Strong encryption is usually a good thing. Until someone else uses it to encrypt your data -- and then asks you to pay a ransom to decrypt it for you.

That's the purpose behind Gpcode.ak, a new virus discovered by Kaspersky Lab yesterday. The virus is a much improved variant of the Gpcode ransomware that Kaspersky helped to crack two years ago, officials say.

Gpcode.ak encrypts files that have popular extensions, such as .doc, .txt, .pdf, .xls, and .jpg, using an RSA encryption algorithm with a 1024-bit key. The author of Gpcode has taken two years to improve the virus: the previous errors have been fixed and the key has been lengthened to 1024 bits instead of 660 bits.

Kaspersky so far has been unable to decrypt files encrypted by Gpcode.ak, since the key is 1024 bits long. The only way to currently decrypt the encrypted files is to use the private key, which only the author has, and which is available for a fee.

"With this new version of Gpcode, we've encountered ransomware which seems impossible to crack during this early stage of detection," said Roel Schouwenberg, senior antivirus researcher at Kaspersky Lab. "Next to running anti-malware solutions, the best measure to fight this kind of malware is to regularly create backups of the files stored on the computer.

"We strongly discourage infected people to pay the ransom, as this will only encourage the author to create new versions" Schouwenberg said. The company is offering to help recover files for users who have been infected.

— Tim Wilson, Site Editor, Dark Reading