Symantec, a manufacturer of anti-virus programs, said on Wednesday on its website (http://www.symantec.com/avcenter/venc/data/trojan.pgpcoder.html) that the attacking program is a "Trojan horse" that enters the computer via holes in the victim's web browser, scans a hard drive and encrypts any text-based documents that it finds.

Unlike a virus, a Trojan horse does not replicate from machine to machine.

If it installs successfully, the new threat wipes out text files then displays a ransom note demanding 200 dollars (153 euros) to supply decryption software that will restore the data back to its original, readable form.

So far, only one case of "ransomware" has come to light, but this could be the start of a new and dangerous generation of malicious software, the British weekly New Scientist says in its next issue dated Saturday.

The algorithm used to scramble the data was not very sophisticated in this case, and the encrypted documents were easily decoded by a computer consultant.

"The danger now is that the virus writers might turn to using strong military-grade encryption systems," leaving the victim with little option but to pay up to decrypt his files, New Scientist warns.

The new program goes under the name of pgpcoder, apparently to besmirch the name of Pretty Good Privacy (PGP), a legal, popular and highly secure encryption product.

The "ransomware" case was spotted by the Californian web-filtering company Websense, which has notified the US Federal Bureau of Investigation (FBI), New Scientist says.

As with extortion everywhere, the weakness of "ransomware" is the line of contact. The demand includes a contact email address and an electronic bank account, both of which can be traced.

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