Simple Blockchain Implementation
This guide demonstrates how to build a basic Blockchain from scratch. It covers the fundamental concepts of chaining blocks together using cryptographic hashes and implementing a simple "Proof of Work" system to simulate mining.
Modules Used:
- hashlib: To create SHA-256 hashes for the blocks.
- json: To serialize block data before hashing.
time: To timestamp each block.
The Code
Save this as blockchain.py.
import hashlib
import json
import time
class Block:
def __init__(self, index, transactions, timestamp, previous_hash):
self.index = index
self.transactions = transactions
self.timestamp = timestamp
self.previous_hash = previous_hash
self.nonce = 0
self.hash = self.compute_hash()
def compute_hash(self):
"""
A function that return the hash of the block contents.
"""
block_string = json.dumps({
"index": self.index,
"transactions": self.transactions,
"timestamp": self.timestamp,
"previous_hash": self.previous_hash,
"nonce": self.nonce
}, sort_keys=True)
return hashlib.sha256(block_string.encode()).hexdigest()
class Blockchain:
def __init__(self):
self.unconfirmed_transactions = []
self.chain = []
self.create_genesis_block()
self.difficulty = 2 # Number of leading zeros required for hash
def create_genesis_block(self):
"""
A function to generate genesis block and appends it to
the chain. The block has index 0, previous_hash as 0, and
a valid hash.
"""
genesis_block = Block(0, [], time.time(), "0")
genesis_block.hash = genesis_block.compute_hash()
self.chain.append(genesis_block)
@property
def last_block(self):
return self.chain[-1]
def proof_of_work(self, block):
"""
Function that tries different values of nonce to get a hash
that satisfies our difficulty criteria.
"""
block.nonce = 0
computed_hash = block.compute_hash()
while not computed_hash.startswith('0' * self.difficulty):
block.nonce += 1
computed_hash = block.compute_hash()
return computed_hash
def add_block(self, block, proof):
"""
A function that adds the block to the chain after verification.
"""
previous_hash = self.last_block.hash
if previous_hash != block.previous_hash:
return False
if not self.is_valid_proof(block, proof):
return False
block.hash = proof
self.chain.append(block)
return True
def is_valid_proof(self, block, block_hash):
return (block_hash.startswith('0' * self.difficulty) and
block_hash == block.compute_hash())
def add_new_transaction(self, transaction):
self.unconfirmed_transactions.append(transaction)
def mine(self):
if not self.unconfirmed_transactions:
return False
last_block = self.last_block
new_block = Block(index=last_block.index + 1,
transactions=self.unconfirmed_transactions,
timestamp=time.time(),
previous_hash=last_block.hash)
proof = self.proof_of_work(new_block)
self.add_block(new_block, proof)
self.unconfirmed_transactions = []
return new_block.index
if __name__ == "__main__":
blockchain = Blockchain()
# 1. Add transactions
print("Adding transactions...")
blockchain.add_new_transaction({"sender": "Alice", "receiver": "Bob", "amount": 10})
blockchain.add_new_transaction({"sender": "Bob", "receiver": "Charlie", "amount": 5})
# 2. Mine the block
print("Mining block 1...")
blockchain.mine()
# 3. Add more transactions
blockchain.add_new_transaction({"sender": "Charlie", "receiver": "Alice", "amount": 2})
# 4. Mine again
print("Mining block 2...")
blockchain.mine()
# 5. Display the chain
print("\n--- Blockchain Content ---")
for block in blockchain.chain:
print(f"Index: {block.index}")
print(f"Timestamp: {block.timestamp}")
print(f"Transactions: {block.transactions}")
print(f"Hash: {block.hash}")
print(f"Prev Hash: {block.previous_hash}")
print(f"Nonce: {block.nonce}")
print("-" * 30)Usage
python blockchain.py