The National Codes Centre at Bletchley Park (who know a thing or two about ciphers) have comprehensive lesson plans, teacher's notes, and student activities for various types of encryption, from simple substitution ciphers (Caesar ciphers) through the infamous Enigma and Lorenz ciphers to modern day methods.

Very detailed but clear explanations of the various types of encryption systems, from simple substitution ciphers (Caesar ciphers) to modern public key encryption systems. The page also explains the types of attacks that can be performed against ciphers, such as known plaintext attacks and frequency analysis attacks. A variety of interactive applets held improve understanding.

Braingle contains extensive notes on many different types of cipher systems from Caesar ciphers to four-square ciphers. There are dozens of examples and some interactives to help students understand the different ways plaintext can be encrypted. This could be a good site to support a lesson starter activity or extension activities for more advanced students who are interested in encryption systems.

A simple interactive that implements a basic Caesar cipher. One great use for this is to highlight just how weak Caesar ciphers are. By giving students examples of ciphertext and asking them to use this tool to break them, they should realise just how easy it is to perform a brute force attack.

An overview of the encryption process, from plain text to cipher text, along with a clear explanation of the Caesar cipher algorithm (which is sufficient for most GCSE Computer Science courses). A good revision resource.