Capstone: Bitcoin-Lite Node

Why this module exists

The real difficulty of blockchain engineering is integration. The capstone requires you to stitch together:

• Serialization and hashing
• UTXO validation
• Script execution
• Mempool selection
• Mining and PoW checks
• Chain selection and reorgs

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1) Validation pipeline

A node must reject invalid data early:

1. Header sanity + PoW
2. Merkle root check
3. Transaction validation (UTXO + scripts)
4. State update

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2) Mining pipeline

1. Select transactions (fee rate + availability)
2. Create coinbase
3. Compute merkle root
4. Search nonce
5. Broadcast block

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3) Reorg safety

Longer chains can arrive after you mine. Your node must rollback and re-apply state deterministically.

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4) Design constraints

We use toy choices to keep code manageable:

• Fixed difficulty
• Simplified sighash
• P-256 instead of secp256k1

These keep the system learnable while retaining the shape of Bitcoin.

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What you will build

A full node + miner that accepts transactions, mines blocks, and maintains a consistent UTXO set across forks.

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Key takeaways

• A blockchain is an execution engine with strict determinism.
• Robustness is about invariants, not just features.
• The capstone is where correctness matters most.