SegWit & Bech32

Segregated Witness (SegWit) moved signature data out of the legacy transaction identifier path. That fixed transaction malleability for modern outputs and changed how block capacity and fees are measured.

Why this matters

Almost all modern wallets use SegWit or Taproot. If your mental model still prices fees in “raw bytes only,” your fee estimates and tx size math will be wrong.

Analogy

Imagine a court filing where the signatures are attached in an envelope marked “witnesses.” The case ID no longer changes if someone fiddles with the signature formatting, and the court weights the witness pages differently when measuring stack size.

What changed

• txid no longer includes witness data (wtxid does)
• Block limits use weight: witness bytes count less than non-witness bytes
• vbytes ≈ weight / 4 for fee rates (sat/vB)

const vbytes = weight / 4;
const fee = Math.ceil(satPerVb * vbytes);

Bech32 addresses

Native SegWit v0 addresses use Bech32 (bc1q...). They are easier to read aloud and detect typos better than Base58 for many cases.

Builder impact

• Lightning and many contracts needed malleability fixes, SegWit enabled safer off-chain protocols
• Explorers should show both txid and wtxid when debugging
• Always estimate fees with weight, not legacy size alone

Common mistakes

• Comparing “byte size” across legacy and SegWit without weight
• Sending to bc1 on the wrong network HRP
• Forgetting older receivers that cannot parse Bech32 (rare now, still possible)

Malleability and Lightning

Before SegWit, third parties could tweak signatures and change txids without invalidating spends in some cases. Second-layer protocols that pre-signed dependent transactions suffered. SegWit removed witness data from txid calculation, enabling safer Lightning-style designs.

Nested vs native

Nested SegWit wraps witness programs inside P2SH (3... addresses) for old wallet compatibility. Native SegWit (bc1q...) is cleaner and usually cheaper. New systems should default to native SegWit or Taproot.

Weight units

• Non-witness byte: 4 weight units
• Witness byte: 1 weight unit
• Block weight limit: 4,000,000 units

Fee rate UX should speak sat/vB consistently.

Worked mental model

Re-read the diagrams in this lesson once out loud in plain language. If you cannot explain the flow to a friend without jargon, pause and revisit Mastering Bitcoin / Mastering Lightning chapters linked in Resources. Chapter references are intentional, not decorative.

Hands-on habit

Every protocol idea should be paired with one local experiment:

1. Reproduce the happy path on regtest (or Polar for Lightning)
2. Break it on purpose (wrong fee, expired invoice, offline peer)
3. Write down what error you saw and which layer produced it (wallet, node, mempool, peer)

That habit turns reading into builder instinct.

Glossary check

Pick three terms from this lesson and define them in one sentence each without opening notes. Weak definitions mean the lesson is not finished yet.

Resource order

Use Resources in order: narrative book chapter first, then BIP/BOLT for precision, then implementation docs for commands. Jumping straight to RPC flags without the mental model creates brittle knowledge.

Next steps

Taproot builds on SegWit v1 with Schnorr and script trees, the current best practice for many new wallets.