Encryption may be necessary for privacy, but it's an aspect of manner of communication.
And time, place, and manner restrictions on expression are limited by the First Amendment (case law gives them slightly less protection than content-based restrictions, intermediate rather than strict scrutiny.)
And encryption restrictions that permit it for certain uses and not for you others are, arguably, content-based restrictions.
The content-based encryption argument is interesting, but I doubt we'll see any "certain use" laws. The problem/benefit of encryption is that if it's well designed then you have no idea what its contents are - mooting any content based restrictions.
We'll see how it goes though. I doubt the courts will be thrilled to do anything that circumscribes their own power.
And time, place, and manner restrictions on expression are limited by the First Amendment (case law gives them slightly less protection than content-based restrictions, intermediate rather than strict scrutiny.)
And encryption restrictions that permit it for certain uses and not for you others are, arguably, content-based restrictions.