Re: [Ntp] Roughtime and Delay Attacks

On Thu, Apr 4, 2019 at 2:00 AM Stewart Bryant <stewart.bryant@gmail.com> wrote:
>
> Sorry I am struggling to understand how the protection works.
>
> If C (who has just woken) sends a request to S, then all C knows is that
> T ~ Ts - 1/2 RTT. C can know that it was S that replied, but C cannot
> possibly know if S was lying or if an on-path router delayed the packet.

Correct.

>
> C can keep asking S, and within the limits of the server delay and the
> variation in the routing path and queuing delays RTT stay sort of
> constant, so C can be suspicious if it changes after it has been running
> or if Ts - 1/2 RTT changes by more than the known drift in its local
> clock. However routing paths do change and there are traffic congestion
> delays in networks.
>
> C can ask S', S'' etc and build up a picture of various servers time,
> but it has to be careful that the paths are disjoint and that S, S' and
> S'' have truly independent and authoritative master clocks.
>
> On the other hand, if the routers are compromised, then there are much
> worse things that can do, so we normally assume that they are truthful.
>
> So how does this design do better than "T ~ Ts - 1/2 RTT assuming S did
> not lie and also was not simply wrong"?

It doesn't. But this is fine. RTTs can be seconds, not hours.

>
> - Stewart
>
>
>
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