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Distributed Blockchain Network

A proof-of-work blockchain built from scratch in C++ with SHA-256 mining, transaction validation, and peer-to-peer consensus

Overview

A proof-of-work blockchain built from scratch in C++17, with SHA-256 mining, transaction validation, and a TCP peer-to-peer network. Nodes connect to each other, exchange chains, and agree on a single history without a central server.

I built it to understand how a blockchain works at every layer, from hashing blocks to sending them over sockets.

How It Works

High level. Each node keeps its own copy of the chain. To mine a block, a node searches for a nonce that makes the block’s hash start with a set number of zeros. Finding one takes a lot of hashing, but checking one is fast. Nodes broadcast new blocks to their peers, and when two chains disagree, the longest valid chain wins.

Low level: a node’s lifecycle.

  1. Mine. mineBlock increments a nonce until the SHA-256 hash meets the difficulty target (about 65,000 hashes on average at difficulty 4). Each block includes a coinbase transaction from SYSTEM that rewards the miner.
  2. Broadcast. mineAndBroadcast locks the chain, appends the block, unlocks, and then sends a NEW_BLOCK message. The lock is released before the network call so other threads aren’t blocked on I/O.
  3. Listen. A background thread accepts incoming TCP connections and starts a separate handler thread for each peer.
  4. Sync. Peers exchange GET_LENGTH and LENGTH messages. If a peer has a longer chain, the node requests it with GET_CHAIN.
  5. Validate and adopt. The node checks every hash and every link in the incoming chain. If the chain is valid and longer, the node replaces its own (Nakamoto consensus through longest-chain fork resolution).
  6. Persist. Chains are saved as JSON and fully re-validated when loaded, so a tampered file is detected.

Balances aren’t stored. getBalance calculates them by replaying every transaction in the chain, which is slower but means balances can never drift from the chain.

What I Built

I wrote every part of the project:

  • Block, Transaction, and Blockchain classes with SHA-256 hashing and adjustable difficulty
  • Merkle roots for tamper detection
  • A TCP networking layer using raw sockets (socket, bind, listen, accept, connect)
  • One thread per peer, with a mutex protecting the shared chain
  • A JSON message protocol (GET_CHAIN, CHAIN, NEW_BLOCK, GET_LENGTH, LENGTH) with a hand-written parser that counts braces to handle nested objects
  • Fork resolution, full-chain validation, and JSON persistence

Stack

  • Language: C++17
  • Networking: POSIX TCP sockets
  • Concurrency: std::thread, std::mutex

Repo

github.com/brockbenton/cpp-blockchain