Keeping secrets is one of the earliest inventions of civilisation, and has become the science of cryptography. The World War II Enigma machine was just lots of scrambling, done in ways that could be understood in principle by a school child though it took daring and powerful computing to crack it. This lecture introduces the key ideas behind conventional cryptography, and explains why it is not good enough for modern applications such as international commerce on the Internet.
From http://www.gresham.ac.uk/event.asp?PageId=108&EventId=114