Would there be anything legally wrong if, when the police force you to unlock your phone with your fingerprint, you intentionally use an unregistered finger multiple times so as to have the phone then require a password?

I have to agree with the circuit court here and I will explain why.  You can be compelled to give DNA, blood, and fingerprints via a warrant because these are things you ARE.  You can be forced to produce the phone because it is something you HAVE.  What you generally can't be forced to do is tell them something you KNOW.  However forcing you to swipe your finger is an example of something you are and nothing would logically prevent them from forcing the swipe.  However I don't think they could demand you tell them WHICH finger to swipe (things you know).  But you would have to swipe whatever finger you were ordered to.Either way, don't use fingerprints to lock something you want to stay locked.  Use a password and not a PIN, yes its a pain but it is more secure with a LOT more permutations.

I assume you have a selection of spare fingers for just such occasions.But a serious point for a moment, I think the answer by Matthe Bohrer is correct if you have already been 'served notice' to unlock your phone and then deliberately do something to frustrate this requirement (why not simply stamp your phone into little bits there and then. I have to say that if I were in that position and someone destroyed their phone in front of me I would probably shrug and say we weren't that interested in it anyway and smirk - but that's just me being cruel).One thing that occurs to me is that when you set up your security in the first place and it asks for your fingerprint to store for comparison, there is nothing stopping you from getting your mate to dab the phone instead, if you were that worried about security - that said, you would have to drag him about with you everywhere just to use it. Thinking about it, if you were required to provide the unlock code, can you refuse on the grounds that you do not wish to incriminate a potentially innocent person who may have no idea what is on your phone?One for the lawyers I think.

In the UK it is illegal not to divulge a password when requested to do so. The choice is then to consider whether the punishment for not divulging a password is worth preventIng whatever evidence that is on the phone from becoming known. For example,  if your phone stored kiddy porn, you'd probably decode that taking the punishment of not giving up your password was worth it, but if all you phone contained was evidence of a misdemeanour  (I don't know, like a video of you putting washing up liquid in the local fountain... ) it's probably better just to pay the small fine!

It wouldn't go well because they would allege, maybe successfully, that you intentionally violated the court order. The fact is that finger prints are not protected constitutionally, so trying to shield a finger print controlled device will be an uphill battle. On the other hand, encryption codes are generally protected, so that is the path you should really go if you don't want the government to find your data.