Date: Tue, 01 Oct 2019 15:35:34 +0000
To: Christian Decker <decker.christian@gmail.com>
From: ZmnSCPxj <ZmnSCPxj@protonmail.com>
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Cc: Bitcoin Protocol Discussion <bitcoin-dev@lists.linuxfoundation.org>,
	"lightning-dev\\@lists.linuxfoundation.org"
	<lightning-dev@lists.linuxfoundation.org>
Subject: Re: [bitcoin-dev] Continuing the discussion about noinput /
	anyprevout
Precedence: list

Good morning Christian,

> > -   A standard MuSig 2-of-2 bip-schnorr SegWit v1 Funding Transaction O=
utput, confirmed onchain
> > -   A "translator transaction" spending the above and paying out to a S=
egWit v16 output-tagged output, kept offchain.
> > -   Decker-Russell-Osuntokun update transaction, signed with `SIGHASH_N=
OINPUT` spending the translator transaction output.
> > -   Decker-Russell-Osuntokun state transaction, signed with `SIGHASH_NO=
INPUT` spending the update transaction output.
>
> That is very much how I was planning to implement it anyway, using a
> trigger transaction to separate timeout start and the actual
> update/settlement pairs (cfr. eltoo paper Section 4.2). So for eltoo
> there shouldn't be an issue here :-)

My understanding is that a trigger transaction is not in fact necessary for=
 Decker-Russell-Osuntokun: any update transaction could spend the funding t=
ransaction output directly, and thereby start the relative timelock.
At least, if we could arrange the funding transaction output to be spendabl=
e directly using `SIGHASH_NOINPUT` or variants thereof.


> > Again, the more important point is that special blockchain
> > constructions should only be used in the "bad" unilateral close case.
> > In the cooperative case, we want to use simple plain
> > bip-schnorr-signed outputs getting spent to further bip-schnor/Taproot
> > SegWit v1 addresses, to increase the anonymity set of all uses of
> > Decker-Russell-Osuntokun and other applications that might use
> > `SIGHASH_NOINPUT` in some edge case (but which resolve down to simple
> > bip-schnorr-signed n-of-n cases when the protocol is completed
> > successfully by all participants).
>
> While I do agree that we should keep outputs as unidentifiable as
> possible, I am starting to question whether that is possible for
> off-chain payment networks since we are gossiping about the existence of
> channels and binding them to outpoints to prove their existence anyway.

* Lightning supports unpublished channels, so we do not gossip some outpoin=
ts even though they are in fact channels underneath.
  * I confess the existence of unpublished channels in the spec fails to su=
mmon any reaction other than incredulity from me, but they exist nonetheles=
s, my incredulity notwithstanding.
* Historical channels that have been cooperatively closed are no longer nor=
mally gossiped, so the fact that they used to be channels is no longer wide=
ly broadcast, and may eventually be forgotten by most or all of the network=
.
  * This means anyone who wants to record the historical use of Lightning w=
ill have to retain the information themselves, rather than delegating it to=
 fullnodes everywhere.

>
> Not the strongest argument I know, but there's little point in talking
> ideal cases when we need to weaken that later again.

The point of ideal cases is to strive to approach them, not necessarily ach=
ieve them.
Just as a completely unbiased rational reasoner is almost impossible to ach=
ieve, does not mean we should give up all attempts to reduce bias.

Outpoints that used to be channels, but have now been closed using cooperat=
ive closes, will potentially no longer be widely gossiped as having once be=
en channels, thus it may happen that they will eventually be forgotten by m=
ost of the network as once having been channels.
But if the outpoints of those channels are specially marked, then that cann=
ot be forgotten, as the initial block download thereafter will have that hi=
story indelibly etched forevermore.

Regards,
ZmnSCPxj