Porn sniffer easily put off scent

Filtering software that could potentially lead to ISPs and user IT departments being searched for illegal pornography appears to be faulty.

Filtering software that could potentially lead to ISPs and user IT departments being searched for illegal pornography appears to be faulty.

Perkeo, the German software being considered as a possible weapon against internet distribution of child porn and other illegal material by New Zealanders, can be defeated, at least in its test version, simply by opening an offending image in graphics program, Paint Shop Pro, and resaving it with the program’s default resolution settings.

The program is one of those being tested by the department of Internal Affairs (DIA) in collaboration with some New Zealand ISPs, so they can scour their server for illegal images, initially, according to the DIA, those posted in Usenet newsgroups.

DIA censorship compliance chief Steve O’Brien says the department has not yet decided how it will use Perkeo or other candidate filtering software, but clearly any identification of supposedly illegal images at an ISP could result in serving of search warrants and possible sanctions against an ISP, with potential disruption of the business affairs of the ISP and its customers.

Computerworld did not have to deal with illegal images in order to test the program. The free test version contains several harmless graphics files in various formats, picturing a dog with its nose to the ground [presumably an allusion to “sniffing out”]. A signature library supplied is tuned to these files and presumably other genuinely offensive ones.

We shifted the test files to various folders in a PC, and Perkeo unfailingly detected their presence wherever they were placed. It detects common illegal files by a “signature” sequence of bits contained within them, rather as virus checkers track their prey. The library contains these signatures, not actual images.

But when we opened one of the dog pictures (a .jpg file) in Paint Shop Pro and simply resaved it, Perkeo did not detect the resaved version.

O’Brien declines to comment on the filter’s apparent shortcomings, saying only “We are testing software and its possible uses. We have not yet determined how or whether we will use it.”

DIA does not yet have a preliminary evaluation yet of the utility, reliability and comprehensiveness of Perkeo and other candidate software, he says.

No false positives (innocent images branded as illegal) were turned up in the tests with the free version, but it is possible that the 1.6MB library included with the test version includes only the signatures of a restricted range of images. Attempts to communicate with the German vendors have so far proved inconclusive.

False positives are not unknown among virus checkers and spellcheckers, which use similar signature or checksum techniques.

ISP use of filters was suggested in the report of a parliamentary select committee on changes to censorship legislation as part of a code of practice suggested to ISPs for voluntary, or, failing that, compulsory adoption (see ISPs should be licensed says report).

Asked whether the filter software might be used on content other than newsgroup postings, such as email attachments, O’Brien declined to comment, citing the preliminary stage of the investigation.

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