Codes and code-breaking are excellent ways to engage pupils in mathematical thinking. Using frequency analysis to break a substitution cipher demonstrates the power of simple statistical analysis. As activities, pupils can develop encryption methods, and encode messages and try to crack each other's ciphers. Pupils can investigate more sophisticated encoding techniques, leading to public-key encryption, which involves using prime numbers and modular arithmetic.
Maths libraries should have The Code Book by Simon Singh and In Code by Sarah Flannery. The first is a history of codes and code-breaking, and explains the maths involved. In Code is about the author, who developed an encryption method. It includes lots of mathematical puzzles.
Why not organise a visit to Bletchley Park, where the Enigma code was cracked (www.bletchleypark.org.uk)?
Simon Singh's website (www.simonsingh.netThe_Black_Chamberhome.html) has activities and simulations on codes and code-breaking.