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2023-06-09 15:07:51 CEST

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📅 Original date posted:2023-01-10
📝 Original message:
Hi list!
This email is a belated Christmas wishlist for interested researchers to solve an open question in lightning.
For context, recently there’s been some discussion about supporting “async payments”[1]. Supporting this feature would mean that e.g. a mobile noncustodial user would be able to receive payments even if they’re regularly offline. See the linked email and preliminary spec PR[2] for details on this scheme.
The open research question relates to how the sender will get an invoice from the receiver, given that they are offline at sending-time. If the receiver trusts their LSP, this is easy – they’ll give their LSP a bunch of invoices ahead of time, and trust the LSP to never provide the same one twice, since doing this would allow the LSP (or some intermediate node on the path) to steal the funds, due to already knowing the preimage.
This trust isn’t ideal, obviously. In the original ML email[1], the solution presented to this problem is PTLCs. However, upon further investigation, it turns out that the current PTLCs design wouldn’t solve this problem: the LSP would be able to steal funds the same as before, see [3].
Note that even if we don’t want to trust LSPs, this problem doesn’t halt async payment progress entirely: the working solution for now is for the receiver’s LSP to provide a reusable keysend[4] invoice to senders. The current thinking is that BOLT12 invoices will add a feature bit to support keysend, and the recipient’s BOLT12 offer will direct senders to its LSP to retrieve the keysend invoice. This isn’t ideal because it loses the proof-of-payment property, but it seems OK as a stopgap.
So more specifically, the research question is: what is a scheme that allows a regularly-offline receiver to create a reusable invoice for their LSP to provide to senders, such that senders have proof-of-payment? This may ideally be built on top of PTLCs. **Note that while the recipient may be offline when the sender initiates the payment, the recipient will come back online some time later to fulfill the incoming payment.**
A possible direction suggested by BlueMatt could be to stick with keysend, but have the sender encode a nonce + the time they sent the payment + the payment amount as a tweak to the keysend PTLC point (as well as in the onion), and make the receiver tweak their point with the same data when fulfilling the payment. Such a scheme may force the receiver to commit to the sender-encoded data[5], which wouldn’t exactly provide proof-of-payment, but would allow the sender to prove “I sent X amount at Y time.” Handwave handwave.
Thoughts welcome!
Cheers,
Val
[1]: https://lists.linuxfoundation.org/pipermail/lightning-dev/2021-October/003307.html
[2]: https://github.com/lightning/bolts/pull/989
[3]: https://github.com/lightning/bolts/pull/989#issuecomment-1325389542
[4]: https://github.com/lightning/blips/blob/master/blip-0003.md
[5]: https://github.com/lightning/bolts/pull/989#issuecomment-1327881563
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2023-06-09 15:06:26 CEST

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📅 Original date posted:2022-06-29
📝 Original message:
Heya Laolu,
From my PoV, adding prepayments to onion messages is putting the cart before the horse a bit, think there's a good amount of recourse before resorting to that.
Seems there are two cases to address here:
1. People are trying to stream GoT over lightning
In this case, just rate limiting should disrupt their viewing experience such that it becomes unusable. Don’t think LN can be compared to Tor here because they explicitly want to support this case and we don’t.
2. An attacker is trying to flood the network with OMs
In this case, IMO LN also can’t be compared to Tor because you can limit your OMs to channel partners only, and this in itself provides a “proof of work” that an attacker can’t surmount without actually opening channels.
Another huge win of backpressure is that it only needs to happen in DoS situations, meaning it doesn’t have to impact users in the normal case.
Cheers —Val
------- Original Message -------
On Wednesday, June 29th, 2022 at 8:22 PM, Olaoluwa Osuntokun <laolu32 at gmail.com> wrote:
> Hi t-bast,
>
> Happy to see this finally written up! With this, we have two classes of
> proposals for rate limiting onion messaging:
>
> 1. Back propagation based rate limiting as described here.
