As Usual, Enough Tools to Equip Your Own Agency
The first option is probably better when there are only a few people you would like to send secure messages too (Lawyers, Accountants, Mistresses). All the tools you need come included in your Ubuntu install, via the GNU Privacy guard.
"GnuPG uses public-key cryptography so that users may communicate securely. In a public-key system, each user has a pair of keys consisting of a private key and a public key. A user's private key is kept secret; it need never be revealed. The public key may be given to anyone with whom the user wants to communicate." From The GNU Privacy Handbook. With this program, you have near military-grade encryption tools at your disposal.
However, since the built-in GnuPG is accessible only through terminal commands in many versions, I would recommend installing one of the graphical interface programs available through the add/remove feature. Enigmail, GNU Privacy Assistant, and Seahorse are all good GNOME options, while KDE users can choose between KGPG and Kleopatra. If for any reason these programs cannot be found through the repositories, simply type sudo apt-get install (app name). Except for GNU privacy assistant, for that one it is sudo apt-get install gpa. GNU privacy assistant has, however, been included in every Ubuntu installation I have ever installed, so unless you installed a custom build or installed one of the stripped-down usb install versions, it is unlikely it will be necessary to install it in this way.