When updating Bitninja on your Debian based server, you experience the errorGPG error: http://apt.bitninja.io/debian bitninja InRelease: The following signatures were invalid: EXPKEYSIG 4AC1BB4C7F8B47DC Heimdall.IO (Heimdall.IO Repository key) packaging@haimdall.io
Solution:
You can update your BitNinja GPG key with this command:
apt-key adv --keyserver keyserver.ubuntu.com --recv-key 7F8B47DC
After that, do an apt update && apt upgrade again.