There is precedent that a court can't compel you give up the combination to a combination lock, because it would be considered testimony. Seems to me that this falls under that precedent.
For these reasons, we cannot accept the Government's submission that respondent's immunity did not preclude its derivative use of the produced documents because its "possession of the documents [was] the fruit only of a simple physical act-the act of producing the documents." Id., at 29. It was unquestionably necessary for respondent to make extensive use of "the contents of his own mind" in identifying the hundreds of documents responsive to the requests in the subpoena. See Curcio v. United States, 354 U. S. 118, 128 (1957); Doe v. United States, 487 U. S., at 210. 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.
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.