[DanBC]

Richard Clayton did some research (some time ago) about cleen feed.

The paper linked on his page here - (http://www.cl.cam.ac.uk/~rnc1/)

> I examined BT's CleanFeed system (its proper name is the BT Anti-Child-Abuse Initiative). This was designed to be a low cost, but highly accurate, system for blocking "child pornography". At first sight it is significant improvement upon existing schemes. However, CleanFeed derives its advantages from employing two separate stages, and this hybrid system is thereby made more fragile because circumvention of either stage, whether by the end user or by the content provider, will cause the blocking to fail.