ca-cacert software package provides CAcert.org root certificates, you can install in your Ubuntu 16.04 LTS (Xenial Xerus) by running the commands given below on the terminal,
$ sudo apt-get update $ sudo apt-get install ca-cacert
ca-cacert is installed in your system.
Make ensure the ca-cacert package were installed using the commands given below,
$ sudo dpkg-query -l | grep ca-cacert *
You will get with ca-cacert package name, version, architecture and description in a table
| Distro | Ubuntu 16 04 lts xenial xerus |
| Section | Miscellaneous |
| Package | ca-cacert |
| Version | 2011.0523-2 |
| Description | CAcert.org root certificates |
| Sub Section | Misc |