I ran "apt-get update" today, and instead of working, it printed this error:

W: An error occurred during the signature verification. The repository is
not updated and the previous index files will be used. OpenPGP signature
verification failed: http://us.deb.devuan.org/merged excalibur-backports
InRelease: The following signatures couldn't be verified because the public
key is not available: NO_PUBKEY B3982868D104092C

And there doesn't seem to be any solution that will make the system work
the way it used to, except reinstalling the entire operating system from
scratch. A chatbot said I could use GPG to get the keys, with a command
like:

gpg --keyserver https://keyserver.ubuntu.com --recv-keys B3982868D104092C

gpg --export B3982868D104092C | gpg --dearmor >
/etc/apt/trusted.gpg.d/devuan-keyring.gpg

Though this retrieved a key file, APT still reports the same error. So
literally nothing works. Remaining suggestions involve adding
"[signed-by=/etc/apt/...]" to every single entry in /etc/apt/sources.list,
but keeping *that* up to date (the new key will surely expire after a short
while) from now on, which is going to be a huge burden, and I didn't have
to have "[signed-by]" on every rule in source.list last week, so I don't
see why I should need to have it now.

This is only happening to my Excalibur system. I find no problems with my
other machine that runs Daedalus.

In the end, I ended up having to create a new file in /etc/apt/apt.conf.d/
containing the following:

Acquire::AllowInsecureRepositories "true";

That is the only way I can install anything.

Is there any real solution to this problem?