by Barbara Bond
The value of the coded letters as a source of potentially useful intelligence information was being steadily realized by other Government departments. They increasingly passed questions to MI9 to be directed through the coded letter system to prisoners of war in particular camps where they were known to work outside the camps alongside Germans and could report on what was going on in the surrounding area. The occurrence of this practice is suggested by the volume of intelligence passed by Julius Green.
There was one reported compromise of the system by British government officials and this occurred when the British Military Attaché (MA) in Stockholm acted unilaterally in sending parcels to prisoners of war and included instructions on how to use one of the codes. Three camps, Stalag Luft III (Sagan), Oflag IX and Oflag VIB, were able to warn MI9 what the MA had done, that it had been discovered by the Germans and the whole system was potentially compromised. Major Winterbotham, an MI9 officer, was then immediately despatched on 7 May 1942 to Stockholm to order the MA to stop sending parcels. It was never clear why he had taken such action in the first place but the extent to which his action potentially compromised the whole system was clear. MI9 immediately stopped using Code III and speeded up the introduction of the Code VI. Code users were able to alert MI9 on more than one occasion to problems such as the discovery of escape aids in parcels. Interestingly, no reported discovery of maps has been found.
THE CONSTRUCTION OF CODED LETTERS
How, then did the codes work and specifically how were the letters constructed with the messages hidden? Coded letters were immediately identifiable by the form of the date: 22/3/42, rather than the more usual form of 22nd March 1942; this numeric date form indicated that the letter contained a coded message. Underlining the signature also confirmed that the letter carried a coded message, as did the inclusion of the word ‘very’ in the closing line, e.g. ‘yours very sincerely’.
The length of the message was determined by multiplying together the number of letters in the first two words of the first line of the letter after the salutation. If the first two words each comprised five letters, then the message was 5 × 5 = 25 words in length and a grid 5 × 5 needed to be constructed. If the words had different numbers of letters in them, then the first word indicated the number of words running horizontally in the grid and the second word indicated the number of words running vertically in the grid.
Using the example of a letter starting with two five-letter words, a square grid was drawn with 25 squares in it, i.e. 5 × 5. The rest of the first line of the letter was ignored.
Moving to the second line of the original letter, it was necessary to use the numerical part of the personal key of the individual coder. Julius Green’s key was 5 6 O, so the fifth and sixth words alternately were selected from the letter and placed in the grid, starting in the top left hand corner and working left to right on each consecutive line as indicated in the grid below.
When the grid was complete, however, the message was read starting bottom right, reading across and then diagonally, across and diagonally, etc., as shown over the page, reading in sequence 1 to 25.
The last word of the message, therefore, went into the first box on the grid and the first word of the message went into the last box of the grid. Different code users were briefed to use different word counts, enshrined in the numerical part of their personal code. That of Julius Green was the fifth word followed by the sixth word, whereas that of John Pryor was the fifth word followed by the fourth word. It is clear that there were variations in the codes given to each code user and MI9 needed to have kept a very detailed record on their card index system.
When the word ‘but’ occurred in the sequence, i.e. the fifth or sixth word in Green’s key, it signified the end of the message which should, of course, also have accorded with the completion of the grid. Words which were hyphenated or had an apostrophe in them were to be treated as one word for the purposes of counting, e.g. ‘two-days’, ‘I’ve’.
When the word ‘the’ occurred in the sequence, i.e. the fifth or sixth word in Green’s key, this indicated that the alphabet code started. Using Green’s key of 5 6 O, his alphabet code consisted of the alphabet starting with O. The alphabet grid was written down in three columns, as shown below, and after Z, a full stop occurred. This gave twenty-seven items and three columns of nine letters, each with an associated three-digit number.
O 111 P 211 Q 311
R 112 S 212 T 312
U 113 V 213 W 313
X 121 Y 221 Z 321
.122 A 222 B 322
C 123 D 223 E 323
F 131 G 231 H 331
I 132 J 232 K 332
L 133 M 233 N 333
Commencing with the next sentence after the occurrence of ‘the’, the first letter of each consecutive word was written down in groups of three. Each letter was then identified on the alphabet grid shown above and then the number of the column (1, 2 or 3) in which the letter occurred was noted down. Thus, if the sentence started ‘Which brings the’ the significant letters were W B T. Each group of three letters was used to signify one letter in the table. For W B T, using the alphabet grid above, W was in column 3, B was in column 3 and T was also in column 3. This represented the number 333 which, using the table again, indicated that the letter required was N.
A word was built up by continuing to take the first letter of consecutive groups of three words until the full stop occurred in the coding (i.e. 122 in the above example). The decoding then continued by moving to the next sentence and reverting to the word count. Again, there were individual variations since it could involve moving to the next paragraph and then reverting to the word count.
