I've just bought the app just to try it out. It works as advertised. But I was even more interested in security.
When you install the Mac application, it will ask for your computer's password. This password is then stored, as I understand, on your phone.
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Quote:
When Knock connects to your Mac, it sends your password over that signal using both Bluetooth's built-in encryption and our own proprietary, 1024-bit RSA encryption.
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I think for many users, this is secure enough. This will probably not do if you are more interesting than an average joe.
What I don't like is that 'proprietary' remark. It almost sounds like they wrote their own crypto implementation of RSA 1024. Again: probably safe enough for home use.
I searched on knock unlock in the IOS app store and found it immediately.
It seems that the Knock application also records usage.
Furthermore, I seem to have troubles with sleep, if the computer does not wake from sleep with this app, I'm not sure if I think it's useful.