I'm getting tired of reading frequent news stories about laptop computers lost, stolen, or left in the back seats of cabs by company or government employees, supposedly exposing untold thousands of social security numbers, insurance records, or other sensitive, private information to identity thieves and other miscreants. What those news reports usually fail to mention is whether the missing data files were securely encrypted.
Making Your Private Data Useless (Except to You) -- Encryption is a mathematical method of scrambling information - one the United States government once tried to classify as a non-exportable munition - so that it can only be unscrambled with the correct password. The best encryption methods make it essentially impossible to decrypt data without that password, no matter what trickery or brute-force methods are applied.
If your Mac stores files you'd rather keep from prying eyes - and chances are it does contain financial or medical records, credit card information, highly personal email, and so on - it's not hard to encrypt that data to protect it when you're logged out or away from your computer. Without the right password, encrypted data is unintelligible digital garbage, so even if someone were to remove your hard disk or copy all your files elsewhere, your protected information would be useless to them.
Better a Safe than a Vault -- Apple includes a feature built into Mac OS X 10.3 and later, known as FileVault, that encrypts your entire home folder. But most experts agree that FileVault is overkill, because of performance issues, the risk of data loss, backup problems, and other reasons that Adam explained in his recent appearance on the MacNotables podcast:
If Apple's tool won't do, what's the alternative? It turns out you can easily use Disk Utility to create an encrypted disk image that behaves just like - and, importantly, can be backed up just like - a regular Mac folder. You can even set it to open automatically (with a password) whenever you restart or log in to your Mac. Then you can protect only those files you really need to, while leaving your iTunes and iPhoto libraries, browser cache files, and less sensitive documents as they are.
Ten Steps to Scramble Your Stuff -- I've posted a step-by-step screenshot tutorial in the form of a photoset and slideshow on the popular photo sharing site Flickr, if you'd like to follow along:
Figure out which files you want to protect, and consolidate them in a single folder. Subfolders are fine, but you want to make sure not only that you have everything you want to protect in one place, but also that you don't miss anything.
Launch Disk Utility (in your /Applications/Utilities folder) and click the New Image button in the toolbar to create a disk image. (You could also choose Images > New > Image from Folder if you want to avoid the manual copying in step 4 below.)
From the Format pop-up menu, choose the Sparse Image format (which is only as big as the data inside, so it doesn't waste any space); under Encryption, choose AES-128 encryption (the only encrypted option); and choose a maximum size from the Size pop-up menu that makes sense - I chose 4.7 GB, so even if I fill the image up, I can burn it to a single DVD-R.
Name your encrypted disk image in the Save As field and choose a location where it will be stored on the hard disk; I saved mine to ~/Documents/. Click the Create button.
When the Authenticate dialog appears, choose a password. Apple's Password Assistant (press the key button) can help you generate a good password, which is strong, secure, and unguessable, and which you will never, ever forget.
I know those two criteria are a bit contradictory, but anyone who can guess your password will have access to your files; on the other hand, if you forget it, chances are that neither you nor anyone else will be able to decrypt them. (I suggest storing a paper copy of your password in your safety deposit box, in case anything ever happens to you and your family needs to access it.)
I also recommend leaving the Remember Password checkbox unchecked, because if you allow your decryption password in your Keychain, anyone who knows your login or Keychain password, or who gets at your computer while you're logged in, may also be able to access your encrypted files.
Disk Utility saves the disk image where you specified and with the name you gave (MyFiles.sparseimage, for example), and also opens it as a virtual disk (MyFiles) on your Desktop. Copy your files into the virtual disk just as you would into a regular removable drive or folder.
Add the encrypted disk image file to your login items. Choose System Preferences > Accounts and click the Login Items tab. Click the plus sign button, find the .sparseimage file, select it, and click Add (or just drag the file into the Login Items tab). Now, whenever you restart or log in to your account, your Mac will ask you for your decryption password and mount the virtual disk on your Desktop. You can eject the virtual disk to protect the files if you're putting your computer to sleep, or even just stepping away from your desk.
For extra convenience, put an alias to the virtual disk where you used to keep the unencrypted files, so it behaves just like the folder it's replacing. If you give it the same name, other programs that expect the unencrypted folder to be there should still continue to work while the volume is mounted. If your encrypted volume isn't mounted, trying to open the alias will prompt you for your password.
Check that everything works. Eject and try to remount the virtual disk. Log out and back in. Open files in the virtual disk to make sure they work properly.
Once you've confirmed your data is safe, erase the unencrypted originals. Choose Secure Empty Trash from the Finder application menu to make sure they're really gone.
Finally, make sure you have a backup strategy that includes regularly copying the encrypted disk image to other media, some of which are stored offsite.
Not for the Truly Paranoid -- This system isn't perfect. Paranoid users would point out that AES-128 encryption isn't the strongest available, would recommend other solutions such as PGP or GPG, and they probably know of other flaws in my process that could expose your secrets to truly determined, crafty, and well-equipped adversaries - maybe. For an introduction to more aggressive encryption techniques, I recommend two recent podcasts (one video, one audio) from host Leo Laporte, at MacBreak and Security Now!
Encrypt that Data -- Let's return to the back seats of those taxicabs. If the nabbed social security numbers and insurance records were encrypted, as they should be, it's not much of a news story, because thieves can't get at them. And if they aren't, the recalcitrant employees (or the IT staff who provided their laptops) should be fired. Laptops are lost and stolen, but there's no excuse for private information to live on them unencrypted.
For most Mac users, an encrypted disk image is secure enough that if you lose your new black MacBook, your despair is lessened slightly, knowing that lurking inside its stealthy case is a once-classified munition keeping your private files out of the wrong hands.
[Derek K. Miller is Communications Manager for Navarik, an Internet software company in Vancouver, Canada. By night, he wears a wig and plays drums in a classic rock cover band, and is also the co-host of the Inside Home Recording podcast. His blog, like his youngest daughter, is six years old in 2006.]