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Date: Tue, 19 Nov 2024 11:35:32 -0800
From: Brandon Black <freedom@reardencode.com>
To: moonsettler <moonsettler@protonmail.com>
Cc: Weikeng Chen <weikeng.chen@l2iterative.com>,
	Bitcoin Development Mailing List <bitcoindev@googlegroups.com>
Subject: Re: [bitcoindev] Multi-byte opcodes
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On 2024-11-19 (Tue) at 16:38:21 +0000, moonsettler wrote:
> I think we should discuss back-porting tapscript instead of coming up with a further divergent set of opcodes.
> 
> To sum up earlier discussion with Brandon, we could:
> 
> * Use a single upgradeable NOP for OP_TAPSCRIPTVERIFY
> * <s1> .. <sn> | <faux-control-block> <tapscript> CTAPV
> * Isolated execution environment
> * Gets the entire stack below the top 2 elements
> * Fails if it fails, does nothing if internal script succeeds.
> * Require the last opcode executed by the script interpreter to be a push opcode, fails otherwise
> 
> The faux control block can be just a few bytes mainly for tapscript version.

Few additional points that came up in discussing this with moon earlier:

* Compared to enabling tapscript without a key spend path (i.e.
	potential for quantum resistance) via a new witnessv1 program length
	or a new witness version, this approach will be similar in weight.
* The biggest challenge with this approach that I came up with is the
	need to clean up the stack after execution (to satisfy witness v0
	clean stack).
* To keep it simple, it probably makes sense to make it a 1-primary
  argument opcode where the argument is `<tapscript||version>` to
	prevent the version from coming from spend-time witness elements.

Best,

--Brandon

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