Re: [bitcoin-dev] Safer NOINPUT with output tagging

[Christian Decker <decker.christian@gmail.com>]

Johnson Lau <jl2012@xbt.hk> writes:
> Correct me if I’m wrong.
>
> For the sake of simplicity, in the following I assume BIP118, 143, and
> 141-P2WSH are used (i.e. no taproot). Also, I skipped all the possible
> optimisations.
>
> 1. A and B are going to setup a channel.
>
> 2. They create one setup tx, with a setup output of the following
> script: <s> CLTV DROP 2 Au Bu 2 CHECKMULTISIG. Do not sign

If we are using a trigger transaction the output of the setup
transaction would simply be `2 Au Bu 2 OP_CMS`. If we were to use a CLTV
in there we would not have an option to later attach a collaborative
close transaction that is valid immediately. Furthermore the timeout of
the CLTV would start ticking down the exact moment the setup transaction
is confirmed, hence whatever effect we are trying to achieve with that
timelock is limited, and we have a limit to the total lifetime of the
channel.

> 3. They create the update tx 0, spending the setup output with NOINPUT
> and locktime = s+1, to the update-0 output with the script: IF 2 As0
> Bs0 2 CHECKMULTISIG ELSE <s+1> CLTV DROP 2 Au Bu 2 CHECKMULTISIG ENDIF

Update 0 is usually what I call the trigger transaction. It takes the
2-of-2 multisig from the setup transaction and translates it into the
two-branch output that further updates or settlements can be attached
to. The settlement transaction attached to the trigger / update 0
reflects the initial state of the channel, i.e., if A added 2 BTC and B
added 1 BTC then settlement 0 will have 2 outputs with value 2 and 1
respectively, with the user's keys (this can also be considered the
refund in case of one party disappearing right away).

The second branch in the script you posted is the update branch, which is
not encumbered by a CSV, while the first branch is the one encumbered
with the CSV and is called the settlement branch since we'll be
attaching settlement txs to it.

The CLTV looks correct to me and ensures that we can only attach any
state >= s+1.

So just to show the output script for state `i` how I think they are
correct:

```
OP_IF
  <timeout> OP_CSV 2 <As_i> <Bs_i> 2 OP_CHECKMULTISIG
OP_ELSE
  <s+1> OP_CLTV OP_DROP 2 <Au> <Bu> 2 OP_CHECKMULTISIG 
```

And the input scripts for the update tx and the settlement tx
respectively would be:

```
OP_FALSE <Sig_Bu> <Sig_Au>
```

and

```
OP_TRUE <Sig_Bs_i> <Sig_As_i>
```

> 4. They create the settlement tx 0, spending the update-0 output with
> As0 and Bs0 using BIP68 relative-locktime, with 2 settlement outputs

If I'm not mistaken the CSV needs to be in the scriptPubkey (or P2WSH
equivalent) since segwit witnesses only allow pushes. Hence the script
in point 3 needs to add that :-)

> 5. They sign the setup tx and let it confirm

They also need to sign (but not broadcast) update_0, in order to allow
either party to initiate the closure if the counterparty become
unresponsive. The order in which settlement_0 and update_0 are signed is
not important by the way, so we can just batch these. The important part
is that signing the setup acts as a commitment.

> 6. To update, they create the update tx 1, spending the setup output
> with NOINPUT and locktime = s+2, to the update-1 output with the
> script: IF 2 As1 Bs1 2 CHECKMULTISIG ELSE <s+2> CLTV DROP 2 Au Bu 2
> CHECKMULTISIG ENDIF and create the settlement tx 1, spending the
> update-1 output with As1 and Bs1 using relative-locktime, with 2
> settlement outputs

The output script of the updates are identical to the ones in the
trigger or update_0 transaction, so they'd also need a CSV (this is why
committing to the script structure with masking still works).

> 7. To close the channel, broadcast update tx 1. Wait for several
> confirmations. And broadcast settlement-tx-1

We have to differentiate 2 cases: collaborative close and unilateral
close. In the collaborative close we come to a mutual agreement that
we'd like to take this latest state and settle. So we create a new
transaction that spends the setup output, and add outputs according to
the state we agreed upon, and we sign it. This transaction is
immediately valid, and does not need to be signed with NOINPUT. So all
the chain sees is a setup transaction with some inputs and one multisig
output (singlesig with Schnorr) and a collaborative close transaction
that spends the setup (also not signed with NOINPUT). About as normal as
transactions in Bitcoin can get.

In the unilateral case, one party isn't there anymore, or refuses to
sign. So we take the trigger transaction (not signed with NOINPUT) and
the latest update_n transaction (signed with NOINPUT) and broadcast
them. Then we wait for the CSV timeout to expire, and then send the
settlement transaction, which gives us the enforcement of the latest
state that we agreed on. The chain sees a setup transaction and a
trigger transaction (normal transactions for all intents and purposes,
except for the output script of the trigger, but we can hide that with
taproot), followed by two more transactions which are signed with
NOINPUT. So 4 transactions in the worst case, of which 2 are special,
and 2 transactions in the good case.


So all in all I think it's a tradeoff between having a larger on-chain
footprint (4 txs vs 3 txs in the worst case) and putting a fixed
lifetime on the channel for the refund case if one party disappears
right away. We'll probably find out what acceptable parameters are for
these and where the cutoff points are :-)