This BIP defines a new structure called a "witness" that is committed to blocks separately from the transaction merkle tree. This structure contains data required to check transaction validity but not required to determine transaction effects. In particular, scripts and signatures are moved into this new structure.
The witness is committed in a tree that is nested into the block's existing merkle root via the coinbase transaction for the purpose of making this BIP soft fork compatible. A future hard fork can place this tree in its own branch.
==Motivation==
The entirety of the transaction's effects are determined by output consumption (spends) and new output creation. Other transaction data, and signatures in particular, are only required to validate the blockchain state, not to determine it.
By removing this data from the transaction structure committed to the transaction merkle tree, several problems are fixed:
# '''Nonintentional malleability becomes impossible'''. Since signature data is no longer part of the transaction hash, changes to how the transaction was signed are no longer relevant to transaction identification. As a solution of transaction malleability, this is superior to the canonical signature approach ([https://github.com/bitcoin/bips/edit/master/bip-0062.mediawiki BIP62]):
#* It prevents involuntary transaction malleability for any type of scripts, as long as all inputs are signed (with at least one CHECKSIG or CHECKMULTISIG operation)
#* In the case of an m-of-n CHECKMULTISIG script, a transaction is malleable only with agreement of m private key holders (as opposed to only 1 private key holder with BIP62)
#* It allows creation of unconfirmed transaction dependency chains without counterparty risk, an important feature for offchain protocols such as the Lightning Network
# '''Transmission of signature data becomes optional'''. It is needed only if a peer is trying to validate a transaction instead of just checking its existence. This reduces the size of SPV proofs and potentially improves the privacy of SPV clients as they can download more transactions using the same bandwidth.
# '''Some constraints could be bypassed with a soft fork''' by moving part of the transaction data to a structure unknown to current protocol, for example:
#* New script system could be introduced without any limitation from the existing script semantic. For example, a new transaction digest algorithm for transaction signature verification is described in [https://github.com/bitcoin/bips/blob/master/bip-0143.mediawiki BIP143]
A new block rule is added which requires a commitment to the witness hashes, the double SHA256 of a transaction including witnesses. The witness hash of coinbase transaction is assumed to be 0x0000....0000.
The commitment is recorded in a scriptPubKey of the coinbase transaction. It must be at least 38 bytes, with the first 6-byte of 0x6a24aa21a9ed, that is:
A scriptPubKey (or redeemScript as defined in BIP16/P2SH) that consists of a 1-byte push opcode (for 0 to 16) followed by a data push between 2 and 32 bytes gets a new special meaning. The value of the first push is called the "version byte". The following byte vector pushed is called the "witness program".
Comparing with a P2PKH output, the witness program equivalent occupies 2 more bytes in the scriptPubKey, while moving the signature and public key from scriptSig to witness.
=== Version 1 witness program ===
The following example is an 1-of-2 multi-signature version 1 witness program.
The '1' in scriptPubKey indicates the following push is a version 1 witness program. The last item in the witness is popped off, hashed with SHA256, compared against the 32-byte-hash in scriptPubKey, and deserialized:
Since the actual program is larger than 32 bytes, it cannot be accommodated in a version 0 witness program. A version 1 witness program allows arbitrarily large script as the 520-byte push limit is bypassed.
The scriptPubKey occupies 34 bytes, as opposed to 23 bytes of P2SH. The increased size improves security against possible collision attacks, as 2<sup>80</sup> work is not infeasible anymore (By the end of 2015, 2<sup>84</sup> hashes have been calculated in Bitcoin mining since the creation of Bitcoin). The spending script is same as the one for an equivalent P2SH output but is moved to witness.
The version 1 witness program is then executed as described in the last example
Comparing with the last example, the scriptPubKey is 11 bytes smaller (with reduced security) while witness is the same. However, it also requires 35 bytes in scriptSig, which is not prunable in transmission. Although a nested witness program is less efficient in many ways, its payment address is fully transparent and backward compatible for all Bitcoin reference client since version 0.6.0.
The new commitment in coinbase transaction is a hash of the witness root hash and a witness nonce. The nonce which currently has no consensus meaning, serves two purposes:
* It allows new commitment values for future softforks. For example, if a new consensus-critical commitment is required in the future, the commitment in coinbase becomes:
:: For backward compatibility, the Hash(new commitment|witness nonce) will go to the coinbase witness, and the witness nonce will be recorded in another location specified by the future softfork. Any number of new commitment could be added in this way.
* Any commitments that are not consensus-critical to Bitcoin, such as merge-mining, may utilize this witness nonce. However, they must not be committed directly as the nonce, or the external system may be forced to hardfork when Bitcoin introduces more consensus-critical commitments. Instead, they should use the nonce as the root of an extensible commitment tree, and should not make any assumption about the location and depth of their commitments in the tree. For example, in the external system, it may use a flag to indicate the actual location of the commitments with the Merkle paths provided.
The optional data space following the commitment also leaves room for metadata of future softforks.
