According to the way I read the literature, a two-tier PKI heirarchy with an offline root CA provides redundancy and an extra layer of security in case your subordinate CA's private key gets compromised. But how does this work?
Let's say I have an offline root CA and use it to sign a subordinate CA, and then use the subordinate CA to sign an internal web server. Now let's say the subordinate CA's key gets compromised. No problem according to the literature. You just burn your subordinate and build a new one. Then you fire up the root CA and sign the new subordinate, and you're back in business.
The problem is that the web server was signed with a now invalid subordinate CA certificate, and the chain of trust is broken. So would I need to also re-sign the web server with the new subordinate's key? If I have to re-sign everything with the new subordinate, then what is the purpose of having the offline root CA in the first place?
Clearly I'm not understanding something here.