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.
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.