Secret messages - thinking about cryptology

The history of secrecy is as old as the history of ideas. Whether for reasons of war, religion, power, jealousy, or, of course, love, people of every culture have always found reasons to keep secrets. Of course, keeping a secret is not difficult until you try to communicate it to someone else. What if someone overhears you whispering the secret? Or worse, what if the secret message is intercepted by an assailant and does not even reach the intended recipient?

Simple model for secure communication between Alice and Bob with adversary Eve attempting to view the romantic missive

Historical attempts to foil Eve's efforts have fallen into three categories:

• Physically secure the message from access to all except its intended recipients. This could include hand delivering a love note to your sweetie at school, hiring a bonded courier to transport contract drafts burned on CDs to your business partner, or hiring an armored car to take a bank vault combination to the main branch for safe keeping. In other words, do not let Eve get her hands on it.
  
• Use steganography - Conceal the message in some other innocuous message. This includes communicating messages in innocent looking classified adds in the newspaper, hiding digital signals in what sounds like background noise on telephone calls, or embed messages in otherwise normal JPEGs, MP3, or other files using steghide. [try it!] In other words, do not let Eve know that a secret is being transmitted.
  
• Mathematically encrypt the message in such a way that it is difficult for an adversary to recover the original message even if she gains access to the entire encrypted message. In other words, even if Eve gets the message, make it difficult for her to decode it. An early example is a method used by Julius Caesar to protect military orders. Now, message encryption has become a sophisticated military practice and is the cornerstone of the modern banking and e-commerce industries. It is the greatest asset of political activists in oppressive regimes and the scourge of law enforcement and national security forces when fighting online threats.

It is this final approach - the mathematical approach to protecting information, the field of cryptology - that I would like to investigate in the next few blog posts. How does it work? What if criminals or enemies of the state use it?

Icons by: Mark James
Photo by: dirtyfeet