Date: Wed, 23 Feb 2022 11:42:54 +0000
To: Antoine Riard <antoine.riard@gmail.com>
From: ZmnSCPxj <ZmnSCPxj@protonmail.com>
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Cc: Bitcoin Protocol Discussion <bitcoin-dev@lists.linuxfoundation.org>,
 Anthony Towns <aj@erisian.com.au>
Subject: Re: [bitcoin-dev] `OP_EVICT`: An Alternative to
	`OP_TAPLEAFUPDATEVERIFY`
Precedence: list

Good morning Antoine,

> TLUV doesn't assume cooperation among the construction participants once =
the Taproot tree is setup. EVICT assumes cooperation among the remaining co=
nstruction participants to satisfy the final CHECKSIG.
>
> So that would be a feature difference between TLUV and EVICT, I think ?

`OP_TLUV` leaves the transaction output with the remaining Tapleaves intact=
, and, optionally, with a point subtracted from Taproot internal pubkey.

In order to *truly* revive the construct, you need a separate transaction t=
hat spends that change output, and puts it back into a new construct.

See: https://lists.linuxfoundation.org/pipermail/lightning-dev/2022-Februar=
y/003479.html
I describe how this works.

That `OP_EVICT` does another `CHECKSIG` simply cuts through the separate tr=
ansaction that `OP_TLUV` would require in order to revive the construct.

> > I thought it was part of Taproot?
>
> I checked BIP342 again, *as far as I can read* (unreliable process), it s=
ounds like it was proposed by BIP118 only.

*shrug* Okay!

> > A single participant withdrawing their funds unilaterally can do so by =
evicting everyone else (and paying for those evictions, as sort of a "nuisa=
nce fee").
>
> I see, I'm more interested in the property of a single participant withdr=
awing their funds, without affecting the stability of the off-chain pool an=
d without cooperation with other users. This is currently a restriction of =
the channel factories fault-tolerance. If one channel goes on-chain, all th=
e outputs are published.

See also: https://lists.linuxfoundation.org/pipermail/lightning-dev/2022-Fe=
bruary/003479.html

Generally, the reason for a channel to go *onchain*, instead of just being =
removed inside the channel factory and its funds redistributed elsewhere, i=
s that an HTLC/PTLC is about to time out.
The blockchain is really the only entity that can reliably enforce timeouts=
.

And, from the above link:

> * If a channel has an HTLC/PTLC time out:
>   * If the participant to whom the HTLC/PTLC is offered is
>     offline, that may very well be a signal that it is unlikely
>     to come online soon.
>     The participant has strong incentives to come online before
>     the channel is forcibly closed due to the HTLC/PTLC timeout,
>     so if it is not coming online, something is very wrong with
>     that participant and we should really evict the participant.
>   * If the participant to whom the HTLC/PTLC is offered is
>     online, then it is not behaving properly and we should
>     really evict the participant.

Note the term "evict" as well --- the remaining participants that are presu=
mably still behaving correctly (i.e. not letting HTLC/PTLC time out) evict =
the participants that *are*, and that is what `OP_EVICT` does, as its name =
suggests.

Indeed, I came up with `OP_EVICT` *after* musing the above link.

Regards,
ZmnSCPxj