Knock uses a combination of an iPhone app (currently offered at an introductory price of $3.99) and a Mac app (available for free at Knock’s website) to enable unlocking of your Mac, wirelessly, by just knocking the back of your iPhone.
It’s a surreal experience. After a couple of minutes of initial setup, you lock your Mac and rap the back of your paired iPhone twice. Your Mac unlocks. The app doesn’t even have to be forefront on your phone, although it does have to be ‘open’ in the multitasking tray. In fact, your phone doesn’t even have to be unlocked. When it senses the Mac is near, a notification appears on the lock screen instructing you to knock. There are some nice UI touches too. For instance, echoes of the ‘sound wave’ appear on OS X’s lock screen in real time as you knock. It’s a subtle visual indicator that the system is actually working.
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The app uses Bluetooth Low Energy to communicate with the target Mac. This is what allows the app to operate whilst backgrounded (using the CoreBluetooth API’s originally introduced in iOS 5) and it also means that battery drain on the phone is minimised. In order to take advantage of this neat effect, naturally, you need a recent Mac and iPhone (4S or later) that has Bluetooth 4 compatibility. Helpfully, the Mac menubar utility will tell you during setup if your computer is supported.
In practice, the system failed on me a couple of times in testing. The phone just couldn’t detect my laptop’s presence. Quitting the app and reopening remedied these connection issues, however.
As a gimmicky party trick, this is the best thing I’ve seen all year. As a tech demo of Bluetooth 4 and iOS 7, it is undeniably pretty cool.
In terms of real-world utility, though, I’m not as enthused. I do not find typing a password for my Mac to be a huge chore, although to be fair I’m not in a setting that necessitates unlocking my machine numerous times a day. Regardless, as it stands, if an malicious individual manages to steal both your iPhone and your Mac, all security is foregone (as the iPhone can unlock the Mac without needing to be unlocked itself).