> nor shall be compelled in any criminal case to be a witness against himself,
I could see compelled decryption being congruent with the constitution if the decrypter is guaranteed immunity--ie, you can be compelled to be a witness against someone else.
Really though, there is a deeper issue here. What if I legitimately forget my passphrase? There is no real way to know whether or not I forgot it; will I be held in contempt of court over my forgetfulness? Will I be accused of perjury? Will I be imprisoned? Might the government accuse me of a crime, point to an encrypted hard drive that I have not used in a year, and then throw me in prison for failing to decrypt it?
Depending on all kinds of criteria, most likely including factors such as what is your known relation to that hard drive, your attitudem, how you look and how good is your lawyer, the argument 'I forgot that passphrase' may turn out both ways.
MRIs might change this.
Who is to say that "It is on the tip of my tongue/fingers, but I can't seem to say/type what it is." does not, under an MRI, appear to be "He knows what it is, but won't say/type it."?
I'm sure you could find expert witnesses willing to be paid to argue either way exactly what the MRI is or could be indicating.
Seems terribly shortsighted, given the points you are arguing from. If there is a disorder, it seems quite likely that we would eventually be able to detect the disorder.
Regardless, I did say "foreseeable future". There may be developments that catch me by surprise, but going at the current rate and direction we are right now, there is no way in hell MRIs will be able to satisfactorily contradict "I am unable to recall" in a courtroom anytime soon.