The 5th Amendment says that you can't be compelled to incriminate yourself, but I don't think that covers everything. If you have a safe in your house and the police have a search warrant for the contents of the safe, I think you are legally required to turn over the combination or key.

I remember hearing about this being used as a justification by a judge in a case relating to encryption. It's just sad that most cases involving encryption / computer crimes are related to child porn charges so people are more worried about punishing the person than they are about protecting the rights of everyone that is brought into the court system.