A few years ago, online magazine “Naked Security” ran a steganography competition. The participants had to hide a given message in an inconspicuous letter using a method developed by British cipher experts in WW2. The results were quite impressing. Can a reader do better anyway?
In the Second World War, British cipher specialists developed a steganographic method that was to be used by soldiers in case they were captured by the Germans. This method allowed for hiding a secret message in an unsuspicious letter a soldier would send to his family back home (British prisoners of war usually were allowed to write such letters; these messages were transported by the Red Cross or similar organisations).
I have blogged about this method before (in German).
How the method works
To explain how the method works, we first need a message. Here’s an example:
WE PLAN ESCAPE NEXT MONDAY AT 8. GOAL IS TO REACH SWITZERLAND BY NOON.
This message consists of 14 words. For clearity, we numerate them:
1WE 2PLAN 3ESCAPE 4NEXT 5MONDAY 6AT 78. 8GOAL 9IS 10TO 11REACH. 12SWITZERLAND 13BY 14NOON.
Now, the order of the words is changed according to a rule based on a rectangle. The following diagram shows how this works with 15 words:
Our message doesn’t have 15 words but only 14. We take the following rectangle instead:
Now, the message is written in the order indicated by the diagram (14, 13, 12, 11, 10, …). In our case, this is equivalent to writing the text backwards:
NOON BY SWITZERLAND REACH TO IS GOAL 8 AT MONDAY NEXT ESCAPE PLAN WE
In the next step, we extend the message with filling words. After the first, third, fifth, seventh etc. word, six filling words are positioned. After the second, fourth, sixth etc. word, three filling words follow (the method allows for other distance patterns as well, for example 5-4-5-4-5…, but this is not relevant here). Now, we get:
NOON xxxx xxxx xxxx xxxx xxxx xxxx BY xxxx xxxx xxxx SWITZERLAND xxxx xxxx xxxx xxxx xxxx xxxx REACH xxxx xxxx xxxx TO xxxx xxxx xxxx xxxx xxxx xxxx IS xxxx xxxx xxxx GOAL xxxx xxxx xxxx xxxx xxxx xxxx 8 xxxx xxxx xxxx AT xxxx xxxx xxxx xxxx xxxx xxxx MONDAY xxxx xxxx xxxx NEXT xxxx xxxx xxxx xxxx xxxx xxxx ESCAPE xxxx xxxx xxxx PLAN xxxx xxxx xxxx xxxx xxxx xxxx WE
Next, we add a line at the beginning. The length of the first two words indicates the size of the rectangle. As we have used a 2×7 retangle, the first word needs to have two and the second needs to have seven letters. I have chosen ON TUESDAY. The rest of the line is irrelevant:
ON TUESDAY xxxxxxxxx
NOON xxxx xxxx xxxx xxxx xxxx xxxx BY xxxx xxxx xxxx SWITZERLAND xxxx xxxx xxxx xxxx xxxx xxxx REACH xxxx xxxx xxxx TO xxxx xxxx xxxx xxxx xxxx xxxx IS xxxx xxxx xxxx GOAL xxxx xxxx xxxx xxxx xxxx xxxx 8 xxxx xxxx xxxx AT xxxx xxxx xxxx xxxx xxxx xxxx MONDAY xxxx xxxx xxxx NEXT xxxx xxxx xxxx xxxx xxxx xxxx ESCAPE xxxx xxxx xxxx PLAN xxxx xxxx xxxx xxxx xxxx xxxx WE
The next step is the most difficult one. The placeholders need to be replaced by words, such that we get a meaningful text. Here is my proposal:
ON TUESDAY WEATHER WAS COLD.
NOON BROUGHT COLD RAIN WHICH GOT WORSE BY LATE AFTERNOON IN SWITZERLAND TEMPERATURES ARE MUCH HIGHER THEY WILL REACH MORE THAN TWENTY TO TWENTY TWO DEGREES TOMORROW WHICH CERTAINLY IS QUITE HOT OUR GOAL IS TO STAY WARM FOR ANOTHER 8 DAYS BECAUSE HERE AT THE CAMP COAL WILL COME NEXT MONDAY OR AT LEAST NEXT TUESDAY I REALLY WISH I COULD NOW ESCAPE FROM THIS WEATHER PLAN TO WRITE AGAIN ON THE NEXT WE
Finally, we make this text look like a letter:
Dear Mary,
on Tuesday, weather was cold.
Noon brought cold rain, which got worse by late afternoon. In Switzerland, temperatures are much higher. They will reach more than twenty to twenty twodegrees tomorrow, which certainly is quite hot. Our goal is to stay warm for another 8 days, because here at the camp coal will come next Monday or at least next Tuesday. I really wish I could now escape from this weather. Plan to write again on the next WE.
Sincerely
John
An authentic example
The following is an authentic message encoded and sent by British soldier John Pryor from a German PoW camp in the Second World War (he used a 5-4-5-4-5… pattern; as an additional rule, after the word “the”, letters instead of words count for the rest of the pragraph):
My Dear Mummy & Daddy,
Last week [4×4 = 16] I received a short letter from Robert.
The envelope had the marks [5] of five of the [9 after the word “the” letters, not words count] RAF censors. I can’t imagine what his new number on the envelope means, maybe he has been turned over to rather different occupations, which of course I can’t know anything about.
I am glad the information [5] I sent you, especially [9] about the Uffa Fox and [13] other books of the [18 after the word “the” letters, not words count] sailing variety, reached you. As regards other possible books, my present desires seem mostly for interesting literature of events in our country’s history. A subject I am unfortunately very weak in.
The “Naked Security” competition
In 2013, online IT security magazine Naked Security held a competition, the participants of which had to encode a message the way I explained it. The words had to be dropped in a 4-5-4-5… pattern. The following message had to be hidden in a cover letter:
INCREASED RAILWAY MOVEMENTS WITH 24 HOUR ARTILLERY AND AIR COVER, GUARD DOUBLED, BUNGALOWS SEARCHED DAILY
This message consists of 15 words. The following order had to be used (without indicating it in the first line):
The following cover letter won the first prize:
I thank God DAILY for your patiance. I’ve SEARCHED for many houses. DOUBLED my efforts and the AIR turned blue one HOUR after we were badly gazumped. BUNGALOWS are really rubbish. GUARD our deposit with care AND I am sure 24 hours will show us MOVEMENTS in the market. COVER your ears until then – ARTILLERY Road is expensive WITH no garage and the RAILWAY use is now INCREASED.
In the Naked Security article you will find a few more cover letters that were submitted to the competition. All in all, this steganographic method appears to work quite well. As it seems, the plan of the British crypto specialists to provide their soldiers a powerful means for secret communications was successful.
If a reader can produce an even better cover letter for the message that was to be hidden in the competition, please let me know.
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Further reading: Als die Briten Geheimbotschaften für Kriegsgefangene im Monopoly-Spiel versteckten
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