Full marks for lateral thinking, but I don't think that works.
It's the use-mention distinction. Uttering a set of words is not the same thing as asserting that they're true. If you say "My password is 'I killed and ate a young girl in Tucson'" then that does not count as a confession, and could not be taken as one by any court (thankfully, because I just typed out that sentence myself...). Therefore, uttering that sentence does not count as testifying against yourself.
I'm presuming that they are true, and that they are a revelation of an actual crime that you committed. Thus, by revealing the passphrase, you are revealing evidence about yourself, and effectively testifying against yourself.
I believe you'd have to be under oath as well.
Further, I believe that this strategy would be employed as an argument to not ever giving up the passphrase. You would tell the judge, or whomever, that the phrase is a literal confession of a crime, and thus, by doing so, invoke 5th amendment protection.
(You may be right, and my idea may not work. I just want to make sure you're not assuming that the confession is for a false crime, when I meant it to be for a real one (though my examples of course, are false.) Which is why I didn't use a murder as an example...)
The court is not interested in your passphrase, they want the information that it is protecting. The court will simply compel you to provide the information in another manner if you claim that revealing your passphrase would be a 5th amendment violation (although I doubt they would even buy that one to begin with.)
1) tell you to provide the pass phrase to your lawyer (which makes it protected via attorney-client privilege) and then tell your lawyer to unlock the system and provide it to the court
2) out-geek you and notify you that since your encryption system does not actually use your passphrase but instead passes it first through a strong hash function you are to provide the court with the hashed passphrase so that they can use a decrypt method which skips the hashing step.
The short version is that claims that a passphrase alone is protected via the 5th is unlikely to succeed.
1) I read about at least one case where that's about what happened. The police asked the defendant to unlock the computer; they didn't ask for the password itself.
Further, I believe that this strategy would be employed as an argument to not ever giving up the passphrase. You would tell the judge, or whomever, that the phrase is a literal confession of a crime, and thus, by doing so, invoke 5th amendment protection.
I'm not assuming the confession is for a false crime, but nonetheless uttering the phrase "my password is $string' where $string is a true confession is still not a confession.
In your strategy, there are two parts to your confession:
1. The information that your password is the string "$s", and
2. Your volunteering of the information to the judge that the strong $s is in fact a confession of a real crime which you did in fact commit.
Part 1 is not testifying against yourself, unless in conjunction with part 2. Since you voluntarily threw Part 2 into the discussion, you're voluntarily testifying against yourself, and there's no rule against that.
A physical equivalent to your strategy would be to write out a letter of confession, leave it in your basement, and tell the police that the existence of this letter means they're not allowed to search your basement. It's not gonna work.
It's the use-mention distinction. Uttering a set of words is not the same thing as asserting that they're true. If you say "My password is 'I killed and ate a young girl in Tucson'" then that does not count as a confession, and could not be taken as one by any court (thankfully, because I just typed out that sentence myself...). Therefore, uttering that sentence does not count as testifying against yourself.