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How to Create a Simple Blockchain using Python

October 13, 2021

This article provides a step-by-step guide on how to create a simple blockchain using Python

This article discussed blockchain technology and its components in detail. In this tutorial, we will focus more on the implementation part.

Prerequisites

To follow along, you should have:

  • Some knowledge of Object-Oriented Programming Language with Python
  • A basic understanding of using Postman Desktop

Tools requirements

We make use of Flask to create a web application containing blockchain technology.

We also require Postman to make requests to interact with our blockchain. The GET request allows us to retrieve the actual state of our blockchain or to mine a block.

Table of contents

Building the architecture

Let’s proceed to build our first blockchain.

We begin by launching our IDE and installing Flask in our virtual environment.

After setting up our environment, we create a new file for our code, and name it blockchain.py.

Initializing packages

In the blockchain.py file, we import the following packages as they are required in building our blockchain:

import datetime
import json
import hashlib
from flask import Flask, jsonify

We use the DateTime library to attach a timestamp to each block that is created or mined.

The hahshlib will be used to hash a block, JSON will be used to encode the block before we hash it.

jsonify from the Flask library will be used to return messages to Postman.

The Genesis block

To start building our blockchain, we create a Blockchain class. The __init__ method will consist of a variable called chain to store a list of all the blocks in the blockchain.

The create_blockchain() method will allow us to create our Genesis block on instantiation of the class.

The create_blockchain() method will take two default arguments which are proof with a value of one(1), and the previous_hash with a value of zero(0).

This aspect of the code shows the importance of having a background on how blockchain works.

In a blockchain, there is always a first block called the Genesis block, and this block does not have any previous_hash.

Our __init__ method should look like this:

class Blockchain:
   def __init__(self):
       self.chain = []
       self.create_blockchain(proof=1, previous_hash='0')

The create_blockchain function

Next, we define a create_blockchain method that extends the Genesis block. The only difference here is that we will pass in three parameters which are self, proof, and previous_hash.

All parameters are without a default value.

Within the create_blockchain function, we include a block variable of type dictionary that will be used to define each block in a blockchain.

The dictionary will take the following key-value pairs:

  • Index: An index key will store the blockchain’s length. It is represented by the chain variable in the __init__ method with an added value of one(1). We will use this variable to access each block in the chain.

  • Timestamp: The timestamp key will take a value of the current Date and Time the block was created or mined.

  • Proof: This key will receive a proof value that will be passed to the function when called. Note that this variable refers to the proof of work.

  • Previous hash: Lastly, the previous hash key takes a value of previous_hash from the function which is equivalent to the hash of the previous block.

By adding these key-value pairs, we then append this block to the chain and return the block itself. Your create_blockchain() function should be similar to the one shown below:

def create_blockchain(self, proof, previous_hash):
   block = {
       'index': len(self.chain) + 1,
       'timestamp': str(datetime.datetime.now()),
       'proof': proof,
       'previous_hash': previous_hash
   }

   self.chain.append(block)
   return block

Accessing previous block

This process is easy. All we have to do is to create a method that gets the previous block in the chain.

We create a new variable and name it last_block and pass in a value of the last block in the list. Then we return the last_block.

Your code for this step should look like this:

def get_previous_block(self):
   last_block = self.chain[-1]
   return last_block

The proof of work function

In the create_blockchain() function, we had a variable called proof. This variable represents the proof of work done to mine a block.

As the programmer of the blockchain, we need to create an algorithm that the miners will solve to mine a block successfully. You can read more about this process here.

We start by creating a new method called proof_of_work() and then we pass two parameters which are self and previous_proof.

In the method, we create a variable to store the proof submitted by miners. We call it new_proof and set the value to one(1).

Next, we create a control statement to check the status of the proof of work, which by default will be False.

Therefore, we create a new variable called check_proof and assign it a False value. At this stage, your code for this function should look like this:

def proof_of_work(self, previous_proof):
   # miners proof submitted
   new_proof = 1
   # status of proof of work
   check_proof = False

Next, we proceed to the algorithm that needs to be solved by the miner. We encapsulate the algorithm in a while statement because this section should repeat until the proof is found.

