The GCHQ — Britain’s secretive agency of intelligence experts — wants to find new spies. To make sure it has a candidate who’s up to scratch, the agency is inviting hobbyist cryptanalysts to try and break a code online.

A website called “can you crack it” is being spread through a viral campaign around social networks like Twitter and Facebook. The site shows a seemingly-senseless jumble of 160 pairs of numbers and letters, and a box to enter some kind of answer.

Though there are no obvious clues that this is related to the GCHQ and nothing to suggest what will happen if you plop in the right...