>The assembly of those documents was like telling an inquisitor the combination to a wall safe, not like being forced to surrender the key to a strongbox. Id., at 210, n. 9. The Government’s anemic view of respondent’s act of production as a mere physical act that is principally non-testimonial in character and can be entirely divorced from its “implicit” testimonial aspect so as to constitute a “legitimate, wholly independent source” (as required by Kastigar) for the documents produced simply fails to account for these realities.

http://www.law.cornell.edu/supct/html/99-166.ZO.html

They're not actually talking about a combination lock, but using that as an example of something that would be covered by the fifth amendment.

Basically, merely forcing one to reveal that one possesses the password can be regarded as self incriminating.