Basically, if the password is a physical thing she has, than the Fifth Amendment does not protect it. But if the password is deemed to be something the defendant knows, it is protected.As the post points out, biometric technologies complicate this further.
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To illustrate the principle, the Supreme Court has previously explained that a witness might be “forced to surrender a key to a strongbox containing incriminating documents,” but not “compelled to reveal the combination to a wall safe.”
The Fifth Amendment guaranty that "No person shall... be compelled in any criminal case to be a witness against himself," applies (outside the military) to those who have already been indicted by a grand jury, are standing trial, and are being asked to assist in their prosecution. The example above doesn't seem to prevent the police from hiring a locksmith to open the wall safe; it merely prevents the police from compelling the accused to help them.
The Fourth Amendment is much more relevant to privacy in the ordinary sense.
The Fourth Amendment guarantees that:
"The right of the people to be secure in their persons, houses, papers, and effects, against unreasonable searches and seizures, shall not be violated, and no Warrants shall issue, but upon probable cause, supported by Oath or affirmation, and particularly describing the place to be searched, and the persons or things to be seized."
"Warrantless mobile device searches" (Google search) are a much hotter digital age privacy issue.
