The secret agent is cool, but isn't clear when reduced in scale. I voted for "something else". How about padlock+ and padlock- ?
I think a padlock, as a symbol, is fairly clear and related to the function. Also, I'd prefer having a + and - instead of entirely different icons since it unifies the two ways to do private. Like it'd be confusing to have one icon to quote someone and another icon to quote multiple people. There should, of course, be a distinction between them, but it should be clear that they are two versions of the same function.