Segregated witness fixes the problem of transaction malleability fundamentally, which enables the building of unconfirmed transaction dependency chains in a trust-free manner.
Two parties, Alice and Bob, may agree to send certain amount of Bitcoin to a 2-of-2 multisig output (the "funding transaction"). Without signing the funding transaction, they may create another transaction, time-locked in the future, spending the 2-of-2 multisig output to third account(s) (the "spending transaction"). Alice and Bob will sign the spending transaction and exchange the signatures. After examining the signatures, they will sign and commit the funding transaction to the blockchain. Without further action, the spending transaction will be confirmed after the lock-time and release the funding according to the original contract. It also retains the flexibility of revoking the original contract before the lock-time, by another spending transaction with shorter lock-time, but only with mutual-agreement of both parties.
Such setups is not possible with BIP62 as the malleability fix, since the spending transaction could not be created without both parties first signing the funding transaction. If Alice reveals the funding transaction signature before Bob does, Bob is able to lock up the funding indefinitely without ever signing the spending transaction.
Unconfirmed transaction dependency chain is a fundamental building block of more sophisticated payment networks, such as duplex micropayment channel and the Lightning Network, which have the potential to greatly improve the scalability and efficiency of the Bitcoin system.
Bitcoin right now only has two real security models. A user either runs a full-node which validates every block with all rules in the system, or a SPV (Simple Payment Verification) client which only validates the headers as a proof of publication of some transactions. The Bitcoin whitepaper suggested that SPV nodes may accept alerts from full nodes when they detect an invalid block, prompting the SPV node to download the questioned blocks and transactions for validation. This approach, however, could become a DoS attack vector as there is virtually no cost to generate a false alarm. An alarm must come with a compact, yet deterministic fraud proof.
In the current Bitcoin protocol, it is possible to generate compact fraud proof for almost all rules except a few:
# It is not possible to prove a miner has introduced too many Bitcoins in the coinbase transaction outputs without showing the whole block itself and all input transactions.
# It is not possible to prove the violation of any block specific constraints, such as size and sigop limits, without showing the whole block (and all input transactions in the case of sigop limit)
# It is not possible to prove the spending of a non-existing input without showing all transaction IDs in the blockchain way back to the genesis block.
Extra witness data can be committed that allows short proofs of block invalidity that SPV nodes can quickly verify:
# Sum trees for transaction fee can be committed making it possible to construct short proofs that the miner does not add excessive fees to the coinbase transaction. Similar for the block size and sigop count limit.
# Backlinks for the outputs spent by the transaction's inputs can be provided. These backlinks consist of a block hash and an offset that thin clients can easily query and check to verify that the outputs exist.
These commitments could be included in the extensible commitment structure through a soft fork and will be transparent to nodes that do not understand such new rules.
Since a version byte is pushed before a witness program, and programs with unknown versions are always considered as anyone-can-spend script, it is possible to introduce any new script system with a soft fork. The witness as a structure is not restricted by any existing script semantics and constraints, the 520-byte push limit in particular, and therefore allows arbitrarily large scripts and signatures.
Examples of new script system include Schnorr signatures which reduce the size of multisig transactions dramatically, Lamport signature which is quantum computing resistance, and Merklized abstract syntax trees which allow very compact witness for conditional scripts with extreme complexity.
The 32-byte limitation for witness program could be easily extended through a soft fork in case a stronger hash function is needed in the future. The version byte is also expandable through a softfork.
Currently there is only one nLockTime field in a transaction and all inputs must share the same value. [https://github.com/bitcoin/bips/edit/master/bip-0068.mediawiki BIP68] enables per-input relative-lock-time using the nSequence field, however, with a limited lock-time period and resolution.
With a soft fork, it is possible to introduce a separate witness structure to allow per-input lock-time and relative-lock-time, and a new script system that could sign and manipulate the new data (like [https://github.com/bitcoin/bips/edit/master/bip-0065.mediawiki BIP65] and [https://github.com/bitcoin/bips/edit/master/bip-0112.mediawiki BIP112]).
As a soft fork, older software will continue to operate without modification. Non-upgraded nodes, however, will not see nor validate the witness data and will consider all witness programs as anyone-can-spend scripts (except a few edge cases in version 0 witness programs which are provably unspendable with original script semantics). Wallets should always be wary of anyone-can-spend scripts and treat them with suspicion. Non-upgraded nodes are strongly encouraged to upgrade in order to take advantage of the new features.
* Sending bitcoin to upgraded wallets using a P2SH address (less efficient)
* Sending bitcoin to upgraded wallets using a native witness program (more efficient) through [https://github.com/bitcoin/bips/blob/master/bip-0070.mediawiki BIP70] payment protocol
*[https://github.com/bitcoin/bips/blob/master/bip-0016.mediawiki BIP16 Pay to Script Hash]
*[https://github.com/bitcoin/bips/blob/master/bip-0142.mediawiki BIP142 Address Formats for Witness Program]
*[https://github.com/bitcoin/bips/blob/master/bip-0143.mediawiki BIP143 Transaction signature verification for version 0 and version 1 witness program]
