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. The lecture also serves as an introduction to the following lecture (28 February) on modern cryptography.

Harold Thimbleby is professor of computer science at Swansea University where he established the Future Interaction Technology Lab. His passion is designing dependable computer systems to accommodate human error. He has been a Royal Society Wolfson Research Merit award holder and a Royal Society Leverhulme Trust Senior Research Fellow. He has also contributed to the Encyclopedia Brittanica and was Gresham Professor of Geometry between 2001 and 2004.
All of Professor Thimbleby's previous lectures may be accessed here.