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About the challenge

The National Cipher Challenge has been run by the University of Southampton Mathematics Department since 2002 and has attracted a wide following. We count Cabinet Ministers, media scientists, journalists, authors and actors among our fans. We even had the pleasure of introducing the Cipher Challenge team from Saint Anne’s School in Southampton to the Duke of Edinburgh who, remembering his work in the second world war immediately fell in love with the competition and gave Harry a reading list for the summer.

The real fans though are the competitors who take part every year until they are too old, by which time it is too late and they are hooked. Many of them go on to careers in cyber security and others follow other paths using the mathematics and computing skills they learned tackling our fiendish challenges.

Julian went on to study Discrete Mathematics and made it to the Grand Final of the UK National Cyber Security Championship in 2013, following in the footsteps of the 2008 National Cipher Challenge winner, Jonathan Millican, who was crowned winner of the UK National Cyber Security Championship the previous year. Naomi Andrew, who operates under a code name as one of the Elves on the site, took part from Year 8 until she was sadly too old to compete, but managed to stay involved as a student at Southampton. She is now studying for her PhD here and we like to think the Cipher Challenge had an important part to play in that journey.

If I were to name one thing which has undoubtedly influenced my academic drive, interests and overall career to date, it would be the National Cipher Challenge. Since being introduced to cryptography and the challenge in Year 8, it has been my one passion and driving force in pursuing further education in maths.

Julian Bahrdwaj

The lesson PowerPoints are great to use with any year group – easy to understand with passages written in code for them to practise using the decryption methods, with little input needed from the teacher. Would recommend – especially if you are starting a Codebreaking club before the competition begins like me.

Sam Barlow, Queen Anne’s School, Caversham

Even if you can’t or don’t want to take part in the competition there is a lot here for you to enjoy.

Alongside the materials we have produced you will find everything you need to be a successful code-breaker.

We have a library full of suggestions of great books, fact and fiction, links to a whole range of videos on topics related to maths, computing and codebreaking, even a guest lecture by one of our alumni describing how he learned python to break the challenge.

We have some basic cipher tools you can use to get started including a Caesar wheel, an affine shift encryption machine and a frequency analyser to help you break down and find patterns in a ciphertext. In our cryptanalyst handbook you can find instructions to build your own cipher machines, including the simple cipher wheel and tis more sophisticated cousin the affine shift cipher wheel. Or you can go a step further and explore the fascinating Enigma machine and its cardboard twin, the Pringle Can Enigma Machine.

It was said by Niels Ferguson, one of the leading cryptographers of his generation, that cryptography was “just about the most fun you can have with mathematics”. As professional mathematicians we can think of a few others too, but we largely agree with that and hope that the Cipher Challenge will convince you too.

This edition of the competition

This is the 21st edition of the competition and takes up where last year’s story finished. It doesn’t matter if you missed it, this story is self contained and you will quickly catch up with what is going on. If you do want to set the scene you can visit last year’s competition site and read the episodes there. In short, BOSS agent Jodie has disappeared and Harry is left wondering whether or not to report her to his superiors. He has known her for four years and at heart he trusts her, but a rogue agent is always dangerous and there could be serious repercussions if he doesn’t tell them. On the other hand, he can’t be sure that she won’t return and reporting her now could put her career or freedom in jeopardy, if not her life.


You can download lessons and notes on codebreaking from the resources page on the competition website. Alongside the materials we have produced you will find links to books, online videos and help guides that contain everything you need to be a successful code-breaker. You can even build your own cipher machines, including the simple cipher wheel and the more complicated Pringle Can Enigma Machine.

Who is the competition for?

The competition is for everyone, the prizes are more restricted! We no longer collect personal information about competitors unless you are in line for a prize, so anyone can take part. The prizes (see below) are restricted to UK residents in full time school or 6th form level education, and if you are a contender for one of these we will be in touch at the end of the competition to verify eligibility. We can’t email you because we don’t ask for your email unless you register as a teacher, however you are able to link your account to a teacher or carer account, either at registration or from within your account pages, so we will be able to contact you that way. We can also send you a message via our inbuilt BOSS comms system, and you can check for those in your account page on the site.

