I've started implementing it in eclair, do you have some test vectors so we can check that we are compatible?
The design seems good to me, but as I've said previously, I think keeping hop payloads and hmacs for 8 nodes only (instead of 27) is enough for almost all use cases and would give us huge size savings.

I don't have test vectors yet, but I can produce them. Will add them to this PR when ready.

Capping the max hops at a lower number is fine to me, but do you have a scenario in mind where this would really make the difference? Or is it to more generally that everything above 8 is wasteful?

@thomash-acinq added a happy fat error test vector.

Added fat error signaling to the PR.

I've spent a lot of time trying to make the test vector pass and I've finally found what was wrong:
In the spec you write that the hmac covers

• failure_len, failuremsg, pad_len and pad.

• The first y+1 payloads in payloads. For example, hmac_0_2 would cover all three payloads.

• y downstream hmacs that correspond to downstream node positions relative to x. For example, hmac_0_2 would cover hmac_1_1 and hmac_2_0.

implying that we need to concatenate them in that order. But in your code you follow a different order:

// Include payloads including our own.
_, _ = hash.Write(payloads[:(NumMaxHops-position)*payloadLen])

// Include downstream hmacs.
var hmacsIdx = position + NumMaxHops
for j := 0; j < NumMaxHops-position-1; j++ {
	_, _ = hash.Write(
		hmacs[hmacsIdx*sha256.Size : (hmacsIdx+1)*sha256.Size],
	)

	hmacsIdx += NumMaxHops - j - 1
}

// Include message.
_, _ = hash.Write(message)

I think the order message + hop payloads + hmacs is more intuitive as it matches the order of the fields in the packet.

Oh great catch! Will produce a new vector.

@thomash-acinq updated vector

Updated LND implementation with sender-picked fat error structure parameters: lightningnetwork/lnd#7139

can't senders opt-in to privacy-preserving random delays via the forward onion?

That could be an interesting trade-off. The sender would include a TLV in the onion to ask relaying nodes to add a random delay? We'd go from delays/batching decided by the intermediate nodes to something that is driven by the senders instead. At first glance, I think it would work.

The downside would be that it's off by default (because most app developers will care more about UX than privacy), while I believe that Matt would like basic privacy features to be on by default at the network level.

I personally think using 100ms precision would be a good trade-off: batch/delays of this order of magnitude still allow for sub-second payment latency for paths that aren't too long, which I believe is good enough in terms of payment experience while offering some privacy if done at the network level.

I'm more in favor of always having 100ms precision (instead of 1ms precision), but if there is a larger consensus for 1ms precision, I'd be ok with it! Not sure how to move forward though: should we just have a small survey among implementers?

Regarding hold time reporting, selection pressure in pathfinding, and its potential negative effect on privacy: can't senders opt-in to privacy-preserving random delays via the forward onion? That way there is no loss of precision and nodes can still serve both privacy-valueing users as well as the ones that aim for the absolute fastest routes.

Well, I don't think this is a good idea as "privacy loves company" and having senders signal "hey, look at me, I want to be more private - I have something to hide" might actually have them stand out more overall.

The preferred outcome would really be that we agree on a reasonable delay threshold that we encourage for all forwarding nodes. If all just do a little, it might introduce sufficient noise to throw on-path and AS-level attackers off.

I'm def. +1 for 100ms buckets, which should be mostly sufficient to cover at least 1-2 RTTs of the handshake, hence introducing at least some degree of uncertainty on the attacker's side.

There are users who don't like privacy delays. They patch their nodes to remove the delay. And surely there will also be users who don't like coarse grained hold times.

Do we really want to force this trade off onto them via the protocol without a way to opt-out, and rule out possibly undiscovered use cases that require ultra-fast payment settlement?

Picking a constant also doesn't look particularly attractive to me.

Do we really want to force this trade off onto them via the protocol without a way to opt-out, and rule out possibly undiscovered use cases that require ultra-fast payment settlement?

If you require ultra-fast settlement, there are always cases where multi-hop payments will fail you: shouldn't you just open a channel to your destination instead?

I still believe in a future of the lightning network where failures aren't tolerated at all, and all routing nodes deliver excellent service. Then it's just a matter of measuring latency and going for the sub-10 ms ones.

Hold time filtering to avoid users doing things we think they shouldn't. OP_RETURN vibes 😂

I still believe in a future of the lightning network where failures aren't tolerated at all, and all routing nodes deliver excellent service.

I agree, but that's orthogonal to payment latency? I honestly don't understand why it's important to aim for minimal latency: is it for performance's sake, to boast about numbers? A payment that takes 500ms to complete is almost indistinguishable from being instantaneous for our brains, so why does it matter that it takes 50ms instead of 500ms if you have to sacrifice privacy for it?

