We use a GPG key to sign our BitNinja packages. These keys have expiry dates to make them secure. When these keys expire you get an error message letting you know that the GPG key is expired.
In such cases, the GPG key needs to be updated.
The above-mentioned error message usually looks like this: An error occurred during the signature verification. The repository is not updated and the previous index files will be used. GPG error: http://apt.bitninja.io/debian bitninja InRelease: The following signatures were invalid: EXPKEYSIG 4AC1BB4C7F8B47DC Heimdall.IO (Heimdall.IO Repository key) <packaging@haimdall.io>
W: Failed to fetch http://apt.bitninja.io/debian/dists/bitninja/InRelease The following signatures were invalid: EXPKEYSIG 4AC1BB4C7F8B47DC Heimdall.IO (Heimdall.IO Repository key) <packaging@haimdall.io>
apt-key adv --keyserver keyserver.ubuntu.com --recv-key 7F8B47DC