My new Macbook Pro has Touch ID, and I've trained it on one of my fingers so I can use it to unlock my machine, which is quite fun. I've been wondering, though, how secure it is really.
To be clear, since I must have already logged in using my password, and you have to be physically present before you can try using Touch ID, Apple doesn't have to guarantee that it will only respond to my finger; it just has to be very unlikely to respond to someone else's finger. That's why, from time to time, the OS will require that my full password be typed in. It's similar to ATM card PIN numbers; four digits makes it difficult enough for a card thief to guess that it's a good-enough confirmation measure, although it would NOT be good enough if the card weren't required.
So, here's my specific question: how likely is it that a random, non-trained finger would be recognized as matching my trained one? Are we talking a 1 in 1000 chance, 1 in 10000, or even less likely? (I doubt I'd get a clear answer from Apple were I to ask them...)