>
> 2. Allowing nodes to express a per-message cost for their forwarding
> services, which is described here [1].
>
> I still need to digest everything proposed here, but personally I'm more
> optimistic about the 2nd category than the 1st.
>
> One issue I see w/ the first category is that a single party can flood the
> network and cause nodes to trigger their rate limits, which then affects the
> usability of the onion messages for all other well-behaving parties. An
> example, this might mean I can't fetch invoices, give up after a period of
> time (how long?), then result to a direct connection (perceived payment
> latency accumulated along the way).
>
> With the 2nd route, if an attacker floods the network, they need to directly
> pay for the forwarding usage themselves, though they may also directly cause
> nodes to adjust their forwarding rate accordingly. However in this case, the
> attacker has incurred a concrete cost, and even if the rates rise, then
> those that really need the service (notifying an LSP that a user is online
> or w/e) can continue to pay that new rate. In other words, by _pricing_ the
> resource utilization, demand preferences can be exchanged, leading to more
> efficient long term resource allocation.
>
> W.r.t this topic, one event that imo is worth pointing out is that a very
> popular onion routing system, Tor, has been facing a severe DDoS attack that
> has lasted weeks, and isn't yet fully resolved [2]. The on going flooding
> attack on Tor has actually started to affect LN (iirc over half of all
> public routing nodes w/ an advertised address are tor-only), and other
> related systems like Umbrel that 100% rely on tor for networking traversal.
> Funnily enough, Tor developers have actually suggested adding some PoW to
> attempt to mitigate DDoS attacks [3]. In that same post they throw around
> the idea of using anonymous tokens to allow nodes to give them to "good"
> clients, which is pretty similar to my lofty Forwarding Pass idea as relates
> to onion messaging, and also general HTLC jamming mitigation.
>
> In summary, we're not the first to attempt to tackle the problem of rate
> limiting relayed message spam in an anonymous/pseudonymous network, and we
> can probably learn a lot from what is and isn't working w.r.t how Tor
> handles things. As you note near the end of your post, this might just be
> the first avenue in a long line of research to best figure out how to handle
> the spam concerns introduced by onion messaging. From my PoV, it still seems
> to be an open question if the same network can be _both_ a reliable
> micro-payment system _and_ also a reliable arbitrary message transport
> layer. I guess only time will tell...
>
>> The `shared_secret_hash` field contains a BIP 340 tagged hash
>
> Any reason to use the tagged hash here vs just a plain ol HMAC? Under the
> hood, they have a pretty similar construction [4].
>
> [1]: https://lists.linuxfoundation.org/pipermail/lightning-dev/2022-February/003498.html
> [2]: https://status.torproject.org/issues/2022-06-09-network-ddos/
> [3]: https://blog.torproject.org/stop-the-onion-denial/
> [4]: https://datatracker.ietf.org/doc/html/rfc2104
>
> -- Laolu
>
> On Wed, Jun 29, 2022 at 1:28 AM Bastien TEINTURIER <bastien at acinq.fr> wrote:
>
>> During the recent Oakland Dev Summit, some lightning engineers got together to discuss DoS
>> protection for onion messages. Rusty proposed a very simple rate-limiting scheme that
>> statistically propagates back to the correct sender, which we describe in details below.
>>
>> You can also read this in gist format if that works better for you [1].
>>
>> Nodes apply per-peer rate limits on _incoming_ onion messages that should be relayed (e.g.
>> N/seconds with some burst tolerance). It is recommended to allow more onion messages from
>> peers with whom you have channels, for example 10/seconds when you have a channel and 1/second
>> when you don't.
>>
>> When relaying an onion message, nodes keep track of where it came from (by using the `node_id` of
>> the peer who sent that message). Nodes only need the last such `node_id` per outgoing connection,
>> which ensures the memory footprint is very small. Also, this data doesn't need to be persisted.
