Mac OS X has a nice login system that keeps your whole computer inaccessible to unwanted users, but have you ever wished that you could keep some of your files safely hidden from prying eyes without locking down your entire system? What if you share your computer and you want other users to have unfettered access, with the exception of just a few files? Wouldn't it be great to keep your banking info hidden from your room mate or some intimate snapshots from your honeymoon away from your teenagers?
With OS X's built in Disk Utility tool and just a few steps that sort of security is easy to achieve.
Open Disk Utility, it should be located in the Utilities folder inside your Applications folder. In the upper left hand corner of the window you should see a "New Image" button, like this:
Click on it. This tells Disk Utility that you want to make a new Disk Image, which is a sort of self contained archive often used to transfer files from one computer to another. If you've ever run across a file with the extention ".dmg" then you've seen a disk image before.
A dialog should pop up asking you to specify a couple things about your new disk image. You'll be able to name it and select the location where you would like it created, and also choose it's size, format and encryption.
Lets say that I'm working on a iMovie project with some footage of my kids that I'm going to give my wife for her birthday. She and I use the same computer and I don't want her to find out what I'm planning, so I want to work on my project from a locked disk image. I name the new image something ambiguous like "lockup" rather than "Secret Bday Movie" and tell it to be created in my Documents folder. I know that iMovie files can get rather large so I use the "size" pulldown to select 2.6 GB. Next I select AES-128 for my encryption. This will ensure that my new image is password protected. I want to be able to access and change data on my image so I will leave the format as "read/write". When I'm done I should see this:
I hit the "create" button the select a password and thats it. Now there's a new file in my Documents folder named "lockup.dmg". I double click on it and enter my password (being careful not to select the "remember in my keychain" option) and presto! A new disk is mounted on my desktop and I've got 2.6 gigs worth of space for my secret iMovie. I save my project inside that image, and when I'm done I simply eject it and all my data is securely locked away until I want it again.