Simple Blockchain Simulation in Python

A lightweight, educational blockchain simulation written in pure Python. It demonstrates core blockchain concepts such as wallets, transactions, proof-of-work mining, and chain validation, and it can save/load the chain to/from a JSON file.

Features

• Wallets – Simulated private/public key pairs (using SHA-256 hashing).
• Transactions – Transfer of coins between addresses, with simulated signatures.
• Blocks – Contain a list of transactions and a link to the previous block.
• Proof-of-Work – Adjustable difficulty (number of leading zeros required in block hash).
• Mining Reward – Coinbase transaction paid to the miner.
• Chain Validation – Checks block hashes, previous block links, and proof-of-work.
• JSON Persistence – Save the entire blockchain to a file and load it back.

Requirements

• Python 3.6 or higher
• No external libraries (uses only standard library modules: hashlib, json, time, secrets)

Usage

1. Clone or download the repository.

2. Run the script:

   python blockchain_sim.py

3. The script will:

   • Create two wallets (Alice and Bob).
   • Simulate two transactions:
     • Alice sends 30 coins to Bob.
     • Bob sends 10 coins back to Alice.
   • Mine two blocks (one per transaction set).
   • Validate the blockchain.
   • Save the chain to blockchain.json.
   • Load the chain from the file and validate it again.

Code Overview

• Wallet – Generates a random private key and a public address (hash of the key). Provides a sign method.
• Transaction – Stores sender, recipient, amount, timestamp, and signature. Can compute its own hash.
• Block – Contains an index, timestamp, list of transactions, previous block hash, nonce, and its own hash. Implements proof-of-work mining.
• Blockchain – Manages the chain, pending transactions, mining, validation, and JSON serialization/deserialization.

Example Output

Alice's address: 3f2a9c...
Bob's address:   8b7d1e...

Mining block 1...
Block #1 mined: 00a1b2c3...
Block added with 2 transaction(s). Reward paid to 3f2a9c...

Mining block 2...
Block #2 mined: 00d4e5f6...
Block added with 2 transaction(s). Reward paid to 8b7d1e...

Is chain valid? True
Blockchain saved to blockchain.json

Loading blockchain from file...
Loaded chain valid? True
Number of blocks: 3

JSON File Structure

The saved blockchain.json contains:

• difficulty – Proof-of-work difficulty.
• mining_reward – Reward per mined block.
• chain – List of blocks, each with:
  • index, timestamp, transactions (list of transaction objects), previous_hash, nonce, hash
• pending_transactions – Transactions not yet included in a block.

Customization

• Difficulty: Modify the difficulty parameter when creating a Blockchain instance (e.g., Blockchain(difficulty=3)).
• Mining Reward: Change mining_reward in the constructor.
• Transaction Simulation: Add more wallets and transactions to see how the chain grows.

Limitations

• This is a simulation for educational purposes only. It does not implement real cryptographic signatures (ECDSA), network communication, or consensus mechanisms.
• Security is intentionally simplified; do not use this code for any real cryptocurrency or production environment.

License

This project is open-source and available under the MIT License.