The infamous Cryptolocker malware, which encrypts your computer files and demands a payment of £534 ($860) to unlock them, may have been sent to "tens of millions" of Brits, Blighty's crime-busters warned today.

According to an alert from the UK National Crime Agency (NCA), a fresh round of ransomware-loaded spam posing as bank notices has been sent out, with small and medium-sized businesses targeted in particular. The messages, described as a "significant risk", carry booby-trapped attachments and claim to be official documents from financial institutions.

Lurking within the attachments is a Trojan called Cryptolocker that, when executed, silently installs itself and quietly begins encrypting documents one by one on the Windows PC using tough-as-nails AES256. When it's finished, it demands a ransom payment of 2 Bitcoins (at least 500 quid or 800 bucks) to decrypt the data, which must be paid within a time limit.

The software nasty is particularly fiendish: The malware first contacts its master's control server, which generates a new public-private 2048-bit RSA cryptographic key pair and sends the public half to the malware.

Then for every file discovered on the computer, Cryptolocker generates a new 256-bit key and uses it to encrypt that document using the virtually unbreakable AES256 algorithm. That AES key is then encrypted using the RSA public key and stored with the obfuscated document.

Only when the victim pays up does the Trojan download the private half of the RSA key, which is used to decrypt the per-file AES keys and ultimately restore all the protected documents. Targeted files include anything with .doc, .docx, .xls, .xlsx, .ppt, .pptx, .dwg, .dxf, .dxg and .jpg extensions and plenty more.

Users are urged to maintain regular backups of their data, kept separate from their computers, as the encryption is essentially uncrackable, and consider using tools to thwart the software nasty. The Trojan infects systems running Windows 8, Windows 7, Vista, and XP.

"The emails may be sent out to tens of millions of UK customers, but appear to be targeting small and medium businesses in particular," the UK's NCA said.

"This spamming event is assessed as a significant risk."

Cryptolocker's operators are also apparently developing a keen sense of economic opportunism, upping their Bitcoin demands at a time when the digital currency's exchange rate has never been higher.

While authorities have yet to finger any suspects behind the Cryptolocker epidemic, the NCA believes the operation is the work of a tech-savvy crime ring.

"The NCA are actively pursuing organized crime groups committing this type of crime," said Les Miles, deputy head of the NCA's National Cyber Crime Unit.

"We are working in cooperation with industry and international partners to identify and bring to justice those responsible and reduce the risk to the public."

In addition to installing and updating trusted security software, users and administrators can protect against infections by using best practices (read: common sense) such as avoiding links and attachments from unknown or suspicious sources and scanning all attached files for malware. ®