Captchas are everywhere. Suppose you're a big company offering Captchas on many sites and you can easily track a user across the web simply by setting a cookie everytime some site loads your captcha engine.
But apart from that, I am wondering if it might even be possible to identify a person by analyzing the way they solve a Captcha.
I am pretty sure that Google's image-based reCaptcha actually analyzes the tiny movements of the cursor while the user is clicking the tiles, how long they take to click adjacent tiles, how the mouse then moves to the "confirm" button etc. On repeated times, I found that solving a Captcha too fast and efficiently will result in me being prompted by more and more Captchas. However, if I play dumb, linger over the tiles and move my mouse a bit randomly like my grandma would, the Captcha gets accepted after the first try. (Which is extremely annoying, by the way.)
So, if we go a step further, it might even be plausible that each person has their own unique characteristics to the way they move their mouse, look at pictures, click fast or slow. Let's call it a cursor fingerprint. By feeding this into a neural network, could it be possible to identify a person?