Once again, you don't have the right to silence in a search (you have to open things if requested), and you aren't incriminating yourself by opening something.
You incriminate yourself by possessing something forbidden, not by opening the box it is in.
You may not prevent the police from doing something, but if you follow the letter and spirit of the human rights rules both from EU and UN you have the right to stay silent whenever questioned by police and so do not have to help them find a key to a thing, you can basically sit/stand there like a statue.
Self-incrimination (under EU, USA, UN definitions) includes furnishing evidence against yourself ... this can be taken as assisting the police to get evidence that can then later be used against you. If you know that a box contains things that will be used against you you have a right to not help the police gain access to that box as if you help them you will furnish them with evidence due to the contents.
The fact that we have strayed away from these rights is shocking really.
You should be allowed, as I said, to simply sit there like a statue whenever police are asking you stuff. It is FOR THEM to gain access to things to find evidence, it is not for you to help them do so.
If they need access to a safe/box/computer file then let them crack/break/etc it.