The fact that he possesses no physical key is the crux of the matter. Any metaphor that tries to use objects like keys is pointless - there would be no controversy if there was a physical key.
Well there are plenty of safes without physical keys that use pin-pads. To to bring that analogy a little closer to reality then, they are trying to force him to enter the pin into the safe.
And again the current argument is not about whether he knows the pin or not, just whether he can refuse to enter it by arguing that this would be "being witness against himself", i.e. protected by the fifth.
