Overview
Segregated Witness (SegWit), activated on August 24, 2017, is one of the most significant upgrades to the Bitcoin protocol. It restructures how transaction data is organized by moving the signature (witness) data into a separate structure. This seemingly simple change solved several long-standing problems: it fixed transaction malleability, enabled the Lightning Network, increased effective block capacity, and introduced a fee discount for witness data.
How SegWit Restructures Transactions
Pre-SegWit Transaction: SegWit Transaction:
┌─────────────────────────┐ ┌─────────────────────────┐
│ Version │ │ Version │
├─────────────────────────┤ ├─────────────────────────┤
│ Inputs: │ │ Marker + Flag (0x0001) │
│ ┌────────────────────┐ │ ├─────────────────────────┤
│ │ Previous TXID │ │ │ Inputs: │
│ │ ScriptSig │ │ │ ┌────────────────────┐ │
│ │ (signatures HERE) │ │ │ │ Previous TXID │ │
│ └────────────────────┘ │ │ │ ScriptSig (empty) │ │
├─────────────────────────┤ │ └────────────────────┘ │
│ Outputs │ ├─────────────────────────┤
├─────────────────────────┤ │ Outputs │
│ Locktime │ ├─────────────────────────┤
└─────────────────────────┘ │ Witness: │
│ (signatures HERE) │
TXID covers entire ├─────────────────────────┤
transaction including │ Locktime │
signatures └─────────────────────────┘
TXID covers everything
EXCEPT witness data
Problems Solved by SegWit
Transaction Malleability Fix
Before SegWit, the transaction ID (TXID) was calculated over the entire transaction, including signatures. Since signatures could be slightly modified without invalidating them (malleated), the TXID could change after broadcast. SegWit excludes witness data from the TXID calculation, making TXIDs immutable. This was a prerequisite for the Lightning Network, which relies on chains of pre-signed transactions referencing stable TXIDs.
Increased Block Capacity
SegWit introduced "block weight" (maximum 4 million weight units) to replace the old 1 MB block size limit. Non-witness data counts at 4 weight units per byte, while witness data counts at 1 weight unit per byte. In practice, this allows blocks to contain roughly 1.7-2.1 MB of total data, depending on the transaction mix.
Witness Discount
The witness discount incentivizes SegWit adoption by counting witness bytes at one-quarter the weight. This results in lower fees for P2WPKH and P2TR transactions compared to legacy P2PKH transactions, benefiting users who upgrade.
SegWit Versions
SegWit introduced a versioning system for future upgrades:
- Witness v0 — P2WPKH and P2WSH outputs using Bech32 addresses (
bc1q...) - Witness v1 — P2TR (Taproot) outputs using Bech32m addresses (
bc1p...) - Witness v2-16 — Reserved for future upgrades, deployable via soft fork
Backward Compatibility
SegWit was deployed as a soft fork, meaning non-upgraded nodes still accept SegWit blocks as valid (they see the witness outputs as "anyone can spend" but never attempt to spend them). This backward compatibility avoided a contentious chain split and allowed gradual adoption.
Common Misconception
SegWit is sometimes described as simply a "block size increase," but this undersells its significance. The malleability fix was arguably more important, as it enabled an entire ecosystem of layer-2 protocols. The capacity increase was a welcome side effect of the new weight-based accounting system.