Introduction
Bitcoin and cryptocurrencies made a lot of noise lately. I have been rather disappointed by the turn the cryptocurrencies took, from an amazing concept to what seems just another way to make quick money ( or not... ).
But I became very interested by the technologies enabling cryptocurrencies, and obviously by the concept of a blockchain. The concept is fascinating, and not limited to Bitcoin and friends. We could imagine many applications for such a technology. So, in a proper developer manner, I decided to code one blockchain, or what I think is a blockchain, to understand better what it is.
A simple project
So, what do we need to create a very simple blockchain?
- A block
A block is what the blockchain is made of. In our case, a block will be composed of a date, an index, some data ( a message in our case ), and the hash of the previous block.
- Cryptography
To keep informations secure, we need to encrypt our data. For our little project, we will use the js-sha256 package. This process will create a string of 64 characters. Ultimately, our blockchain will be a series of hashes, each composed of 64 characters. As I said earlier, we use the hash of the previous block to encrypt a new block ( that is why we call it a chain ).
- Difficulty and nonce
We don't just create one hash per block and that's it. A hash must be valid. In our case, a hash will be valid if the first four characters of our hash are 0. If our hash starts with '0000......', it is considered valid. This is called the difficulty. The higher the difficulty, the longer it takes to get a valid hash.
But, if the hash is not valid the first time, something must change in the data we use right? If we use the same data over and over, we will get the same hash over and over and our hash will never be valid. You are right, we use something called nonce in our hash. It is simply a number that we increment each time the hash is not valid. We get our data (date, message, previous hash, index) and a nonce of 1. If the hash we get with these is not valid, we try with a nonce of 2. And we increment the nonce until we get a valid hash.
- Genesis block
Their must be a first block in our chain. It is called the genesis block. Of course, this block can't use the hash of the previous block because it doesn't exist. We will just give it some arbitrary data to create its hash.
And that is pretty much what we need for our blockchain.
The methods
We will need a few methods to make a functional blockchain:
- initialize our blockchain => creates the genesis block
- hash our blocks => a function responsible for creating a valid hash
- check the validity of a hash => does our hash starts with 'OOOO' ?
- get the last hash => we need the previous hash to create a new block
- add a new block => We need to do that at one point, if we want a chain
THE COOOOODE !!
Let's get coding now.
For this little project, I will create two files, one called index.js and another called blockchain.js. The second one will hold our little module to create a blockchain. It's straightforward, let's take a look at it:
const sha256 = require('js-sha256').sha256
const blockchain = (function(){
const blocks = []
const initBlockchain = () => {
const data = 'Hello World!'
const timestamp = new Date()
const previousHash = 0
const index = 0
hashBlock(data, timestamp, previousHash, index)
}
const hashBlock = (data, timestamp, prevHash, index) => {
let hash = '', nonce = 0
while( !isHashValid(hash) ){
let input = `${data}${timestamp}${prevHash}${index}${nonce}`
hash = sha256(input)
nonce += 1
}
console.log(nonce)
blocks.push(hash)
}
const getLastHash = blocks => blocks.slice(-1)[0]
const isHashValid = hash => hash.startsWith('0000') // Difficulty
const addNewBlock = data => {
const index = blocks.length
const previousHash = getLastHash(blocks)
hashBlock(data, new Date(), previousHash, index)
}
const getAllBlocks = () => blocks
return {
initBlockchain,
getLastHash,
blocks,
getAllBlocks,
addNewBlock
}
})()
module.exports = blockchain
So, in this module, I have a few methods. At the top, I import the module that will handle the cryptography part. I have an empty array that will hold my blockchain's blocks, called blocks.
initBlockchain: This method starts the blockchain by creating the first block, the genesis block. I give it a timestamp, a message, the block's index in the blockchain ( 0 ) and a arbitrary previous hash because there are no previous blocks in the chain yet. With all these informations, I can now create the hash for the genesis block.
hashBlock: This method takes all the block's data and creates a hash. As you can see, the first time we run the function for a specific block, the nonce is set to 0. We encrypt our block and check if the hash is valid with isHashValid. In our case, a hash is valid if the four first characters are 0. This is called the difficulty. This is the problem we have to solve to make sure the block can be part of the blockchain. Once the hash is valid, we add it to our blocks array.
addNewBlock: This method is responsible for creating a new block. We only need to give it the message as an argument, because all the other arguments ( index, previousHash, and timestamp) can be found in the blockchain. The method calls hashBlock with the data to create and validate the new block.
getLastHash: The method I call to get the previous hash. We always need the previous hash to create a new block.
getAllBlocks: Just returns all the blocks currently in the blockchain
Great, so let's move to index.js to use our new blockchain!
const blockchain = require('./blockchain')
blockchain.initBlockchain()
blockchain.addNewBlock('First new block')
blockchain.addNewBlock('I love blockchains')
blockchain.addNewBlock('Make me a new hash!!')
console.log(blockchain.getAllBlocks())
We initialize our blockchain, then we create three new blocks. When I run this, I get the following chain in response:
Initializing the blockchain
139355
30720
68789
51486
[ '0000d87875f12e8c00d60cdfc8c21c4867eb1e732d3bb0e4d60bd0febcfafbaf',
'0000331d80f4e83461bad846e082baa08c5e739edfa19a4880c1dcbe4eed1984',
'00000dcab247410050e357158edc20555cc0110429023fdadb1d8cda3e06da5e',
'0000a16968811cf75c33d877e99f460d396c46b5485f669c8e55b193b862106d' ]
The array represent the four blocks. As you can see, every single one of them starts with four zeros, so every single hash is valid. If one of those hashes didn't start with four zeros, I would know right away the hash was invalid, therefore, the data in the corresponding block should probably not be trusted.
