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Hand UK
Manual WWII cipher system
SLIDEX
is a paper-based hand-held manual cipher system,
used by the British Army
during WWII
and during part of the Cold War,
for sending tactical field messages. It is based on a pre-defined matrix of
letters, words and common expressions, of which the row and column IDs
are used. The system is also referred to as SLIDEX R/T CODE, in which
R/T 1 is short for Radio Telegraphy [1].
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When closed,
Slidex measures approx. 245 x 157 mm and consists of two parts
that are folded together at the short side like a wallet.
When opened, the
unit is about 49 cm wide.
The right half
is the actual cipher device, which
consists of a paper card with a 12 by 17 grid or matrix,
resulting in a total of 204 cells, plus two rulers: one at the left
for the identification of the rows and one at the top to identify the columns.
The left half
acts as a pocket in which additional grid cards, rulers
and operating instructions can be stored.
The pocket is closed by a plastic flap.
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In addition to words and phrases, each cell of a grid also
contains a single letter or number. This was used for spelling words,
coordinates, frequencies, times, etc., that were not available in any of the
existing cells. It required the use of a so-called SWITCH code
in order to toggle between spelling (SWITCH ON)
and fixed expressions (SWITCH OFF). In order to hide their frequency
from potential eavesdroppers, multiple
SWITCH ON and SWITCH OFF cells were present on each card.
Slidex was introduced to the British Army in 1943.
It was used heavily during Operation Overlord,
the Allied Invasion of Europe on D-Day, and long thereafter,
first by the British and later also by American and
Canadian troops [1].
Apparently the cipher was not very safe and was
easily broken
by German interceptors [2]. In the period leading up to the
Allied Invasion, the Germans had been able to reconstruct most of the
vocabulary cards based on pre-invasion exercises in the UK [3].
Being relatively unsafe but easy to use, SLIDEX was replaced around
1980, by the more advanced BATCO system
and by voice encryption systems.
Like SLIDEX, BATCO was a paper-based manual cipher system, but had
provisions for authentication and call-sign protection.
For many years BATCO was used as a backup
that would be used in case any of the existing equipment failed.
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The abbreviation R/T is sometimes translated as Radio Telephone
or Radio Telephony.
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The right half of the device has a metal frame that can hold a paper
card of approx. 195 by 137 mm. The card contains a grid with 204 cells,
organised as 17 rows by 12 columns. At the top is a metal gutter that can
hold a celluloid 1 strip. It allows the strip to slide horizontally.
To the left of the card is another metal gutter that holds a shorter
celluloid strip that can slide vertically.
Each day, a unique combination of the letters of the alphabet (i.e. the key)
is written onto the rulers with a pencil, as per cipher instructions.
Each Slidex was supplied with three sets of rulers marked at the end
with red, green and black lines, in order to identify the
different key levels.
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With a later (post-war) version of Slidex, the carton back panel was
replaced by a strong 2.5 mm thick brown Pertinax 2 panel with rounded
corners. It made the device more robust. The image on the right shows
the upgraded version.
With this version, the card holder is made of aluminium, which is less
prone to corrosion.
The rest of the device remained unchanged, but it should be noted that
the grid cards that were used during the
Cold War were different from the
WWII ones.
Click here to view the post-war cards.
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Celluloid is one of the earliest types of plastic. ➤ Wikipedia
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Pertinax is a flame retardant synthetic resin bonded paper, a
composite material made of paper impregnated with a plasticized phenol
formaldehyde resin. Also known as Paxolin or FR-2.
➤ Wikipedia
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Over the years, the operating instructions for the Slidex code were revised
a number of times. The original manual that was issued in January
1944 is available for download below. It was updated in March 1944
and this version was in force during the landings in Normany on
D-Day. The image on the right shows the revised operating
instructions that were issued in December 1944.
➤ Original instructions of January 1944
➤ Update of March 1944
➤ Revised instructions of December 1944
In the years following WWII, the instructions were updated and revised
several times.
One of the most important improvements in the operating procedure,
was to allow multiple letters the be used for the identication of
the selected row and column of each cell, rather than a single letter.
As a result, all letters of the alphabet were used, rather than just
10, which made the cipher less predictable and improved overall
cipher security. This procedure is further explained below.
