I have been running a Debian server here at work for a several years now mostly as a web server, but also as a way for me to have quick access to some of the old cli tools like whois, dig, etc. While running updates I ran across this error:
Fetched 199 B in 1s (122 B/s) Reading package lists... Done W: GPG error: http://debian.opennms.org stable Release: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 57801F6F5B9EFD43
You can fix this by simply re-add the key by running the follow command at the command line:
wget -O - http://debian.opennms.org/OPENNMS-GPG-KEY | sudo apt-key add -
Enjoy!