That is the third article in a sequence deep diving into particular person covenant proposals which have reached some extent of maturity meriting an in-depth breakdown.
TXHASH and CHECKTXHASHVERIFY (TXHASH), put ahead by Steven Roose and Brandon Black with a BIP quantity at the moment unassigned, is a “template based mostly” covenant that may conceptually seen as an extension or extra superior model of CHECKTEMPLATEVERIFY (CTV).
Earlier than entering into the nitty gritty of how TXHASH works, let’s refresh on the items of knowledge in a Bitcoin transaction.
At a excessive stage you have got the outputs, the inputs, and the witness (or script sig for non-Segwit transactions within the enter).
International transaction fields:
Model Marker, indicating Segwit with a flag worth Flag, indicating Segwit with a flag worth Enter depend Output depend nLocktime, used for timelocks
Every enter incorporates:
TXID of the earlier transaction VOUT (index) of the output from that transaction being spent ScriptSig measurement ScriptSig (if a non-segwit transaction) Sequence quantity (used for RBF flagging and relative timelocks).
Every output incorporates:
Quantity of satoshis assigned to the output ScriptPubKeySize, the scale of the locking script ScriptPubkey, the precise locking script
We are able to ignore the witness subject when contemplating TXHASH or CHECKTXHASHVERIFY as neither opcode constrains the witness subject to retain sure properties.
How TXHASH Works
Each TXHASH (tapscript solely) and CHECKTXHASHVERIFY (legacy script and tapscript) have completely different behaviors on the stack due to the variations between legacy script and tapscript. For the needs of this text, these variations are usually not materials, so we’re merely going to disregard them.
If CTV is a covenant opcode that constrains a bitcoin output to solely be spent in a singular and precisely outlined method, TXHASH is a supercharged model of CTV that allows you to decide and select precisely what items of a transaction are constrained and have to be spent within the precisely pre-defined method, and which items of a transaction might be no matter somebody desires at spending time.
It offers you the perfect of each worlds, requiring one thing be finished when spending a covenant restricted coin, however then permitting a consumer to do no matter they need with the remainder of the funds out there to them or the transaction they’re crafting.
That is achieved utilizing the ‘TxFieldSelector’.
CTV merely makes use of a single hash of the pre-defined transaction with a view to confirm at spending time. With TXHASH, you want a solution to talk what items of data that hash is committing to, and what items of data it isn’t. That’s the TxFieldSelector’s job.
TxFieldSelector is actually a sequence of bytes (that may be variable in size), with every bit speaking what fields in a transaction are dedicated to by the hash that will likely be verified towards. This lets you choose particular fields of the transaction, nLocktime, model and many others. It lets you choose particular fields of the inputs and outputs, i.e. embody or not the sequence quantity, or the earlier output id, or the taproot annex (a datafield particular to taproot scripts). The outputs, whether or not to decide to the ScriptPubkey, the quantity values, each, or neither. You too can determine precisely which outputs and inputs these restrictions apply to.
There’s some complexity and adaptability in how the TxFieldSelector is put collectively, and you may learn all of the finer particulars right here within the proposed BIP in the event you’re interested by these, however the chief level to remove is it lets you decide precisely which components of the transaction are restricted by the covenant when somebody goes to spend the encumbered output, and which components are usually not, to a really granular diploma.
What Is TXHASH Helpful For
Firstly, TXHASH lets you do all the things you could with CTV. So all the worth supplied by CTV to optimize the coordination prices of something at the moment doable with pre-signed transactions can be supplied by TXHASH. However it supercharges that functionality massively. As a substitute of getting to decide to the whole lot of a transaction, you’ll be able to commit to simply the components you care about.
This has two large advantages in principle proper off the bat. To start with, in band payment administration for layer twos turns into simpler to take care of. At the moment the usage of anchor outputs is required to fee-bump layer two settlement transactions with Little one Pays For Mum or dad, the place a transaction spending an output from an unconfirmed one can add to the online charges for each. TXHASH allows you to decide to solely your counterparties outputs in a multiparty transaction, and go away yours free to do no matter you need with (caveat right here that different issues have to be finished to make this secure so a 3rd occasion can’t burn your whole funds to charges), together with decrementing barely to RBF the transaction.
Second, the door is now open for multiparty protocols to permit granular ensures about what off-chain transactions are committing to. Some customers can now obtain a assure about how their cash will likely be spent, however not should care about what another group of customers does with theirs. I can make sure that one TxFieldSelector ensures my cash are dealt with correctly, and I don’t should care about the place anybody else’s cash go.
Together with CHECKSIGFROMSTACK (CSFS), TXHASH can facilitate a very generalized SIGHASH system. The SIGHASH flag is part of a signature that communicates what components of the transaction to examine the signature towards. They’re at the moment:
SIGHASH_ALL – indicators all inputs and outputs SIGHASH_NONE – indicators all inputs and no outputs SIGHASH_SINGLE – indicators all inputs and the output with the identical index as this enter
None of those SIGHASH flags enable including any new inputs to a transaction with out invalidating them, however every has an ANYONECANPAY model that solely indicators its personal enter and the suitable outputs, permitting anybody else to then add new inputs, and new outputs for the ANYONECANPAY model of SIGHASH_NONE and SIGHASH_SINGLE.
By having the ability to “sideload” new TxFieldSelectors utilizing CSFS, customers can emulate a SIGHASH system that enables them to choose and select precisely which particular person items of a transaction the signature commits to or not.
TXHASH additionally permits implementing equality between the worth of inputs and outputs through the use of particular person TxFieldSelectors that commit solely to a single worth subject of an enter or output you want to examine, after which guaranteeing their hashes are the identical on the stack.
Closing Ideas
TXHASH is a possible supercharging of CTV, enabling an extremely granular diploma of introspection of the spending transaction which might be extremely highly effective, particularly together with one thing like CSFS.
Nevertheless, that energy is expressive sufficient that it opens up the door to an extremely giant design area. One that might probably have a cloth impact on the general incentives of Bitcoin. Issues like guaranteeing quantity equality throughout outputs or inputs is getting very near the territory of what’s wanted for trustless automated trade on-chain. That could be a critical supply of Miner Extractable Worth (MEV), which has been a really critical incentive and centralization drawback for different blockchains to take care of.
TXHASH ought to completely not be dismissed, because it supplies extremely highly effective primitives for protocol builders to reap the benefits of, however the potential second order implications of what folks will construct with it must be weighed towards the positives.
