Using secure DMGs to store your files on Mac OS X

by Milind Alvares

Using secure DMGs to store your files on Mac OS X

by Milind Alvares on November 14, 2008

Time to rid yourself of the days of storing your files inside of hidden folders. They are not only unsecure, but also tend to get lost when you do a reformat or sell off your Mac (since you’ve hidden the folder so well). Keeping your files inside disk images not only keeps your data secure, but you can easily move them around and store them in places which might otherwise be considered public.

What’s the big deal about disk images?
Disk Utility, which comes built into Mac OS X, lets you create upto 256bit AES encryption, which I’m told is as secure as it gets. So how exactly do these DMGs work? If you have ever installed an application on the Mac, you know what a DMG or Disk Image is. The image is a single file, which once mounted turns into a virtual storage device where you can add, delete or modify files.

Oh wow! Let’s create one then!
To begin, launch Disk Utility (Applications » Utilities » Disk Utility). After launch, go to File » New » Blank Disk Image.

Note, that while the file name of the disk image can be changed later, the Volume name (which is the label shown after mounting the disk image).

Disk Utility will not only allow you to create fixed sized disk images, but also sparse images that expand as you add files to them. So choose your volume size according to the amount of data you will be using. The volume format is best left at Mac OS Jounaled, as a PC will not be able to open a FAT32 formatted disk image anyway.

The Encryption can be set to 128-bit, as cracking that itself would be close to impossible for any single person. The NASA computer farms might be able to crack through that, so if you have earth shattering information in your files, make it 256-bit encryption.

The partition format changes according to the size you have selected, so leave that unchanged. As for the Image Format – leave read/write selected. If you want the image to be ever expanding, select “Sparse disk image” and it will create a relatively small file and expand to a maximum size of what you set in the “Volume Size” option. Furthermore, selecting “Sparse bundle disk image” will allow Time Machine to backup only those bits inside the disk image that have changed, rather than backing up the entire disk image. (Thanks to John for this tip).

Of course, since you have selected that you want this encrypted, Disk Utility will ask you for a password. You can either enter any regular old password or use the password generator to get a new one.

Give them a password to think about
To get the password generator, click the blue coloured “Password Strenth” text string. You can then either create a memorable one or generate a totally random one. You then decide how long you want that password to be. The pass is then automatically filled into your password fields, and added to the Keychain (which I’d suggest you uncheck).

That’s it. Once you double click the disk image saved on your hard drive, it will mount on your desktop and Finder sidebar. Now you can add/delete/modify any files as you normally would! When you are done, just eject the disk by dragging the icon to the trash.

Everything is safe and secure
Finally, you can be assured that no one can get into those confidential recipes passed on to you from your great great grandmother. Of course, better not lose that password — because last I heard, NASA has stopped allocating their resources for the recovery of delicious family recipes.

Thanks for Andy and John for pointing out a huge error on my part.

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