Organisations are being offered an unusual new anti-child porn system that can identify and block real, individual images by comparing them to one of the world's largest databases of such pictures.

Working from a 400,000 image and video database drawn mainly from those collected by the Swedish police, NetClean Technologies' ProActive system creates a digital signature for each image using a similar method to that adopted by anti-virus companies.

If the system - which can be deployed as a gateway and on the PC itself - detects a signature from the core database, an admin is notified and copying of the image to an external device such as a USB stick is blocked. The PC software monitors external drives, which are typically used in child porn distribution.

NetClean sees the strength of the system that it can detect what is a statistically rare but hugely serious offence, that of using a business network to distribute child porn. The company estimates that around one person is detected for every 1,000 licenses sold, far below the detection rate for adult porn access, which is nevertheless easier to spot using web filtering.

The technology's key feature is its ability to spot actual images rather than simply setting out to block images based on some form of generalised heuristic approach. In theory, this should reduce the occurrence of false positives.

Anecdotally, child porn appears to be different from other types of banned content in that offenders are highly likely to exchange images among one another. This explains why file-sharing P2P networks have become the medium of choice.

"Many collectors of child abuse images carry the images with them when they go to the office as they are often family men who think it is safer to view these images at work, because current filters only look for material coming off the internet," said NetClean CEO, Christian Sjöberg.

NetClean has launched ProActive in the UK for the first time where it hopes to attract the interest of government and perhaps banks, which it sees as being motivated to deploy the technology beside other types of security product. Longer term, the company is looking at alliances to have the technology integrated into third-party security products.

The system is aimed at larger installations, with the price quoted for a 1,000 user base quoted as being around £8,000 ($12,100), or £8 per PC, per annum. The company also produces products for performing the same monitoring for ISPs and law enforcement.