Prof Randell continued to badger civil servants and his efforts were helped by the appearance of several books about Bletchley and code-cracking that mentioned the machines used to read messages sent by the Wermacht, Luftwaffe and Hitler's generals.

BBC: The road to uncovering a wartime Colossus

He created the genetic algorithm that runs the app after reading programming books and forums, writing the code during his summer vacation and sending mail shots to the tech press to get coverage.

FORBES: Interview: Teenager Sells His Mobile Startup To Yahoo! For $30 Million

Visit any bookstore and there's likely to be a table of books with titles like, Exploring the Da Vinci Code, Breaking the Da Vinci Code, and The Truth Behind the Da Vinci Code, many released by Christian publishers.

NPR: Religious Groups Respond to 'Da Vinci' Movie Release

It would be quite wrong if fictional writers were to have their writings pored over in the way The Da Vinci Code has been pored over in this case by authors of pretend historical books to make an allegation of infringement of copyright.

BBC: ON THE BACKGROUND TO THE CASE

In the prison library, where inmates most ask for Westerns and romance novels, employees scour pages of returned books just to make sure there's not a message or code that could be passed along to another inmate.

CNN: Reporters get first look inside mysterious Supermax prison

But because the code is freely distributed, there are potentially millions of users who need additional tools, books and applications to enhance the software.

CNN: Celebrated freeware OS Linux grows up

Brown makes a point of visiting the locations he describes, and since "The Da Vinci Code" published in 2003, his fans have obsessively tried to discern where his next books might take place and what they're about.

WSJ: Dan Brown returns to Europe for 'Inferno'

So, the PGP source code was only available in a 43-volume, 65, 000-page set of books sold in a Palo Alto bookstore called Printer's Ink.

CNN: Technology - Security flaw discovered in Network Associates PGP software