Hello Matt,
I think it is not too late to add a new feature to BOLT-11. In any
case, the belief that BOLT-11 is ossified should not be a reason to
make interactive something that fundamentally does not require more
interactivity than what BOLT-11 already offers. Technical decisions
should be dictated by technical needs, and I am a minimalist when it
comes to adding new messages to protocols.
I believe that two major implementations have an incentive to support
this proposal (although I cannot speak for them):
- Lightning Labs could potentially offer their Loop service to
non-LND users.
- ACINQ would be able to open channels to Phoenix users without
requesting the preimage first. This would put them on the safe side
of the upcoming MICA regulation; I cannot emphasize enough how
important that is.
In addition, you could certainly decide to support that feature in
LDK, and I can speak for Electrum :-)
It is the first time I suggest a change to the Lightning protocol, and
what I am proposing is really a tiny change. All we need is a new
invoice feature, that describes the prepayment of a fee using a
different preimage. This feature does not need to be set on all
invoices, and it could be made optional during a transition period.
Here is how that feature could possibly made optional:
- a new feature bit is defined, BUNDLE_PREPAYMENT
- two extra fields are defined: prepayment_amount, prepayment_hash
- if the sender does not support BUNDLE_PREPAYMENT and the feature is
optional, it ignores the new fields
- if the sender support BUNDLE_PREPAYMENT:
- sender sends (amount - prepayment_amount) with payment_hash
- sender sends prepayment_amount with prepayment_hash
The decision to make this feature required or optional remains with
the service provider. I can see how submarine swap providers who are
already exposed to the mining fee griefing attack could decide to make
it optional for a transition period.
cheers,
Thomas
Regarding your question (a) about the distinction between splice-out
and submarine swaps: Submarine swaps make it possible to add receiving
capacity to a channel.
On 14.06.23 19:28, Matt Corallo wrote:
I think the ship has probably sailed on getting any kind of new interoperable
change in to BOLT-11.
We already can't get amount-less BOLT-11 invoices broadly supported, rolling
out yet another new incompatible version of BOLT-11 and expecting the entire
ecosystem to support it doesn't seem all that likely.
If we're working towards specifying some "standard" way of doing swaps, (a) I'd
be curious to understand why the need isn't obviated by splice-out, and (b) why it
shouldn't be built on OMs so you can do it more privately.
Matt
On 6/13/23 1:10 AM, Thomas Voegtlin wrote:
Good morning list,
I would like to propose an extension to BOLT-11, where an invoice can contain
two bundled payments, with distinct preimages and amounts.
The use case is for services that require the prepayment of a mining fee in
order for a non-custodian exchange to take place:
- Submarine swaps
- JIT channels
In both cases, the service provider receives a HTLC for which they do not have
the preimage, have to send funds on-chain (to the channel or submarine swap
funding address), and wait for the client to reveal the preimage when they
claim the payment. Because there is no guarantee that the client will actually
claim the payment, the service providers need to ask prepayment of mining fees.
In the case of submarine swaps, services that use dedicated client software, such as Loop
by Lightning Labs, can ask for a prepayment, because their software can handle it (this
is called "no show penalty" on the Loop website). However, competitors who do
require a dedicated wallet, not such as the Boltz exchange, cannot do that. Their website
shows an invoice to the user, whose wallet that is agnostic about the swap, and it would
be unpractical for them to show two invoices to be paid simultaneously. This creates a
situation where Boltz is vulnerable to DoS attacks, where the attacker forces them to pay
on-chain fees.
In the case of JIT channels, providers who want to protect themselves against
this mining fee attack need to ask the preimage of the main payment before they
open the channel. I believe this is what Phoenix does (although their
pay-to-open service is not open-source, so I cannot really check). The issue is
that a service that asks for the preimage first becomes custodian. From a legal
perspective, it does not matter whether they open the channel immediately after
receiving the preimage, the ordering of events makes their service custodian.
In Europe, such a service will fall within the European MICA regulation.
Competitors who refuse to offer custodian services, such as Electrum, are
excluded from that game.
In order to solve that, it would be beneficial to bundle the prepayment and the
main payment in the same BOLT-11 invoice.
The semantics of bundled payments is as follows:
- 1. the BOLT-11 invoice contains two preimages and two amounts: prepayment
and main payment.
- 2. the receiver should wait until all the HTLCs of both payments have
arrived, before they fulfill the HTLCs of the pre-payment. If the main payment
does not arrive, they should fail the pre-payment with a MPP timeout.
- 3. once the HTLCs of both payments have arrived, the receiver fulfills the
HTLCs of the prepayment, and they broadcast their on-chain transaction. Note
that the main payment can still fail if the sender never reveal the preimage of
the main payment.
Of course, nothing in my proposal prevents the service provider from stealing
the pre-payment, but that is already the case today.
I believe this proposal would level the field in terms of competition between
lightning service providers. Currently, you need to use a dedicated client in
order to use Loop, and competitors who do not have an established user base
running a dedicated client are exposed to the mining fee attack. I also believe
that ACINQ would benefit from this, because it would make it possible for them
to make their pay-to-open service fully non-custodian. My understanding is that
in its current form, the 'pay-to-open' service used by Phoenix will fall into
the scope of the European MICA regulation, which they should consider as a
serious issue.
Finally, I believe that such a change should be implemented in BOLT-11, and not
using BOLT-12 or onion messages. Indeed, my proposal does not require the
exchange of new messages. Some of the initial feedback I received was that this
is a use case for BOLT-12 or OM, but I think that this is making things
unnecessarily complicated. We should not add new messages when things can be
done in a non-interactive way.
Cheers,
ThomasV
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