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Re: [ntpwg] Parsing NTP packets regarding MACs and EXTs.

"Salz, Rich" <rsalz@akamai.com> Thu, 23 June 2016 05:44 UTC

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From: "Salz, Rich" <rsalz@akamai.com>
To: Harlan Stenn <stenn@ntp.org>
Thread-Topic: [ntpwg] Parsing NTP packets regarding MACs and EXTs.
Thread-Index: AQHRy6DQX/6oVm/mW0S5teuNJd2+v5/zwAiAgAEJXwCAALwZUIAAyWQAgAAIqhA=
Date: Thu, 23 Jun 2016 02:39:11 +0000
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References: Message from Harlan Stenn <stenn@ntp.org> of "Tue, 21 Jun 2016 03:25:32 -0000." <E1bFCJh-000G0C-Bf@stenn.ntp.org> <20160621093932.BD9A7406057@ip-64-139-1-69.sjc.megapath.net> <f4f6f8f969ac49ff819ccae06ec2e3db@usma1ex-dag1mb1.msg.corp.akamai.com> <E1bFURY-000H2p-G8@stenn.ntp.org> <70060988e192497caf2402dec106e8b2@usma1ex-dag1mb1.msg.corp.akamai.com> <E1bFqCc-000IMW-90@stenn.ntp.org>
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Subject: Re: [ntpwg] Parsing NTP packets regarding MACs and EXTs.
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> What do you mean by "protect"?
> 
> Seems to me the two concerns are:
> 
> - prevent spoofing
> - detect tampering
> 
> and we need to identify ways to do this that are not too expensive.

Yes.  Tamper-protection can be done via HMAC/SHA256 or AES-GCM depending on plaintext or encrypted. Outside of the state issue of a shared key, which Daniel has talked about, the load this would impose on a modern server would not be a factor.

As for preventing spoofing, another way to state this issue is how you authenticate/verify the other side. For peer-to-peer server configuration, or perhaps S1 to S2, pre-shared keys might be reasonable. For larger fan-out, the state of the art is X509 certificate based. 

Hope this helps.
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