[Ntp] Antw: [EXT] Re: New Version Notification for draft‑gruessing‑ntp‑ntpv5‑requirements‑03.txt

[Ulrich Windl <[e-mail]>]

>>> Miroslav Lichvar <[e-mail]> schrieb am 19.10.2021 um 09:16 in
Nachricht <YW5w4OPTrVLVQPdA@localhost>:
> On Mon, Oct 18, 2021 at 06:47:13PM +0200, James wrote:
>> > On 18 Oct 2021, at 16:33, Miroslav Lichvar <[e-mail]> wrote:
>> > For NTPv5 to be successful in replacing NTPv4, I think it needs to
>> > support no authentication, symmetric keys and NTS.
>> 
>> As I said to you last year, no authentication is fine but the protocol MUST

> prevent downgrade of it ‑ to be explicit, an untrusted adversary removing
it.
> 
> The format of the message itself should make it impossible for the
> authentication to be removed? I think there always needs to be an
> authenticator field, which can be removed or zeroed out by the
> attacker. If the implementation does not check the field correctly, it
> will accept an unauthenticated message. I think that's an
> implementation issue, not a protocol issue.

So it seems the authentication being requested has to be authentic ;-)
Unfortunately as the key to use is the key to "sign" the message (classic
symmetric keys), you cannot request key 0 (for unauthenticated) signed with key
0, because everybody can create such a "signature".
At the moment the server configuration determines the authentication to be
accepted, not the on-wire protocol.

Regards,
Ulrich