Abstract: For nearly two decades, CAPTCHAS have been widely used as a means of protection against bots. Throughout the years, as their use grew, techniques to defeat or bypass CAPTCHAS have continued to improve. Meanwhile, CAPTCHAS have also evolved in terms of sophistication and diversity, becoming increasingly difficult to solve for both bots (machines) and humans. Given this long-standing and still-ongoing arms race, it is critical to investigate how long it takes legitimate users to solve modern CAPTCHAS, and how they are perceived by those users...

As an actual human and not a bot, I had no idea how to answer. Is this a joke? (Seems not.) Is it a Magritte-like existential question? (It’s not a bicycle. It’s a drawing of a bicycle. Actually, it’s a photograph of a drawing of a bicycle. No, it’s really a computer image of a photograph of a drawing of a bicycle.) Am I overthinking this? (Definitely.) I stared at the screen, paralyzed, for way too long.

It’s probably the best CAPTCHA I have ever encountered; a computer would have just answered...