Basic_Blockchain_Implementation

A minimal, from-scratch Blockchain implementation in Python demonstrating:

• SHA-256 based block hashing
• Proof-of-Work (PoW) mining with adjustable difficulty
• Chain validation
• A simple P2P network demo using Flask nodes (longest-chain consensus)

Project structure

basic-blockchain/
├── block.py          # Block class + hash computation
├── blockchain.py      # Blockchain class: PoW, validation, consensus
├── app.py             # Flask REST API node for P2P networking
├── test_local.py       # Standalone demo without networking
├── requirements.txt
└── README.md

Setup

git clone https://github.com/vedant-4747/basic-blockchain.git
cd basic-blockchain
python -m venv venv
source venv/bin/activate   # venv\Scripts\activate on Windows
pip install -r requirements.txt

1. Local demo (no networking)

python test_local.py

This creates the genesis block, adds transactions, mines two blocks with PoW, and prints the full chain plus a validity check.

2. P2P network demo (multiple nodes)

Start two or more nodes in separate terminals:

python app.py 8000
python app.py 8001

Register node 8001 with node 8000 (this syncs the chain too):

curl -X POST http://127.0.0.1:8001/register_with \
  -H "Content-Type: application/json" \
  -d '{"node_address": "http://127.0.0.1:8000/"}'

Submit a transaction to node 8000:

curl -X POST http://127.0.0.1:8000/new_transaction \
  -H "Content-Type: application/json" \
  -d '{"author": "Vedant", "content": "Send 5 coins to Asha"}'

Mine it on node 8000 (this broadcasts the new block to registered peers):

curl http://127.0.0.1:8000/mine

Check the chain on node 8001 — it should now include the newly mined block:

curl http://127.0.0.1:8001/chain

How it works

Hashing

Every block's contents (index, transactions, previous hash, timestamp, nonce) are serialized to JSON and hashed with SHA-256 to produce a unique fingerprint. Changing any field changes the hash completely.

Proof-of-Work

difficulty = 4 means a valid block hash must start with four zeros. The nonce is incremented repeatedly until this condition is met — this is the "work" that secures the chain against tampering.

Chain validation

is_chain_valid() walks the chain and checks that each block's previous_hash matches the prior block's hash, and that each block's stored hash actually satisfies the PoW target.

P2P consensus

Each Flask node keeps its own copy of the chain and a set of peer URLs. When a node mines a block, it POSTs that block to every peer's /add_block endpoint. If a node discovers a longer valid chain from a peer, it adopts it (longest-chain rule), which is how the network resolves conflicts.

Possible extensions

• Digital signatures for transactions (public/private key pairs)
• Wallet balances and UTXO tracking
• Dynamic difficulty adjustment based on block time
• A simple web UI to visualize the chain