Many computer users are concerned about the security (or lack therof) of personal data on their computer. Whether it be passwords, account numbers, bank records, personal correspondences, or any other number of private details, there needs to be a secure way to keep them from prying eyes.

Yea, this post is about making encrypted .dmg files; however, I feel I must first say a few things about security in general. For more detailed information about security, see HERE.

Minimally, if you’re the least bit concerned about protecting data on your computer, you should disable the Automatic Login feature in the Accounts System Preferences (Apple menu > System Preferences > Accounts). This way, when you turn on your computer, you’ll be required to enter your password at the login screen.

If you are ever away from your computer when others are around, you should also take a look at the Security settings in System Preferences, and set them accordingly.

Speaking of Security settings in System Prefs, there is a function in Mac OS called “FileVault”. This is a method of securing data by encrypting everything on your computer. When you access something, FileVault decrypts it on the fly, and encrypts it again when you’re finished. Although this method has been proven effective in thwarting prying eyes, it has its cons. Search Google for “FileVault pros and cons” for more information. Personally, I don’t use it.

Now, on to making encrypted disk images.

Most of you are probably familiar with disk images. Those are the files that look like a document with a pic of a hard drive on it. You may have seen these files when downloading software; distributors commonly use this format to bundle their products. When you double-click on a .dmg file, it “mounts” just as any other disk does; and, it shows up on your desktop and in the sidebar of Finder windows.

An encrypted disk image works just the same as any other, except a password is required to mount the image. Encryption uses 128-bit security — if you forget the password, you’ll need the help of the NSA and a few thousand hours of computing time to get your data back.

Begin by opening Disk Utility, in the Applications > Utilities folder. Click the “New Image” button at the top of the window. In the window that drops down, give your .dmg file a name. Also give it a “Volume Name”. This is what will show as the name when it is mounted. The Volume Size can be whatever you wish. You may want to choose a standard size so that it can be easily burned to an optical disk. Set the encryption to 128-bit AES encryption. All other settings can be left at the default. When you click the “Create” button, you will be presented with a password dialog box. Enter any password of your liking; but, BE SURE to UNCHECK “Remember password in my keychain”!! If someone knows your keychain password, they can bust right through the encryption.

When you click OK, you image is created and automatically mounted. It appears as a normal disk drive icon on the desktop and in Finder window sidebars. Now, just copy anything you wish to secure to the mounted image. You can keep filling it up until you’ve reached the capacity you set when creating the image. When you are finished, eject the mounted image by dragging it to the trash, or by clicking the eject icon in the sidebar. The mounted image will disappear, leaving you with a nicely encrypted .dmg file. To access your secure data, double-click the .dmg file. You will be prompted for the password before it will mount.

Voila! Your files are magically decrypted! You can add to or remove items from the mounted image at any time. Eject it when you’re done!