We start by creating a while statement and set the condition of check_proof as False. If the program hasn’t checked the proof of work, then it has to run through the body of the while loop.

Next, in the while loop, we define the problem/algorithm to be solved which will be based on the previous proof that was successful and the new proof submitted by the miner.

In this aspect of the program, you can define how complex you want this problem to be. In this tutorial, we will make it simple to test if our code works.

Moving forward, we create a new variable called hash_operation, and we assign a value of hashlib.sha256(str(the algorithm).encode()).hexdigest().

This is how we encode the problem in a cryptographic hexadecimal digit with the use of the SHA256 hash library.

Now, we need to replace the algorithm for mining a block.

Below is my algorithm that needs to be solved for mining a block:

new_proof ** 2 - previous_proof ** 2

The algorithm takes the new proof submitted by the user and raises it to the power of 2, then subtracts it from the exponent of the previous proof raised to the power of 2.

Checking the miner’s Solution to the problem

Next, we evaluate the miner’s solution to the problem by checking the hash_operation first 4 characters.

For our code, we check if the first 4 characters are equal to zeros.

If the check returns True, then we’ve checked the proof, and it’s valid. We can then assign it a value of True.

If the result is False we increment the new_proof by 1 which gives the miner another chance to try again and then we return the new proof.

Your codebase should look this:

def proof_of_work(self, previous_proof):
   # miners proof submitted
   new_proof = 1
   # status of proof of work
   check_proof = False
   while check_proof is False:
       # problem and algorithm based off the previous proof and new proof
       hash_operation = hashlib.sha256(str(new_proof ** 2 - previous_proof ** 2).encode()).hexdigest()
       # check miners solution to problem, by using miners proof in cryptographic encryption
       # if miners proof results in 4 leading zero's in the hash operation, then:
       if hash_operation[:4] == '0000':
           check_proof = True
       else:
           # if miners solution is wrong, give mine another chance until correct
           new_proof += 1
   return new_proof

Why do we need four leading zeros for hash operation

Now, why four(4) leading zeros? why not five(5) or six()6 or more.

Note that the more leading zeros we require the more difficult it will be to mine a block. For the sake of this tutorial, we have it at 4 to ensure that we can mine a block faster.

Also, the hash_operation algorithm we pass in must be non-symmetrical such that if the order of operation were to be reversed it wouldn’t result in the same value. For instance, a+b is equal to b+a, but a-b is not equal to b-a.

Lastly, in our algorithm, we require both the new proof submitted and the previous proof from the existing block to effectively define an algorithm for miners to solve. The process is, therefore, continuous and linked to the existing blocks.

Generating a hash

In this step, we generate a hash for the entire block itself. We create a hash() method that takes two parameters (self and block).

In this function, we use JSON dumps to encode the block and return a cryptographic hash of the entire block.

Your code should look like this:

# generate a hash of an entire block
def hash(self, block):
   encoded_block = json.dumps(block, sort_keys=True).encode()
   return hashlib.sha256(encoded_block).hexdigest()

Checking the validity of the blockchain

In this step, we create a function that checks if the entire blockchain is valid.

This step is crucial in maintaining the integrity of our blockchain to ensure that none of our blocks is corrupt.

We start by creating a is_chain_valid method that takes self and chain as parameters.

We then retrieve the first block in the chain, which serves as the previous block to the current block:

previous_block = chain[0]

Next, we set an index value of 1 for the blocks in the chain for the sole purpose of iteration:

block_index =1

Here comes the complex part of this process.

We start by creating a while loop that takes a conditional statement of True if the block_index is equal to the length of the chain and False if the block_index is less than the length of the chain.

