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

Daniel Franke <dfoxfranke@gmail.com> Wed, 22 June 2016 15:20 UTC

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From: Daniel Franke <dfoxfranke@gmail.com>
Date: Wed, 22 Jun 2016 11:07:19 -0400
Message-ID: <CAJm83bALASspCE3CTE7G_AnP0JoX_MsqSg+T+CB=1Oc5OgHADg@mail.gmail.com>
To: "Salz, Rich" <rsalz@akamai.com>
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Subject: Re: [ntpwg] Parsing NTP packets regarding MACs and EXTs.
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On 6/22/16, Salz, Rich <rsalz@akamai.com> wrote:
>
>> But again, how thoroughly do you want to "protect" this 48-byte message?
>
> The size of the message is not relevant; the importance of it is what
> counts.
>
> I want to protect network-based time synchronization with best-practice
> security.

To expand on Rich's point: MD5 is broken and SHA-1 is weak, for
reasons that mostly have *nothing to do* with their digest length.
MD5's digest length is 128 bits. That's not enough these days for
collision resistance, but for a MAC it's plenty; the problem is
elsewhere! As I've already pointed out earlier in this thread, MACs
may be truncated. HMAC-SHA-256 with its output truncated to 128 bits
is just fine.
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