I am able to sign documents with bEID identity card certificate:
Linux eID software installation | eID software
But when viewing the signed document it displays:
"The signature is OK, but the certificate could not be validated."
I tried to resolve this by downloading the belgium rootcertificates from here
http://certs.eid.belgium.be/belgiumrca.crt http://certs.eid.belgium.be/belgiumrca2.crt http://certs.eid.belgium.be/belgiumrca3.crt http://certs.eid.belgium.be/belgiumrca4.crt
and installing them like this
sudo cp Downloads/belgiumrca*.crt /usr/share/ca-certificates/extra sudo dpkg-reconfigure ca-certificates
And the ca-certificates package shows the new certificates, but even after reboot libreoffice seems unable to use them.
I tried to import them via seahorse, I can see the contents with seahorse but was not able to import the certificates.