To allow access to classified information.
keys to the kingdom
A resource, usually information or knowledge, the possession of which …
open the door
To allow access or facilitate.
read between the lines
To infer a meaning that is not stated explicitly.
read someone the riot act
To scold or berate somebody; to reprimand.
read out
To read something and say the words to inform other people; to call.
read someone's lips
To pay close attention (often imperative).