Re: [Ntp] The NTP WG has placed draft-schiff-ntp-chronos in state "Call For Adoption By WG Issued"
Harlan Stenn <stenn@nwtime.org> Tue, 10 September 2019 09:14 UTC
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From: Harlan Stenn <stenn@nwtime.org>
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Date: Tue, 10 Sep 2019 02:14:45 -0700
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Subject: Re: [Ntp] The NTP WG has placed draft-schiff-ntp-chronos in state "Call For Adoption By WG Issued"
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Hi Neta, I want to see this sort of information either in the actual RFC or in related RFCs, even for Informational documents. I still have other issues with some of the core assumptions, and I'm not currently able to elaborate on those now. H On 9/10/2019 1:16 AM, Neta R S wrote: > Hi Harlan, > > Please see a brief description of Chronos threat model, Chronos > algorithm and security analysis below. > > *_Threat model: > _*Chronos considers a powerful form of man-in-the-middle (MitM) > Byzantine attacker, capable of determining precisely the values of the > time samples gathered by the Chronos client from > a subset of the NTP servers in its server pool (up to one-third of the > pool). > > *_A short reminder of Chronos' algorithm: > _*- Chronos client choose m servers at random, queries them, orders > their m samples according to their value, drop the d lowest and highest > samples. > - Then it tests the remaining set, and use its average as the new > client's clock only if the two conditions are satisfied: > 1 - All the samples are close (up to 2w) > 2 - Their average is close to the clients clock (up to ERR+2w) > Otherwise - the client resamples (choose again m servers at random, > queries them, orders them from low to high and drop the d lowest and the > d highest and tests the remaining set again). > After the client resamples (fails in the tests) K times, it moves to the > panic mode where all the servers are sampled. > > *_Security analysis > _*Chronos draft is based on the Chronos paper, which provide a security > analysis. > It was proved mathematically, that since Chronos use (slightly) more > servers to update, and choose them at random, the probability of > successful MitM attack decreases dramatically compares to NTPv4. > The improvement factor depends on the number of servers queries by > Chronos client at each update. For example, if the attacker controls 1/7 > of the servers in the pool, while querying 14 servers per update, > Chronos client can reduce its probability for timeshifthimg by a factor > of approximately 1000. > > Moreover, we considered the spectrum of feasible attack scenarios in > order to evaluate the > effectiveness of Chronos in thwarting timeshifting attacks. > The scenarios depend on how many malicious servers were queried. > We considered two scenarios: > > _- Scenario I: Less than m-d of the queried servers are under the > attacker’s control._ > It means, that there are more than d ''good'' samples (defined as up to > w away from the UTC). > In this scenario, there are two options: > > 1. There is at least one ''good'' sample in the remaining set, and then > the others should be close to it (according to condition 1, otherwise > the client resamples). > Then, the average of the samples in the remaining set (which is used to > update the client's clock) is close to the UTC. > > 2. There are no ''good'' samples in the remaining set. Thus, since there > are more than d ''good'' samples, the values of remain set are bounded > by ''good'' samples. > Thus, the average of the remaining set is also close to the UTC > > Therefore, these attack strategies are ineffective. > > _- Scenario II: More than m-d of the queried servers are under the > attacker’s control._ > In the worst case of this scenario, all the samples in the remaining set > are ''bad'' (more than w away from the UTC). > However, we proved in the paper that the probability of this scenario is > extremely low (since it requires malicious servers to be randomly chosen > at a much higher ratio than their ratio in the population – the pool).. > Thus, the probability of repeated time shift is even exponentially lower > – negligible. > Consequently, a significant time shift is practically infeasible > > > Moreover, we examined the probability of DoS attack on the servers by an > attacker who cause many clients to resample and reach the panic mode > multiple times. > We found that the probability of this attack is extremely low, even if > the client moves to panic mode directly after 3 times of resampling. > > *_Next step: > _*We are continuing to evaluate Chronos's performance and security for > different attack strategies and at different locations. > > Detailed discussions (along with the parameters evaluation can be seen > in the full paper: > https://www.ndss-symposium.org/wp-content/uploads/2018/02/ndss2018_02A-2_Deutsch_paper.pdf) > > > Neta > > _______________________________________________ > ntp mailing list > ntp@ietf.org > https://www.ietf.org/mailman/listinfo/ntp > -- Harlan Stenn, Network Time Foundation http://nwtime.org - be a Member!
- [Ntp] The NTP WG has placed draft-schiff-ntp-chro… IETF Secretariat
- Re: [Ntp] The NTP WG has placed draft-schiff-ntp-… Harlan Stenn
- Re: [Ntp] The NTP WG has placed draft-schiff-ntp-… Neta R S
- Re: [Ntp] The NTP WG has placed draft-schiff-ntp-… Harlan Stenn
- Re: [Ntp] The NTP WG has placed draft-schiff-ntp-… Neta R S
- Re: [Ntp] The NTP WG has placed draft-schiff-ntp-… Hal Murray
- Re: [Ntp] The NTP WG has placed draft-schiff-ntp-… Neta R S
- Re: [Ntp] The NTP WG has placed draft-schiff-ntp-… Marcus Dansarie