In order to alter old data, you would have to modify not only the target block, but every block that comes after it.

You will implement your blockchain with three related ES6 classes:

  • Transaction: A signed transfer of funds from one public key to another
  • Block: A hashed collection of transactions with a previous hash
  • Blockchain: An ordered collection of blocks, with a method to calculate balances

03 Validation

Module: validation.js

Blockchain validation is a huge and varied topic. In Sawtooth, validation is so important that the central component is named a "validator". For this section, you get to be your own validator and finally verify all of the cryptographic boilerplate that you've been including with your blocks and transactions.

You can use your signing.verify method to ensure that none of your transactions have been tampered with. You should get a similar assurance for your blocks if you recreate the hashes and make sure they match. Also, of course, the chain of previousHashes should be unbroken all the way back to your genesis block.

You'll be completing four methods, one to validate each data structure you made, and one more just for fun, to try to tamper with your own blockchain:

  • isTransactionValid
  • isBlockValid
  • isChainValid
  • breakChain

Extra Credit

While the next module can help you gain a deeper understanding of consensus algorithms, and Proof of Work in particular, you can feel free to move on to part two at this point. This section is strictly optional.

To run the tests for this extra credit, remove the .skip from the wrapping describe block on Line 14 of tests/04-ExtraCredit-Mining.js.

04 Mining

Module: mining.js

All this validation stuff is great, but what is to stop someone from coming in and replacing huge sections of the blockchain with their own valid blocks and transactions? That is where consensus comes in. A good consensus algorithm like Proof of Work or Proof of Elapsed Time (one of the algorithms available to Sawtooth), feature Byzantine Fault Tolerance: they not only ensure correctness, but also prevent bad actors from taking over the system and rewriting large sections of the blockchain. Zulfikar Ramzan does an excellent job of explaining how this works for Bitcoin, but the short version is that if you randomize who gets to create new blocks, and always select the longest chain available, a bad actor would need to take over 51% of the network to be effective. In a highly decentralized network, this is hopefully impractical.

So now it's your turn. You will create some tweaked versions of your original blockchain structure around a new method: MineableChain.mine. This method will allow your miners to solve the same cryptographic problem that Bitcoin miners do, building your blockchain as they go, and rewarding themselves for their efforts. Bonus: This means there is now a source for new funds on your blockchain, so you can also check that no one has a negative balance.

Three new classes:

  • MineableTransaction
  • MineableBlock
  • MineableChain

One new validation method:

  • isMineableChainValid