Fixing Transaction Malleability

Transaction Malleability was a "Bug" in the original Bitcoin protocol that allowed the identification number of a transaction (the txid) to be changed before it was confirmed. SegWit's biggest achievement was solving this problem once and for all.

1. How Malleability Worked

In ECDSA signatures, there are multiple ways to represent the same mathematical proof.

• An attacker could slightly modify the signature data in a transaction sitting in the mempool.

• The signature remains Valid, so the transaction is still accepted by the network.

• The Catch: Because the signature was part of the data hashed to create the txid, the ID of the transaction changed.

2. The Nightmare Scenario

Imagine you are waiting for a transaction with ID A.

1. The transaction is malleated in the mempool and confirmed as ID B.

2. Your software is looking for ID A, so it thinks the payment never happened.

3. If you had another transaction that depended on ID A, that second transaction is now Invalid because ID A technically does not exist in the blockchain.

3. The SegWit Solution

By moving the signatures into the Witness field and excluding that field from the txid calculation, SegWit made the txid immutable.

• If someone tweaks the signature, the txid stays exactly the same.

• The Result: We can now safely sign a transaction that depends on a previous, unconfirmed transaction.

4. Enabling the Lightning Network

The Lightning Network works by exchanging "Commitment Transactions" off-chain.

• These transactions are chained together: Transaction #2 spends from Transaction #1.

• Without SegWit, if Transaction #1 was malleated, Transaction #2 would become worthless paper.

• With SegWit, the chain is rock-solid. This is why the Lightning Network only became possible after SegWit activation in 2017.

5. Third-Party Malleability vs. Owner Malleability

• Legacy: Anyone (even a miner or a node) could malleate a transaction.

• SegWit: Only the owner (the person with the private key) can change the signature, but even then, they cannot change the txid.

Issue	Legacy	SegWit
ID Modification	Possible by Anyone	Impossible
Layer 2 Support	Weak / Dangerous	Native / Secure
Integrity	Variable	Fixed

In the next section, we will discuss The Witness Discount & Block Weight.