Companies and individuals in Japan are finding their computers are increasingly targeted by ransomware — programs that bar victims from accessing important files unless they pay money.

“Attacks on Japanese businesses have been particularly large in number,” said Masakatsu Morii, a professor of information and telecommunications engineering at Kobe University’s Graduate School of Engineering.

“The attackers may have come to know that Japanese would pay money,” he added.

Ransomware typically infects computers when its user opens a file attached to spam mail from a sender pretending to be a legitimate entity such as a parcel delivery company, according to the government-affiliated Information-Technology Promotion Agency.

The malicious programs encrypt the infected computers’ files, and users can only open them after paying the perpetrators money to obtain a special key to unlock them.

Yoshihito Kurotani, a researcher at the agency’s engineering department, said the programs employ basic encryption technologies. Kurotani’s agency has received numerous inquiries asking for help from victims who cannot access their photos or business files.

The bogus emails “used to be written in English or unnatural Japanese, but we have seen increasing attacks using natural Japanese recently,” Kurotani said. “Japan has evidently been a target.”

Computer security firm Trend Micro Inc. said it received 2,810 reports of ransomware attacks nationwide in 2016 — a 3.5-fold jump from the previous year.

“Tactics are expected to be even more sophisticated in 2017,” a Trend Micro official said.

A survey conducted by the firm last June shows that about 60 percent of companies that were attacked paid ransoms. The payment in one case exceeded ¥10 million ($88,000).

The extortion and the transactions in the ransomware programs themselves have become a profitable business for cybercriminals. The programs are traded on online black markets that cannot be accessed without the use of special software.

In the “dark web” networks, various programs are sold, including multilingual ones and one that can be used for a “lifetime” for just $39. The people who post the programs make profits by taking a share of ransoms collected.

Personal data are also sold on the dark web. Firms undertaking the delivery of unsolicited emails do business there, too.

Katsuyuki Okamoto, a security “evangelist” at Trend Micro, said it has become easier and easier to be involved in or become a victim of cybercrime.

Cybersecurity experts warn that users should protect their computers by always keeping operating systems and anti-malware software up-to-date and should constantly back up their data.

They said victims should never pay ransoms as there is no guarantee their files will actually be restored.

“If you pay money to the criminals, that will only help them create a new virus,” Okamoto said. “So you should never pay them.”