Date: Sun, 08 May 2022 02:19:57 +0000
To: Nadav Ivgi <nadav@shesek.info>
From: ZmnSCPxj <ZmnSCPxj@protonmail.com>
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Cc: Bitcoin Protocol Discussion <bitcoin-dev@lists.linuxfoundation.org>
Subject: Re: [bitcoin-dev] Speedy covenants (OP_CAT2)
Precedence: list

Good morning shesek,

> On Sat, May 7, 2022 at 5:08 PM ZmnSCPxj via bitcoin-dev <bitcoin-dev@list=
s.linuxfoundation.org> wrote:
> > * Even ***with*** `OP_CAT`, the following will enable non-recursive cov=
enants without enabling recursive covenants:
> >=C2=A0 * `OP_CTV`, ...
> > * With `OP_CAT`, the following would enable recursive covenants:
> >=C2=A0 * `OP_CHECKSIGFROMSTACK`, ...
>
> Why does CTV+CAT not enable recursive covenants while CSFS+CAT does?
>
> CTV+CAT lets you similarly assert against the outputs and verify that the=
y match some dynamically constructed script.
>
> Is it because CTV does not let you have a verified copy of the input's pr=
evout scriptPubKey on the stack [0], while with OP_CSFS you can because the=
 signature hash covers it?
>
> But you don't actually need this for recursion. Instead of having the use=
r supply the script in the witness stack and verifying it against the input=
 to obtain the quine, the script can simply contain a copy of itself as an =
initial push (minus this push). You can then reconstruct the full script qu=
ine using OP_CAT, as a PUSH(<script>) followed by the literal <script>.

    <OP_PUSH_length-of-script> OP_SWAP OP_DUP OP_CAT OP_CAT <rest of script=
...>

Ha, yes, looks like you are correct here.

`OP_CAT` makes *all* covenant opcodes recursive, because you can always qui=
ne using `OP_CAT`.

By itself it does not make recursive covenants, but with probably any opcod=
e it would.

Looks like `OP_CAT` is not getting enabled until after we are reasonably su=
re that recursive covenants are not really unsafe.

Regards,
ZmnSCPxj