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SLIDEX
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].

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.
  
Slidex with grid card and sliders installed

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.

  1. The abbreviation R/T is sometimes translated as Radio Telephone or Radio Telephony.

Front Rear Slidex Slidex with grid card and sliders installed Wallet Spare cards and sliders Slidex manual cipher system with spare cards Manufacturing codes
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Front
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Rear
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Slidex
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Slidex with grid card and sliders installed
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Wallet
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Spare cards and sliders
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Slidex manual cipher system with spare cards
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Manufacturing codes

Features
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.

Post-war version
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 the grid cards that were used during the Cold War were different from the WWII ones. Click here to view the post-war cards.
  
Pertinax back

  1. Celluloid is one of the earliest types of plastic.  Wikipedia
  2. 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

Front side Rear side Post-war Slidex wallet with message cards and sliders Post-war Slidex Pertinax back Grid card installed Spare cards and sliders, stored inside wallet Grid cards
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Front side
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Rear side
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Post-war Slidex wallet with message cards and sliders
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Post-war Slidex
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Pertinax back
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Grid card installed
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Spare cards and sliders, stored inside wallet
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Grid cards

Instructions
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.
  

Use in other countries
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:

Netherlands
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].

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.
  
Dutch instruction booklet

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:

Example of a complete Dutch Slidex system

  1. 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

Dutch Slidex variant Complete Dutch Slidex device Dutch Slidex detail Rulers Dutch cards Dutch card A/10 Blank card B/5 Dutch instruction booklet
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Dutch Slidex variant
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Complete Dutch Slidex device
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Dutch Slidex detail
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Rulers
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Dutch cards
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Dutch card A/10
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Blank card B/5
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Dutch instruction booklet

West-Germany   BRD
Slidex was also used by the Bundeswehr (Army) of West-Germany (BRD) until the late 1970s, where it was known as Tarntafel (cover table).

East-Germany   DDR
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

  
The matrix on page 1

Known cards
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.

Example of a wartime British Slidex card. Click to view all known cards.

Known British WWII cards
Series Card Title Remark No Date
A 1 OPS/SIGS   1 1944
A 2 RE   1 1944
A 4 MED   1 1944
A 5 UNIT  Example 150 OCTU R Sigs 1 1944
A 6 RA   1 1944
A 7 RAC/REME   1 1944
A 8 Q(a)   1 1944
A 9 Q(b)   1 1944
A 15 ASSU   1 1944

 View all wartime British cards


Post-war British cards
Series Card Title Remark No Date
A 19 Home Defence   1 1978 1
A 20 - Missing    
A 21 OPS.   1 1978 1
A 22 RE   1 1978 1
A 23 MED   1 1978 1
A 24 UNIT   1 1978 1
A 25 RA   1 1978 1
A 26 REME   1 1978 1
A 27 AQ(A)   1 1978 1
A 28 AQ(B)   1 1978 1
A 29 TPT & MOV   1 1978 1
A 30 PROVOST   1 1978 1
A 31 - Missing    
A 32 Signals   1 1978 1
A 33 Army Air   - 1978 1
A 34 Logistic Support Group   - 1978 1
A 35 RE   2 1978 1

 View all post-war British cards

  1. This is an educated guess, based on the expressions on the cards.

Known Dutch cards
Series Card Title Translation No Date
A 3 Luchtdoel artillerie Air artillery ? 1952
A 4 MGD Medics ? 1952
B 5 - Blank ? 1952
A 6 Veld-artillerie Field artillery ? 1952
A 7 Cavalerie Cavalry ? 1952
A 8 GS Personeelskaart Personnel file ? 1952
A 9 Logistiek Logistics ? 1952
A 10 Koninklijke Marechaussee Military Police ? 1952

 View all Dutch cards


Operating procedure
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].

Sliding rulers
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:

  • ◼ Red
    Army headquarters
  • ◼ Black
    Divisional headquarters
  • ◼ Green
    Batallion and other units
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. Kindly provided by Karsten Hansky [6].
Example of a real key sheet for 23-29 July 1944 [6].


Rows and columns
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.

Later improvements
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.

Creating the message
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



Example of a message
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:

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



Breaking Slidex
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].


Documentation
  1. Instructions for the Use of SLIDEX RT CODE 1
    (B43/193) 96000 1/44 G.S.St. C.1338. January 1944. Booklet, 8 pages.
     Centerfold page

  2. Instructions for the Use of SLIDEX RT CODE (amendment No. 1) 1
    (B43/193) 100000 3/44 W.O.P. 17128. March 1944. 1 page.

  3. Instructions for the Use of SLIDEX R/T CODE, 1944 1
    (B44/200) 175000 12/44 W.O.P. 18611. December 1944. 8 pages.
     Centerfold page

  4. Example of a grid card
    Series A, Card 1, OPS/SIGS. Date unknown.

  5. Korte verklaring Slidex
    Short instructions for using Slidex (Dutch).
    Date unknown, but probably 1947/48.

  6. Beschrijving van het Slidex-Systeem 1
    Operating instructions (Dutch). Voorschrift nr 1200, 2e druk.
    Ministerie van Oorlog (War Ministry), 22 October 1952.
     Changes and corrections (14 June 1956)
     Centerfold page
  1. 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'.

References
  1. Louis Kruh, The Slidex RT Code
    Cryptologia, Volume 8, Issue 2, April 1984, p. 163-172.

  2. Gary Jones, Radio Operator in the British Army
    Via Jerry Proc's website. Retrieved April 2016.

  3. Christos Triantafyllopoulos, The Slidex code
    Christos military and intelligence corner, 23 July 2012.

  4. Wikipedia, Celluloid
    Retrieved April 2016.

  5. 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

  6. Karsten Hansky, Layout of British Slidex Cards
    Retrieved June 2016.
  1. Declassified by NSA on 23 October 1998. EO 12958.

Further information
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Crypto Museum. Created: Friday 13 August 2010. Last changed: Sunday, 24 September 2017 - 15:47 CET.
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