Subject: Re: [bitcoindev] What's a good stopping point? Making the case for the capabilities enabled by CTV+CSFS

From: Anthony Towns <[email-masked]>
Date: Thu, 3 Jul 2025 13:55:28 +1000

On Tue, Jun 24, 2025 at 11:54:02AM -0400, Matt Corallo wrote:
> > which
> > warrants a compelling demonstration that arbitrary transaction introspection
> > does enable important use cases not achievable with more minimal capabilities.
> I'm somewhat skeptical that showing this isn't rather simple,

I think the BitVM/CTV idea posted on delving [0] is one such simple demo?

I gave an example in that thread of how you'd implement the desired
construct using bllsh's introspection primitives, but the same could
equally well be done with Rusty's as-yet unpublished OP_TX, something
like:

  DUP 0x1011 TX 0x00000002 EQUALVERIFY 0x1009 TX 0x0809 TX EQUALVERIFY

where:

 * "0x1011 TX" pops an input index from the stack and gives the four-byte
   vout index of that input's prevout
 * "0x1009 TX" pops an input index from the stack and gives the txid of that input's
   prevout
 * "0x0809 TX" gives the txid of the current input's prevout

(this encodes "this utxo can only be spent (via this path) if its sibling
output at index 2 is also being spent in the same transaction")

Cheers,
aj

[0] https://delvingbitcoin.org/t/how-ctv-csfs-improves-bitvm-bridges/1591

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