So to be clear right-off-the-bat: I think re-designing modern encryption to include governmental "backdoors" is, on balance, a quite bad idea, for any number of good reasons. Nor (FWIW) do I actually think laws mandating that will actually get enacted, when all is said and done (in the U.S., anyways). But my question is not the policy or political aspects of the encryption debate; it's about a technological aspect to this that is more interesting.
Setting aside whether you should do so, could you adapt current major encryption standards & implementations to allow one authorized third-party (for e.g., the U.S. government) to monitor encrypted communications between Party A and Party B and decrypt those conversations without necessarily making it significantly easier for another, unauthorized third-party to do so as well?
In the past weeks I've read too many statements to count basically answering: "No. No chance." But it's usually been unclear about whether by that answer it is meant that (a) such modifications cannot be made without fundamentally weakening common encryption, or (b) that technical feat might be doable, but the "authorized" government would just inevitably lose control of whatever secrets they possessed and allow the Bad Guys free reign against personal & org info.
I do still remember the 1990's debacle where the NSA tried to sort-of start to do this in the Fortezza/"clipper chip" initiative via a key escrow system; it went exactly nowhere outside of government use. And to my understanding key escrow wouldn't really be scalable enough for usage today anyway. And certainly it's easy to create "backdoored" encryption ... if you're not worried about weakening that encryption vs. every attacker. But if those are not viable are there any alternative technical approaches (looking at things from a 30,000ft level of technical detail) that might be used to create encryption systems/implementations as robust as current ones against "unauthorized" third-party decryption. Or is that really--to our current knowledge--impossible?