Just because a document from a colour laser printer doesn't carry your name doesn't mean no one can trace it back to you, privacy advocates warn.

The Electronic Frontier Foundation says it has cracked the tracking codes embedded in Xerox's DocuColor colour laser printers. Such codes are just one way manufacturers employ technology to help governments fight currency counterfeiting.

Researchers found patterns of yellow dots arranged in 15 by 8 grids and printed repeatedly over every colour page, said Seth Schoen, a staff technologist at the San Francisco-based civil-liberties group.

Consider two documents, one carrying the author's name and one meant to be anonymous. By comparing the codes, it could be determined whether the two documents came from the same printer, even if Xerox reveals nothing about a customer's serial number, Schoen said.

Certainly, counterfeit currency (and other official thingies) is to be actively discouraged, but to put general privacy at risk in this way is quite horrific.