In Swift 2 you can do it in this way:
var dictionary: NSDictionary = ...
/* NSDictionary to NSData */
let data = NSKeyedArchiver.archivedDataWithRootObject(dictionary)
/* NSData to NSDictionary */
let unarchivedDictionary = NSKeyedUnarchiver.unarchiveObjectWithData(data!) as! NSDictionary
In Swift 3:
/* NSDictionary to NSData */
let data = NSKeyedArchiver.archivedData(withRootObject: dictionary)
/* NSData to NSDictionary */
let unarchivedDictionary = NSKeyedUnarchiver.unarchiveObject(with: data)