Problem first showed up 3 days ago during a regular apt-get update, and is currently showing as this error:

 

Err:5 http://netcup.armbian.com/apt bookworm InRelease                                                                                    
  The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 93D6889F9F0E78D5

 

This seems to be (matching final part of the fingerprint of) armbian's GPG public key present in /usr/share/keyrings/armbian.gpg

 

I've followed the suggestion to refresh the key, as posted in response to my inquiry on reddit: https://www.reddit.com/r/Armbian/comments/1im4uka/broken_mirrors/

 

I've verified that the machine is still ntp-synchronizing and in the proper timezone.

 

So, for some reason, it seems that this key is no longer picked up during apt-get. There's no .asc equivalent of this key present in /etc/apt/trusted.gpg.d:

 

# ls -l
total 84
-rw-r--r-- 1 root root 11861 Jul 30  2023 debian-archive-bookworm-automatic.asc
-rw-r--r-- 1 root root 11873 Jul 30  2023 debian-archive-bookworm-security-automatic.asc
-rw-r--r-- 1 root root   461 Jul 30  2023 debian-archive-bookworm-stable.asc
-rw-r--r-- 1 root root 11861 Jul 30  2023 debian-archive-bullseye-automatic.asc
-rw-r--r-- 1 root root 11873 Jul 30  2023 debian-archive-bullseye-security-automatic.asc
-rw-r--r-- 1 root root  3403 Jul 30  2023 debian-archive-bullseye-stable.asc
-rw-r--r-- 1 root root 11093 Jul 30  2023 debian-archive-buster-automatic.asc
-rw-r--r-- 1 root root 11105 Jul 30  2023 debian-archive-buster-security-automatic.asc
-rw-r--r-- 1 root root  1704 Jul 30  2023 debian-archive-buster-stable.asc

 

Can anybody confirm my suspicion that there's a missing armbian.asc file here?

Problem solved by adding an ASCII version of armbian.gpg in /etc/ap/trusted.gpg.d/; now the only remaining mystery is how that key disappeared from there.