Knock is an app that turns your iPhone into an incredibly secure password for your Mac. Knock isn't like other apps. It's always ready, so you can use Knock even when your phone is in your pocket. Just knock twice, and your Mac unlocks. That's it. Knock uses Bluetooth Low Energy, which means your iPhone's battery won't feel a thing. Knock is available for an introductory price of $3.99 in the App Store, and as a free Mac app at knocktounlock.com.