"Ransomware" last seen in 2006 has reappeared and is trying to extort US$300 from users whose files the malware has encrypted, a Russian security researcher said Monday.

GpCode, a Trojan horse which last made a run at users last summer, has popped up again, said Aleks Gostev, senior virus analyst with Moscow-based Kaspersky Lab, in a posting to the research center's blog.

Noting the long quiet time, Gostev added: "So you can imagine our feelings this weekend, when some of our non-Russian users told us their documents, photos, archive files etc. had turned into a bunch of junk data, and a file called 'read_me.txt' had appeared on their systems."

The text file contained the "ransom" note.

"Hello, your files are encrypted with RSA-4096 algorithm. You will need at least few years to decrypt these files without our software. All your private information for last 3 months were collected and sent to us. To decrypt your files you need to buy our software. The price is $300."

So-called ransomware typically follows the GpCode pattern: malware sneaks onto a PC, encrypts files, and then displays a message demanding money to unlock the data.

Gostev hinted that the blackmailer was likely Russian. "The e-mail address is one that we've seen before in LdPinch and Banker [Trojan horse] variants, programs which were clearly of Russian origin," he said.

The blackmailer's claim that the files were enciphered with RSA-4096 -- the RSA algorithm locked with a 4,096-bit key -- is bogus, said Gostev. Another oddity, he added, was that the Trojan has a limited shelf life: from July 10 to July 15.

"Why? We can only guess," said Gostev.

Kaspersky is working on a decryption scheme to recover the files; that process has been the usual salvation -- and solution -- for users attacked by ransomware. "[But] we'd just like to remind you, if you've fallen victim to any type of ransomware, you should never pay up under any circumstances.

"Contact your antivirus provider, and make sure you back up your data on a regular basis."