Workshop on blockchain - Scipy 2021

blockchain.py

"""
Exercises:
Try to change difficulty level in proof of work algorithm and see how long does it takes to compute nonce.
Write a method to validate the chain.
Reward the mining node.

"""

# blockchain.py
import hashlib
import json

class Blockchain:
	def __init__(self):
		self.chain = []
		self.pending_transactions = []

		# create genesis block
		self.add_new_block(0, 100)

	def add_transaction(self, sender, receiver, amount):
		transaction = {
			"sender": sender,
			"receiver": receiver,
			"amount": amount
		}
		self.pending_transactions.append(transaction)

	def add_new_block(self, previous_block_hash, nonce):
		block = {
			"transactions": self.pending_transactions,
			"previous_hash": previous_block_hash,
			"nonce": nonce
		}
		self.chain.append(block)
		self.pending_transactions = []

	@staticmethod
	def hash(block):
		block_string = json.dumps(block, sort_keys=True)
		block_bytes = block_string.encode()
		hash_obj = hashlib.sha256(block_bytes)
		return hash_obj.hexdigest()

	@property
	def last_block(self):
		return self.chain[-1]

	def proof_of_work(self, previous_block_hash):
		"""
		given a p, identify a value x such that hash has 4 leading zeros
		"""
		nonce = 0
		while True:
			if self.valid(previous_block_hash, nonce):
				break
			nonce += 1
		return nonce

	@staticmethod
	def valid(previous_block_hash, nonce):
		guess = f'{previous_block_hash}{nonce}'.encode()
		hash_value = hashlib.sha256(guess).hexdigest()
		return hash_value[:4] == "0000"
	
	
# app.py
from flask import Flask, jsonify, request
from blockchain import Blockchain

app = Flask(__name__)

blockchain = Blockchain()

@app.route('/chain')
def view_chain():
	return jsonify(blockchain.chain)

@app.route('/transaction')
def add_transaction():
	sender = request.args.get('sender')
	receiver = request.args.get('receiver')
	amount = request.args.get('amount')
	blockchain.add_transaction(sender, receiver, amount)
	return "Transaction is recorded, will be added soon to chain"

@app.route('/mine')
def add_new_block():
	# Get previous block
	last_block = blockchain.last_block
	# Need to compute its hash
	previous_block_hash = blockchain.hash(last_block)

	# I need to figure out nonce
	nonce = blockchain.proof_of_work(previous_block_hash)

	# mine the block
	blockchain.add_new_block(previous_block_hash, nonce)

	return "A new block is mined"

if __name__ == "__main__":
	app.run(debug=True)