It's good to be paranoid, and the best way to protect your thoughts and data is with powerful encryption.
Encryption is a vital tool with which all Ubuntu users should familiarize themselves. One of the best ways to encrypt emails and files is with GPG, the GNU Privacy Guard. Like its inspiration PGP, GPG utilizes public-key cryptography. In essence, each user owns two keys: a private key used to decrypt messages sent to the user and a public key others use to encrypt messages they send to that user. In order for this system to work, therefore, folks who want to send encrypted messages to each other must exchange public—never private!—keys.
Once you have GPG up and running, there are several things you can do with it:
Encrypt and decrypt email messages and attachments
Encrypt and decrypt files
Sign a file with your electronic signature or verify the authenticity of a file by checking its digital signature
Verify or sign other users' public keys
As I said in the first paragraph, you need two keys, but where do they come from? If you've been using GPG for a while, you can move your keys from machine to machine, as needed, or you can create a new key pair from scratch.
There are GUIs for virtually all of the commands I'm going to show you in this hack. For KDE, KGpg is an excellent tool (from the package kgpg). GNOME users should check out Seahorse (from the package seahorse, and shown in GNOME menus as Encryption Key Manager).