India, Feb. 20 -- The game Doom has, over time, turned up everywhere. It's been retrofitted, by fans, for iPods and digital cameras, ATM machines and calculators; within Excel spreadsheets and even in the display screen of a pregnancy test.

So it isn't surprising that there's a kind of CAPTCHA in which the user plays Doom to prove they are human. The premise is simple: kill three enemies.

But it isn't as easy as it looks. Move forward, and you may be destroyed by fireball-throwing imps. Far better to camp at starting position and wait for the enemies to approach. I did this, and it took me two tries to clear the test.

The game, devised in 2021 by programmer Miquel Camps Orteza as a way to showcase his skill, is the simplest in a small ...