Installing CA Certificates

When using a school or work network, you may be required to install a Root CA certificate to allow you to browse to secure websites (those that use HTTPS, which includes most websites nowadays!). The Root CA certificate is required because the traffic is being decrypted by the firewall or filter, then re-encrypted. Internet browsers will rightly detect this as a man-in-the-middle (MITM) attack, and will therefore throw up errors preventing you from continuing to load the page. 
Installing the required CA certificate will remove certificate errors, but you must be aware that this means all traffic will be viewable by your school or employer.
Copying the certificate
Ubuntu looks in specific folders for CA certificates. The exact location depends on the version - to do this on 20.04 (Focal) you will need to use a command similar to this. This copies the CA and changes it's extension from .cer (or .crt) to .pem:
sudo cp mycert.cer /usr/share/ca-certificates/local/mycert.pem
Older versions of Ubuntu may look in the following locations
/usr/share/ca-certificates/
/usr/local/share/ca-certificates/
Enabling the certificate
Our certificate is now within one of the required locations, but due to the nature of certificate trust, we will need to enable it via dpkg:
sudo dpkg-reconfigure ca-certificates
Updating the certificate cache
After enabling the certificate via dpkg, we must run the update-ca-certificates comand
sudo update-ca-certificates
Configuring git to use the certificate
Git has a separate setting for CA certificates, so we must set it using this command:
git config --global http.sslCAInfo /usr/share/ca-certificates/mycert.pem