PHILADELPHIA — If you use the Web or have a smartphone with Google email accounts, you probably have encountered an annoying invention called a CAPTCHA.
They’re the squished-up, stretched and squiggled, color-blotched collections of letters that often must be deciphered before sending an email, posting a comment, or buying a ticket.
Is that an “i” or an “l”? you wonder. A zero or the letter “O”? Maybe you see three letters where it seems there should only be two. You tilt your head. You scoot your chair back and squint. You wonder if you need new glasses.
You might also wonder if these things are getting harder – maybe too hard for people with aging eyes and brains.
The CAPTCHA was created at Carnegie Mellon University in 2000. The name is short for Completely Automated Public Turing test to tell Computers and Humans Apart. Websites need CAPTCHAs to guard against the “bots” of spammers and other computer underworld types.
“Anybody can write a program to sign up for millions of accounts, and the idea was to prevent that,” said Luis von Ahn, a Carnegie Mellon professor who was part of the CAPTCHA team. The little puzzles work because computers are not as good as humans at reading distorted text. Google says that people are solving 200 million CAPTCHAs a day.
Over time, though, the bad guys’ computers have been getting smarter and, well, people have not. The CAPTCHAs have to get harder for users, because they’re easier for the computers.
“It’s an arms race between site owners and spammers; users lose,” said Jeremy Elson, a researcher at Microsoft Research who has developed a CAPTCHA called Asirra. It uses pictures of dogs and cats.
Von Ahn said there were now “probably hundreds” of different kinds of CAPTCHAs. He worked on one of the biggies, reCAPTCHA. Google bought that one and now offers it for free. Users have to decipher two words for reCAPTCHA. One of them, usually the easier one, is lifted from an old book. A computerized scanner has failed to read it properly, and reCAPTCHA users get a chance to do the job right, thereby helping Google digitize books.
Von Ahn said he thinks some kinds of CAPTCHA have been getting harder. ReCAPTCHA is harder than it was in 2000, but it has been at about the same difficulty level for the past two years. On average, he said, people spend nine seconds solving a reCAPTCHA, and 92 percent of them get it right. In 2000, the success rate was 97 percent. The letters will be made more distorted when too many spammers start getting in.
Von Ahn said he did not know how many people give up when they see a hard CAPTCHA or ask for new words. He also did not know whether older people had more trouble than young, but there’s reason to wonder.
Robert Sergott, a neuro-ophthalmologist at Wills Eye Hospital in Philadelphia, said seniors were more likely to have cataracts, glaucoma and macular degeneration – eye diseases that can make vision blurry. Older people read best when there’s high contrast and more space between letters, pretty much the opposite of what some CAPTCHAs offer.