I'm torn on this one, since in one way it's not really much different than requiring someone to open their door so police can search their home.
The thing is you aren't under obligation to open your door, if you don't open the door they just break it down. The same thing applies here. If you don't give them your encryption keys, they still can try to break them. IMO, it's a violation of the 5th for them to force you into providing encryption keys.