From: Rusty Russell <rusty@rustcorp.com.au>
To: Jeremy Rubin <jeremy.l.rubin@gmail.com>
In-Reply-To: <CAD5xwhi4y1NiZ__c1WY-rCV3XBzN5yxY1Zox6Mc1FTjxUhXK9A@mail.gmail.com>
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Date: Wed, 16 Feb 2022 12:56:14 +1030
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Cc: Bitcoin Protocol Discussion <bitcoin-dev@lists.linuxfoundation.org>
Subject: Re: [bitcoin-dev] TXHASH + CHECKSIGFROMSTACKVERIFY in lieu of CTV
	and ANYPREVOUT
Precedence: list

Jeremy Rubin <jeremy.l.rubin@gmail.com> writes:
> Hi Rusty,
>
> Please see my post in the other email thread
> https://lists.linuxfoundation.org/pipermail/bitcoin-dev/2022-February/019886.html
>
> The differences in this regard are several, and worth understanding beyond
> "you can iterate CTV". I'd note a few clear examples for showing that "CTV
> is just as powerful" is not a valid claim:
>
> 1) CTV requires the contract to be fully enumerated and is non-recursive.
> For example, a simple contract that allows n participants to take an action
> in any order requires factorially many pre-computations, not just linear or
> constant. For reference, 24! is about 2**80. Whereas for a more
> interpretive covenant -- which is often introduced with the features for
> recursion -- you can compute the programs for these addresses in constant
> time.
> 2) CTV requires the contract to be fully enumerated: For example, a simple
> contract one could write is "Output 0 script matches Output 1", and the set
> of outcomes is again unbounded a-priori. With CTV you need to know the set
> of pairs you'd like to be able to expand to a-priori
> 3) Combining 1 and 2, you could imagine recursing on an open-ended thing
> like creating many identical outputs over time but not constraining what
> those outputs are. E.g., Output 0 matches Input 0, Output 1 matches Output
> 2.

Oh agreed.  It was distinction of "recursive" vs "not recursive" which
was less useful in this context.

"limited to complete enumeration" is the more useful distinction: it's a
bright line between CTV and TXHASH IMHO.

> I'll close by repeating : Whether that [the recursive/open ended
> properties] is an issue or not precluding this sort of design or not, I
> defer to others.

Yeah.  There's been some feeling that complex scripting is bad, because
people can lose money (see the various attempts to defang
SIGHASH_NOINPUT).  I reject that; since script exists, we've crossed the
Rubicon, so let's make the tools as clean and clear as we can.

Cheers!
Rusty.