Is it safe to keep the (secure and random) PSK provided with your router?

Question

It is very common (at least where I am) for routers to come with a WiFi-PSK on the bottom of the router.

And I don’t mean something like Wifi-Link-2G, I mean something random and (cryptographically) secure.

For example, my one came with the PSK: Cooks177;8264!eGG798:2$, which would be pretty secure if chosen myself.

But even though it is cryptographically secure, is it safe to keep this PSK as my password, that is printed on the bottom of my router?

Obviously this is assuming that my family is trusted (and also already knows it.)

Also, if a burglar broke in, I wouldn’t have though that this is a risk because proximity prevents use of a local network.

Answer 1

Just because the PSK looks secure doesn't mean it actually is.

For example, the router vendor might have used an insecure random number generator to derive the keys. Or they could have derived all PSKs from a common seed which, once leaked, makes it trivial to find your particular PSK. Or maybe the PSKs have been knowingly or unknowingly logged, so that an attacker can potentially obtain them.

And there are probably a lot more people who could read the PSK than just your friends and family: neighbors, service contractors, acquaintances, ...

So the secure approach is to generate your own PSK with a cryptographically secure random number generator. The only valid use case for the predefined PSK is that it prevents laymen who don't know anything about wi-fi security from choosing an even worse PSK (a word from the dictionary, the name of their pet etc.).

Answer 2

It's easy to imagine attacks like what you describe, involving the print on the router. If someone in your house (family member, burglar, acquaintance etc) goes and looks they can then utilise this key for malicious activities whenever they are in wifi range again. This is not as big of a concern if you don't have many guests you don't trust much, and the attack can be easily mitigated by just making the print unreadable.

Another risk is that this key was generated from a weak seed/predictable PRNG/insecure PRF. This could mean that the key can be guessed by any potential attacker within wifi range.

Then there's also the risk that this key could be stored somewhere else. E.g. somewhere in the packaging that an attacker could take from the recycling bin if thrown away, or maybe in a database at the manufacturer that could be published in the case of a data breach.

In all these cases, generating your own key securely helps mitigate any possible attacks.