Re: [Ntp] NTP Security (was NTPv5: big picture)

[Magnus Danielson <[e-mail]>]

Hi,

On 2021-01-18 17:25, Miroslav Lichvar wrote:
> On Mon, Jan 18, 2021 at 04:46:22PM +0100, Magnus Danielson wrote:
>> 6) If verified time matches that of the initial handshake request (as
>> updated using local time progess), then unauthenticated time in step 2
>> and 3 was valid, and thus the verification was valid and progress, else
>> fail.
>>
>> So, we see how unauthenticated time can be used to bootstrap the
>> handshake. Notice that this is not used to initiate node time, but only
>> for that handshake. Only after things is secured, you accept time as
>> being secured.
> I don't quite follow. If A was compromised at some point, how do you
> know you are not talking to the attacker intercepting your requests
> and giving you old signatures? Isn't the whole point of the validity
> period to prevent such attacks? If you don't know the current time,
> how can you validate anything that's not supposed to be trusted
> forever?
>
You actually do not trust the time you get initially. You validate it.
If your DNSSEC is invalid because of time, then the DNSSEC does not
validate. If your DNSSEC is invalid because false data, the DNSSEC does
not validate. Then, if and only if DNSSEC validate, only then you get
time but using the data-validated path, and only then you validate if
the original time and the validated time match up. If they don't, then
you do not trust either. If you can spoof the DNSSEC to validate, you
are toast anyway, if you have put your trust there.

Now, one *could* argue that the validated time from A can be trusted
even if the non-validated initial time mismatch. This could be used for
relaxed rules, but should result in warning.

If someone feeds you DNS-records that is wrong, DNSSEC should not validate.

If someone feeds you initial non-validated time that is wrong, that is
caught later. It may be "good enough" for the DNSSEC to be valid, but we
do not trust that value any further, it's only used in the boot-strap
procedure to hand-off to the secured form.

Cheers,
Magnus