Re: [Ntp] NTPv5 client cookie size

[Miroslav Lichvar <[e-mail]>]

On Fri, Nov 04, 2022 at 12:03:38PM +0100, David Venhoek wrote:
> 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.

Against what attacks would the increased cookie length help?

The cookie is not a key that an attacker could be cracking offline.
It's supposed to protect against off-path attacks. The packets need to
be received by the client. A typical client running on modern HW
cannot handle more than million responses per second. That's about
300000 years on average to guess a single 64-bit value.

Even if we assume an infitely fast CPU and network speed of 400Gb/s,
it's still about 500 years of guessing.

If we consider a client using random source port, those numbers need
to be multiplied by at least 10000 or more depending on the OS.

-- 
Miroslav Lichvar