Creating an Encrypted Disk Image

When I travel with a computer, I am very careful about making sure important data is hidden away, and in some cases even encrypted. This is to protect my personal data in case the Powerbook is lost or stolen, or even if someone tried to access my computer over an unprotected public wi/fi network.

First, I’ve created a unique user on my computer, simply called “travel.” I only log in as this user while traveling. This user does not have access to the same Address Book, IMAP and POP email, documents folder or even iPhoto library as my regular log in. The user “travel” has it’s own password assigned to it.

If a thief has physical access to a Mac, it is fairly easy for them to get around the computer’s password. However, it would be almost impossible for the thief to open a password-protected, encrypted disk image, especially if you have selected a difficult password.

When I have folders I want to lock up as encrypted disk-images, I often use a program called Knox. Knox locks data into secure, password-protected disk images. It’s a great, easy to use program that I continue to recommend.

However, it’s possible to make a secure disk image without third party software – Disk Utility, which is part of OS X, can do this for you. I used OS 10.4 for this article; it should work just the same for 10.3 and even 10.2.

To make an encrypted, password-protected disk image, browse to the Disk Utility program, which by default is stored in the Utilities Folder. The Utilities Folder is typically found in the Applications Folder. Launch Disk Utility and select File> New> Disk Image From Folder. Browse to the folder you want to encrypt. A dialog box will come up, asking you to name the disk image you are about to create. At the bottom of the dialog box are two drop-down tabs. Select the “Encryption” tab and choose “AES-128 encryption.” This is a very robust and fairly flexible encryption scheme. It allows passwords up to 255 characters. It is important to note that, if you forget your password, all data stored in the disk image will be forever locked up.

Name this Disk Image and select “Save As. ”

Disk Utility will create the encrypted disk image, and typically will leave the new encrypted disk image on your desktop. When you try and open the disk image, a window will pop up, asking you for your password (unless you have saved the password to your keychain, in which case the image will just pop open.)

This method quickly creates password-protected disk images, but you can’t go back later and add new items to the disk image. Also, as far as I know, you can’t later go back and change the password on an encrypted disk image.