I updated the draft BIP with a proposed reference implementation and a
link to an implementation of a caching strategy.
It shows that it's possible to achieve TXHASH in a way that after each
large tx element (scripts, annexes) has been hashed exactly once,
invocations of TXHASH have clear constant upper limits on the number of
bytes hashes.
Link to the draft BIP in above e-mail and link to the cache impl here:
https://github.com/stevenroose/rust-bitcoin/blob/txhash/bitcoin/src/blockdata/script/txhash.rs
On 9/30/23 12:44, Steven Roose via bitcoin-dev wrote:
Hey all
The idea of TXHASH has been around for a while, but AFAIK it was never
formalized. After conversations with Russell, I worked on a
specification and have been gathering some feedback in the last
several weeks.
I think the draft is in a state where it's ready for wider feedback
and at the same time I'm curious about the sentiment in the community
about this idea.
The full BIP text can be found in the attachment as well as at the
following link:
https://github.com/bitcoin/bips/pull/1500
I will summarize here in this writing.
*What does the BIP specify?*
* The concept of a TxFieldSelector, a serialized data structure for
selecting data inside a transaction.
o The following global fields are available:
+ version
+ locktime
+ number of inputs
+ number of outputs
+ current input index
+ current input control block
o For each input, the following fields are available:
+ previous outpoint
+ sequence
+ scriptSig
+ scriptPubkey of spending UTXO
+ value of spending UTXO
+ taproot annex
o For each output, the following fields are available:
+ scriptPubkey
+ value
o There is support for selecting inputs and outputs as follows:
+ all in/outputs
+ a single in/output at the same index as the input being
executed
+ any number of leading in/outputs up to 2^14 - 1 (16,383)
+ up to 64 individually selected in/outputs (up to 2^16 or
65,536)
o The empty byte string is supported and functions as a default
value which commits to everything except the previous
outpoints and the scriptPubkeys of the spending UTXOs.
* An opcode OP_TXHASH, enabled only in tapscript, that takes a
serialized TxFieldSelector from the stack and pushes on the stack
a hash committing to all the data selected.
* An opcode OP_CHECKTXHASHVERIFY, enabled in all script contexts,
that expects a single item on the stack, interpreted as a 32-byte
hash value concatenated with (at the end) a serialized
TxFieldSelector. Execution fails is the hash value in the data
push doesn't equal the calculated hash value based on the
TxFieldSelector.
* A consideration for resource usage trying to address concerns
around quadratic hashing. A potential caching strategy is outlined
so that users can't trigger excessive hashing.
o Individual selection is limited to 64 items.
o Selecting "all" in/outputs can mostly use the same caches as
sighash calculations.
o For prefix hashing, intermediate SHA256 contexts can be stored
every N items so that at most N-1 items have to be hashed when
called repeatedly.
o In non-tapscript contexts, at least 32 witness bytes are
required and because (given the lack of OP_CAT) subsequent
calls can only re-enforce the same TxFieldSelector, no
additional limitations are put in place.
o In tapscript, because OP_TXHASH doesn't require 32 witness
bytes and because of a potential addition of operations like
OP_CAT, the validation budget is decreased by 10 for every
OP_TXHASH or OP_CHECKTXHASHVERIFY operation.
*What does this achieve?*
* Since the default TxFieldSelector is functionally equivalent to
OP_CHECKTEMPLATEVERIFY, with no extra bytes required, this
proposal is a strict upgrade of BIP-119.
* The flexibility of selecting transaction fields and in/output
(ranges), makes this construction way more useful
o when designing protocols where users want to be able to add
fees to their transactions without breaking a transaction chain;
o when designing protocols where users construct transactions
together, each providing some of their own in- and outputs and
wanting to enforce conditions only on these in/outputs.
* OP_TXHASH, together with OP_CHECKSIGFROMSTACK (and maybe OP_CAT*)
could be used as a replacement for almost arbitrarily complex
sighash constructions, like SIGHASH_ANYPREVOUT.
* Apart from being able to enforce specific fields in the
transaction to have a pre-specified value, equality can also be
enforced, which can f.e. replace the desire for opcodes like
OP_IN_OUT_VALUE.*
* The same TxFieldSelector construction would function equally well
with a hypothetical OP_TX opcode that directly pushes the selected
fields on the stack to enable direct introspection.
*What are still open questions?*
* Does the proposal sufficiently address concerns around resource
usage and quadratic hashing?
* *: Miraculously, once we came up with all possible fields that we
might consider interesting, we filled exactly 16 spots. There is
however one thing that I would have liked to be optionally
available and I am unsure of which side to take in the proposal.
This is including the TxFieldSelector as part of the hash. Doing
so no longer makes the hash only represent the value being hashed,
but also the field selector that was used; this would no longer
make it possible to proof equality of fields. If a txhash as
specified here would ever be used as a signature hash, it would
definitely have to be included, but this could be done after the
fact if OP_CAT was available. For signature hashes, the hash
should ideally be somehow tagged, so we might need OP_CAT, or
OP_CATSHA256 or something anyway.
* A solution could be to take an additional bit from each of the
two "in/output selector" bytes, and assign to this bit "commit
to total number of in/outputs" (instead of having 2 bits for
this in the first byte).
o This would free up 2 bits in the first byte, one of which
could be used for including the TxFieldSelector in the
hash and the other one could be left free (OP_SUCCESS) to
potentially revisit later-on.
o This would limit the number of selectable leading
in/outputs to 8,191 and the number of individually
selectable in/outputs to 32, both of which seem reasonable
or maybe even more desirable from a resource usage
perspective.
* General feedback of how people feel towards a proposal like this,
which could either be implemented in a softfork as is, like
BIP-119 or be combined in a single softfork with
OP_CHECKSIGFROMSTACK and perhaps OP_CAT, OP_TWEAKADD and/or a
hypothetical OP_TX.
This work is just an attempt to make some of the ideas that have been
floating around into a concrete proposal. If there is community
interest, I would be willing to spend time to adequately formalize
this BIP and to work on an implementation for Bitcoin Core.
Looking forward to your thoughts
Steven
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