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(Date: Sun, 28 Jun 1998 22:43:09 +0200)
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During the early days of WW2, routine and random security checks were inserted into all allied messages to verify that the sender was who he said he was, and to check whether or not he was transmitting under duress. Routine checks often comprised 'errors', such as the 3rd and 17th letter of a message misplaced by four places in the alphabet. Because the enemy quickly learned that these checks were inserted, the agents carried two codes, one to mislead the foe and one for messages to friends.
Because these checks weren't very safe, random checks were inserted into the messages. A three-letter code like 'wun', would be inserted at the begin and end of each message in normal situations. If the sender was transmitting under duress, he would not insert the codes so that the receiver would know that he was in trouble. The code word at the beginning of the message would be disguised by advancing its letters by an agreed number of letters in the alphabet, while the same code placed at the end of the message, would be substituted by letters four places ahead in the alphabet.
Example: code word = WUN, substituted by letters three places ahead in the alphabet. Result: 'ZXQ' (W=Z, U=X, N=Q) and substituted by letters four places ahead 'AYR'(W=A, U=Y, N=R). Two extra letters were added to camouflage the words as normal five-letter code words, e.g. ZXQ becomes AZXQW and AYR becomes BAYRP.
Only too often the security codes were forgotten by the field agent, or the home office thought that the agent had forgotten to add it, while in fact the agent did not add the code because he transmitted under duress. In short, this was not very safe and the allies phased this check out in 1942.
Other security checks included the use of phrases. These phrases were also used for short information exchanges. They often look funny, like 'the car needs a new engine', meaning that something was wrong, while 'there is no need for more coffee' could mean that everything was ok. Another one, noted on a British Army net: 'Zeppelins in the night sky' Reply: 'Pop them with a drawing pin'
The subject was discussed on Spooks a while ago. One of the comments came from Simon who says ''I read in one particular publication on the history of modern espionage that Radio Swan, the mysterious anti-Castro station sent "strange" messages on the day of the Bay of Pigs invasion in the form of coded sentences that made little sense, similar to the ones discussed in other postings.''
The other postings that Simon mentions include phrases like "Les sanglots longs des violons de l'automne percent mon coeur d'une langueur monotone" and "The water in the Seine is dirty". You can often hear sentences like these in WWII movies or TV-series. Some may be real, others a phantasy of the writer.
The Playfair code system had its origin in the UK. It was based on a phrase from a poem or song. The agent had to learn this line which was then transposed into blocks of five letters. The rest of the alphabet was used to fill the 5x5 letter square in an agreed order, the letters 'i' and 'j' counted as one. Double letters were omitted.
Example: the phrase 'the numbers and oddities newsletter'would appear as:
| T | H | E | N | U | The actual message was sent in bigrams (= groups of two letters). The bigrams were encoded by taking the two opposite corners of the rectangle it formed in the square, eg DA becomes ML. |
| M | B | R | S | A | |
| D | O | I | W | L | |
| P | F | K | G | C | |
| X | Y | Q | V | Z |
If both letters of the bigram are in the same line, then the next letters below are used; if both are in the same column, the next letters to the right are used, eg IW becomes KG and RI becomes SW.
The message 'spooks newsletter' in bigrams: SP OO KS NE WS LE TT ER and encoded: MG FF GR SR AL IU MM NS. After that the bigrams are grouped in five letter groups: MGFFG RSRAL IUMMN SZUQX. If the final group is too short, which is the case here, it is filled with dummy letters to complete it.
Because the system was too easy to break, it was replaced by other, more sophisticated systems in the early years of WWII.
Many other systems, a.o. Delastelle -based on Fairplay- were used until the one-time pad was introduced. The one-time pad is one of the most successfull inventions of the spy-world. The system is unbreakable. It is easy to use and is in fact a very simple way of encryption, and very effective at the same time.
A one-time pad is a list of code groups, mostly five figure groups -but also other codes are used- printed on paper, silk handkerchiefs, or supplied on microfilm. Each group represents a certain phrase.
Example:
Guns and backup required - GB; Out of toiletpaper - OP. The two-letter codes should be repeated to avoid errors, eg GBGB and OPOP.
A more common way is the use of code groups, being a vocabulary of several hundreds words. The first five-digit group gives the index code; eg an incoming message starting with group 81114 would tell the agent that the decoding begins at page 81 line 114 of the code list. The next five-digit blocks are made up of three-letter codes, representing the various phrases.
When an agent wants to send a message, he must use the codes for the relevant phrases. Example: the message 'I will cross the border tonight' has three relevant words, each with its own three-digit code.
cross = 321 The encrypted text would now read:
border = 551 321 551 873, or using five-digit groups:
tonight = 873 32155 18730 (the '0' is a filler here).
Next is the transposition cycle. The agent would pick a page and line of the code list to encrypt the message itself, eg page 23 line 4. The code line could look like this: 67554 23075 33687 18873 33109 99841
He now has to transpose the numbers of his message as follows. Subtract the lower number from the higher number without carrying across.
67554 23075 The first group of the message would
32155 18730 indicate the page and line: 23004
----- ----- followed by the encrypted message
35499 15345 35499 15345
---------
That's it for now. Next time we'll focus on other encryption techniques.
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