The emergence of a variant on a virus that encrypts the victim's data with a strong 1,024-bit algorithm so the victim can't unscramble it without paying a ransom has begun to spread, potentially posing a major threat, according to the antimalware firm which discovered it.

Kaspersky Lab says the new variant of the Windows-based encryptor virus Gpcode, which hasn't been spotted for about 1 ½ years, is more of a threat than it was in the past because this time it is using strong encryption that so far has defied efforts to crack it.

"Up until now, we were able to crack the algorithms," says Roel Schouwenberg, senior antivirus researcher at Kaspersky Lab.

Earlier versions of Gpcode - which first appeared about 3 ½ years ago - used far weaker encryption than what it has today, plus it wasn't well implemented, making it fairly easy to crack, Schouwenberg says.

But Gpcode.AK, with its RSA 1,024-bit encryption, is proving hard to break. He adds that computer users should be making an effort to back up their data vigorously in the face of this new threat.

The Gpcode.ak is hard to detect because it attempts to self destruct after encrypting, according to Kaspersky Lab. So far only a handful of computers with files that have been maliciously encrypted have been identified so far. Most evidence about it is originating in Russian-speaking countries, Europe and Africa, he says, but it may be spreading further.

So far, the primary means it uses to spread is unclear, but Kaspersky Lab believes it's a form of "social engineering" that may involve trickery to induce computer users to make use of software they shouldn't.

The text file that the criminals leave tells the victim that the file has been encrypted and offers to sell them a "decryptor." Kaspersky Lab would advise against yielding to blackmailers in any ransomware situation.

Kaspersky Lab says efforts are continuing along with others in the antivirus industry to analyze Gpcode.ak further for technical weaknesses, but that users should now be extra careful in opening files and Web activity.