TMO Quick Tip - Protecting Files with an Encrypted Disk Image

by , 7:30 AM EST, November 28th, 2007

I recently showed how to keep Leopard's pre-built Smart Folders from snooping where you don't want them. A couple of readers pointed out encrypted disk images are really effective at keeping your private documents private, too, and they are absolutely right. If you don't know how to create an encrypted disk image, however, they won't do you much good at all.

Here's how to make your own encrypted disk image:

Launch Disk Utility. You can find it hiding in Applications / Utilities.
Click New Image.

Select New Image in Disk Utility.

Enter a name for your image file in the Save As field.
Enter the name you want to use for the disk image when it is mounted in the Volume Name field. This can be different that the image file name.
Set the maximum size for your disk image volume with the Volume Size pop-up menu.

Be sure to choose 256-bit AES encryption and sparse disk image.

Choose 256-bit AES encryption from the Encryption pop-up menu.
Select sparse disk image from the Image Format pop-up menu. That way your image file can grow or shrink to the size of the documents it holds.
Click the Create button.
Enter a password for your disk image in the password dialog. Be sure to uncheck Remember password in my keychain for an added level of security.

Password protect your disk image to keep nosey people out.

To mount your disk image on your Desktop, double-click the disk image file and enter the password you created when asked. You can add and remove files from the mounted image just as if it was a real disk connected to a FireWire or USB port.

Your disk image file (left), and the mounted disk image (right).

To keep people from seeing the files stored in your disk image, simple eject the mounted image from your Desktop. All of the documents stored there will be safely locked away and encrypted. The only way to see what's hiding inside the disk image file is to double-click it again and enter the associated password -- even Spotlight can't look inside to see what's there when your image isn't mounted.