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

[Miroslav Lichvar <[e-mail]>]

On Tue, Jan 19, 2021 at 01:24:37PM +0100, Magnus Danielson wrote:
> RTC is not a trusted time source. I can be just as much a false-ticker
> as any source you see. Do not trust the RTC to be near true time, it's
> failed before and it will fail again. It's just another source of time,
> it can be helpful to bootstrap things, but until validated you do not
> trust it. This is really about doing a trust analysis.

Yes, RTC can fail, but it's the only source of time you usually have
for time validation of certificates, DNSSEC records, etc. The idea is
that you check or set it manually once, and then allow it to be
synchronized to secure time sources only, so it can be always be
trusted to be close to the true time.

If you cannot trust the RTC and you don't have a different trusted
time source, it's game over. You cannot perform a full validation. You
can accept the security implications and disable the time checks, but
that's not generally acceptable.

> In this process we only assumed that the original timestamp was valid
> durign the data-validation phase, and we already stated that we trust
> the data validation part, and later test that assumption.

In this process you have only verified consistency of the time and
other data you have received. It was valid some time ago, but it may
not be valid now. If the attacker captured all that data, it can be
reused later in a MITM attack. You need to know the current date to
avoid that. That's why certificates and DNSSEC records have a validity
period.

You have a loop in your trust analysis.

-- 
Miroslav Lichvar