[Ntp] NTPv5 client cookie size

[David Venhoek <[e-mail]>]

Dear all,

After having implemented the NTPv5 spec, I feel that the current size
for the client cookie it gives is on the small side. Although 64 bits
is probably still hard to guess for an out-of path attacker, I think
we should seriously consider increasing its size given that 64 bits of
security is usually regarded as insufficient.

128 bits seems fairly straightforward to do if we move the server
cookie needed for interleaved mode into an extension field. This seems
like a reasonable option to me as it is already an optional part of
the draft, and would also eliminate one of the flags in the flag
field.

Beyond that, reaching a 256 bit size in some way could also be
interesting, because at that point we essentially have a large enough
cookie that a separate NTS cookie would no longer be needed in NTS
requests/responses. However, I see no reasonable way to achieve this
outside of extending the header. Although I don't see any right now,
such an extension might have unintended effects with regards to
backwards compatibility.

Any thoughts?

Kind regards,
David Venhoek