>A back door marked ‘staff only’ doesn’t actually know who is staff and who isn’t.

Which is my you use cryptography instead of just a sign.

What one can make, another can break. Skeleton keys are impossible to secure.

By that logic so is the two parties' decryption keys. Can we make a third party whose decryption key is more secure than the others? I suspect it is possible.

It’s a fool’s errand chasing after a fool’s reward.