The MI9 Code for Captured and Detained Personnel

The MI9 Code originated in WW-II and a version of this code is still taught in SERE, allowing captured and detained personnel to transmit a hidden message in otherwise ordinary and mundane correspondence.

First, you are going to need the letter writer’s secret number which in this example is 1. (This number is per-arranged, and listed in the EPA.) 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.

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 secret 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.

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