[DanBC]

It doesn't happen like that.

They know he downloaded files which have the same hash values as known images of child sexual abuse.

They arrest him and search him. They can't find these images but they find an encrypted hard drive.

In some countries (England) they can ask for the keys, and not handing over the keys is its own offence (with 2 or 5 year sentence depending on the supposed contents).

I have no idea what the legal situation is in any other countries.

[kobeya]

The US has a provision on the bill of rights of its constitution saying, loosely, that you can never be compelled / coerced into being a witness against yourself. E.g. "Admit you did this crime or we send you to jail for life." Keep in mind that the bill of rights was written by rebelling colonials who were explicitly countering what they saw as abuses in the English system of law--in your example the ability of a judge to charge you with essentially the "crime" of not helping the prosecution's case. The crux of this case is over whether revealing a password to unlock a drive is the same as admitting to guilt -- which the court cannot compel you to do. There are arguments for both sides, honestly.