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From: Johnson Lau <jl2012@xbt.hk>
In-Reply-To: <87woo3uo4m.fsf@gmail.com>
Date: Sat, 22 Dec 2018 00:21:42 +0800
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To: Christian Decker <decker.christian@gmail.com>
Cc: bitcoin-dev <bitcoin-dev@lists.linuxfoundation.org>
Subject: Re: [bitcoin-dev] Safer NOINPUT with output tagging
Precedence: list


> On 21 Dec 2018, at 7:15 PM, Christian Decker =
<decker.christian@gmail.com> wrote:
>=20
> Johnson Lau <jl2012@xbt.hk> writes:
>=20
>> I think the use of OP_CSV (BIP112) is not needed here (although it
>> doesn=E2=80=99t really harm except taking a few more bytes). All you =
need is
>> to sign the settlement tx with a BIP68 relative locktime. Since this
>> is a 2-of-2 branch, both parties need to agree with the relative
>> locktime, so it is not necessary to restrict it through OP_CSV
>=20
> I keep forgetting about BIP68, but you're right, that should be
> sufficient for our use-case and would safe us a few bytes.
>=20

With taproot, this actually saves a lot more than a few bytes. For each =
update, you will make 3 signatures. One is a SIGHASH_ALL spending the =
setup TXO with no locktime. One is a NOINPUT spending a previous update =
TXO with absolute locktime. One is a NOINPUT spending the latest update =
TXO with relative locktime. For the first and third signatures, you will =
just sign directly with the scriptPubKey, without revealing the hidden =
taproot script. The second signature will reveal the taproot script, but =
it is needed only when someone published an outdated update tx.