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Toen onze mop een mopje was
The Poem Code is a simple insecure
cryptographic method, that was used during
World War II by the British
Special Operations Executive (SOE),
to communicate with their agents in Nazi-occupied Europe [2].
The variant featured here, was used by the Dutch clandestine
organisation Ordedienst (OD),
for communication with the Dutch Government and intelligence service
in exile in London.
A full description of this variant, in the Dutch language, was given by
Chief Telegraph Operator of the OD,
Ton van Schendel, in a
7-page document [A]
that he issued after the war, as part of a more
complete account
of his involvement with the OD and his capture by the Germans [1].
Van Schendel
and his men used it for communication with the
Dutch Intelligence Service in London (BI)
– under supervision of the SOE –
until it was replaced by more secure codes, like CODE 101.
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For a proper and secure exchange of messages between the Radio Service (BR)
of the OD in occupied Netherlands and the Dutch intelligence service (BI) in London,
the Code Officer (CO) needed two things:
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- A pre-agreed poem
- A secret number
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The secret number had been pre-arranged with London, and was different for
each resistance organisation. It had usually been
supplied by a courier, for example via the so-called Swedish Route
(passed to the diplomatic service in Sweden and then by boat to the
Netherlands). In this example we will use the secret number 58265,
and use Dutch poem at the top of this page.
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The code is based on the first 20 words of the famous Dutch poem
'Toen onze mop een mopje was', as shown above.
Note that the exact wording may vary over time and place,
and may differ from the version that
people in The Netherlands learned as a child.
Each word is now numbered:
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- TOEN
- ONZE
- MOP
- EEN
- MOPJE
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- WAS
- HET
- AARDIG
- HEM
- TE
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- ZIEN
- NU
- BROMT
- HY
- ALLE
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- DAGEN
- EN
- BYT
- NOG
- BOVENDIEN
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The message can now be prepared. A full stop (.) is replaced by the word STOP.
Spaces are omitted, but in case the result is ambiguous, it may be replaced by
the letter 'X'.
Suppose the following text has to be encrypted:
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READY FOR ACTION NOW STOP CONFIRM GO AHEAD AS PLANNED BY BROADCASTING THE
AGREED MESSAGE VIA RADIO ORANGE STOP WAITING FOR FURTHER INSTRUCTIONS STOP
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Three randomly selected letters must be added to the end of the message.
They will be used as the callsign on the next transmission. In this case,
we will choose the callsign UXV and add it at the end of the message.
The complete message now looks like this:
READYFORACTIONNOWSTOPCONFIRMGOAHEADASPLANNEDBYBROADCASTINGTHE
AGREEDMESSAGEVIARADIOORANGESTOPWAITINGFORFURTHERINSTRUCTIONSSTOPUXV
It is important that the total length of the message (including the callsign)
is a multiple of five, so that the message can be sent in 5-letter groups.
The above message has a total length of 128 letters, which means that we
have to add two letters in order to obtain a multiple of five (130).
To make it simple, we will add the letters QW at the beginning
of the message:
QWREADYFORACTIONNOWSTOPCONFIRMGOAHEADASPLANNEDBYBROADCASTINGTHE
AGREEDMESSAGEVIARADIOORANGESTOPWAITINGFORFURTHERINSTRUCTIONSSTOPUXV
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We will now pick a random number of words from the poem,
for example: ONZE WAS ZIEN ALLE, which together form the KEY of
our message: ONZEWASZIENALLE. In this case, the key is 15 characters long.
Furthermore, we will remember the positions
of the words ONZE (2), WAS (6), ZIEN (11) ALLE (15), so that they can later
be used to construct the message indicator.
As the KEY is 15 characters long, we will use a grid with 15 columns,
and write out the KEY at the top. Next, we will sort the letters of the
KEY in the order of the alphabet and number them sequentially.
Duplicate letters simply get the next sequential number. The first occurence
of the letter 'A' is '1', the second occurance is '2'. B, C and D do not
occur. The first 'E' is '3', the second 'E' is '4', the third 'E' is '5',
and so on, until each column has a unique number, as follows:
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O
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N
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Z
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E
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W
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A
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S
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Z
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I
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E
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N
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A
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L
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L
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E
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11
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9
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14
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3
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13
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1
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12
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15
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6
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4
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10
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2
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7
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8
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5
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Our complete message is now written out in the grid from left to right,
using 9 rows of the grid and leaving the last 5 positions blank.
