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'We've found that the dots from at least one line of printers encode the date and time your document was printed, as well as the serial number of the printer,' said Staff Technologist Seth David Schoen of the Electronic Frontier Foundation (EFF).

The group looked at printers from many big name brands, such as Xerox, Canon, Epson, Dell and HP, and found that the printers printed these codes onto every page. It focussed on Xerox DocuColor printers and found that by comparing the subtle changes in the position of the dots it was possible to decode this information.

The group said that currently only

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the US Secret Service and now itself (in the case of Xerox DocuColor printers) had the ability to decrypt the imprint. It said that although the Secret Service claims to use this information only for cornering counterfeit crimes, there is no legal framework to prevent the information being put to other uses. The only defence the owner can fall back on is any privacy policy that is included with the device itself.

'Underground democracy movements that produce political or religious pamphlets and flyers, like the Russian samizdat of the 1980s, will always need the anonymity of simple paper documents, but this technology makes it easier for governments to find dissenters,' said EFF Senior Staff Attorney Lee Tien. 'Even worse, it shows how the government and private industry make backroom deals to weaken our privacy by compromising everyday equipment like printers. The logical next question is: what other deals have been or are being made to ensure that our technology rats on us?'

The EFF is looking to provide evidence that more printer lines are involved. You can get involved in the project at the group's website.