On his website David Kriesel explains at length and in detail how he stumbled upon the anomalies in Xerox Workcentre 7535 and 7556 machines, and analyses what causes the problem – a compression algorithm randomly replaces patches of pixel data in an almost unnoticeable way. Apparently Xerox machines use JBIG2 for compression, an algorithm that creates a dictionary of image patches it considers similar. As long as the error generated by these patches is not too high, the machine reuses them instead of using the original image data. This also would explain why the error occurs when letters or numbers are scanned in moderate resolution, yet still readable for the human eye. When the letter size is close to the patch size of JBIG2, complete letters and even blocks of letters that look similar to the machine are replaced.

Xerox says it will release a software patch to tackle the fact that scans made with its Workcentre copiers can result in numbers and letters being changed in saved files.

It says the issue only occurs if people alter its machines' default settings.

However, it acknowledges that referring to the affected mode as "normal" on the devices' selection screens needs to be reconsidered.

There had been claims that the flaw could put lives in danger.

Xerox says the problem is the result of a compression technique widely used in the industry.

I'm kind of glad this got news coverage as it meant I found out about Jbig2. It's pretty cool, but people need to understand when it's being used and possible consequences.

I'm kind of glad this got news coverage as it meant I found out about Jbig2. It's pretty cool, but people need to understand when it's being used and possible consequences.