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Trying to understand blockchain by making one!

Damien Cosset on December 12, 2017

Introduction Bitcoin and cryptocurrencies made a lot of noise lately. I have been rather disappointed by the turn the cryptocurrencies t...

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Benoit Coste • Dec 17 '17

Thanks for this "Blockchain made easy" article. Really interesting.

Ivan Babak • Dec 18 '17

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?

tintybot • Oct 7 '18

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.

Anatoli • Jan 14 '18

Actually the point of the blockchain is to secure the immutability of data, not to encrypt it.

Pierre Bouillon • Jan 8 '18

I'm very interested about an answer to your question. How would you implement it in the current example?

Ivan Babak • Jan 8 '18

I'm interested in an answer from the author before proposing mine.

Pierre Bouillon • Jan 8 '18

Too bad we can't tag people

Ezzat • Mar 3 '18

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

import hashlib
import time

blocks = []


def encrypt_string(hash_string):
    sha_signature = \
        hashlib.sha256(hash_string.encode()).hexdigest()
    return sha_signature


def isValidhash(hash):
    if hash.startswith("0000"):
        return True
    return False


def hashBlock(data, timestamp, previoushash, index):
    _hash = ""
    nonce = 0
    while not isValidhash(_hash):
        _input = data + str(timestamp) + str(previoushash) + str(index) + str(nonce)
        _hash = encrypt_string(_input)
        nonce += 1
        print(nonce)
    blocks.append(_hash)


def getLastHash():
    return blocks[len(blocks) - 1]


def addNewBlock(mmessage):
    _index = len(blocks)
    timestamp = time.time()
    previousHash = getLastHash()
    hashBlock(mmessage, timestamp, previousHash, _index)


def getAllBlocks():
    for i in range(0, len(blocks)):
        print(blocks[i])


def initBlock():
    data = "hello world"
    timestamp = time.time()
    previoushash = 0
    index = 0
    hashBlock(data, timestamp, previoushash, index)

the second file

main.py

#!/bin/python3


import blockchainLib as bl

if __name__ == "__main__":

    bl.initBlock()
    bl.addNewBlock("hello world")
    bl.addNewBlock("hello world 2")
    bl.getAllBlocks()

Vincent Grovestine • Dec 19 '17 • Edited

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:

José Miguel Malaca • Jan 3 '18

thanks for the article ;)
in ruby:

require "digest"

class Blockchain

  def initialize(difficulty = "00")
    @blocks = []
    @difficulty = difficulty
    hash_block("I'm the Genesis block")
  end

  def add_new_block(data)
    hash_block(data, last_hash, @blocks.length)
  end

  def all_blocks
    @blocks
  end

  private

  def hash_block(data, previous_hash=0, index=0)
    hash = ""
    nonce = 1

    while(!valid_hash? hash)
      input = "#{data}#{timestamp}#{previous_hash}#{index}#{nonce}"
      hash = encode input
      nonce += 1
    end

    @blocks << hash
    {hash: hash, nonce: nonce}
  end

  def encode(input)
    Digest::SHA256.base64digest input
  end

  def valid_hash?(hash)
    hash.start_with? @difficulty
  end

  def last_hash
    @blocks.last
  end

  def timestamp
    Time.now.getutc
  end
end

eerk • Dec 19 '17

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?

Damien Cosset • Dec 20 '17

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 :)

Alan Languirand • Dec 17 '17 • Edited

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...

Alan Languirand • Dec 17 '17

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.

SkillSlide • Dec 13 '17

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?

Stefan Dorresteijn • Dec 13 '17

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.

Valentin Silvestre • Dec 14 '17

I don't really get it. You can explain a bit more please ?

The Dark Side • Jan 25 '18

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.

Shawn Leberknight • Dec 13 '17 • Edited

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() returned undefined. I got it to work by updating that function to const getAllBlocks = () => blocks.

Damien Cosset • Dec 13 '17

You are right, I updated the code in the article. Thanks for the catch!

Dhruv • Jan 15 '18

So a blockchain is an encrypted Linked List?

Damien Cosset • Jan 16 '18

Mhmm, I guess you could call it like that.

The Dark Side • Jan 25 '18

the blockchain itself is not encrypted, everyone can read (search: blockchain explore), it would be better to say that the blockchain is a list of concatenated hashes.

eva • Jan 8 '21

That is absolutely brilliant, Damien. Nothing like seeing it with your own eyes in order to understand it. Many thanks :-)

To help out anyone who is not at all familiar with node (hello, anyone there? ;-) ) here's a very simple way to run this:

Create a folder.
In it, create both files index.js and blockchain.js as instructed in the article. Copy & paste respective code.
Get a command prompt or a terminal to the folder that contains both files.
Run npm init -y (this will create the package.json file)
Run npm install js-sha256 (this will install the required module and update package.json)
Just run the project with node index.js

Hope it helps.

Alexander • Dec 18 '17

Thanks for this article, but i think the devil is in details. Blockchain itself is a simple pattern, but it has nothing to do with real world blockchain powered p2p networks. In real application you need to spend much more code and time with such things like establishing secure socket connection, managing resources and implementing an idea of your cryptosystem (what it is and why it should help people live their lives in happiness).

P.S. there is no need in self-executing function if you use a node's require/exports system.

