In a case like this, there would (normally) only be one secret to find. (IVs, or at least the information IVs are derived from, and so forth would be stored wit

In a case like this, there would (normally) only be one secret to find. (IVs, or at least the information IVs are derived from, and so forth would be stored with the data.) That doesn't necessarily make it easy to break if the key was securely handled, but it does make it catastrophic if the key is determined. Apple's email essentially says, "we don't think they have the key, but..." And a complete investigation, along with changes to the system and an opportunity for users to change data as soon as possible under the new system, is the right way to go about it.