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Although SLIDEX was a British development, and was predominatly used by
the British Army, it was also used by other NATO
countries, in partcular
for low-grade tactical traffic, until it was replaced by
BATCO and
voice encryption systems
in the late 1970s. Here are some examples:
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The Dutch Army used SLIDEX for low-grade traffic for many years
during the Cold War,
starting immediately after WWII had ended.
Initially the device was used with all text and instructions in
English [A], with a one-page hand-typed carbon copy 1
containing short instructions in Dutch
[E].
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Around 1952, it was replaced by a Dutch variant of the Slidex
which was physically identical to the original British version,
but was not stamped with the Crown symbol and did not have
any manufacturing markings.
It was supplied with a set of printed
cards in the Dutch language.
Different cards were available for different Army Units, as shown in the
table below.
Additional blank cards were available for ad-hoc creation
of codes for specific missions.
It came with a small A5-size booklet with
operating instructions in Dutch
[F], as shown in the image on the right.
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The set was also supplied with a small notebook in which the (encrypted)
messages were written down.
The Dutch instructions were revised on 14 June 1956, and existing users
in the field were issued a
four-page inlay sheet
with applicable changes [F]. The complete Dutch set is shown here:
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At the time, multiple copies of a document were created simultaneously
by means of a typewriter, using thin sheets of white paper with carbon
paper in between, hence the name Carbon Copy (CC). ➤ Wikipedia
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Slidex was also used by the Bundeswehr (Army) of West-Germany (BRD)
until the late 1970s, where it was known as Tarntafel (cover table).
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Although not actually a variant of the British Slidex system,
the National People's Army (NVA) of the former DDR (East-Germany)
used a nearly identical system throughout the Cold War.
It is a small red book with a 10 x 10
grid (i.e. 100 cells) with common phrases and expressions,
that was also used for low-grade tactical messages.
The device was known as Sprechtafeln (speech tables) and
as Parolen- und Gesprächstabelle
(expressions and conversation table).
➤ More information
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Below is a non-exhaustive list of Slidex grid cards that we've seen
over the years, or that have been reported to us. Please note that
the cards were changed a number of times during Slidex' lifetime,
and that blank cards were issued for special operations, allowing
hand-written codes.
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Series
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Card
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Title
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Remark
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No
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Date
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A
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1
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OPS/SIGS
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1
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1944
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A
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2
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RE
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1
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1944
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C
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2
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RE
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1
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?
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A
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4
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MED
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1
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1944
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A
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5
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UNIT
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➤ Example 150 OCTU R Sigs
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1
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1944
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A
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6
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RA
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1
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1944
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A
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7
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RAC/REME
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1
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1944
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A
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8
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Q(a)
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1
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1944
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A
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9
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Q(b)
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1
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1944
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A
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11
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S&T
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1
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1944
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4
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12
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Place names Ed. 4
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1
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1944
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FE-4
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13
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Internal Security Ed. 4
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1
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1944
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A
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15
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ASSU
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1
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1944
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A
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16
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Airborn
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1
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1944
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This is an educated guess, based on the expressions on the cards.
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MGD = Militaire Geneeskundige Dienst (Military healthcare service).
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Series
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Card
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Title
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Translation
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Date
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B
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1
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OPS
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1956
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D
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4
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MED No 1
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Medics
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1956
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D
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8
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AQ(A)
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1956
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B
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9
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AQ(B)
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1956
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FE 1
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1
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OPS-Edition 5
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1956
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FE 1
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3
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Q-Edition 4
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1956
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The cards stayed in effect for relatively long periods of time.
The handwritten order of the letters of the alphabet on the two
rulers however, was changed regularly. The contents of the two
rulers and their relative position to the card, is known as the key.
During periods of little action, the key was changed weekly,
but during an operation it was changed daily and in some cases
even twice a day. Generally speaking a new key came into effect when the
call-sign was also changed [A].
