The MI9 competition: How to decipher a letter encrypted with MI9 code

Try your hand at deciphering this MI9-inspired coded letter for a chance to win Helen Fry’s books on intelligence history including her latest bestseller, MI9: A History of the Secret Service for Escape and Evasion in World War Two

Have you ever wondered how WWII prisoners communicated information to organisations such as MI9? In her latest book, bestselling historian Helen Fry explores how MI9 engineered the escape of Allied forces from behind enemy lines.

Colonel Albert Clark, an American trained by MI9, managed to send a message to MI9 while imprisoned. His coded message was sent from Stalag Luft III at Sagan, later inspiring the scene of the ‘great escape’ of March 1944. Using the same code, we re-imagined what his message to MI9 might have looked like when he initially approached them as a POW in Stalag Luft III – can you decode what information he communicated?


The letter to decode

Hover your cursor over the letter and click on the square to bring it up full screen


Guidelines: how to break code V. of MI9

First, you are going to need the letter writer’s number which is 1. Then, you need to find the keyword in the encrypted letter which will give you the positions of the words that comprise the hidden message. Read on for further codebreaking instructions, and if you need a bit of inspiration, scroll down to the bottom of this post for an example letter!

Find the keyword

Count the characters of the first two words of the letter and add the number of the letter writer. In the example embedded below, the first words are ‘I’m afraid’, hence 8 characters. We add 1 (the letter writer’s number), 8+1=9. The ninth word, in this case ‘keenly’, is our keyword!

Find the codeword frequency

Convert the keyword into numbers based on the letters’ position in the alphabet and eliminate numbers 1 to 3. In the example, K=11, E=5, E=5, N=14, L=12, Y=25, so KEENLY turns into 1155141225. After deleting numbers under 4, we get the frequency of the coded words: 5545. 

Find the coded words

The frequency numbers give us the positions of the coded words, starting from the first full line of the letter. In our example the first coded word is ‘defeat’, the second ‘of’, the third ‘talking’ and the fourth ‘depressed’. Once the numbers of the frequency are over, we restart the sequence: in the example, the fifth word is ‘very’, found five words after ‘depressed.’ The positions of the coded words are called ‘frequency positions.’

Find the coded numbers

To write a number as part of the coded message, the code writer uses the article ‘a’ or ‘an’ in one of the frequency positions. The initial letter of the word in the following frequency positions must be turned into a number based on the following system: 

A=1, B=2, C=3, D=4, E=5, F=6, G=7, H=8, I=9, J=0, AJ=10, AB=12, CD=34 

Numbering has finished when another ‘a’ or ‘an’ is used in the next frequency position. Note that to write numbers over 10, you need more than one coded word.  

Hiding words more carefully

The letter writer might not be able to use the word they need in full in their letter and might have to spell it. If the article ‘the’ appears in a frequency position, we only use the initial letter of the word in the following frequency positions until the article ‘the’ appears again in a subsequent frequency position. The initial letters of those words will spell the hidden coded word of the message. 

Ending the message

The message ends when the word ‘but’ appears in a frequency position. Remember that ‘a’, ‘an’, ‘the’ and ‘but’ cannot be part of the message. 

Read the hidden message

The message is coded from the end to the beginning, so you will need to read the coded words in reverse order. 

Top tip

Keep a notepad and pencil and write down the code words as you go along.

Good luck!

Click on the document below to view our example letter, in which the letter writer’s number is also 1


MI9 by Helen Fry

‘Fry has undertaken prodigious research…The book is a fitting tribute to the hundreds of men and women who risked their lives in assisting Allied escapees, and a welcome salute to those who broke out of their PoW camps that they might be returned to the battlefront.’
—Giles Milton, The Sunday Times

‘Helen Fry’s engrossing tale M19…details the exploits of the secret organisation that rescued allied troops from behind Nazi lines.’
—Martin Chilton, The Independent


To enter the competition, submit your answer by email to yalebooknews@yaleup.co.uk with the subject line “The MI9 Competition” by Sunday 8th November 2020.

Runner-up prize: 1 hardback copy of MI9 signed by the author
Grand prize: 1 signed hardback copy of MI9, plus copies of Helen’s previous spy history books The London Cage and The Walls Have Ears in paperback.

Terms & Conditions for the Competition

Please read these terms and conditions carefully. By taking part in the competition, all participants will be deemed to have read and accepted them.
We will not sell or share your data to any other organisations for their own purposes. To read the full Yale University Press, London Privacy Policy click here.

  1. The closing date for entry to the competition will be Sunday 8th November 2020. Entries must be received no later than 11.59pm (BST) on the closing date. Entries sent past this date will not be considered.
  2. Each Entrant may submit one entry only.
  3. The prizes are non-transferable, non-refundable and cannot be exchanged for a cash alternative in whole or in part.
  4. This competition is not open to employees of Yale University Press or their families.

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