For me it is two-fold:

• I don't want to rule out potential future uses cases that somehow do require minimal latency.

• It is unclear to me how much an arbitrary 100 ms granularity is going to help with privacy today and also in a future in which we are stuck with this hard-coded value in the protocol. Yes it probably helps, but it may not be substantial.

Furthermore I think that an instant instant payment with no observable latency at all is just cool too indeed.

Are we talking about introducing actual time delays here, or just bucketing the reported value of the hold time?

If we're talking about further delaying the actual forwarding of the HTLC then I'm very much against it. Keep in mind this is done per payment attempt so if I need to try out 20 different routes this will accumulate quite fast, significantly killing ux.

Seems like the concern is that the accurate reporting of hold times does not directly ruin the privacy for the nodes reporting the hold times, but will eventually lead to intense competition around latency, contributing to further centralization in the network?

If we're talking about hold times leaking information about the routing nodes (fingerprinting?) then this is already too noisy to produce consistent results. Every node has different hardware and internet connectivity configuration, I wouldn't expect all LNDs or LDKs to have some entropy in their reports, and there exist better ways to fingerprint anyway.

I don't think we should pollute the protocol with this kind of safeguard/filter against more precise reports. If we assume that a user is willing to get their hands dirty to somehow signal lower hold times, they'll find a way. Also let's not underestimate the power of what implementations default to.

By having implementations default to scoring all hold times <=100ms the same, or reporting buckets of 100ms by default, most of the concerns seem to be eliminated. Of course, if we assume the majority of the network to be running custom software then what's even the point of this discussion?

I’m very much against the intentional slowing down of HTLC resolves.

Every milli second a HTLC is unresolved it poses a risk to the node runner.
One of the common causes of force closures are stuck HTLCs.

A HTLC gets stuck when a node goes offline after the HTLC passed that node.
So the failure or success can not propagate backwards through it.
The chances of a HTLC getting stuck is proportional to the time the HTLC needs to resolve.

So if you intentionally slow down the HTLC resolve speed, you also increase the number of force closures.

I would consider the intentional hold of a HTLC of more than 10ms as an jamming attack.

A HTLC gets stuck when a node goes offline after the HTLC passed that node.
So the failure or success can not propagate backwards through it.
The chances of a HTLC getting stuck is proportional to the time the HTLC needs to resolve.
So if you intentionally slow down the HTLC resolve speed, you also increase the number of force closures.

That's not at all a valid conclusion. HTLCs get stuck and channels force-close because of bugs, not because nodes go temporarily offline or introduce batching/delays. That's completely unrelated.

I 100% agree with you that all bugs that lead to stuck HTLCs or force-closed channels must be fixed and should be the highest priority for all implementations. But that has absolutely nothing to do with whether or not we should introduce delays or measure relay latency in buckets of 100ms.

That's not at all a valid conclusion. HTLCs get stuck and channels force-close because of bugs, not because nodes go temporarily offline or introduce batching/delays. That's completely unrelated.

I 100% agree with you that all bugs that lead to stuck HTLCs or force-closed channels must be fixed and should be the highest priority for all implementations. But that has absolutely nothing to do with whether or not we should introduce delays or measure relay latency in buckets of 100ms.

I do agree, that it is usually a bug or sometimes negligence, that is the root cause of most force-closes.

I also totally agree that there should be an easy way to measure relay latency.
Currently the only way to measure relay latency is by aggregating a lot of data (100’000+ payments) and calculate the time for each channel from that data.

In fact I do exactly this to generate the list you can find here on the “node speeds” tab
https://docs.google.com/spreadsheets/d/1gn2TY3zDoPdk46yYQfRhGb2_YDepQEn7l3qtHWCxov4
And I would love to have simpler and more accurate way to do this. And 100ms accuracy in the reported time would also be good enough.

But saying that adding a relay delay would not have an impact of the number of force-closes is - in my opinion - wrong.

If a channel with active HTLC goes offline and does not come online again before the HTLC reaches its timeout, this will lead to a force-close.
The chance of a channel going offline with an active HTLC on it increases with the time a HTLC lingers on the channel.

I send out more than 100’000 HTLCs each day.
0.1s extra per hop would add hours of exposure each day.

Wrote up some thoughts on delving that I think are worth considering when we think about latency measurements and privacy: https://delvingbitcoin.org/t/latency-and-privacy-in-lightning/1723.

I did a rough implementation in LDK of hold times for the success case. It turned out to be straight-forward, mostly reusing code from the failure case. Test vectors in #1261