Inside our while loop, we retrieve the current block from the chain:

while block_index < len(chain):
block = chain[block_index]

Next, we check if the current block’s previous hash field is not similar to the hash field of the previous block. If it does, then it returns True else False:

if block["previous_hash"] != self.hash(previous_block):
   return False

Next, we get the proof from the previous block, and we also get the proof from the current block. Both values are required for our hash operation in our algorithm:

previous_proof = previous_block['proof']
current_proof = block['proof']

We further proceed by running the proof data through the algorithm, then we check if the first four leading characters are not equal to four zeros.

If the hash operation is invalid, then we return False, else, we set the previous_block value to the current block that just completed its check, and we increment the block index by 1, then return True as a positive validation check.

Your complete code should look like this:

# check if the blockchain is valid
def is_chain_valid(self, chain):
   # get the first block in the chain and it serves as the previous block
   previous_block = chain[0]
   # an index of the blocks in the chain for iteration
   block_index = 1
   while block_index < len(chain):
       # get the current block
       block = chain[block_index]
       # check if the current block link to previous block has is the same as the hash of the previous block
       if block["previous_hash"] != self.hash(previous_block):
           return False

       # get the previous proof from the previous block
       previous_proof = previous_block['proof']

       # get the current proof from the current block
       current_proof = block['proof']

       # run the proof data through the algorithm
       hash_operation = hashlib.sha256(str(current_proof ** 2 - previous_proof ** 2).encode()).hexdigest()
       # check if hash operation is invalid
       if hash_operation[:4] != '0000':
           return False
       # set the previous block to the current block after running validation on current block
       previous_block = block
       block_index += 1
   return True

By following the above steps, we’ve built our blockchain. However, we need to interact with it to mine our block and display some details using Flask and Postman.

Mining our blockchain

To mine our block, we create a web application entry point with Flask as follows:

app = Flask(__name__)

Next, we create an instance of the Blockchain class:

blockchain = Blockchain()

Now, we can proceed to mine the block.

Mining a new block

We start by creating a Flask route of mine_block with a GET method, then we define the view

@app.route('/mine_block', methods=['GET'])
def mine_block():

Inside the mine_block() view, we fetch the data we need to create a block. This includes the following:

  • The previous block
  • The previous block proof
  • The proof of work
  • The previous hash

The following data points have their methods defined in the Blockchain class. All we need is to connect them, as shown below:

# get the data we need to create a block
previous_block = blockchain.get_previous_block()
previous_proof = previous_block['proof']
proof = blockchain.proof_of_work(previous_proof)
previous_hash = blockchain.hash(previous_block)

Next, we create the block and store it in the block variable:

block = blockchain.create_blockchain(proof, previous_hash)

Then, we return a jsonify response that contains a message, index, timestamp, proof, and the previous hash.

response = {'message': 'Block mined!',
           'index': block['index'],
           'timestamp': block['timestamp'],
           'proof': block['proof'],
           'previous_hash': block['previous_hash']}
return jsonify(response), 200

Now, we have all the code to mine a block. Let’s go a step further to write the feature for retrieving all blocks in the chain.

Fetching the chain

We start by creating a new get_chain route with a GET method. We then return a response that contains the blockchain’s length and the blockchain within the chain.

@app.route('/get_chain', methods=['GET'])
def get_chain():
   response = {'chain': blockchain.chain,
               'length': len(blockchain.chain)}
   return jsonify(response), 200

Setting the application port

Lastly, we need to run our app, so we add this line of code beneath our existing code:

app.run(host='0.0.0.0', port=5000)

At this stage, you should have the complete blockchain code, as highlighted below:

import datetime
import json
import hashlib
from flask import Flask, jsonify


class Blockchain:
   def __init__(self):
       self.chain = []
       self.create_blockchain(proof=1, previous_hash='0')

   def create_blockchain(self, proof, previous_hash):
       block = {
           'index': len(self.chain) + 1,
           'timestamp': str(datetime.datetime.now()),
           'proof': proof,
           'previous_hash': previous_hash
       }

       self.chain.append(block)
       return block

   def get_previous_block(self):
       last_block = self.chain[-1]
       return last_block

   def proof_of_work(self, previous_proof):
       # miners proof submitted
       new_proof = 1
       # status of proof of work
       check_proof = False
       while check_proof is False:
           # problem and algorithm based off the previous proof and new proof
           hash_operation = hashlib.sha256(str(new_proof ** 2 - previous_proof ** 2).encode()).hexdigest()
           # check miners solution to problem, by using miners proof in cryptographic encryption
           # if miners proof results in 4 leading zero's in the hash operation, then:
           if hash_operation[:4] == '0000':
               check_proof = True
           else:
               # if miners solution is wrong, give mine another chance until correct
               new_proof += 1
       return new_proof

