⛓️ Java-Blockchain — Bitcoin-Style UTXO Engine

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📌 Overview

A production-grade Bitcoin-style blockchain engine built from scratch in Java, implementing the complete UTXO transaction lifecycle — from coinbase issuance through mempool staging, PoW mining, and full chain validation. Replicates real-world consensus mechanics including dynamic difficulty retargeting, Merkle root integrity, and ECDSA cryptographic signing.

Feature	Status
UTXO Transaction Model	✅
ECDSA Wallets (BouncyCastle)	✅
Proof-of-Work Mining	✅
Mempool (Transaction Staging)	✅
Coinbase Transaction + Mining Reward	✅
Dynamic Difficulty Retargeting	✅
Merkle Root Integrity	✅
Full Chain Validation	✅
Swing GUI Dashboard	✅

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🧠 Core Concepts

🔐 Wallets & Cryptography

• ECDSA key pair generation via BouncyCastle (prime192v1 curve)
• Private key signing + public key verification per transaction
• Base64 key encoding via StringUtil crypto layer

💰 UTXO Model

• Full Bitcoin-style Unspent Transaction Output model
• Dynamic balance calculated by scanning UTXO map — no stored balance
• Double-spend prevention: spent inputs removed from global UTXO set immediately
• Input-output equality enforced on every processTransaction() call
• Change outputs automatically returned to sender

⛏️ Full Bitcoin Transaction Lifecycle

Coinbase TX (miner reward)
    ↓
Wallet.sendFunds() → Mempool (pending)
    ↓
mineNextBlock() → Block (coinbase first, then mempool TXs)
    ↓
PoW solved → Block appended to chain → UTXO set updated

🏦 Mempool

• Transactions submitted via sendFunds() enter a global pending pool
• Miner drains the full mempool into each new block
• Mempool cleared atomically after successful mine — no double-processing

🪙 Coinbase Transaction

• First transaction in every mined block — no inputs, creates coins
• Miner wallet receives miningReward coins per block
• Only legal mechanism for new coin issuance — prevents arbitrary coin creation
• Skips signature verification (no sender by design)

🎯 Dynamic Difficulty Retargeting

• Difficulty stored per block at mine time (not globally — fixes validation bug)
• Every retargetInterval blocks: compares actual vs target block time
• Increases difficulty if mining faster than target, decreases if slower
• Minimum difficulty floor of 1 enforced

🌳 Merkle Root

• Merkle root computed from all transaction IDs in a block
• Stored in block header and included in calculateHash()
• Updated on every addTransaction() call
• Any tampered transaction invalidates the root — detected by isChainValid()

🔎 Full Chain Validation

• Hash chain integrity: each block's previousHash must match prior block's hash
• Per-block hash recalculation with correct stored difficulty
• ECDSA signature verification on every non-coinbase transaction
• UTXO reference validation: inputs must exist and match stored values
• Coinbase transactions skipped correctly (index 0, null sender)

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🏗️ Architecture

Wallet → sendFunds() → Mempool → mineNextBlock() → Block (Coinbase + TXs) → Chain → isChainValid()

6-Layer Design

             ┌────────────────────┐
             │    Dashboard GUI   │  Swing dashboard — live balances,
             └─────────▲──────────┘  mempool count, difficulty, block height
                       │
             ┌─────────┴──────────┐
             │      Noob.java     │  App layer — blockchain state, mempool,
             └─────────▲──────────┘  mineNextBlock(), isChainValid(), difficulty retarget
                       │
      ┌────────────────┼────────────────┐
      │                │                │
 ┌────┴────┐   ┌───────┴──────┐   ┌────┴────┐
 │ Wallet  │   │ Transaction  │   │  Block  │
 │         │   │  + Mempool   │   │+ Merkle │
 └────▲────┘   └──────▲───────┘   └────▲────┘
      │               │                │
      └───────────────┴────────────────┘
                       │
               ┌───────┴────────┐
               │   StringUtil   │
               │  SHA-256       │
               │  ECDSA Sign    │
               │  Base64 Encode │
               └────────────────┘

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📦 Project Structure

src/
├── Noob.java              ← Main class: blockchain state, mempool, mining, validation
├── Block.java             ← Block: hash chain, nonce, merkle root, per-block difficulty
├── Transaction.java       ← UTXO TX: inputs, outputs, signing, merkle tree
├── TransactionInput.java  ← UTXO input reference
├── TransactionOutput.java ← UTXO output: recipient, value, ID
├── Wallet.java            ← ECDSA keypair, balance scan, sendFunds()
├── StringUtil.java        ← SHA-256, ECDSA sign/verify, Base64
└── Dashboard.java         ← Swing GUI: live stats, send, mine, validate

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⚙️ Requirements

• Java JDK 17+
• GSON 2.10.1
• BouncyCastle bcprov-jdk15on-1.70
• JUnit Jupiter 5.10.0 (test scope)

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🚀 How to Run

# 1. Clone
git clone https://github.com/Gaurav77Kumar/Basic-BlockChain
cd Basic-BlockChain

# 2. Add dependencies via Maven or IntelliJ Project Structure → Libraries

# 3. Run blockchain simulation
# Execute Noob.java — opens Dashboard GUI + prints full lifecycle to console

# 4. Run tests
mvn test
# or right-click BlockchainTest.java → Run in IntelliJ

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📸 Output