Key escrow
A third party holds a copy of the key, so authorities can read along when entitled to — and everyone else when they manage to break in.
The idea is that encryption should be strong against everyone except a trusted party holding a spare copy of the keys. The best-known implementation was the Clipper chip, where every device transmitted a field letting authorities reconstruct the session key. The proposal returns at regular intervals, most recently under headings like "exceptional access" and client-side scanning.
The technical objection has not changed since the 1990s: the escrow store becomes a target of extreme value where a single breach exposes everything at once, while any such mechanism adds complexity exactly where the system can least afford it. Fifteen cryptographers summarised the position in the 2015 report Keys Under Doormats.