A very interesting World War II cipher came to light today, as letters home from a British captive passed through German censors and made their way to MI9 (British Intelligence). The explanations for the cipher may be difficult for some to understand, it took me 30-minutes to come up with a better angle. I thought I might shed additional light on the cipher to help readers figure this stuff out and get the most enjoyment from it.
The cipher deserves to be explained from how it was probably originally intended at MI9. Think of the number 3 as a series for everything. This is can be insightful from the fact that people are equipped to more easily remember things in series of threes than with any other number-based mnemonic device. This important subtlety may have served to help operatives teach the cipher, remember it and aid in encoding / decoding speed.
A great fact about the number 3 is you can more easily construct an alphabet grid with it than with other numbers. An alphabet grid using the number 4, for example, requires double the stopgap characters. Five requires 4 stopgaps, or alternatively either combining or dropping letters altogether. You only need one stopgap for the alphabet encoded into threes, and MI9 chose the period ‘.’ to complete it, making it divisible by 3 (27 characters). This is the alphabet used to encode and decode the messages:
A B C D E F G H I J K L M N O P Q R S T U V W X Y Z “.” (stopgap).
Using three, you can form 9 rows or columns. Using 6 and other divisions could provide additional complexities that are not necessary and were not used in these letters. Here we have a terrific example of when human ingenuity has beaten man-made machines. When we rely on substitution ciphers, we must always be wary of machine power which can be applied. A human method can fool computer-aided solutions by virtue of using a logic system that falls outside their domain. Think of the number three as usable for a multitude of purposes.
A series of ideas around the number three, when combined with the letter writing method kept this cipher hidden. The method is a crafty recipe for writing the letter: Start with a signal that the letter contains an encoded message using date format or underline. After an obvious salutation, encode the grid size using the next two words by their length, which dictates the size of the message. “Last week” translates to 4 columns, 4 rows and “lucky again” translates to 5 columns, 5 rows. Fill the grid with every fifth then fourth word of sentences, and so on, then read it all in reverse. The article does a good job demonstrating this.
Words like “mark” could be confidently written in cleartext, saving space, governed by the fifth then fourth word recipe above. Unlike the number grid system used here to encode the most sensitive words, this works out to simply jumble and toil cleartext intermixed with plain words as written in letter content. An additional cipher was used to encode potentially more sensitive words, which went through an intermediary number system and back again, used for names of operatives (“Elder”) and telltale nouns (“Renten.”). A word “the” in this recipe would mark the start, or “but” would mark the end of a string of words where the letters would form a greater sequence encoded additionally.
"I can’t imagine what his new number on the envelope means, maybe he has been turned over to rather different occupations," the first letters of which are used for encoding the word: "Renten." The letters are broken into groups of 3 so "I can’t imagine" works out to by ICI and so on: ICI WHN NOT EMM HHB TOT RDO. The actual letter that ICI represents translates through an intermediary number "333" for the letter "R" and so on to spell: Renten. The key at this point is how MI9 built the lookup table using numbers and its alphabet.
Using three columns and a number series pattern, the alphabet is arranged into 3 columns and 9 rows. Each letter has a unique three-digit number associated with it, where each three letter combination encodes and decodes to the corresponding number by their appearance in columns 1, 2 or 3. This three digit number reveals that ICI represents the letter “R” arriving at the three digit number by column number and is the key to encoding and decoding this cipher. You only need know how numbers 1, 2 and 3 are applied and in which sequence to ensure the unique three digit for each of the 27 letters in the alphabet.
Go down each column and write numbers. The first number in each column counts left to right: 1s all the way down, 2s, then 3s all the way down 9 rows. The second number will go down in a pattern of 3 1s, 3 2s and 3 3s: 111, 222, 333. The last digit will go down: 123, 123, 123 each and all the way down. The result is you have a 27 letter alphabet (A-Z plus “.”) and each letter having a unique three digit number pattern. Rotate the alphabet and encrypt it with any start letter you like. Below, the alphabet starts with “A” as per normal. In the article letter, the alphabet starts with the letter “S” for the actual message. Officers can learn and recall the cipher around the number 3, and rotate the alphabet by any start letter, which can be deciphered on the fly:
A-111 J-211 S-311
B-112 K-212 T-312
C-113 L-213 U-313
D-121 M-221 V-321
E-122 N-222 W-322
F-123 O-223 X-323
G-131 P-231 Y-331
H-132 Q-232 Z-332
I-133 R-233 .-333
To encode and decode ICI, use the table in the article to lookup which column these letters appear in the table. The above table will only give you the number 111 which corresponds to the letter “A” by illustration. The letter “I” actually appears in the third column, as does the letter “C” to arrive at the number 333. Find 333 corresponds to the letter “R” for “Renten.” You can see the letter “R” for “Elder” is encoded separately with: OUC. Note that all those letters also appear in the third column, which decodes to the letter “R” once again by virtue of the number: 333.
Once you know how to read this pattern, you will be able to encrypt and decrypt messages using the MI9 method provided to soldiers and used successfully in WWII. One can only imagine that this cipher at MI9 utilizes 9 rows to encode the most sensitive words in a memorable way, by way of a series in threes. Could that number of 9 have had any special meaning to MI9 back then? I won’t imply anything knowing about the “number 9” lyric looped by John Lennon in one of his Beatles songs. That’s a British invasion of an entirely different sort. Good fun to be had today from a time that is finally able to be celebrated out in the open.
For more fun and mystery by way of Bletchley Park, find the third episode of Bletchley Circle airing tonight on PBS.