Archive for April 17th, 2018

by Tom Nelson

The Mac’s Disk Utility app supports a number of capabilities that make managing the Mac’s storage system easier. But one set of features seems to get overlooked a bit: the creation and management of encrypted disk images.

Disk images have many benefits; they can be used to distribute apps and data to users, for creating master image files for various media types, such as CDs and DVDs, and for creating archives and backups, as well as quite a few additional creative uses.

Encrypted disk images allow you to protect the content of the images from prying eyes. Encrypted disk images can’t be mounted, viewed, or accessed unless you know the password associated with the image file.

In this Rocket Yard Guide, we’re going to look at how to create encrypted disk images. We’ll start with an overview of the basics of disk images and encryption, and then show you how to actually create various types of disk images.

Encryption Type
Disk images support two types of encryption: 128-bit AES (Advanced Encryption Standard) and 256-bit AES. The two levels of encryption refer to the size of the keys used in the encryption/decryption process. The 256-bit encryption is considered more secure than the 128-bit encryption, but the 256-bit encryption also takes longer to encrypt and decrypt. The 128-bit encryption will likely meet the needs of most people, while the 256-bit encryption is a better choice for data that needs a higher level of protection.

Mounting an Encrypted Disk Image
Before you can make use of a disk image, it needs to be mounted, so your Mac can work with the data within it. Mounting an encrypted disk image isn’t much different than mounting a normal disk image; simply double-click the disk image file, or right-click (control-click) the disk image file, and select Open from the popup menu.

Before the image is mounted, your Mac will display a window that asks you to provide the password to grant access to the information stored within. Enter the password, and click the OK button.

You can also automate the task of providing the password by selecting the option to “Remember password in my keychain.” When this option is selected, either during the encrypted image file creation (OS X Yosemite and earlier), or when you’re asked for the password when mounting the image (all versions of the Mac OS), the password will be stored within your keychain and used automatically the next time you mount the image file.

Unmounting an Encrypted Disk Image
Unmounting an encrypted disk image returns the image file to an encrypted state, preventing access to the data stored within. You can unmount the image by dragging the mounted image (not the image file) to the trash, or right-clicking on the mounted image and selecting Eject from the popup menu.

Image Formats
Disk Utility supports creating a number of disk image formats that can be used for various projects. Not all of the following formats are available in every version of Disk Utility, or with every method of creating a disk image.

Disk Utility supports a number of image formats. The formats that are available can change with the version of the OS, and the method used to create a disk image. Screen shot © Coyote Moon, Inc.

Read only: Allows the content of the mounted image to be viewed, and any files it contains to be opened and read. Additions to the image or changes to any of the files are not allowed. The read only option is only available when creating an image from a folder or drive, or when converting from one image format to another.

Compressed: Similar to the read only option, but any free space within the image is first removed to reduce the size of the image file. The compressed option is only available when creating an image from a folder or drive, or when converting from one image format to another.

Sparse image: This type of image format allows the image size to grow and shrink, to accommodate the amount of data stored in the image. The maximum size the image can grow to is set during the image creation process. Sparse image files have the file extension: .sparseimage

Sparse Bundle disk image: This type of disk image is made up of multiple small files, usually 1 MB, 2 MB, 4 MB, or 8 MB in size. When data stored on this type of image is changed, only the file(s) that contains the changed data needs to be changed, created, or deleted. Just like the sparse image format, a sparse bundle disk image has a flexible size that grows or shrinks to accommodate the data within. The sparse bundle disk image is used extensively with Time Machine. Sparse bundle image files have the file extension: .sparsebundle

Read/Write disk image: This image format allows you to add files to the image after it is created. The size of the image file is predefined, and can’t be expanded or reduced once created. Read/Write image files have the file extension: .dmg

DVD/CD master: This image type is used for mastering CDs or DVDs. If you’re using OS X El Capitan or later, when this format is selected, the image size field will change to a dropdown menu with 177 MB (CD 8 cm) selected. You can use the dropdown size menu to select any of the standard DVD/CD sizes. If you’re using OS X Yosemite or earlier, you must manually change the size field to one of the standard DVD/CD sizes. DVD/CD images have the file extension: .cdr

Hybrid image (HFS+/ISO/UDF): This image format is used for creating a single image whose files can be used on multiple platforms.

Note: The two sparse image formats have a maximum size that you set during creation. This is the size the image file will appear to have when mounted on your desktop. The actual image file (the .sparsebundle or .sparseimage file) will only use the amount of space needed to hold the data within.

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