Looking for a job? Britain is hiring spies, and their job listing is online. A bewildering array of letters and numbers at Can You Crack It invites aspiring cryptographers to solve the code. At least 50 people have done so since it went live last month. The agency is one of the oldest surveillance organizations in the world: the Government Communications Headquarters, which works closely with the American National Security Agency. When you solve the puzzle, the site presents you with a keyword that leads you to a Web address with a congratulatory note signed by a group called the Cyber Security Specialists, a newly formed organization dedicated to counterespionage. Then the hard part begins: you have to submit an actual job application.