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Mac® Security Bible by Joe Kissell

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13.3. Working with Encrypted Disk Images

Mac OS X makes frequent use of disk images, special containers that look and act like single files when they're closed but when opened mount on your desktop like hard drives, revealing any number of other folders and files inside. Disk Utility, included with Mac OS X, lets you create and modify disk images of several types, each of which can optionally be protected with AES-128 or AES-256 encryption. By definition, any file you store on an encrypted disk image is encrypted, so you can use a single encrypted container to protect many files without having to worry about passwords or other settings for each one individually. Using the encrypted disk image requires merely double-clicking it and then typing your password; locking it again is as simple as ejecting the disk.

Because Mac OS X can create highly secure encrypted disk images without any extra software, it's a popular way to get this capability. Another popular option is PGP Desktop, which employs a different (and, arguably, easier-to-use) mechanism to create disk images with comparable capabilities. In addition, several third-party tools provide other ways of creating and using encrypted disk images (using Apple's disk format in some cases and a proprietary format in other cases).

13.3.1. Encrypting disk images with Disk Utility

Disk Utility can make robustly encrypted disk images that enable you to conveniently store any number of files securely and lock or unlock them almost instantly. ...

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