Installing a CA certificate authority system-wide

I'm sorry for the bump but, really noone can help me ? :(

You can do it like this:

sudo cp certificate-name.crt /usr/share/ca-certificates/
sudo dpkg-reconfigure ca-certificates

Just make sure that the certificate has crt extension.

Didn't read your post carefully. If you tried to follow the guide you probably didn't run the commands as root.

To solve this you can prepend sudo to the commands like this:

sudo wget -O /usr/local/share/ca-certificates/autistici-ca.crt http://autistici.org/static/certs/ca.crt
sudo update-ca-certificates

Of course, this needs to be executed in a terminal.

You really shouldn't grab a certificate over an unencrypted connection then add it to system-wide.

@BugRep I've tried both tips you suggested but nothing seems to change.

Could you at least provide messages that were shown to you, so I can determine what failed?

Maybe you're not testing it properly. To test if you included the certificate you can try running this:

curl https://www.autistici.org/en/stuff/man_ca/linux.html

If you see an error message that means it failed. However if you see the source of the page it means it works.