If you are running a cipher club this term or considering building some cipher cracking into your lessons then you could take a look at the six lesson plans we provide, along with other useful materials, on our resources page. They were written in collaboration with our friend and colleague, Hugh Evans, erstwhile Head of Mathematics at Sholing Girls School (Now the Sholing Oasis Academy).
If you have any suggestions to improve them, or indeed to add to them, and would like to work with us to provide additional resources then please get in touch. We would love to hear from you.
Lesson 1, the Caesar shift cipher and pattern detection
The simplest cipher, but a great introduction to both the implementation and cracking of a code.
Lesson 2, frequency analysis
Cracking substitution ciphers depends on finding hidden patterns in the text. Frequency analysis is your friend and is where the mathematics begins.
Lesson 3, the affine shift cipher
This is an introduction to modular arithmetic and builds on the first two lessons.
Lesson 4, breaking the affine shift cipher using modular inverses
A look at the mathematics behind the cipher, how to solve equations in modular arithmetic, and how that helps us break the affine shift cipher using inverses in modular arithmetic.
Lesson 5, the keyword cipher
Examines how to implement a much richer family of field ciphers based on an agreed keyword, and considers how to crack one in which the word structure is given, using cribs.
Lesson 6, cracking a keyword cipher using frequency analysis and cribs
This lesson takes what we know about frequency analysis and cribs and uses it to crack a much harder example of a keyword cipher in which the word structure is hidden.