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Re: [Ntp] Roughtime and Delay Attacks

<Greg.Dowd@microchip.com> Thu, 04 April 2019 19:16 UTC

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From: Greg.Dowd@microchip.com
To: watson=40cloudflare.com@dmarc.ietf.org, stewart.bryant@gmail.com
CC: ntp@ietf.org
Thread-Topic: [Ntp] Roughtime and Delay Attacks
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Subject: Re: [Ntp] Roughtime and Delay Attacks
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Actually, T ~ Ts - RTT.  Asymmetry could be up to 99+% in malicious or broken environment.


Greg Dowd
Principal Engineering Technologist, FTD
Microsemi 
3870 N. First St. | San Jose | CA 95134 | USA
Office: 408.964.7643
Email: greg.dowd@microchip.com
Company Website:  www.microsemi.com



-----Original Message-----
From: ntp [mailto:ntp-bounces@ietf.org] On Behalf Of Watson Ladd
Sent: Thursday, April 4, 2019 9:55 AM
To: Stewart Bryant <stewart.bryant@gmail.com>
Cc: ntp@ietf.org
Subject: Re: [Ntp] Roughtime and Delay Attacks

External E-Mail


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
>
>
>
> _______________________________________________
> ntp mailing list
> ntp@ietf.org
> https://www.ietf.org/mailman/listinfo/ntp

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