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이관호(Gwanho LEE)
이관호(Gwanho LEE)

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🚀 How I Added SegWit to My Rust Bitcoin Wallet (and Why You Should Too)

#bitcoin #rust #blockchain #segwit

Why This Post?

When I first built my Bitcoin CLI wallet in Rust, it worked — but it was stuck in the past.
It only supported legacy addresses (1...), which meant:

  • Higher fees 💸
  • Slower confirmations 🐢
  • No Lightning Network ⚡ support

Then I integrated SegWit (Segregated Witness), and everything changed:
transactions got 35% cheaper, more secure, and finally ready for the Lightning era.

In this post, I’ll break down:

  • What SegWit really does (without the fluff)
  • How I implemented it in Rust step-by-step
  • Real-world results (performance, fees, and UX)
  • What this unlocks next (Lightning, Taproot, and beyond)

If you’re building wallets, apps, or just curious about Bitcoin internals, this is your practical roadmap to SegWit.

🧩 SegWit in 60 Seconds

SegWit is a Bitcoin upgrade (2017) that separates witness data (signatures) from the main transaction.

Why it matters:

  • Fixes transaction malleability (critical for Lightning + smart contracts)
  • Makes transactions smaller & cheaper (up to 35% fee savings)
  • Expands block capacity without hard forks

Think of it like moving signatures into a side folder: the transaction stays valid, but the “heavy” parts don’t clog up the main block.

⚖️ Legacy vs SegWit: What Changes?

Feature Legacy (P2PKH) SegWit (P2WPKH)
Address format 1... bc1...
Signature placement In scriptSig In witness data
Bytes per input ~148 ~68 (vbytes)
Transaction malleable? ✅ Yes ❌ No
Fees Higher ~35% lower

That’s the practical win: less bytes = less fees = happier users.

🛠 Implementing SegWit in Rust

1. Address Generation 

// Legacy
let legacy = Address::p2pkh(&compressed, Network::Testnet);

// SegWit (bech32)
let segwit = Address::p2wpkh(&compressed, Network::Testnet);
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🔑 Key step: add bech32 encoding and witness program support.

2. Transaction Signing 

// Legacy signing
let sighash = cache.legacy_signature_hash(
    input_index,
    &script_pubkey,
    EcdsaSighashType::All as u32,
)?;

// SegWit signing
let sighash = cache.p2wpkh_signature_hash(
    input_index,
    &script_code,
    Amount::from_sat(utxo.value),
    EcdsaSighashType::All,
)?;
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⚡ Difference: SegWit stores signatures in witness data, not in the main script.

3. Fee Calculation with vBytes 

let num_p2wpkh_inputs = selected_utxos.iter()
    .filter(|(addr, _)| addr.address.starts_with("tb1q"))
    .count() as u64;

let num_p2pkh_inputs = selected_utxos.len() as u64 - num_p2wpkh_inputs;

let estimated_size = num_p2pkh_inputs * 148 + num_p2wpkh_inputs * 68 +
                     dest_out_sz + change_out_sz + 10;
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SegWit inputs get a 75% discount in size → lower fees.

🎯 Real-World Results

After adding SegWit:

  • Transactions shrank from ~250 bytes → ~180 vbytes
  • Users paid 35% less in fees
  • Transactions confirmed faster (blocks fit more txs)
  • Wallet code became cleaner + ready for Lightning

This wasn’t just an optimization — it was a gateway to future Bitcoin features.

⚡ What’s Next?

SegWit laid the groundwork. Next stops:

  1. Lightning Network integration
  2. Taproot support (more privacy + efficiency)
  3. Advanced scripts and multi-sig flows

🔗 Resources

👉 If you’re building in Bitcoin, add SegWit today. It’s not optional anymore — it’s the bridge to the future of Bitcoin.

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