Re: [Ntp] The NTP WG has placed draft-schiff-ntp-chronos in state "Call For Adoption By WG Issued"

Hi Harlan,

Please my comments inline.

On Tue, Sep 10, 2019 at 12:14 PM Harlan Stenn <stenn@nwtime.org> wrote:

> Hi Neta,
>
> I want to see this sort of information either in the actual RFC or in
> related RFCs, even for Informational documents.
>
We will consider the best way to present this information.

I still have other issues with some of the core assumptions, and I'm not
> currently able to elaborate on those now.
>
Sure, we are currently working on extending Chronos and we will be glad to
hear you thoughts.
I understand you can't elaborate now. It can be done after the adoption.

Neta

> 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!
>
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