Encrypt Your Sensitive Data: Macintosh Systems

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IT Help Center
Last Updated: 8/17/05
Copyright 2005
University of Delaware

Encrypt your Macintosh files

To encrypt information on a Macintosh computer, you must first create a new disk image. You will then save files you want to encrypt to the disk image. To create a disk image:
  1. On your hard drive, click Applications, then click the Utilities folder.
  2. Double-click Disk Utility. The window below shows this process for a person who is viewing the hard drive as columns. (View menu, select "as columns")

    Start disk utility

    You will see a window like the one below.

  3. On the File menu, select New, then click Blank Disk Image:

    New menu

  4. In the New Blank Image window's Save As box, type a name for the disk image. Then, use the Where drop-down menu to select where you want to save the disk image. (If will you have many files, save the disk image to your hard drive rather than to the desktop.)

    New Blank Image  dialog box

  5. From the Size drop down menu, select the size you would like for your disk image. (Make sure you have that much space on your computer.)

    Size menu in Save As box

  6. Change the Encryption option to AES-128 (recommended). Leave the Format set to read/write disk image.

    Encription option in Save As box

  7. Click Create:

    Create button in Save As box

  8. When you are prompted to authenticate, type a password that you would like to use for securing the data in this file; then type it again in the Verify text box

    Authentication dialog box

  9. Uncheck the box next to Remember password (add to Keychain).

    uncheckf Remember password box

  10. Click "OK."

You have now created a disk image file with the extension .dmg in the location where you chose to save the file. To see the contents of the disk image, you must first mount it. To do this,

  1. Double-click the .dmg file to mount the disk image.
  2. Type the password you created for the disk image.
  3. The disk image appears in the Finder sidebar and a window opens to show its contents.

Save files you wish to encrypt to the disk image. When you shut down the computer, the disk image is automatically dismounted.

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Back up encrypted files

To back up the disk image you have create to a CD or DVD:

  1. In the disk utility program, select the disk image you have created. (If the disk image doesn't appear in the list, drag the disk image to the Disk Utility window and then select it.)
  2. Click the Burn icon on the toolbar.
  3. Insert a recordable CD or DVD into your computer's disk drive and follow the directions on the screen.

You should take measures to protect the data on the backup disk image. Secure the media physically; for example, lock it in a file cabinet or desk.

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