[ntp:hackers] A stop-gap authenticated time service

Miroslav Lichvar mlichvar at redhat.com
Tue Nov 10 10:09:16 UTC 2015

On Tue, Nov 10, 2015 at 07:58:05AM +0000, Poul-Henning Kamp wrote:
> The only variable left for the attacker to tamper with is the RTT
> which I can reliably measure and thus detect said tampering.

I don't think delay added in a MITM attack can be realiably detected.
The attacker can increase the delay by a small amount and slowly, so
it looks like the network is getting congested or the HTTPS server is
overloaded and it needs more time to respond.

Sure, the admin could set a very tight limit on the delay, but then it
will drop measurements when the network really is congested or the
server is overloaded.

> If the clock was already suitably disciplined, it will take days
> before the accumulated phase error becomes prohibitive, and that
> is plenty of time to raise an alarm where it matters.

I think that depends on what error is acceptable. The attacker doesn't
need to delay the packets much in order to throw the client's
frequency off and let it drift away. The frequency error can be much
larger than what it would be if the synchronization was just suddenly
stopped. This is why it's important to monitor if the servers are
reachable and the clock is still being updated.

Miroslav Lichvar

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