If you have any questions please contact Harry at [email protected].

How to register and join in

There is no charge to register or take part, and all you need to get involved is a reasonably modern web browser. We publish news about the competition on Twitter and you can keep up to date by following us there.

Entrants can take part alone or in teams of any size. To take part you will need to register for an account on the website, and you can find out more about that on the Registration Instructions and Joining a Team pages. You can also find out more about this in the BOSS Trainer’s Manual.

Your account will also allow you to join in on the BOSS Forums, where you can discuss a whole range of things connected to the competition, and quite a few that are totally unrelated.

Competition schedule

Registration will open on September 15th and the first practice round for the competition will be published at 3pm on October 6th. The first three rounds on 6th, 13th and 20th October are designed as a warm up, and while we will publish leader boards, the marks for those challenges won’t count towards the final competition standings. The main competition starts with episode 4 on November 3rd, with the remaining challenges published weekly until December 15th. NOTE: all times stated on the site refer to current UK time. The clocks change back from British Summer time at around 2am on Sunday October 30th and our competition clock will reset to GMT.

Once registration starts you will find a schedule for the release of challenges on the Challenge page

Scoring the Challenge

There are two parallel competitions, part A and part B, and you can take part in one or both (or neither, but why would you?) Competition B is scored for speed and accuracy. We use the Damerau-Levenshtein distance to determine how accurate you are and break up the time into bands each worth a certain number of points. For each round you can submit more than once, and we mark each of your submissions. We then take your most accurate submission and award the appropriate time points to give a pair of numbers (accuracy out of 100, time points) and then use this to rank team entries. Accuracy is ALWAYS more important than speed. Speed does matter, but you do not have to rush to download the first challenges immediately as you have a day or two in which you can still get top marks. In later challenges speed will become important, and the full schedule of marks is published on the Challenge pages so you can see how quickly you will need to get started in each round.

Challenge A is intended for less experienced code breakers, so typically the challenges are a bit easier. You will be able to download certificates showing you how well you did in each round as well as your placing in the overall competition after each round.

The first three challenges should be thought of as a “warm-up” exercise and will not count in the final leader board rankings, however it is still worth tackling them as they give excellent practice and they do develop the storyline. You can submit multiple times to each round and will find feedback on each submission in your account pages, though we sometimes delay that to keep things interesting. Resubmission can never lower your mark and can improve it, so don’t be disheartened if you make a mistake. Have another go!.

At the end of each round you will be able to download a certificate recording your team’s performance.

Rewards and Prizes

This year we have a prize fund of £4,000 pounds supported in part by our very generous sponsors at GCHQ, IBM and Trinity College Cambridge. We will be awarding four prizes each to the tune of £1,000 as follows

The Alan Turing Prize and Ada Lovelace prizes for the best two individual codebreakers

The Hut 8 and Hut 33 prizes for the two best teams of codebreakers.

To be eligible for any of the prizes the winners must be in full time school or sixth form college level education in the UK. Homeschoolers also qualify, but proof of age may be required. It is also a condition that prizewinners agree that their names and affiliations may be published on the competition website. We encourage winners to publish details to the media, but this is not a requirement. These rules apply both to individual winners and to ALL members of any winning team.

Potential winners will be contacted at the close of the competition and asked to confirm eligibility.

Finally, for the integrity of the competition it is important that winners are able to show that their entry is the result of their own work and the prize committee reserve the right to ask for reasonable evidence of this. Even if this wasn’t needed, it is absolutely fascinating to hear how you tackled the competition and is one of the highlights of the whole thing for Harry and the team! You can see some of the recent accounts submitted on the resources page.

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