There is an issue describing Sybil-like NTP-level attack.
The document describes a draft proposal to solve the problem.
NB We describe correlated faults and centralization in a bit more details in the document (Work in progress).
The essence of the issue is correlated time-level faults. Validator node clocks can fail (to be synchronized) for various reasons:
If such failures are independent, it's a problem of affected validators, but not of the beacon chain system, since it's very unlikely that many of such faults occur at the same time.
However, if such failures are correlated then the probability of many simultaneous faults is much higher and, thus, the beacon chain cannot be considered a reliable system.
My assumption is that there are likely to be many low-stake validators which do not have enough resources and/or expertise to set up a reliable time service by themselves. Thus they will use the simplest solution, which is NTP + NTP pool (or NTP servers from Public Time Server Lists), leading to an implicit dependence on the single time source. Thus, one cannot assume time faults are independent and there is significant probability that an adversary can organize successful Sybil-like NTP-level attack (described in the issue above or in the separate document).
NTP can withstand certain faults like "falseticker" time servers, however, if it's configured to use several NTP servers which only looks like independent, then it can be compromised (one see more details here).
It's easy to detect the problem, if there is an independent time source. The problem is that for many validators setting up an independent time source can be a problem. While it's not necessarily expensive, assuming using GNSS receiver is an acceptable option.
Advising validators on NTP setup should help, however, it does not eliminate the risk of correlated faults. Basically, recommendations are not requirements. It would be much better if beacon chain clients provided a default (that means "cheap" in many senses) solution, which would mitigate the problem.
Considering that Ethereum 2.0 is to be much more scalable than Ethereum 1.0, it's capitalization is expected to be proportionally higher (much more transactions per second means much more value). And due to a network effect can be even higher.
Thus more expensive and complex attacks can be profitable, thus security requirements also should be stricter.
Summarizing the notes above, I've formulated requirements that a solution to the problem should satisfy:
Dependence on UTC/IAT (world time standard) cannot be eliminated.
Beacon chain is also semi-centralized regarding to UTC(k) and the public entry points to access them, i.e. GNSS, Radio Clock stations and stratum 1 NTP Time Servers.
However, graceful degradation is possible: if the root time suppliers fail, validator clocks can be still synchronized over IP, while clocks cannot be syntonized/synchronized to a world standard only.
We believe the dependence is inevitable and concentrate on resolving the potential dependence on centralized Time Server Lists, like NTP pool or others.
GNSS/Radio Clock/Stratum 1 faults are presumed to be independent and very rare.
If most validators were able to properly configure time service, in the sense that the faults of time service are independent, then the time-level faults can be tolerated by the beacon chain protocol.
Thus, any ways to ease NTP configuration, including choosing of time servers will be helpful. However, choosing time service is still a problem, since everybody will be picking them from the same list.
Additionally, it's better to pick NTP time servers from different lists. E.g. typically, NTP configuration includes four NTP servers, so one can be taken from NTP pool, but other three should be chosen from different lists, including corporate controlled time servers.
The measure will reduce correlation, it will be more difficult to "attack" many public lists. which will help tolerate attacks.
However, the measure will reduce centralization, but does not eliminate it.
Every node has a relatively stable RT clock on board, which is impossible to be attacked (at least, extremely difficult to attack on board RTC clocks of many validators). In short term, it can be used to filter out outlier clock corrections (e.g. results from malicious Time Servers).
This will limit the ability of an adversary to induce clock drift via malicious time corrections, since if such a correction exceeds several times the expected clock drift, then it can be considered as an outlier.
A more complex scheme is to calibrate clock rate using a robust regression approach, filtering out outlier external time corrections. That can help to reduce the ability even more, since calibrated clocks are more predictable (compared to predictions based on a nominal clock rate).
Outlier Filtering/Clock Calibration alone won't be able to prevent time-level attack, because clock skew can grow unbounded over time, but it limits the induced clock drift, which gives more time to detect an attack. And simplify other counter-measures.
Each validator node should use few time sources to correct local clocks. A form of BFT or "weak" BFT Clock Sync protocol can be used to disseminate the information between validator nodes.
Thus, each node will have an indirect access to additional "stratum n+" time servers, which can be used to withstand the NTP pool attack.
The main requirements are that majority of time sources are correct and there are not that much network-level faults.
As we expect that there will be a lot of nodes (up to 10K), communication can be a huge problem. Thus, a full BFT solution can be an overkill, even if it can piggyback the existing beacon chain protocol.
So, some lightweight "weak" BFT protocol may be required. We will describe BFT approaches in a separate document.
However, we will outline a simple lightweight approach below.
As Vitalik Buterin suggests in Network adjusted timestamp, one can use implied timestamps of messages.
E.g. an honest proposer should send a block at the beginning of a slot. And attesters should send attestations with an offset of one third of slot duration.
A minimum and maximum message delay bounds should be specified (exceeding the bounds will be a network-level fault). In p2p network, a message will pass through transient nodes, so, in general, the bounds are path-dependent. A simplified version is that bounds are proportional to amount of hops.
Messages should be cryptographically signed so that an adversary is not able to forge amount of hops or other time-related info.
As Vitalik suggests, a median of implied timestamps can be used to derive a "consensus" time.
However, a slightly more complex approach is possible.
First, one can convert the implied timestamps into clock offset bounds.
For example,
genesis time \(t_g = 1000000\), \(slot\_duration = 12 secs\), message delay (plus time uncertainty) \(\delta \in [0.1,2] secs\).
At local time \(t_r = 1000065\), a block for a slot \(n = 5\) arrived.
Thus, clock offset between the sender and the receiver is \(\in [3,4.9] secs\) (ignoring the clock skew accrued during the message transmission).
Then, one can use Marzullo's algorithm, instead of a median, to select the best time offset.
The uncertainty and the Marzullo's algorithm allows to include validator node own local time and time server corrections into account. Thus, the result time is bound to NTP time.
One more step is possible, if a sender can periodically attach time corrections that it received from its NTP servers, adjusted for elapsed time and uncertainty.
Then each validator node has an indirect access to time servers of all validators.
If correct time sources dominate and there are not that much network (and clock) faults, then the correct time sources will form a coherent majority, which can be used to adjust calibrate local clocks.
Summing the above, the solution is:
Many validators will take time sources from Internet, that means NTP time servers may appear several times in the time source pool. Moreover, each node has a varying number of validators behind it. This raises a question how time sources should be aggregated and how it will affect BFT properties.
If validators exchange with time server ids too (i.e. IP address), then a common time server can be used to measure a delay between two nodes. It also can help detecting two-faced clocks, if the time delay is too big and growing.
An additional idea is each validator may periodically sample time servers from the common pool, so that delays between nodes can be measured.
Validator node can compare different time servers in the pool and some form of reputation can be designed, e.g. based on deviation from the beacon chain time. One can sort time servers by the metric and throw away worst ones.
That can additionally help to withstand such NTP pool attacks.
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