Copy Protection On Currency
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Bank Note Copy Protection

Adobe has recently admitted that they have added banknote copy protection to their image manipulation software. This follows Xerox adding similar copy protection to the firmware of their photocopiers. The copy protection has apparently been developed by the Central Bank Counterfeit Deterrence Group (CBCDG) which is a collection of international banks. So how does this copy protection work?

The answer is quite simple and was discovered by Markus Kuhn from Cambridge University, who calls it the EURion Constellation. The answer lies in clusters of circles printed on the note. These are about 1mm in diameter and arranged roughly in the format of dots on the the 5 side of a die. This formation is approximately 7mm at its widest. These circles are printed in yellow, red or green and are best viewed by looking at the blue channel. Clusters are printed several times and rotated by various angles. In the following table I have reproduced 100x100 pixels of the blue channel from several bank notes I just happen to have in my wallet.

5, taken from the front side, the bottom left of the apparently empty oval 10, taken from the same position as the 5 note 20, taken from the front side, the short piece of music near the top of the note 20 Euro, taken from the front of the note to the left of the arches

On first sight the circle formation is not obvious. In the next image I have joined the circles from two clusters on the 5 pound note. Note that these two clusters share one circle.

Once you recognise the shape of the clusters it is easy to identify them on the other banknotes. Markus Kuhn says that he has found clusters like these on the new US $20.

This raises several issues. Firstly, how accurate is the algorithm, how easy is it to create a false positive? If I create a page of 1mm black circles placed randomly on the page at a density of, say, 10 per square centimeter I am likely to get an appropriate cluster somewhere on the page. Will the Xerox photocopier refuse to copy the image? If so, then this could be a relatively cheap way of copy protecting any document: if the 'blank' page consists of many yellow circles and then the document is printed over the top, the clusters are likely to be recognised by the photocopier. For example, this could be an effective way of preventing a fraudster from photocopying the signature on a letter.

As a copy protection measure on bank notes, I am less convinced. For counterfeiting to be successful the fraudster only has to convince an ordinary person in a shop, and does not have to convince an expert. Most people will check the watermark on th banknote or other noticeable features like the metal strip that goes through the Bank of England notes. If you look at the image of the 10 you'll notice a mark near the top of the image. A bank teller had written on the note. I have removed some of this writing (to make the circle clusters more visible), but it could have affected an algorithm looking for the clusters. The colour and the texture of the paper are important too. Banks are very careful about the availability of the paper that they use. For these reasons I think that there is little to gain having Xerox and PhotoShop implementing this algorithm: if the counterfeiter does not have the right paper, the note will be instantly identified. Furthermore, the algorithm could be fooled by applying some extra image on a real banknote using a colour that is less visible to the human eye, but is picked up by the algorithm in the blue channel. In particular, if you take a 10 and carefully colour-in the oval on the front with a yellow (the compliment of blue) or an orange pencil (the bank note is orange) the algorithm would be fooled that the clusters are not on the note.

However, this technique is more useful for machines that read banknotes, and I suspect that the counting machines used in clearing banks would use it, and also vending machines that take banknotes. In both of these cases it would be just one of several techniques used to verify a note's authenticity.

Finally, it is worth pointing out that it is not Xerox or Adobe's responsibility to catch lawbreakers as explained in this eWeek article. At the moment those products from those companies are checking for counterfeiting and then refusing to make a copy of the banknote. How long will it be before those products report the offender? Before you judge that this would be a good thing remember false positives.

(c) 2006 Richard Grimes