My understanding of the fifth is that it only applies to information that can't be collected under a warrant.
For example, if you have a lockbox with incriminating documentation, and the police can provide sufficient evidence for a warrant, you can be required to unlock the box. However, you can still plead the fifth if a lawyer is asking about your "intentions" for the contents of the incriminating box.
So I think there is a valid question of whether the FBI had the right to force the lock on "the box" of encryption if they didn't have a warrant already. That's like the police breaking and entering to seize evidence; it would be thrown out in court because it wasn't collected properly.
They have to have evidence of a crime before they can get a warrant. But once they have a warrant, they have the right to open "the box" of encryption.
I believe that also means they have the right to demand the keys to the box: your passwords.
IANAL, but this is nearer the truth than other comments earlier in the thread, I think.
Having read not just THIS article, but others about Feldman's case, I learned that the prosecution has found, on Feldman's personal computer, unencrypted indexes of file names that are present on the encrypted drives. Many of these files have explicit names about the subject matter contained within, giving them reasonable suspicion that these files actually contain depictions of underage children engaging in sexual activity.
Because the prosecution has this list of file names on the encrypted drives, Judge Callahan determined that Feldman's 5th amendment right no longer applied, as he would not be incriminating himself -- they know what is on the drives, they just need to have them unencrypted to enter them as evidence.
Make no mistake, this man is being accused of perversions that, if true, deserve prosecution to the full extent of the law. However, that does not give the government of this country or any country free reign to ignore his rights as a citizen and as a human being. I am in one sense extremely glad to see the judicial process being carefully applied, so that the accused is properly and legally convicted, and does not get off on a technicality. In another sense, I am also watching this with much trepidation, as precedents set too hastily could erode another one of our rights.
I believe, having read the motions presented in court that are publicly available, that in this case, because they already have evidence that there are files on the encrypted drives which have pornographic names, referencing the depiction of pre-teens in sexual acts, for instance, then the prosecution is justified in forcing the defendant to decrypt the drives and provide the court with the contents of those files.
If they didn't have the unencrypted index, I would probably have to see more proof to feel comfortable with what they are doing, however.