I couldn’t do a ‘sudo apt-get update’ on my Pi 3 running Raspbian as I got this error:
Fetched 74.9 kB in 31s (2,381 B/s) Reading package lists... Done
W: GPG error: http://security.debian.org jessie/updates InRelease: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 9D6D8F6BC857C906 NO_PUBKEY AA8E81B4331F7F50
To import the missing keys:
sudo apt-key adv –keyserver keyserver.ubuntu.com –recv-key AA8E81B4331F7F50
sudo apt-key adv –keyserver keyserver.ubuntu.com –recv-key 9D6D8F6BC857C906
Tips from here.