Subject: Re: [bitcoin-dev] [Lightning-dev] On the scalability issues of onboarding millions of LN mobile clients

From: Richard Myers <[email-masked]>
Date: Tue, 12 May 2020 12:09:34 +0200

Thanks for sharing your thoughts ZmnSCPxj. I think I can summarize your
concern as: A node without direct internet connectivity can not rely on an
opportunistically incentivized local network peer for blockchain
information because the off-grid node's direct LN peers could collude to
not forward the payment.

On Mon, May 11, 2020 at 7:44 AM ZmnSCPxj <[email-masked]> wrote:

> > 2) a light client can query an ISP connected full node on the same
> unmetered local WiFi network and exchange differences in block headers
> opportunistically or pay for large missing ranges of headers, filters or
> full blocks using a payment channel. Cost is reduced and privacy is
> enhanced for the light client by not using a centralized ISP. Bandwidth for
> running the full node can be amortized and subsidized by payments from
> light clients who they resell data to.
>
> A relatively pointless observation, but it seems to me that:
>
> * The light client is requesting for validation information, because...
> * ...its direct peers might be defrauding it, leading to...
> * ...the money it *thinks* it has in its channels being valueless.
>
> Thus, if the light client opportunistically pays for validation
> information (whether full blocks, headers, or filters), the direct peers it
> has could just as easily not forward any payments, thus preventing the
> light client from paying for the validation information.
>
> Indeed, if the direct peer *is* defrauding the light client, the direct
> peer has no real incentive to actually forward *any* payments --- to do so
> would be to reduce the possible earnings it gets from defrauding the light
> client.
> ("Simulating" the payments so that the light client will not suspect
> anything runs the risk that the light client will be able to forward all
> its money out of the channel, and the cheating peer is still potentially
> liable for any funds it originally had in the channel if it gets caught.)
>

One question I had is: how can a malicious direct payment peer "simulate" a
successful payment made by an off-grid victim peer to an information
source?  The censoring peer wouldn't be able to return the preimage for a
payment they failed to forward. Also, since the information provider and
off-grid node can presumably communicate via their local network
connection, it would be obvious if all of the victims LN peers were failing
to forward payments (whether maliciously or due to routing failures) to an
information provider they could otherwise communicate with.

Any LN payments not monitored by a watchtower that are received by the
eclipsed off-grid victim node would be at risk in this attack scenario.
Likewise any layer 1 payments they received should be buried under
sufficient valid block headers before being relied on.

I don't see how a LN node one-step removed from a direct internet
connection is at more risk than an internet connected node eclipsed by
their ISP, for example. In both cases, failure to get timely blockchain
info should trigger warnings to stop accepting payments.

> What would work would be to use a system similar to watchtowers, wherein
> the validation-information-provider is prepaid and issues tokens that can
> be redeemed later.
> But this is not suitable for opportunistic on-same-WiFi where, say, a
> laptop is running a validation-information-provider-for-payment program on
> the same WiFi as a light-client mobile phone, if we consider that the
> laptop and mobile may have never met before and may never meet again.
> It would work if the laptop altruistically serves the blocks, but not if
> it were for (on-Lightning) payment.
>

There's another problem if we can't rely on a recurring relationship with
an information provider besides not being able to prepay for validation
information doesn't make sense. We don't want an information provider to
collect payments for serving invalid information. Maybe for very small
payments this isn't a problem, but ideally validity could be coded into the
HTLC.

For example, an alternative HTLC construct that only paid for valid 81 B
headers that hash to 32 B values with a number of leading zeros committed
to by the HTLC. It would make more economic sense for an internet gateway
node to serve valid mined header to nodes on their local WiFi network than
to create bogus ones with the same (high) amount of work.

> So it seems to me that this kind of service is best ridden on top of
> watchtower service providers.
>

Public watchtowers or some sort of HTTP proxy data cache similar to a
watchtower makes the most sense to me because they would be expected to be
economically motivated and LN payment aware. Full nodes could potentially
be incentivized to exchange new data with other nodes in a tit-for-tat way,
but I don't expect them to be incentivized by light clients using LN
micropayments in a server-client arrangement.

Network agents that monetize full node information services beyond channel
monitoring would be more than just a "Watchtower" for light clients. Would
they be more like incentivized Electrum servers? Are there still privacy
concerns when they serve generic/un-personalized headers/filters/blocks to
light clients? A personal, altruistic or friends and family watchtower is
also possible, but I'm thinking about how light clients without this
possibility can be served.

Happy new epoch,

  -- Richard

-- 
Richard Myers
Decentralized Applications Engineer, goTenna
gotenna.com
@gotenna