That is sort of like the old "Scotty with the phaser" explanation for transporters. FMRI extraction of text has already been performed in a controlled setting.

That is sort of like the old "Scotty with the phaser" explanation for transporters. FMRI extraction of text has already been performed in a controlled setting. No one has demonstrated it with an adversarial subject in the open literature, yet. An interesting consideration for the extraction of cryptographic keys is that the extraction could be very noisy and yet still be a massive speedup for a search. If the attacker can try a billion combinations per second they may not need to get that many bits of search space reduction from the subject.