Then you give the word. The immunity only covers the word, not the things that they find from the words. Can be introduced in court. Only word and fact that you knew the word that can t be introduced in court. Equivalent of turning over a record.
Analogy is to safe deposit box cases.
Everything seems to be policy based, not taking tech into account. Programs not perfect. This is one of the fundamental questions 2. Answer is that you can t secure it. When you build a backdoor in it is impossible to keep it from being hacked.