Ask HN: Which mathematics subject should I base my book's plot on?

I'm about to resume my book project I had to stop in 2008. The plot is about a journalist unraveling a secret society in Rio de Janeiro whose members communicate only through numbers. What do you think would be an interesting subject within mathematics for the plot? Something that the journalist would eventually find out, research and explore in order to know more about that group of people.

edit: changed term "hidden society" to "secret society"