In particular note the precedent from Hoffman v. United States, which you can find in context at http://caselaw.lp.findlaw.com/scripts/getcase.pl?navby=case&..., which says The privilege afforded not only extends to answers that would in themselves support a conviction under a federal criminal statute but likewise embraces those which would furnish a link in the chain of evidence needed to prosecute the claimant for a federal crime.
Providing the password obviously could furnish a link in the chain of evidence needed to prosecute the claimant for a federal crime. Therefore it is protected.
In particular note the precedent from Hoffman v. United States, which you can find in context at http://caselaw.lp.findlaw.com/scripts/getcase.pl?navby=case&..., which says The privilege afforded not only extends to answers that would in themselves support a conviction under a federal criminal statute but likewise embraces those which would furnish a link in the chain of evidence needed to prosecute the claimant for a federal crime.
Providing the password obviously could furnish a link in the chain of evidence needed to prosecute the claimant for a federal crime. Therefore it is protected.