What mechanisms exist to detect compromised default certificate private signing keys?

Question

This is partially a follow-up to the these questions:

Is visiting HTTPS websites on a public hotspot secure?

Can free Wi-Fi hotspot providers snoop on HTTPS communications?

I understand of course there this exists other ways to compromise your data when connected to a public hotspot (trick you into trusting unsafe certs, DNS redirects, downgrade attacks, etc).

I'm asking about the probability of your HTTPS data being compromised over an internet connection via the very specific attack vector of a compromised private key belonging to either a root or intermediate certificate authority that is already in your device's default trusted certificates (that is, the trusted certificates provided automatically by Mac, Windows or Android, for example).

Please correct me if my base assumptions are wrong, but my understanding is that if the private key of one of the trusted certificate authorities already on my device was compromised, than a malicious actor in possession of that key could perform a MITM attack on my HTTPS traffic when I'm connected to the network of that malicious actor.

Furthermore, my assumption is that upon detection the private key has been compromised, such a private key would need to be revoked. Any certs signed with that private key would become invalid until signed with a new, valid key, and my device would warn me when accessing a site using the revoked key.

Therefore my main inquiry is what mechanisms exist to detect that a root or intermediate key has been compromised? As a follow-up, how well protected are the private keys for my default trusted certs? Are there any metrics as to how long it would take for the revocation to reach my device (that is, how long the malicious actor with the compromised key could operate) or any evidence that such attacks via this vector are common on public hotspots?

Answer 1

what mechanisms exist to detect that a root or intermediate key has been compromised?

If the attacker uses the key to decrypt the traffic only, then there is no direct way to detect it. You can only suppose that it might have been compromised if you can observe events that with high probability are possible only if a third party obtained access to your communication data.

If the attacker uses the key to issue new certificates, this can be detected in some cases. There is a Certificate Transparency ecosystem, which can help the CA to check if there are any certificates issued in the name of this CA but not known to it. Also it can help domain owners to check if there are certificates for their domains that they have not requested.

how well protected are the private keys ... ?

One of protection measures is usage of HSM. See details here.