When trying to import a certificate into seahorse/gnome-keyring on Ubuntu 18.04, seahorse GUI application shows the 'import' button is greyed out, while mouse hovering the "import" button shows the message "Cannot import because there are no compatible importers".
This problem doesn't occur on Ubuntu 16.04 LTS (Seahorse 3.18.0), as I've just tested on my wife's laptop, but happens in my Laptop with Ubuntu 18.04 LTS (Seahorse 3.20.0-5).
Because that problem, it's not possible to digitally sign documents with LibreOffice.
Seahorse 3.20.0-5 / Ubuntu 18.04 LTS / GNOME 3.28.1
When trying to import a certificate into seahorse/ gnome-keyring on Ubuntu 18.04, seahorse GUI application shows the 'import' button is greyed out, while mouse hovering the "import" button shows the message "Cannot import because there are no compatible importers".
This problem doesn't occur on Ubuntu 16.04 LTS (Seahorse 3.18.0), as I've just tested on my wife's laptop, but happens in my Laptop with Ubuntu 18.04 LTS (Seahorse 3.20.0-5).
Because that problem, it's not possible to digitally sign documents with LibreOffice.