As follows:
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O
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N
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Z
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E
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W
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A
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S
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Z
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I
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E
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N
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A
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L
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L
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E
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11
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9
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14
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3
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13
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1
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12
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15
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6
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4
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10
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2
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7
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8
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5
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Q
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W
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R
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E
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A
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D
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Y
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F
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O
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R
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A
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C
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T
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I
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O
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N
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N
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O
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W
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S
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T
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O
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P
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C
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O
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N
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F
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I
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R
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M
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G
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O
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A
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H
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E
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A
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D
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A
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S
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P
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L
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A
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N
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N
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E
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D
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B
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Y
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B
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R
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O
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A
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D
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C
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A
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S
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T
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I
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N
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G
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T
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H
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E
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A
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G
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R
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E
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E
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D
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M
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E
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S
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S
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A
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G
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E
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V
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I
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A
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R
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A
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D
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I
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O
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O
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R
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A
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N
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G
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E
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S
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T
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O
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P
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W
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A
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I
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T
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I
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N
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G
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F
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O
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R
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F
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U
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R
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T
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H
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E
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R
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I
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N
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S
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T
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R
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U
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C
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T
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I
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O
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N
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S
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S
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T
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O
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P
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U
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X
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V
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We will now read each column of the grid from top to bottom,
in the order of the numbers written under the KEY — 1, 2, 3, ... 15 — and write them down
in groups of five letters each. As follows:
DTAOR AAROC FATSA FUEWH BAAPH SROPA MONTV OMEGG EFIOC SCDOI
SXTIN ISNOC IRNNA GRTWN OBHVT RNANL SERGR QNGDT ESUOY ODAED
IIPAS ERGRW ETROA YEIOT SFPAD EITNU
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We will have to inform the recipient of the words we have choosen to
create the KEY. This is done by taking the position numbers of the words that
form the KEY, adding a secret (pre-arranged) number to each of them, and
converting the result to five letters. This is done as follows:
The position of each of the words ONZE WAS ZIEN ALLE in the poem,
are 2, 6, 11 and 15, whilst our scret number is 58265.
The digits of the secret number are now used as separate numbers and
added are to the KEY positions,
after which the result is converted to the letters of the alphabet,
using the following substitution table (which has the order of the alphabet):
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1
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A
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⇆
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11
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K
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21
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U
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2
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B
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⇆
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12
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L
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22
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V
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3
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C
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⇆
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13
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M
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23
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W
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4
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D
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⇆
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14
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N
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24
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X
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5
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E
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⇆
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15
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O
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25
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Y
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6
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F
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⇆
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16
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P
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26
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Z
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7
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G
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⇆
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17
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Q
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8
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H
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⇆
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18
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R
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9
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I
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⇆
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19
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S
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10
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J
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⇆
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20
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T
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This result in a message indicator of five letters, that will
be added to the beginning of our message. In order to detect transmission
errors, a check group will be added at the end of our message. This check
group is constructed from the message indicator by converting it using the substitution table above, swapping the letters from the first two columns
only. This means, for example that 'C' becomes 'M' and that 'R' becomes 'H'.
The letters U to Z are not converted.
Position of the KEY words 2 6 11 14
Secret number 5 8 2 6 5
—————————————— +
Result 5 10 8 17 20
Convert to letters ↓ ↓ ↓ ↓ ↓
Message Indicator E J H Q T
Check group O T R G J
We will now place the Message Indicator at the beginning of our message
and the check group at the end. Our message will now look like this:
EJHQT DTAOR AAROC FATSA FUEWH BAAPH SROPA MONTV OMEGG EFIOC
SCDOI SXTIN ISNOC IRNNA GRTWN OBHVT RNANL SERGR QNGDT ESUOY
ODAED IIPAS ERGRW ETROA YEIOT SFPAD EITNU OTRGJ
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When sending the first message in morse code,
the stations will use the default callsigns HLD (Holland) and OZX (UK).
For each message that follows the initial one, the callsign is used as specified in the last three letters
of the previous message. Each message is prefixed by a start sign,
followed by a sequential message number and the number of groups that follow.
Call OZX OZX de HLD OZX OZX de HLD ...
Terminator (+) · — · — ·
End (k) — · —
Answer HLD HLD de OZQ HLD HLD de OZQ ...
Terminator (+) · — · — ·
End (k) — · —
Start sign — · — · —
Prefix nr 1 28 gr
Separator (=) — · · · —
Message EJHQT DTAOR AAROC FATSA FUEWH ... OTRGJ
Terminator (+) · — · — ·
End (k) — · —
If the recipient has taken the message without problems, it will be
confirmed as follows:
R R nr 1 R R · — · · — · nr 1 · — · · — ·
If certain groups were received with errors, the recipient will ask these
groups to be resent. For example, when groups 12 and 16 were not taken
correctly, the recipient will send the following message, followed by a regular
confirmation:
rpt gr 12 gr 16 k
For decoding a message, the above precedure has to be reversed.
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Copy of the original document kindly provided by Cor Moerman [3].
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- Leo Marks, Between Silk and Cyanide
1998. ISBN 987-0-684-86780-9.
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© Crypto Museum. Created: Saturday 02 January 2021. Last changed: Wednesday, 29 December 2021 - 23:56 CET.
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