Segregated Witness

Bitcoin's original design capped block size at 1 MB, limiting transaction throughput to roughly 3–7 transactions per second. As usage grew, this became a bottleneck, leading to high fees and delayed confirmations. The Segregated Witness proposal, first outlined by Bitcoin Core developer Pieter Wuille in December 2015, aimed to address this by separating the signature data (witness) from the transaction data. This separation allows more transactions to fit in each block without increasing the block size limit in the traditional sense.

The upgrade was activated on August 24, 2017, via a user-activated soft fork (UASF) after months of contentious debate within the community. It was designed as a backward-compatible soft fork, meaning older nodes could still validate new blocks, though they would not see the full benefits.

SegWit introduces a new transaction format where signature data is moved to a separate witness structure, which is committed to via a Merkle root in the coinbase transaction. This allows the block size limit to be effectively increased to 4 MB, with a new weight-based limit of 4 million weight units. The base block size remains 1 MB, but the witness data is counted separately, enabling more transactions per block.

The upgrade also fixes transaction malleability, a long-standing issue where transaction IDs could be altered by third parties before confirmation. By separating the witness, the transaction ID is computed only from the non-witness data, making it immutable. This was a critical prerequisite for second-layer solutions like the Lightning Network, which rely on unchangeable transaction IDs to build payment channels.

Following activation, SegWit adoption grew gradually. By 2021, over 70% of Bitcoin transactions used SegWit addresses (starting with 'bc1'). The upgrade reduced transaction fees by allowing more transactions per block, and it enabled the development of the Lightning Network, which facilitates instant, low-cost payments.

SegWit also paved the way for other protocol improvements, such as Taproot, which further enhances privacy and smart contract capabilities. The upgrade demonstrated the feasibility of soft forks in Bitcoin, influencing subsequent governance debates.

One lesser-known aspect is that SegWit was initially proposed as a solution to transaction malleability, with the block size increase being a secondary benefit. The term 'segregated witness' itself was coined by Wuille, and the concept was first discussed on the Bitcoin developer mailing list in 2015.

Another edge case is that SegWit introduced a new address format (Bech32) that is case-insensitive and more error-resistant, but it was not immediately supported by all wallets and exchanges, causing some initial compatibility issues. Additionally, the upgrade required a signaling period where miners indicated support; the activation was triggered by a threshold of 95% of blocks signaling readiness, which was achieved in August 2017.

SegWit also had a subtle impact on the block reward: because the witness data is not included in the block size calculation, miners could include more transactions and thus collect more fees, but the block subsidy remained unchanged.

The SegWit activation was highly controversial, leading to a split in the Bitcoin community. Some factions opposed the upgrade, arguing it was a partial measure that did not solve the scalability problem. This opposition culminated in a hard fork in August 2017, creating Bitcoin Cash, which increased the block size to 8 MB without implementing SegWit.

Despite the controversy, SegWit has been widely adopted and is now considered a foundational upgrade. It also influenced other cryptocurrencies; for example, Litecoin activated SegWit in May 2017, and several other coins have implemented similar solutions.

Soft fork

A backward-compatible upgrade to a blockchain protocol where old nodes still accept new blocks.

Witness data

The signature and script data in a Bitcoin transaction that proves ownership of the inputs.

Malleability

The ability to alter a transaction's ID without invalidating it, which can cause issues in multi-step protocols.

Lightning Network

A second-layer payment protocol that enables instant, low-cost transactions by creating off-chain payment channels.