   # generate a hash of an entire block
   def hash(self, block):
       encoded_block = json.dumps(block, sort_keys=True).encode()
       return hashlib.sha256(encoded_block).hexdigest()

   # check if the blockchain is valid
   def is_chain_valid(self, chain):
       # get the first block in the chain and it serves as the previous block
       previous_block = chain[0]
       # an index of the blocks in the chain for iteration
       block_index = 1
       while block_index < len(chain):
           # get the current block
           block = chain[block_index]
           # check if the current block link to previous block has is the same as the hash of the previous block
           if block["previous_hash"] != self.hash(previous_block):
               return False

           # get the previous proof from the previous block
           previous_proof = previous_block['proof']

           # get the current proof from the current block
           current_proof = block['proof']

           # run the proof data through the algorithm
           hash_operation = hashlib.sha256(str(current_proof ** 2 - previous_proof ** 2).encode()).hexdigest()
           # check if hash operation is invalid
           if hash_operation[:4] != '0000':
               return False
           # set the previous block to the current block after running validation on current block
           previous_block = block
           block_index += 1
       return True


app = Flask(__name__)

blockchain = Blockchain()


@app.route('/mine_block', methods=['GET'])
def mine_block():
   # get the data we need to create a block
   previous_block = blockchain.get_previous_block()
   previous_proof = previous_block['proof']
   proof = blockchain.proof_of_work(previous_proof)
   previous_hash = blockchain.hash(previous_block)

   block = blockchain.create_blockchain(proof, previous_hash)
   response = {'message': 'Block mined!',
               'index': block['index'],
               'timestamp': block['timestamp'],
               'proof': block['proof'],
               'previous_hash': block['previous_hash']}
   return jsonify(response), 200


@app.route('/get_chain', methods=['GET'])
def get_chain():
   response = {'chain': blockchain.chain,
               'length': len(blockchain.chain)}
   return jsonify(response), 200



app.run(host='0.0.0.0', port=5000)

Next, we proceed to run our app.

Running The Application

There are two ways to test-run our blockchain code. The first way is to use the Postman desktop app, while the other method is via the web browser.

Running Via The Web Browser

Let’s run our app through the web browser. When we run the application on Pycharm, we get a local address to run our app from the terminal:

Terminal Image

From my terminal, the URL address is 172.20.10.2:5000.

In the browser, we start by mining a block, and we do this by calling the mine_block route function we created:

Mine Block Image

To call the mine_block function, we add the function name after the forward-slash, and it runs the code instruction under the block.

From the result, we can see the index of the block, a message showing that the block is mined, the previous hash, the proof of work, and the timestamp.

Every time we refresh the page, it mines a new block. Now, let’s get the chain data by replacing mine_block with get_chain. You should have something similar to the one below:

Blockchain Image

The above functions display the blocks in the chain and their index as intended. Now, let’s repeat this process on the Postman desktop application.

Running via Postman

To mine a block, we copy the mine_block URL and paste it into the request URL bar. Make sure that the request type is GET before sending the request.

The result should be similar to the ones shown below:

Postman Mine Block Image

Next, let’s get the chain data:

Postman BlockChain Image

Conclusion

In this article, you’ve successfully built your first blockchain. You can mine a block as well as view the list of blocks in the chain.

The codebase for this article can be found here.

You can do more practice by creating a new function to check if the blockchain is valid and return a response to the request.


Peer Review Contributions by: Adrian Murage