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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.

Powerpoint slides

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.

Powerpoint slides

Lesson 3, the affine shift cipher

This is an introduction to modular arithmetic and builds on the first two lessons.

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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.

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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.

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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.

Powerpoint slides

Simon Singh's Black Chamber

A rich technical resource for beginners

How to crack Challenge 10B of The Kompromat Files

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Programming for cryptologists, 101

A fantastic short introduction to programming for beginners interested in using computers to break codes and ciphers.

Julian Bhardwaj, long time participant in the National Cipher Challenge and finalist in the UK Cyber Security Championship, wrote these notes and kindly offered them to us.


Download programming guide


There is a rich a variety of careers for mathematicians in just about every aspect of the economy. Here are a few that might appeal to you if you have enjoyed the Challenge

GCHQ have a number of technical roles which require strong mathematics skills, an enquiring m1280px-government_communications_headquarters_logo-svgind and dedication. You can find out more about some of them here

ibm-logo-blueIBM run a GAP year program, and you can register your interest for that online by clicking on their logo here.

The Code trilogy

Our good friend Jo Welch has written some books that we think you might enjoy (with the third volume due out this Autumn!). Head over to Amazon to take a look and sign up on Twitter to follow her.

Novels for code breakers!

Codes and ciphers in the movies

A short introduction

The Lost Legion - National Cipher Challenge 2017

Last year's challenge