I much as I'd like to sell you a Teensy, PJRC does not recommend buying Teensy for this purpose.
Some time ago, an individual not in any way affiliated with PJRC made a YouTube video showing a proof-of-concept demo of "brute force" guessing a 4 digit numerical password by trying all 10000 combinations. Or at least about a dozen combinations were attempted in the video. I believe it might have been using an iPad? I'm honestly not very familiar with the video. I don't believe I even have the link to find it again. Remember, this video didn't come from PJRC.
My understanding is the author did not publish his software. PJRC does not provide such password guessing software. If you want to try this, you must write the software yourself. Teensyduino and Arduino attempt to make developing your own USB-based projects easier, but it still involves writing software using the Arduino IDE. Teensy is about DIY electronic projects. DIY stands for "Do It Yourself", and if you buy Teensy for this application, you will need to write the software yourself. Teensy doesn't come with password guessing software, and PJRC won't write it for you, or even help much. You really are on your own if you try this.
At least one person who purchased a Teensy (against my advice) reported he wrote the software and attempted all 10000 combinations, without success. Perhaps he made a mistake? Perhaps it works on some products, but not others? Perhaps Apple's products are able to detect brute force guessing, where a dozen attempts can succeed but correct entry after hundreds or thousands may fail? Apple does employ many of the world's smartest engineers. Their products also tend to auto-update if connected to the internet, so if Apple were made aware of any vulnerability (say, perhaps by a widely known YouTube video), it's quite likely they would fix it.
So, once again, I do not recommend using Teensy for this purpose. There is one credible report that it does not actually work in practice, so if you don't heed this advice and go to all the trouble to try anyway, odds for success are very slim.
If you buy a Teensy, write the software and it doesn't work, PJRC will not give you a refund simply because Apple's products are secure.
