If you are new to codes and ciphers then this fantastic podcast with Janna Levin is a nice place to start. It appeared in the magazine Quanta in August, just in time for the start of the competition (thank you Quanta) and gives a good overview of how mathematics helps us keep secrets safe.
The podcast has a transcript if you prefer to read rather than listen, and If you have other suggestions for podcasts, articles, books, films or tv that you think the National Cipher Challenge community would enjoy then you can post them in The Intelligence Room.