>>
>> Let's walk through an example to illustrate this mechanism:
>>
>> * Bob receives an onion message from Alice that should be relayed to Carol
>> * After relaying that message, Bob stores Alice's `node_id` in its per-connection state with Carol
>> * Bob receives an onion message from Eve that should be relayed to Carol
>> * After relaying that message, Bob replaces Alice's `node_id` with Eve's `node_id` in its
>> per-connection state with Carol
>> * Bob receives an onion message from Alice that should be relayed to Dave
>> * After relaying that message, Bob stores Alice's `node_id` in its per-connection state with Dave
>> * ...
>>
>> We introduce a new message that will be sent when dropping an incoming onion message because it
>> reached rate limits:
>>
>> 1. type: 515 (`onion_message_drop`)
>> 2. data:
>> * [`rate_limited`:`u8`]
>> * [`shared_secret_hash`:`32*byte`]
>>
>> Whenever an incoming onion message reaches the rate limit, the receiver sends `onion_message_drop`
>> to the sender. The sender looks at its per-connection state to find where the message was coming
>> from and relays `onion_message_drop` to the last sender, halving their rate limits with that peer.
>>
>> If the sender doesn't overflow the rate limit again, the receiver should double the rate limit
>> after 30 seconds, until it reaches the default rate limit again.
>>
>> The flow will look like:
>>
>> Alice Bob Carol
>> | | |
>> | onion_message | |
>> |------------------------>| |
>> | | onion_message |
>> | |------------------------>|
>> | | onion_message_drop |
>> | |<------------------------|
>> | onion_message_drop | |
>> |<------------------------| |
>>
>> The `shared_secret_hash` field contains a BIP 340 tagged hash of the Sphinx shared secret of the
>> rate limiting peer (in the example above, Carol):
>>
>> * `shared_secret_hash = SHA256(SHA256("onion_message_drop") || SHA256("onion_message_drop") || sphinx_shared_secret)`
>>
>> This value is known by the node that created the onion message: if `onion_message_drop` propagates
>> all the way back to them, it lets them know which part of the route is congested, allowing them
>> to retry through a different path.
>>
>> Whenever there is some latency between nodes and many onion messages, `onion_message_drop` may
>> be relayed to the incorrect incoming peer (since we only store the `node_id` of the _last_ incoming
>> peer in our outgoing connection state). The following example highlights this:
>>
>> Eve Bob Carol
>> | onion_message | |
>> |------------------------>| onion_message |
>> | onion_message |------------------------>|
>> |------------------------>| onion_message |
>> | onion_message |------------------------>|
>> |------------------------>| onion_message |
>> |------------------------>|
>> Alice | onion_message_drop |
>> | onion_message | +----|
>> |------------------------>| onion_message | |
>> | |--------------------|--->|
>> | | | |
>> | | | |
>> | | | |
>> | onion_message_drop |<-------------------+ |
>> |<------------------------| |
>>
>> In this example, Eve is spamming but `onion_message_drop` is propagated back to Alice instead.
>> However, this scheme will _statistically_ penalize the right incoming peer (with a probability
>> depending on the volume of onion messages that the spamming peer is generating compared to the
>> volume of legitimate onion messages).
>>
>> It is an interesting research problem to find formulas for those probabilities to evaluate how
>> efficient this will be against various types of spam. We hope researchers on this list will be
>> interested in looking into it and will come up with a good model to evaluate that scheme.
>>
>> To increase the accuracy of attributing `onion_message_drop`, more data could be stored in the
>> future if it becomes necessary. We need more research to quantify how much accuracy would be
>> gained by storing more data and making the protocol more complex.
>>
>> Cheers,
>> Bastien
>>
>> [1]
>> https://gist.github.com/t-bast/e37ee9249d9825e51d260335c94f0fcf
>>
>> _______________________________________________
>> Lightning-dev mailing list
>> Lightning-dev at lists.linuxfoundation.org
>> https://lists.linuxfoundation.org/mailman/listinfo/lightning-dev
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