Apt¶
The following signatures were invalid: BADSIG 54404762BBB6E853¶
https://security.debian.org/debian-security/README.security
https://www.reddit.com/r/Proxmox/comments/1hiamg8/apt_wont_update_keys_missing_or_invalid/
If you are using apt you can use this entry in /etc/apt/sources.list:
deb http://security.debian.org/debian-security stable-security main