Even if you can't authenticate evidence (chain of custody, disclosure of technical process, etc), you can still use the fruits of the analysis as long as acquisition of the phone wasn't illegal and the fruits can be proven independently after the fact. I imagine that in most situations law enforcement can make their case once information on the phone points them in the right direction, especially in high-profile cases where the government would spend a lot of money on a secret process.

Authentication is necessary because the prosecutor has the burden of proof, and part of meeting that burden of proof is making a facially sound case about the authenticity and reliability of each piece of evidence. But unlike, say, an illegal search, failure to meet that burden doesn't poison derivative evidence as long as that evidence is independently submissible.

Importantly, you don't need to authenticate evidence _before_ getting a warrant to take possession of the phone; at least not to the extent required at trial. And as far as I know there are no laws limiting how the government can extract information from a phone it legally possesses for investigatory purposes, which means any technical process would be entirely irrelevant to the legality of the search. So there's no way to force the government to divulge the process as long as they don't try to submit the information gained by that process directly as evidence.

But maybe I'm missing something.