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Each Slidex was supplied with three sets of sliders, marked with
red, green and black lines. One side of each ruler was marked with
a dotted coloured line, whilst the reverse side had a solid line
on it. The key for the first part of the day was writted on the
side with the dashed line. The reverse side was used for the
second part of the day. The colours had the following meaning:
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◼ Red Army headquarters ◼ Black Divisional headquarters ◼ Green Batallion and other units
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The horizontal slider has 16 cells, four more than the actual
number of columns. The vertical ruler has 21 cells, again four more
than the actual number of rows. This was done to allow sliding keys
with 5 possible starting positions for each ruler. For this reason,
the last four cells were a repeat of the first four cells,
as will be demonstrated in the example below.
It was also possible to use fixed keys, in which case the last
four letter groups (i.e. the repeated groups) were omitted.
Example of a real key sheet for 23-29 July 1944 [6].
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In 1944, the keys were issued for one full week in advance, as shown
in the example above. In this case, the keys for the week of 23 to 29
July 1944 are specified as two rows of letters: one for the horizontal
ruler and one for the vertical ruler. Let's use the key of 26 July 1944
as an example:
Date → 26 July 1944
Hor → H I F L E A B C G J K D H I F L
Vert → B E F H K M Q N J P A O L G D C I B E F H
Key → 04
Note that the first four letters are repeated at the end, which is why
we have underlined them. This was done to allow sliding keys.
According to the 1944 instructions [A], only the letters A-L were used
for the horizontal ruler and only the letters A-Q for the vertical ruler.
The letters of the supplied key are now written onto the selected rulers,
using a soft pencil so they can be washed off later. The key rectangle
specifies which grid cell is used to inform the other party of the ruler
offsets, in case sliding keys are used. It specifies the figure
group or letter that is printed in the top left corner of the cell.
As letters occur more than once, the instance may be given as well.
When sliding keys are used, the originator of a message can give
each of the rulers five possible positions, or offsets. In this
case the other party has to be informed of the position of the key
rectangle so that he can give his rulers the same offset. In the
example above the key rectangle is specified as 04. With our
randomly chosen position of the rulers, the coordinates of cell 04
are LC (indicated by the red circle). This is then sent as the first
letter group of the coded message.
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One improvement was to write the numbers in the first five cells of
each slider in a randomized order, as specified on the key sheet.
Rather than 12345 the order could be something like 52413. The scrambled
order had to be specified as part of the key, as in the example below.
In the above procedure, not all letters of the alphabet were used to
identify a row or column position. This was later considered to be
a cryptographic weakness, after which the procedure was changed into
using all letters of the alphabet and using one or two letters for
each row or column. The operator then had multiple two-letter
combinations to choose from. An example:
Date → 28 February 1952
Hor → 5 2 4 1 3 CV BW FU JX AR LT ES IP DQ KM HN GO CV BW FU JX
Vert → 2 3 5 1 4 IR C HS BZ K AX G JW NV FT O Q MY PU D L E IR C HS BZ
Note that the first four letter groups are repeated at the end,
which is why they are underlined. The letters of the supplied key are
now written onto the selected rulers, using a soft pencil so they
can be washed off later. The five numbers at the start of the key
are written into the upper right corner of the first five cells
of each ruler.
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A (radio) message was usually sent in
morse code.
If it contained a slidex code, it should be prefixed
by the word 'slidex' followed by the number of the card (only if
it was different from the default one) and the position of the
horizontal and vertical sliders respectively, for example:
SLIDEX 1 1 5
This means that card number 1 is used, the horizontal ruler is shifted
so that the cell with the number '1' in it is lined up with the first
column of the card, and the vertical ruler is shifted so that the
cell with the number '5' in it is lined up with the first row, which
may look like this:
The message can now be created by specifying the column and row of
each cell that holds a particular word or phrase. Note that some
cells can be identified by multiple letter-combinations.
Suppose we want to send the message 'ATTACK IMMEDIATELY'. First
located the cell that holds the word ATTACK and write down
the one of the letters of the column and the row. Now do the same
for the word IMMEDIATE(LY). This would yield the following message:
AB CF
As each row and column can be identified by more than one letter,
different combinations would yield the same message.
This is very useful, as it hides the frequency at which some often
used expressions appear in the message. In this case, the above
message could also be sent as:
RZ VT
AZ VF
RB CF
AB CT
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More complex messages can also be created this way.
