updated 04:35 pm EST, Tue November 5, 2013
Requires recent Mac, iPhone hardware
A new product, Knock, promises to let Mac owners unlock their computers by knocking twice on their iPhone. Users first have to download a $4 iPhone app and a free Mac client. Once both are installed, knocking on an iPhone sends a command to a Mac via Bluetooth, triggering an automatic login.
Because the link relies on Bluetooth Low Energy to save an iPhone's battery life, only certain Macs and iPhones are compatible. This includes Macs released in 2011 or later, and the iPhone 4S, 5, 5s, and 5c. If an iPhone is lost or stolen, Knock users can remove it from login access through the Mac app.