How can I easily encrypt a file?

You can use the Archive Manager to zip the file and password protect the zip file.

That is probably the closest thing to right clicking and entering a password that you describe.

To do this right click on the file and choose "Compress" then choose zip as the archive type and in "Other options" you have the option to enter a password.

This is simple to do and stops the problem of someone mounting the file system from a live CD and getting the file that way.

Also you can easily email the file or copy to USB stick, etc without having to worry about having the means to unencrypt the files at the other end, you just need the password.

As many pointed out, access control based on user id and encrypted filesystem is the only real way of securing user data. If, however, all that is stopping you from using Truecrypt is because you don't have a free partition / filesystem that you can use exclusively for storing encrypted data, then you can still make an encrypted file-system inside a file within your existing filesystem.

For this you need to have "sudo" rights, i.e., you must be able to run sudo.

1. Get the latest version of truecrypt
2. Open TrueCrypt (normally found in Applications -> Accessories)
3. Using the gui you can create a new volume contained in a file. You can choose the location of this file.
4. Steps 1-3 are one-time setup. After this whenever you mount this file-system using truecrypt GUI, you will see it in nautilus.
5. You can move the sensitive files and directories within this filesystem.
6. When "you are leaving your computer alone", unmout this using the "dismount" option in the truecrypt GUI.
7. It is also important to use a good password (more than 20 characters at least, as recommended by the developers).

If you want to encrypt a lot of files that you access regularly, an encrypted filesystem is the way to go. But if you have single files that you want to encrypt/decrypt quite rarely (say, a list of passwords) you can do it very easily with a right-click in nautilus:

• Install the seahorse-plugins package
• Create a new key for GPG/PGP (Applications - Accessories - Passwords and Encryption Keys)
• After a restart of nautilus (enter nautilus -q in a terminal or simply log out of your GNOME session) you have two new entries in your right-click menu: Encrypt and Sign, respectively Decrypt for encrypted files