Oh, and they can only be retrieved by the current FBI Director, in person.
At this point, anytime I see an "expert" claim that it's possible to pull this off, I assume that this "expert" is secretly a black hat hacker trying to make it easier for them and their black hat buddies to break encryption.
I'd guess Backpage could have challenged the constitutionality of the law the second it was signed. All of the noise made about SESTA / FOSTA targeting Backpage specifically may have given them standing.
With Backpage already gone, it will be more difficult for anyone else to challenge the law until it's used against them.
Hell, the feds won't even need to charge anyone under the law, just being able to say "it'd be a shame if your nice little website violated SESTA / FOSTA" will probably be enough to make any site "voluntarily" take down whatever the government wants.