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To: ZmnSCPxj <ZmnSCPxj@protonmail.com>,Nadav Ivgi <nadav@shesek.info>
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Cc: Bitcoin Protocol Discussion <bitcoin-dev@lists.linuxfoundation.org>
Subject: Re: [bitcoin-dev] Speedy covenants (OP_CAT2)
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> Looks like `OP_CAT` is not getting enabled until after we are reasonably =
sure that recursive covenants are not really unsafe.

Maybe we should use OP_SUBSTR instead of OP_CAT. Or even better: OP_SPLIT. =
Then, we could have OP_SPLIT <n> <pos1> <pos2> ... <posN> that would split =
a string N times (so there will be N+1 pieces). Or we could have just OP_SP=
LIT <pos> to split one string into two. Or maybe OP_2SPLIT and OP_3SPLIT, j=
ust to split into two or three pieces (as we have OP_2DUP and OP_3DUP). I t=
hink OP_SUBSTR or OP_SPLIT is better than OP_CAT, because then things alway=
s get smaller and we can be always sure that we will have one byte as the s=
mallest unit in our Script.

On 2022-05-08 04:20:19 user ZmnSCPxj via bitcoin-dev <bitcoin-dev@lists.lin=
uxfoundation.org> wrote:
> 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:
> >  * `OP_CTV`, ...
> > * With `OP_CAT`, the following would enable recursive covenants:
> >  * `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
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