He said the students typically work two and a half to three hours a day from their homes and make at least $6 every 15 days; they earn more the faster and the more accurate they are. It is not a lot of money, he acknowledged, but it requires little effort and can help supplement their pocket money.

Image The going rate for solving captchas like this ranges from 80 cents to $1.20 for each 1,000, according to online exchanges.

Mr. Shaon, who agreed to speak to a reporter only over an Internet chat, said he gets the work on Web sites and is paid through Internet money transfer services.

He does not know the identities of the people paying him, nor does he have any interest in finding out. If he asks them, he said, “they maybe do not give me my payments.”

Another operator in Bangladesh who goes by the screen name Workcaptcha on Freelancer.com boasts on his profile page that his firm has 30 computers, up from just five a year ago. Three shifts of workers allow the operation to hum 24 hours a day, seven days a week. On the site, Workcaptcha has 197 reviews from other users, the vast majority of them positive.

It was not possible to verify the claims made by Workcaptcha and Mr. Shaon, but Mr. von Ahn said it was clear that Bangladesh had become a hub for paid captcha solving, as have China and India. The completed captchas help spammers open new online accounts to send junk e-mail and carry out other mischief.

Executives at Internet companies like Google say they do not worry a lot about people being paid to decode captchas because they are one of several tools that Web sites use to secure themselves. Some sites, for instance, might also send confirmation codes as text messages to cellphones, which then have to be entered into a separate verification page before new e-mail accounts are activated.

“It can’t be helped that paid human solvers will be able to solve captchas,” said Macduff Hughes, an engineering director at Google. “Our goal is to make mass account creation less attractive to spammers, and the fact that spammers have to pay people to solve captchas proves that the tool is working.”