[ntpwg] Unlinkability formulations in merged draft "NTS-4-NTP"

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Hello all,

we have made a few changes to the merged draft (have been pushed to the 
repository at https://github.com/dfoxfranke/nts) after some discussion 
with Aanchal.
These changes are about the formulation of the text regarding 
unlikability.
We would welcome any comments or suggestions.


The relevant text in Section "Objectives":

- Privacy: NTS preserves unlinkability, i. e. it does not leak data that 
would allow an attacker to track mobile NTP clients when they move between 
networks. Note that unlinkability is not guaranteed for devices that 
function as servers as well as clients, see [link to Section "Privacy 
Considerations"].

The relevant text in Section "Privacy Considerations":

Unlinkability shall prevent that a device can be tracked when it changes 
networ adresses (e.g. because said device moved between different 
networks). This is to say that an attacker shall be unable to link a new 
address with one that was formerly used by the device, because of 
recognizable data that the device persistently sends as part of an 
NTS-secured NTP association. This is the justification for continually 
supplying the client with fresh cookies, so that a cookie never represents 
recognizable data in the sense outlined above.

Note that the objective of NTS regarding unlinkability is merely to not 
leak any additional data that would cause linkability. NTS does not 
rectify legacy linkability issues that are already present in NTP. To 
minimize the risk of being tracked by a passive adversary the NTP client 
has to minimize the information it transmits within a client request (mode 
3 packet) as described in the draft "draft-dfranke-ntp-data-minimization"

Also, the objective only holds for actual time synchronization traffic, as 
opposed to key exchange traffic. This implies that it cannot be guaranteed 
for devices that function not only as time clients, but also as time 
servers (because the latter can be externally triggered to send 
authentication data).


What do people here think?

Best regards,
Kristof

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