Words that do not appear on the card can be spelled out
as literal text, using the SWITCH ON and SWITCH OFF
commands in combination with the red text printed on
each card. Numbers are also sent in this manner. An example:
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Expect parachute counter-attack WALTER brigade.
Attack tomorrow 0500 hours.
Assume continuous watch on frequency 7135.
Leave bridgehead intact.
Using the above card and keys, this message could be sent as follows:
SLIDEX 1 1 5
HJ EU WA DQ EQ XS KH LL JM FB RA RK
RZ CI OL JE XA FC HF XZ CE GN PA KP
WO UZ LO
For clarity, the SWITCH ON and SWITCH OFF commands are printed
in bold in the above example. Anything in between these two commands
should be seen as literal text. Note that there are multiple
occurrences of these commands. Again, this is done to hide their
frequency. At the other end, the recipient writes down the message
that was received in
morse code,
sets up his Slidex as required and decodes the message:
SLIDEX → Slidex encoded message to follow
1 → Use Slidex card number '1'
1 → Shift horizontal ruler to position '1'
5 → Shift vertical ruler to position '5'
HJ → Expect
EU → parachute
WA → counter-attack
DQ → SWITCH ON
EQ → W
XS → A
KH → L
LL → T
JM → E
FB → R
RA → SWITCH OFF
RK → brigade
RZ → Attack
CI → tomorrow
OL → SWITCH ON
JE → 05
XA → 00
FC → SWITCH OFF
HF → hour(s)
XZ → Assume continuous watch on frequency
CE → SWITCH ON
GN → 71
PA → 35
KP → SWITCH OFF
WO → Leave
UZ → bridgehead
LO → intact
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According to former German interceptors that were interviewed after the
war had ended, most British low-grade manual ciphers, and Slidex
in particular, were broken by them relatively easy in a couple of hours.
Such traffic was designated Englische Code or EC by the Germans.
The letters EC were generally followed by a number that identified
the type or variation of the cipher. Known IDs were EC 5, 12, 23, 24, 30/3
and 30/20, with the last two probably referring to Slidex [5].
The first German successes with respect to Slidex
were achieved in late 1943 and early 1944,
when the Allies were preparing for the Allied invasion of continental
Europe. All training traffic was successfully solved by the German
codebreakers and gave them a good feeling of how the cipher was used.
After D-Day, Slidex was also used by the American military police,
who supplied a steady unintended stream of intelligence
about all troups that passed by their check points [5].
Because of this, breaking Slidex was given a high priority by the
German High Command (OKW). The Germans were usually able to break a Slidex
key in one to three hours if the contents of the cards were known,
and five to six hours if the cards were not known. Apparently,
about 65% of all traffic was encrypted with the existing cards [5].
For further information, please refer to [3].
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In order to preserve the original patina of the pages,
these documents have been scanned in full-colour at 150 dpi,
so that you will see the faded paper, stains, earmarks and rusty staples.
As most booklets contain an example of a grid card on the
centre pages, these pages have been scanned separately
as 'centerfold'.
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- Louis Kruh, The Slidex RT Code
Cryptologia, Volume 8, Issue 2, April 1984, p. 163-172.
- Gary Jones, Radio Operator in the British Army
Via Jerry Proc's website. Retrieved April 2016.
- Christos Triantafyllopoulos, The Slidex code
Christos military and intelligence corner, 23 July 2012.
- Wikipedia, Celluloid
Retrieved April 2016.
- National Security Agency (NSA), European Axis Signal Intelligence in WWII
Volume 4: The Signal Intelligence Service of the Army High Command. pp. 149-151.
1 May 1946. Document ID: 3486746. Retrieved from
NSA website
April 2016. 1
- Karsten Hansky, Layout of British Slidex Cards
Retrieved June 2016.
- Gordon Bubb, British Slidex card Series B, OPS No. 1 with amendment
Received April 2021.
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Declassified by NSA on 23 October 1998. EO 12958.
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© Crypto Museum. Created: Friday 13 August 2010. Last changed: Friday, 17 March 2023 - 08:19 CET.
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![Example of a real key sheet for 23-29 July 1944. Kindly provided by Karsten Hansky [6].](img/302294/010/full.jpg)