Piotr Szymaszek • Jan 7 '18

Two things:

In the "Genesis block" section you have "Their must be", it should be "There must be".
"check the validity of a hash => does our hash starts with 'OOOO'" weirdly this gets formatted like it has four letters "O" instead of zeros?

Aside from that, it is a good article and thank you for posting it :)

vishnuvizz • Oct 14 '19

I have a doubt!! Bitcoin!!!!
If people from multiple mining pool trying to find nounce for certain block, then how are they going to agree to validate the same transaction in that block? if they didn't agree to mine with same transactions, then how is blockchain going to verify that they did valid mining(I meant not cheating). There is still a possibility that they can cheat right? If other mining pool people took diff set of transaction to mine for a block, then obviously they ll get diff hash and so they can't compare with them with actual hash. and If only one pool would get incentive (12.5BTC) then how after one mining a block, some other pool is going to waste its compute power in verifying the transaction. Even if it did so, how is it going to resist to change as change is already done in the blockchain.

Someone, please help me out...

Abigail • Dec 19 '17

this is pretty cool. thanks to Blockchain

Pratik Ambani • Dec 25 '17

Would anybody like to volunteer to write the same in Java?

Maciej Przerwa • Feb 16 '18 • Edited

import java.security.MessageDigest;
import java.security.NoSuchAlgorithmException;
import java.text.SimpleDateFormat;
import java.util.ArrayList;
import java.util.Calendar;
import java.util.List;

public class BlockchainExample {

//list of hash values
List<String> blocks = new ArrayList<String>();

public void initBlockchain() {
    String msg = "Hello world";
    String timeStamp = new SimpleDateFormat("yyyyMMdd_HHmmss").format(Calendar.getInstance().getTime());
    String previousHash = "0";
    String index = "0";

    hashBlock(msg, timeStamp, previousHash, index);
}

public void hashBlock(String msg, String timestamp, String prevHash, String index) {

    String hash = "";
    Integer nonce = 0;

    while(!isHashValid(hash)) {
        String input = msg+timestamp+prevHash+index+nonce.toString();
        hash = sha256(input);
        nonce+=1;
    }
    System.out.println(nonce.toString());
    blocks.add(hash);   
}

public String getLastHash() {
    return blocks.get(blocks.size()-1);
}

public boolean isHashValid(String hash) {
    if (hash.startsWith("0000")) return true;
    else return false;
}

public void addNewBlock(String msg){
    Integer in = blocks.size();
    String previousHash = getLastHash();
    String timeStamp = new SimpleDateFormat("yyyyMMdd_HHmmss").format(Calendar.getInstance().getTime());

    hashBlock(msg, timeStamp, previousHash, in.toString());
}


public void getAllBlocks() {
    for(int i=0;i<blocks.size();i++){
        System.out.println(blocks.get(i));
    } 
}


public String sha256(String password) {
    MessageDigest md = null;
    try {
        md = MessageDigest.getInstance("SHA-256");
    } catch (NoSuchAlgorithmException e) {
        // TODO Auto-generated catch block
        e.printStackTrace();
    }
    md.update(password.getBytes());

    byte byteData[] = md.digest();

    StringBuffer sb = new StringBuffer();
    for (int i = 0; i < byteData.length; i++) {
     sb.append(Integer.toString((byteData[i] & 0xff) + 0x100, 16).substring(1));
    }

    return sb.toString();
}


public static void main(String[] args) {

    BlockchainExample be = new BlockchainExample();

    be.initBlockchain();
    be.addNewBlock("First new block");
    be.addNewBlock("Second new block");

    be.getAllBlocks();
}

}

Eric7Giants • Apr 5 '19

New to Java and I was referencing this for a Security course. I'm getting BlockchainExample cannot be resolved to a type.

Maciej Przerwa • Apr 10 '19

Hi there, could you specify in which line you are getting this error?
And also, did you copy all the code? Not only part on black? There are lines at the top

import java.security.MessageDigest;
import java.security.NoSuchAlgorithmException;
import java.text.SimpleDateFormat;
import java.util.ArrayList;
import java.util.Calendar;
import java.util.List;

public class BlockchainExample {

and also one
}
at the end.
I have checked it minute ago - code works.

Grey • Dec 16 '17

Neat!!! Thanks for the the simplistic approach to explaining what one formerly considered complex. Cheers!

The Dark Side • Jan 25 '18

it is not complex, it is complicated, the blockchain is complex in nature because a block is linked to the previous one. I hope I have made the concept.

Vien • Jan 1 '18

Great article in simplifying a complicated process re: blockchain

Pierre Bouillon • Jan 8 '18

Very helpful article !
Not sure about it but I tried my own in Python3

Maciej Przerwa • Feb 16 '18

I don't get one thing. How to retrieve information from this Blockchain (the data Strings?)

Kartik Arora • Dec 19 '17

This post did what all whitepapers couldn't - Explain the concept!. Great article Damien, thank you so much!

Salim MAHBOUBI • Dec 17 '17

Great article.
I just have one small remark, the header image you used illustrates a kind of cyclic graph, which has nothing to do with blockchains as far as I know. This can be misleading.

eternalprofits • Jul 10 '18

Thank you for the article! very helpful.
Here is a block application building hackathon coming on this weekend July 14, 15. Let me know if anyone else is joining: bit.ly/2L2olS9