There is no public key available for the following key IDs:
If you’re getting an error like the above when attempting to upgrade to Debian 9 (the new Stable version), it’s an easy fix:
sudo apt-get install debian-keyring debian-archive-keyring
Complaints about keys scare me something chronic, as it can be a sign that the repo (or worse, your OS) has been compromised… so I always look for a way to solve it.
Fortunately this one was easy.