The difference is that testimony is revealing the contents of your brain ("I saw X, I did or did not do Y, I felt Z") and evidence is revealing the contents of your car trunk. Evidence does not have its own opinion of what did or did not happen, evidence does not decide what is or is not the truth. Evidence simply exists; it is for others (giving testimony) to give evidence context and relevance.

The password is among the contents of your brain.

It's not the password they want, it's the evidence in side the "safe" that your password protects.

You're compelled to provide access. You could probably make a legitimate case for not actually revealing the password if you provided access for them.

damn. that's a really good point, but i still think it'd fall under the 4th. A password is closer to providing a key to a locked door rather than testimony.

It would be interesting to know if there was ever a hybrid case where there was a password-protected door with a numeric keypad, and someone refused to give the pw. I'd assume in a case like that, however, the cops would just smash shit out of it.