Digital signature in Papers

Hi, I’m trying to use the digital signature feature of the new Papers app. In the application repo there is some information: Add digital certificates signing support (!296) · Merge requests · GNOME / Papers · GitLab

My question is, if someone knows which database is Papers trying to read, because I tried to use .pki/nssdb and the app still says “Certificate required”. If I add this database in KDE’s Okular, it finds my auto-signed certificate.

What I did was to import my p12 certificate to nssdb:

pk12util -d .pki/nssdb/ -i my_certificate.p12

which I generated with

openssl req -newkey rsa:2048 -nodes -keyout private_key.pem -x509 -days 365 -out my_certificate.pem
openssl pkcs12 -inkey private_key.pem -in my_certificate.pem -export -out my_certificate.p12

I could not find much documentation on this feature, so it would be helpful if someone knows how to read the generated certificates.

EDIT: Somehow, importing the certificate in Firefox, to the mozilla database, Papers finds it correctly, and the PDF can be signed. It would be useful if one could specify the database path as in Okular