How to check for unchecked comboBoxes in an array?

I have the following form and I need to check for unchecked boxes

<input type="checkbox" name="permission[]" value="user.create">Create</input>
<input type="checkbox" name="permission[]" value="user.update">Update</input>
<input type="checkbox" name="permission[]" value="user.delete">Delete</input>
<input type="checkbox" name="permission[]" value="user.read">Read</input>
<input type="submit" name="submit" value="Submit"/>

The way that it is right now It gives me an array like permission['user.create'.'user.read'.'user.delete'] What I need is another array that says for example unSelected['user.update']