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For the past two years, a mysterious online organization has been setting the world’s finest codebreakers a series of seemingly insolvable problems. But to what end?

One evening in January last year, Joel Eriksson, a 34-year-old computer analyst from Uppsala in Sweden, was trawling the web, looking for distraction, when he came across a message on an internet forum. The message was in stark white type, against a black background.

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“Hello,” it said. “We are looking for highly intelligent individuals. To find them, we have devised a test. There is a message hidden in this image. Find it, and it will lead you on the road to finding us. We look forward to meeting the few that will make it all the way through. Good luck.”

The message was signed: “3301.”

A self-confessed IT security “freak” and a skilled cryptographer, Eriksson’s interest was immediately piqued. This was – he knew – an example of digital steganography: the concealment of secret information within a digital file. Most often seen in conjunction with image files, a recipient who can work out the code – for example, to alter the colour of every 100th pixel – can retrieve an entirely different image from the randomized background “noise.”