Subject: Re: [bitcoin-dev] CTV BIP Meeting #8 Notes

From: Billy Tetrud <[email-masked]>
Date: Sun, 8 May 2022 11:32:45 -0500

>  This requires *recursive* covenants.

Actually, for practical use, any walled-garden requires *dynamic*
covenants, not recursive covenants. CTV can get arbitrarily close to
recursive covenants, because you can have an arbitrarily long string of
covenants. But this doesn't help someone implement visacoin because CTV
only allows a specific predefined iteration of transactions, meaning that
while "locked" into the covenant sequence, the coins can't be used in any
way like normal coins - you can't choose who you pay, the sequence is
predetermined.

Even covenants that allow infinite recursion (like OP_TLUV and OP_CD
<https://github.com/fresheneesz/bip-efficient-bitcoin-vaults/blob/main/cd/bip-constraindestination.md>)
don't automatically allow for practical walled gardens. Recursion
definitely allows creating walled gardens, but those gardens would be
impractically static. You could add millions of potential addresses to send
to, which would "only" quadruple the size of your transactions, but if
anyone creates a new address you want to send to, you wouldn't be able to.
Everyone would have to have a single address whitelisted into every
government-bitcoin output. If someone lost their key and needs to create a
new wallet, suddenly no one would be able to pay them.

In order to really build a wallet garden, infinite recursion isn't really
necessary nor sufficient. You need to be able to dynamically specify
destination addresses. For example, if you were a government that wants to
make a walled garden where you (the government) could confiscate the funds
whenever you wanted, you'd have to have a covenant that allows the end-user
to specify an arbitrary public key to send money to. The covenant might
require that user to send to another covenant that has a government spend
path, but also has a spend path for that user-defined public key. That way,
you (the government) could allow people to send to each other arbitrarily,
while still ensuring that you (the government) could spend the funds no
matter where they may have been sent. Even without recursive covenants, you
could have arbitrarily long chains of these, say 1 million long, where at
the end of the chain the user must send your coins back to the government
who can then send them back with another million-long chain of covenants to
work with.

OP_CHECKOUTPUTVERIFY <https://fc16.ifca.ai/bitcoin/papers/MES16.pdf> can do
this kind of dynamicness, and OP_PUSHOUTPUTSTACK
<https://github.com/fresheneesz/bip-efficient-bitcoin-vaults/blob/main/pos/bip-pushoutputstack.md>
can
enable it for things like OP_TLUV and OP_CD. I personally think dynamic
covenants are a *good* thing, as it enables more secure wallet vaults,
among other things. And I'm not worried about a government creating a
in-bitcoin visa-coin. Why? Because they can already do it today. They have
been able to do it for 9 years already. How?

Replace the covenant above with a multisig wallet. The government has 2
keys, you have 1 key. Every time you make a transaction, you request the
government's signature on it. The government then only signs if you're
sending to a wallet they approve of. They might only sign when you're
sending to another multisig wallet that the government has 2 of 3 keys for.
Its a very similar walled garden, where the only difference is that the
government needs to actively sign, which I'm sure wouldn't be a huge
challenge for the intrepid dictator of the land. You want to add
demurage fees? Easy, the government just spends the fee out of everyone's
wallets every so often.

On the other hand, OP_CTV *cannot* be used for such a thing. No combination
of future opcodes can enable either recursion or dynamicness to an OP_CTV
call.

On Sat, May 7, 2022 at 5:40 PM ZmnSCPxj via bitcoin-dev <
[email-masked]> wrote:

> Good morning Jorge,
>
> > I think people may be scared of potential attacks based on covenants.
> For example, visacoin.
> > But there was a thread with ideas of possible attacks based on covenants.
> > To me the most scary one is visacoin, specially seeing what happened in
> canada and other places lately and the general censorship in the west, the
> supposed war on "misinformation" going on (really a war against truth imo,
> but whatever) it's getting really scary. But perhaps someone else can be
> more scared about a covenant to add demurrage fees to coins or something, I
> don't know.
> > https://bitcointalk.org/index.php?topic=278122
>
> This requires *recursive* covenants.
>
> At the time the post was made, no distinction was seen between recursive
> and non-recursive covenants, which is why the post points out that
> covenants suck.
> The idea then was that anything powerful enough to provide covenants would
> also be powerful enough to provide *recursive* covenants, so there was no
> distinction made between recursive and non-recursive covenants (the latter
> was thought to be impossible).
>
> However, `OP_CTV` turns out to enable sort-of covenants, but by
> construction *cannot* provide recursion.
> It is just barely powerful enough to make a covenant, but not powerful
> enough to make *recursive* covenants.
>
> That is why today we distinguish between recursive and non-recursive
> covenant opcodes, because we now have opcode designs that provides
> non-recursive covenants (when previously it was thought all covenant
> opcodes would provide recursion).
>
> `visacoin` can only work as a recursive covenant, thus it is not possible
> to use `OP_CTV` to implement `visacoin`, regardless of your political views.
>
> (I was also misinformed in the past and ignored `OP_CTV` since I thought
> that, like all the other covenant opcodes, it would enable recursive
> covenants.)
>
>
> Regards,
> ZmnSCPxj
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