Bitcoin VerityOpen comparator ↗
Bitcoin Verity/Education/88

Level 9 · Advanced technical layer

SegWit, witness and transaction malleability

What Segregated Witness separated, how identifiers changed and why signatures did not disappear from the blockchain.

Article
88
Reading time
15 minutes
Reviewed
10 September 2026

In a nutshell

SegWit moved signatures and unlocking data into a witness structure and introduced versioned outputs. It stabilised txid for fully SegWit-spent transactions, improved signing and introduced block weight, while witness remains a verified part of every relevant block.

01

What was segregated

BIP 141 defines a witness for each input. txid excludes witness, whereas wtxid includes it. A commitment in the coinbase binds the block to its witness data, which upgraded full nodes download and verify.

02

Why txid could change

Legacy signature or scriptSig encodings could sometimes be altered without redirecting funds, changing txid and breaking dependent unconfirmed transactions. For SegWit inputs, third-party witness changes do not alter txid.

03

Weight and compatibility

Block weight counts base bytes more heavily than witness bytes and is capped at four million units. Old nodes could accept the new witness programs under their looser rules, while upgraded nodes enforced the added conditions, enabling a soft-fork deployment.

04

More than capacity

BIP 143's SegWit v0 digest commits to the spent amount and avoids problematic repeated hashing. Versioned witness programs also supplied an upgrade path later used by Taproot.

Level 9 · Advanced technical layer

Terms to know

SegWit
Segregated Witness, the rules and formats separating witness data.
Malleability
Changing a transaction representation without changing its intended payment.
Vbyte
A virtual byte derived from transaction weight for fee rates.

Common misconception

SegWit removed signatures from the blockchain.

A more accurate explanation

Witness is relayed, committed in the block and verified by upgraded full nodes; it is simply serialized and weighted differently.

A more accurate explanation

Was the capacity change a hidden hard fork?

Old nodes still accepted the blocks under their rules, while new nodes enforced stricter witness conditions. That is the compatibility pattern of a soft fork, though old nodes do not verify the new signatures.

88

Key takeaways

  1. 01txid excludes SegWit witness; wtxid includes it.
  2. 02Witness is committed and verified.
  3. 03Witness bytes cost less weight, not zero.
  4. 04SegWit created room for later script versions.

A child-friendly recap

In very simple terms

SegWit put signature data in a separate part so the payment's basic fingerprint stays stable. The signatures remain in the block and full nodes still check them.

Reviewed: 10 September 2026

Sources and further reading

Sources support particular facts and definitions; listing one does not mean the editors endorse every view of its author.

01
BIP 141: Segregated WitnessBitcoin Improvement Proposals
bips.dev ↗02
BIP 143: new signature digestBitcoin Improvement Proposals
bips.dev ↗03
BIP 144: witness relayBitcoin Improvement Proposals
bips.dev ↗

Educational material, not an investment recommendation.