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From: Brandon Black <freedom@reardencode.com>
To: Rusty Russell <rusty@rustcorp.com.au>,
Bitcoin Protocol Discussion <bitcoin-dev@lists.linuxfoundation.org>
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Subject: Re: [bitcoin-dev] Examining ScriptPubkeys in Bitcoin Script
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On 2023-10-20 (Fri) at 14:10:37 +1030, Rusty Russell via bitcoin-dev wrote:
> I've done an exploration of what would be required (given
> OP_TX/OP_TXHASH or equivalent way of pushing a scriptPubkey on the
> stack) to usefully validate Taproot outputs in Bitcoin Script. Such
> functionality is required for usable vaults, at least.
So you're proposing this direction as an alternative to the more
constrained OP_UNVAULT that replaces a specific leaf in the taptree in a
specific way? I think the benefits of OP_UNVAULT are pretty big vs. a
generic construction (e.g. ability to unvault multiple inputs sharing
the same scriptPubkey to the same output).
> TL;DR: if we have OP_TXHASH/OP_TX, and add OP_MULTISHA256 (or OP_CAT),
> OP_KEYADDTWEAK and OP_LESS (or OP_CONDSWAP), and soft-fork weaken the
> OP_SUCCESSx rule (or pop-script-from-stack), we can prove a two-leaf
> tapscript tree in about 110 bytes of Script. This allows useful
> spending constraints based on a template approach.
I agree that this is what is needed. I started pondering this in
response to some discussion about the benefits of OP_CAT or OP_2SHA256
for BitVM.
Personally I'd use OP_TAGGEDCATHASH that pops a tag (empty tag can be
special cased to plain sha256) and a number (n) of elements to hash,
then tagged-hashes the following 'n' elements from the stack.
Best,
--Brandon
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