Re: [Ntp] Antw: Re: Follow-up to yesterday's mic comment about PTP security

Harlan Stenn <stenn@nwtime.org> Thu, 25 July 2019 06:24 UTC

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From: Harlan Stenn <stenn@nwtime.org>
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Date: Wed, 24 Jul 2019 23:24:52 -0700
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Subject: Re: [Ntp] Antw: Re: Follow-up to yesterday's mic comment about PTP security
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On 7/24/2019 10:44 PM, Ulrich Windl wrote:
>>>> Daniel Franke <dfoxfranke@gmail.com>; schrieb am 24.07.2019 um 16:09 in
> Nachricht
> <CAJm83bA74UiYbVbfYOHk4Vsw=go0d5P70uwbJ7CDvFrkdtcbfw@mail.gmail.com>;:
>> On Wed, Jul 24, 2019 at 3:00 AM <kristof.teichel@ptb.de>; wrote:
>>> 1.
>>> It is established in general (and I have a proof lying around for a model
> of 
>> NTS in particular) that a client performing a request-response exchange with
> 
>> NTS and using all relevant checks gets a strong guarantee that the error in
> 
>> its measured offset is no larger than half the added flight times of the 
>> packets (plus some negligibly small delta accounting for frequency 
>> instability of the clocks used on client and server side).
>>> For anyone wondering why we bothered to prove this again: this guarantee is
> 
>> 100%, and the new part is "no matter what a Man-in-the-Middle attacker did
> in 
>> the process".
>>
>> The "half the RTT" error bound can be improved somewhat if you know
>> the physical distance between yourself and the server. In which case
> 
> The probem is the "distance" here, as it definitely refers to "cable length",
> or more specifically "route length". Back in the time when the WWW started,
> connections inside Germany were routed via USA sometimes (that was the time
> whan a "good" transferrate was near 5kB/s). To make matters worse, you cannot
> safely assume that the route (length) from A to B will be the same as from B to
> A.

A MITM can still do at least a delay attack.

>> the error bound is ±(RTT/2 - distance / speed_of_light), because it is
>> safe to assume that the network adversary does not have a wormhole
>> through which to route your packets. (If you do have one, Akamai wants
>> your resume).
>>
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>> https://www.ietf.org/mailman/listinfo/ntp 
> 
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-- 
Harlan Stenn <stenn@nwtime.org>;
http://networktimefoundation.org - be a member!