A Xerox engineer says that color printers from Xerox and other companies print faint information in the background of printed-out pages, to identify the model and serial number of the printer that printed the pages. According to a story, the information is represented as a set of very small yellow dots. (We already knew that some printers did this. The article tells us more about how it’s done.)
We have a Xerox color printer here (a Phaser 860). We tried printing out a page and looking for the dots, but we couldn’t find them, even with the aid of a magnifying glass and blue LED light. If anybody can find the dots on their output, please let me know.
There are still several unanswered questions about this scheme:
Do they use encryption, and if so, how? Even if we can find the dots and read out the digital bits they represent, we may not be able to tell what information those bits are encoding. They might be putting the model and serial number onto the page in such a way that we can learn to read them. Or perhaps they are encrypting the information so that we can’t read out the identifying information but we can at least recognize whether two pages were printed on the same printer. Or perhaps they encrypt the information so that we can’t tell anything without having some secret key.
If there is a secret key, who knows it? The key might be disclosed to the government so that they can extract the model and serial number from a page at will. (And if the U.S. government has the key, which other governments do?) Or the key might be known only to the printer vendor, so that the government needs the vendor’s help to decode the dots. If they use public-key cryptography, then the decoding key might be known only to the government and not to the printer vendor.
Do they try to track who buys each printer? If they can extract the serial number, they might want to know who has that printer. They could try to track the passage of each individual printer through the supply chain, to get an idea of who might have bought it. They might also build a database of information gleaned through service calls and warranty registrations.
What we know already is enough to make privacy advocates itchy. It’s probably possible to design a system that raises fewer privacy issues, while still allowing certain limited use of printer-specific marks as courtroom evidence. For example, one could build a system so that somebody who has physical possession of a printer, and physical possession of a printed page, and access to a special crypto key, can tell whether or not that page was printed by that printer, but can’t learn anything else.