In the same way that there were variations in the word count, it was proved by Professor McMullan that there were also variations in the alphabet code. He identified the letter S as the missing alphabet key in John Pryor’s code by a lengthy elimination process of all other letters of the alphabet. The S alphabet code is shown below.
S 111 T 211 U 311
V 112 W 212 X 312
Y 113 Z 213 . 313
A 121 B 221 C 321
D 122 E 222 F 322
G 123 H 223 I 323
J 131 K 231 L 331
M 132 N 232 O 332
P 133 Q 233 R 333
Whichever alphabet code was used, it was based directly on a form of modular arithmetic where the pattern of three is key to constructing the coded message and deciphering it.
While MI9 did initially describe their codes as ‘simple’, it is clear that, from a layman’s perspective, they were far from simple and, in order to avoid the searching oversight and scrutiny of German censors, it is hardly surprising. The code user was essentially required to reverse engineer his message. He started with the message to be encoded, constructed the grid, numbered it, placed those numbers/words in the correct order in his letter with the requisite number of spaces alternating between, reflecting his own personal numerical code, and then sought to construct a sensible sounding but totally innocuous letter linking it all together. The whole exercise required considerable intellectual application, time and commitment to plan and construct the message ahead of time. Coders were then in a position to maximize the use of the limited opportunity they were given by their German captors to write letters home, and commit that opportunity to a covert purpose.
It must have been a considerable morale booster for the prisoners of war to know that, despite their loss of liberty, they were still in a position to contribute in a very significant way to the war effort by providing valuable intelligence information, and also to have the reassurance of knowing that an entire organization was working at home to aid their attempts to escape and to ensure that their families were being kept briefed about their continuing contribution. In addition, it would have relieved their considerable boredom during long months and years spent in captivity with ample ‘leisure’ time on their hands.
Three British prisoners of war produce news sheets during their ‘l
eisure time’ at Stalag Luft III.
Staff at the Army Post Office ‘somewhere in the Midlands’, 1944. Letters sent to prisoners in the camps were sent through here.
DECODING A HIDDEN MESSAGE
As an example, over the page there is the text of a letter dated 7/5/42 which John Pryor wrote home. The form of the date, the underlined signature and the inclusion of the word ‘very’ in the last line of the letter indicate that this is a coded letter. The exercise to decode the letter is marked up on the accompanying version, where significant words and letters are highlighted in red. The following method of decoding was employed to discover the hidden message.
John Pryor’s coded letter
7/5/42
My Dear Mummy & Daddy, Last week I received a short letter from Robert. The envelope had the marks of five of the 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 I sent you, especially about the Uffa Fox and other books of the 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 gardens are improving, borders of wire which require a net to keep the football off. Possibly a move to Marlag Nord in the late summer may prevent getting all advantages from some of the later plants, but we hope not. During the course of the last days we obtained some chairs, from local sources I believe, also some tables, which give the recreation room a much better appearance. A few weeks ago we arranged a rather useful scheme, so men could get “legergeld” by remitting money at home for that received here. After another year – year and a half my clothing requirements etc. will be but a mere trifle.
As I have received all your clothing parcels sent off up to the end of 1941. The suitcase in the last one arrived a bit battered but still quite usable. Private cigarette and tobacco parcels seem to becoming in better now, so I am hoping to get another shortly as my last one arrived in November. Now I must stop, hoping you are all as well as I am. Remember me especially to Marj. Your very loving son.
John.
John Pryor’s coded letter with hidden message highlighted
7/5/42
My Dear Mummy & Daddy, Last week (4x4 grid) I received a short letter from Robert. The envelope had the marks (5) of five of the (4 – start alphabet code) 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 (4) about the Uffa Fox and (5) other books of the (4 – start alphabet code) 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 gardens are improving, borders (5) of wire which require (4) a net to keep the (5 – start alphabet code) football off. Possibly a move to Marlag Nord in the late summer may prevent getting all advantages from some of the later plants, but we hope not. During the course of (4) the last days we obtained (5) some chairs, from local (4) sources I believe, also some (5) tables, which give the (4 – start alphabet code) recreation room a much better appearance. A few weeks ago we arranged a rather useful scheme, so men could get “legergeld” by remitting money at home for that received here. After another year – year and (5) a half my clothing (4) requirements etc. will be but (5 – indicates end of message) a mere trifle.
As I have received all your clothing parcels sent off up to the end of 1941. The suitcase in the last one arrived a bit battered but still quite usable. Private cigarette and tobacco parcels seem to be coming in better now, so I am hoping to get another shortly as my last one arrived in November. Now I must stop, hoping you are all as well as I am. Remember me especially to Marj. Your very loving son.
John.
The first two words after the salutation are ‘Last week’: a 4 × 4 grid is, therefore, constructed. Moving to the second line of the letter and using Pryor’s numerical code of 5 and 4, the fifth word is ‘marks’, so this goes into the top left box of the grid and is the final word of the message.
