Could you be more specific? Under the 5th ammendment, you can refuse to talk to the police. If a judge orders you to reveal it, or be held in contempt of court, is that not a "Testimony"? And thus, wouldn't giving your passphrase literally be expressing a confession to a crime (presuming it was in the form of a confession)?
The question is really about whether the content of the password itself can be entered into evidence, and in most cases, the answer is no, though it would, of course, depend on the scope of the search warrant (if that's the context in which it is being evaluated) and several other factors. But in practice, a password is understood to be something rather arbitrary, so it is not regarded as having strong evidentiary weight. Of all the things I can do, setting my password to "ikillednicolebrownsimpson" is least likely to implicate me, even with a great deal of related supporting evidence.
Secondly, they can get around this quite easily by telling you to just type it in without disclosing it to them. Your refusal to comply will be regarded the same way as a refusal to open a safe or something similar, and you can be held (indefinitely, in many jurisdictions) for contempt of court. The difference, of course, is that the police can open a safe if they have to.
The 5th amendment does not release you from an obligation to cooperate with police, though, as you rightly point out, you are not required to disclose evidence to them or to aid their investigation apart from complying with their request to look around. To what extent divulging a password or passphrase enters into this scope is a matter of ongoing court battles. They have gone both ways.
On the other hand, by admitting that you have knowledge of the passphrase, you may be associating yourself with a piece of machinery or its contents more conclusively than would otherwise be possible to prove, which may be a non-trivial piece of information in itself. That, in turn, may be covered by 5th amendment protections.
