[LINK] Rudd online porn-free plan questioned

[Irene Graham]

On Wed, 02 Jan 2008 10:38:03 +1100, Rick Welykochy wrote:
[...]
>> In Britain where a clean feed policy is being pursued, only between
>> 200 and 1000 child pornography sites have been included on a
>> blacklist.
>
> No wonder Britain can claim that their safe feeds are not slowed down
> by blocking. Although, if you think about it, a list of, say, 1000000
> domains can be searched with a max 21 lookups using binary search, or
> on average about 11 lookups. Think about that. Every web request, every
> domain, ftp, email, etc request, will require an extra step of on
> average 11 lookups in a table of 1 million names.

BT's 'cleanfeed' is a hybrid/two stage system. People with technical 
understanding/knowledge may find the following paper interesting:

"Failures in a Hybrid Content Blocking System"
Richard Clayton, University of Cambridge, Computer Laboratory
http://www.cl.cam.ac.uk/~rnc1/cleanfeed.pdf

	"Abstract. Three main methods of content blocking are used on the In-
ternet: blocking routes to particular IP addresses, blocking specific URLs
in a proxy cache or firewall, and providing invalid data for DNS lookups.
The mechanisms have different accuracy/cost trade-offs. This paper ex-
amines a hybrid, two-stage system that redirects traffic that might need
to be blocked to a proxy cache, which then takes the final decision. This
promises an accurate system at a relatively low cost. A British ISP has
deployed such a system to prevent access to c***d pornography. However,
circumvention techniques can now be employed at both system stages to
reduce effectiveness; there are risks from relying on DNS data supplied
by the blocked sites; and unhappily, the system can be used as an oracle
to determine what is being blocked. Experimental results show that it is
straightforward to use the system to compile a list of illegal websites."

Richard also remarks:
	"Although legal and ethical issues prevent most experimentation at 
present, the attacks are extremely practical and would be straightforward 
to implement. If CleanFeed is used in the future to block other material, 
which may be distasteful but is legal to view, then there will be no bar to 
anyone assessing its effectiveness. It must be expected that knowledge of 
how to circumvent the system (for all material) will then become widely 
known and countermeasures will become essential."


Irene