One of my desktop box I recently got this message when I wanted to update it via
Fetched 1448 B in 8s (180 B/s) Reading package lists... Done W: An error occurred during the signature verification. The repository is not updated and the previous index files will be used. GPG error: http://xxxxxxx ./ Release: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 0000000000
Solution for it is very straight forward but lets write it down here since I always forgot how to do this. Here it is:
gpg --keyserver subkeys.pgp.net --recv-keys keyId gpg -a --export keyId | sudo apt-key add -
Of course you will need to replace
keyId with a real GPG key which is missing/uknown to APT.
Additiona resources - SecureApt Debian wiki page