There are four numbers here: 139355, 30720, 68789, 51486. These are the nonce for each block. I printed them out to see how many times the function hashBlock ran to come to a valid hash.
The first block, the genesis block, ran 139355 times before having a valid hash! The second, 30720 times. The third 68789 times and the fourth 51486 times.
Conclusion
This is a very simple example of a blockchain. I'm pretty sure I missed a few things here. I also kept things pretty simple because hey, I'm learning! This little project made me understand a few things:
If one person decides to modify a previous block, she would have to change every single block after that one. Each block inherits from its parent ( previous hash ), so trying to cheat a blockchain seems complicated.
But if a majority of the blockchain's users decide to cheat, they could modify a previous block and all agree to change the rest of the blockchain accordingly. A blockchain seems to work only if the majority decides to follow the rules. Or you could end up with two different blockchains, one where the users decided to stick with the original data, and the other where the users decided to use the modified blockchain.
I've heard about the Bitcoin enormous use of power where it came to mining. Mining is the concept of solving the difficulty problem when you encrypt the data. You get the transaction and you try to find a valid hash for that block. As a reward for your effort, you get some bitcoin. I can only imagine the amount of power you would use when the blockchain becomes huge.
Well, that's about what I got from that. It made things a lot clearer for me. Feel free to correct me if i got things wrong!
Top comments (47)
Thanks for this "Blockchain made easy" article. Really interesting.
I don't quite get one part: from your example, only the hash of the data is stored in the blockchain; isn't the data supposed to be decrypted later? Where are the data messages ("transactions") stored then, if not in the block chain?
well hash defines a unique signature of a block, in real practice, the block also contains an index value, a data(record of transactions), a timestamp , and also the golden nonce value.The cryptographic hash dont account for decryption. Its like the address part of a data-structure, and the data is stored in the data part of the data-structure.
Actually the point of the blockchain is to secure the immutability of data, not to encrypt it.
I'm very interested about an answer to your question. How would you implement it in the current example?
I'm interested in an answer from the author before proposing mine.
Too bad we can't tag people
thank you for the post it really cleared many concepts i was struggling to understand.
here is the implementation in python
first file
blockchainLib.py
the second file
main.py
Been dabbling in cryptocurrency mining lately, and never really understood what was going on behind the scenes until reading this. Much thanks!
On a lark, I was inspired to draft a really simple implementation in PHP:
thanks for the article ;)
in ruby:
Awesome explanation, really helpful! I'm just wondering why a hash is only valid if it starts with 0000 characters. I'm also not clear on how to use this blockchain for, say creating my own cryptokittens or some other application. How are the hashes in a blockchain connected to actual data?
The 0000 characters are called the difficulty. I chose those characters. They do not mean anything in particular, I could have picked anything. In Bitcoin for example, there is a concept called mining, where you have to 'solve' this difficulty by finding a valid hash. If the answer is found too quickly, the difficulty will be augmented for the next blocks( say 5 zeroes instead of 4 ).
I'll make another article to explain the connexion between hashes and data, I am getting a few questions about this :)
Nice article! To help me learn this I pulled out the js-sha256 to rely on node's crypto module directly and made some async/functional updates of my own along the way. Could be an interesting comparison for anyone who wants to compare imperative vs. functional and brings up the issue seen in real cryptocurrencies where the first miner to get a valid proof of work causes others to start over now that their 'last hash' value has changed. Code is here:
github.com/alanguir/blockchain/blo...
The revolutionary part is the distributed ledger - i.e. decentralizing one block chain to many clients so that any transaction on any client can be verified against the history of transactions on other clients.
Thanks for the breakdown. It seems simple enough. What's the revolutionary part of blockchains then, the part that Satoshi figured out? Did Satoshi just put out a proof that this is a valid way to do things?
The revolutionary part of blockchain technology (or at least one of its revolutionary features) is that you never have to trust a single user of the network. You only have to trust the technology because if that works, it's impossible to cheat the system.
I don't really get it. You can explain a bit more please ?
I think it's about decentralization, in a p2p network everyone has a copy of the blockchain (in case) and if one changes its pair this will generate an incongruity with the other pairs scattered in the nodes (computer, device) of the network, and your fake couple it would be excluded, therefore, no one should worry if a node is honest because he himself would denounce and punish himself by excluding himself from the network.
Great article! I've been wanting an good intro into block chain and this was perfect. However, when I ran the index.js file,
blockchain.getAllBlocks()
returnedundefined
. I got it to work by updating that function toconst getAllBlocks = () => blocks
.You are right, I updated the code in the article. Thanks for the catch!