The fourth word after this is ‘the’, which indicates that the alphabet code starts at this point. Pryor’s alphabet code started at S and is shown below.
S 111 T 211 U 311
V 112 W 212 X 312
Y 113 Z 213 . 313
A 121 B 221 C 321
D 122 E 222 F 322
G 123 H 223 I 323
J 131 K 231 L 331
M 132 N 232 O 332
P 133 Q 233 R 333
Beginning with the next sentence, the first letter of each consecutive word is written down in groups of three. Thus, the decoder moves to the sentence which starts ‘I can’t imagine’ and writes down:
I C I
I is in column 3 in the table above, as is C, so:
I C I = 333
In the grid the number 333 represents R, so the decoder records:
I C I = 333 = R
The decoder lists the first letter of every word in groups of three, which gives the following:
I C I = 333 = R
W H N = 222 = E
N O T = 232 = N
E M M = 211 = T
H H B = 222 = E
T O T = 232 = N
R D O = 313 = .
The letters spell ‘renten’, the name of the currency in Nazi Germany, which becomes the second word on the grid.
At the point where the full stop occurs, the decoder reverts to the 5 4 sequence at the start of the next sentence in the letter while maintaining the correct alternating rhythm. Having finished on the fourth word at the previous stage, this time the fifth word is counted. This gives the word ‘information’ which becomes the third word on the grid. The following fourth word is ‘especially’ which becomes the fourth word on the grid. The fifth word is ‘and’ which becomes the fifth word on the grid.
The next fourth word is ‘the’, signalling that the alphabet code starts again at the beginning of the next sentence. Taking the first letter of each word and setting them out in groups of three produces the following:
A R O = 133 = P
P B M = 121 = A
P D S = 111 = S
M F I = 133 = P
L O E = 332 = O
I O C = 333 = R
H A S = 211 = T
I A U = 313 = .
It contains a spelling error but is nonetheless recognizable as the word ‘passport’, which becomes the sixth word on the grid.
Moving to the start of the next complete sentence in the letter and picking up the 5 4 rhythm, the fifth word is ‘borders’ which becomes the seventh word on the grid. The following fourth word is ‘require’ which becomes the eighth word on the grid. The following fifth word in the letter is ‘the’ which indicates that the alphabet code starts again, as follows over the page:
P A M = 111 = S
T M N = 212 = W
I T L = 323 = I
S M P = 111 = S
G A A = 111 = S
F S O = 313 = .
The ninth word on the grid is ‘swiss’.
Moving to the start of the next complete sentence in the letter and keeping the 5 4 rhythm, the fourth word is ‘of’ which becomes the tenth word on the grid. The following fifth word is ‘obtained’ which becomes the eleventh word on the grid. The next fourth word is ‘local’ which becomes the twelfth word on the grid. The following fifth word is ‘some’ which becomes the thirteenth word on the grid.
The following fourth word is ‘the’, which indicates that the alphabet code starts again at the beginning of the next sentence. This produces the following:
A F W = 132 = M
A W A = 121 = A
A R U = 133 = P
S S M = 111 = S
C G L = 313 = .
The fourtee
nth word on the grid is ‘maps’.
Moving to the start of the next complete sentence in the letter and keeping the 5 4 rhythm, the fifth word is ‘and’ which becomes the fifteenth word on the grid. It is worth noting at this point the deliberate repetition of the word ‘year’ which makes no sense in English but is clearly designed to ensure that the fifth word is ‘and’ so that the hidden message will make sense. The following fourth word is ‘clothing’ which becomes the sixteenth and final word on the grid. To reinforce the point, the fifth word after this point is ‘but’ which confirms the end of the coded message: the introduction of the apparently superfluous ‘etc.’ ensured that ‘but’ occurred as the fifth rather than the fourth word. Placing all these words in their correct consecutive position on the grid produces the following:
Starting in the bottom right corner and reading across and diagonally in sequence, the message reads:
CLOTHING AND LOCAL MAPS OBTAINED REQUIRE SOME OF BORDERS ESPECIALLY SWISS PASPORT INFORMATION AND RENTEN MARKS
Leaving aside the misspelling of ‘passport’, it is a clear request for maps of the Swiss border, and information which would allow them to produce passports. It also carries an indication that they have already been able to obtain local maps and clothing. At the time of this letter (May 1942) Pryor was at the Westertimke camp near Sandbostel in northern Germany. Asking for maps of the Swiss border is perhaps surprising, since the distance that would need to be covered was considerable. Escape via the Baltic ports, which is what Pryor and others subsequently attempted (see Chapter 7), would have been more likely. The message does, however, serve as a stark reminder of the raison d’être of MI9 and the extent to which prisoners of war had taken on board the escape philosophy.
