* Security is ensured using codes and encryption, made possible by an application of number theory and involving the use of very large prime numbers. Understanding the basis of such techniques is within the capabilities of upper secondary students.
* Codes and encryption are a rich source of maths from junior school upwards and have a colourful history, relating to the transfer of secret military and political information, as well as in finance and business. Two good books covering this area, both containing many ideas that could be adapted for the classroom, are: Simon Singh's The Code Book (a history of codes and code-breaking) and Sarah Flannery's In Code, which tells the story of a 16-year-old Irish girl who won the 1999 young European scientist of the year award for inventing a system for encrypting data on the internet. This contains lots of entertaining maths suitable for secondary pupils. Simon Singh also has a website that gives information on codes and encryption: www.simonsingh.net, and he has produced an interactive CD suitable for classroom use.