Date: Thu, 8 Jul 2021 18:44:16 +1000
From: Anthony Towns <aj@erisian.com.au>
To: Jeremy <jlrubin@mit.edu>,
 Bitcoin Protocol Discussion <bitcoin-dev@lists.linuxfoundation.org>
Message-ID: <20210708084416.GB1339@erisian.com.au>
References: <CAD5xwhgO5p2Ldy1P1rUuqDHcJ0opSe7Tg_mX_rb+ZpLOxa47cA@mail.gmail.com>
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Cc: lightning-dev <lightning-dev@lists.linuxfoundation.org>
Subject: Re: [bitcoin-dev] Eltoo / Anyprevout & Baked in Sequences
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On Wed, Jul 07, 2021 at 06:00:20PM -0700, Jeremy via bitcoin-dev wrote:
> This means that you're overloading the CLTV clause, which means it's impossible
> to use Eltoo and use a absolute lock time,

It's already impossible to simultaneously spend two inputs if one
requires a locktime specified by mediantime and the other by block
height. Having per-input locktimes would satisfy both concerns.

> 1) Define a new CSV type (e.g. define (1<<31 && 1<<30) as being dedicated to
> eltoo sequences). This has the benefit of giving a per input sequence, but the
> drawback of using a CSV bit. Because there's only 1 CSV per input, this
> technique cannot be used with a sequence tag.

This would disallow using a relative locktime and an absolute locktime
for the same input. I don't think I've seen a use case for that so far,
but ruling it out seems suboptimal.

Adding a per-input absolute locktime to the annex is what I've had in
mind. That could also be used to cheaply add a commitment to an historical
block hash (eg "the block at height 650,000 ended in cc6a") in order to
disambiguate which branch of a chain split or reorg your tx is valid for.

Cheers,
aj