Disk utility's encrypted drives and user permissions

Question

I've decided against using filevault based on some of the feedback and research I have conducted. I am still interested, however, in encryption of some documents. I have used the Disk Utility to create a 128 bit encrypted .dmg image. I mount it using my user account, but only as r-x. I see that root has write permission, and so I assume it is safe to write to these drives. What is the best way to make it so that my user account has rwx permissions? I could manually do a chmod each time I mount the drive, but that seems silly. Also I noticed that apple doesnt seem to mount drives using the same files as linux, so I'm a little confused. Could someone please provide some insight to this. Also, *put on tin foil hat* how secure is this? When I access the drive is it writing unencrypted data to the swap file? thanks,

Answer 1

Hi, iMacGyver.
A. You wrote: " I have used the Disk Utility to create a 128 bit encrypted .dmg image. I mount it using my user account, but only as r-x. I see that root has write permission, and so I assume it is safe to write to these drives."Were you logged in as root when you created the dmg? Or did you create a read-only image from a folder containing the documents?
To create a disk image to which you can add/remove documents and on which you will have Read/Write permissions:
1. Log in to your account.
2. Launch Disk Utility.
3. Select File > New > Blank Disk Image...
4. In the New Blank Image window, type a name for the new disk image in the Save As field, then select a location for saving the disk image, the desired Size, Encryption (select AES-128 in the pop-up menu), and leave Format as "read/write disk image."
5. Click Create.
6. Provide a password for the disk image when prompted. If you want extra security, de-select the option to save the password in your keychain, but be sure to never forget the password if elect to not save it in your Keychain.
You can then double-click the disk image you just created, type the password when prompted, the disk image will mount, and you can add files to it. Ejecting the disk image will unmount it and all the content you added will be encrypted.
One can use Disk Utility > File > New > Disk Image from Folder... to create either a Read Only or Read/Write encrypted disk image of an existing folder of documents, but if you elect to make it Read/Write there isn't much spare capacity for adding documents as the size of the disk image is based on the size of the folder.
Also, if your comfortable with Terminal, you can look into the hdiutil command for creating and managing disk images.
For example, if in step 4 above you selected the format as "sparse disk image", you'd later need to use the hdiutil command to compact the sparse image to recover disk space from files deleted from the spares disk image. Spare disk images are always Read/Write and only occupy as much hard drive space as the data they contain, e.g. a 100 MB sparse disk image containing 30 MB of files only uses 30 MB of hard disk space: they grow as files are added. A conventional R/W disk image of 100 MB in size takes up 100 MB of hard drive space even if it is empty.
However, when you delete files from a sparse disk image, the amount of hard disk space it occupies does not decrease until you compact it using the compact verb of hdiutil. Disk Utility does not expose all of hdiutil's functions.
B. You wrote: "Also I noticed that apple doesnt seem to mount drives using the same files as linux, so I'm a little confused. Could someone please provide some insight to this. "I don't understand what you mean. You'll need to be more specific.
C. You wrote: " Also, *put on tin foil hat* how secure is this? When I access the drive is it writing unencrypted data to the swap file? thanks"To use encrypted swap files, select "Use secure virtual memory" in System Preferences > Security. Depending on the applications you use, there may be a slight performance reduction with encrypted swap.
D. Your questions imply you're new to Mac. While you may be familiar with Linux, there are differences, especially in doing things through the GUI. If that's the case, my "Learning About Mac OS X" FAQ has a number of resources that you will find helpful including books, online training, and more.
Good luck!
Dr. Smoke
Author: Troubleshooting Mac® OS X
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