Create a random string of n chars in swift

Random string

func randomStringWithLength (len : Int) -> NSString {
        
        let letters : NSString = "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789"
        
        var randomString : NSMutableString = NSMutableString(capacity: len)
        
        for (var i=0; i < len; i++){
            var length = UInt32 (letters.length)
            var rand = arc4random_uniform(length)
            randomString.appendFormat("%C", letters.characterAtIndex(Int(rand)))
        }
        
        return randomString
    }