You are correct, sir. And in this case, you have the choice to turn over the encryption key or go to jail. I guess the major difference is that it's impossible to build a front door that the cops can't break down, whereas you can have encryption that is unfeasible to break. If the cops have a warrant to search your documents, then they can look through your diary to make sure that there aren't any of the things they're looking for in there. I agree if they've got a warrant to look for, say, a stolen stereo, they can't just riffle through your personal papers.