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Transvertex HC-9
Portable mechanical cipher machine

HC-9 was a mechanical cipher machine built by AB Transvertex in Stockholm (Sweden) in the early 1950s. It was developed from a 1940s design by company CEO Vigo Waldemar Lindstein, for use by the Swedish Army. The HC-9 resembles the design of the Hagelin pin-wheel machines.
The image on the right shows a typical HC-9 machine. The identical military version of the machine was known as KRYAPP 301.

Contrary to wheel-based cipher machines of the same era, such as the German Enigma machine, the HC-9 belongs to the class of pin-wheel machines, just like the early Hagelin machines.

Unlike the Hagelin machines however, it features a punched card (inserted at the back) that replaces the pins of the pin-wheels. It also has a window with a rotating alphabet ruler.
Transvertex HC-9

The machine measures 18 x 15 x 7 cm; roughly the same size as a Hagelin M-209. For storage and transport it was usually packed inside a black tight-fit rubber transit case. The machine was used for low-level tactical messages and remained in service until the late 1970s.

The image below shows a top view of the HC-9 with the location of all controls and special features. The terminology used in the description below is quite different from the original machine description released in the 1960s [6] [7], but is more in line with the terminology used with other cipher machines.
Transvertex HC-9 controls

Unlike the Hagelin M-209, the HC-9 does not print the enciphered text directly to paper. Instead each letter has to be translated by reading it from a window at the front of the machine. Behind the window is a drum with 16 reciprocal alphabets. Depending on the current cryptographic key, one of the 16 alphabets will be rotated into place when the trigger knob is activated. Setting the key, involves four steps:
  1. Internal setting 1: Insert alphabet sheet (drum)
  2. Internal setting 2: Insert punched card (rear)
  3. Zeroize counter
  4. External setting: Set the 5 wheels to the desired starting position
Before starting to encrypt or decrypt, the character counter (top right) has to be zeroized. This is done by pushing a knob, to the right of the wheels, towards the rear. Whilst holding the knob in that position, turn the wheel axle (black knob at the right side of the machine) until the counter reads 0000. Then set the 5 cipher wheels to the desired position (external key).

The randomized alphabet shown in the window, lines up with a fixed (ordered) alphabet above and below the window. The alphabet is self-reciproke, which means that the translation is reversable. Take for example the two red arrows in the drawing above. Fixed letter I translates to random letter S, but fixed letter S also translates to random letter I.

When encrypting a text, the trigger knob at the left has to be rotated clockwise and is then released. One of the 16 alphabets will then rotate into place. Now look up the first letter from the plain text on the fixed (ordered) alphabet and write down the corresponding letter from the drum. Next, activate the trigger knob, translate the second letter, etc.

Due to the reciprocal nature of the alphabets, decrypting is done in exactly the same way as encrypting. After activating the trigger knob, the first letter from the cipher text is looked up on the fixed (ordered) alphabet. Then write down the corresponding letter from the drum. And so on, until the text is deciphered.
Zeroizing the counter Rotating the trigger knob clockwise Releasing the trigger knob Placing the drum with the new alphabet Adjusting the cipher wheels Setting the external key to 'RBKAE'

Sending a message
Sending an excrypted message requires a number of steps to be taken [9]. First of all the interior key needs to be set up. This requires a punched card and an alphabet sheet to be fitted inside the machine. This can be considered the daily, weekly or monthly key.

Next the operator randomly selects a 5-letter message key, e.g. VOMPF. This key is unique for each message and has to be sent to the receiver at the start of a message. Although this could be done en-clair (in the clear), it would make sense to send the message key encrypted. At present, the exact message procedure is not known.

After zeroizing the counter, the operator sets the machine to his chosen message key (VOMPF) and starts encrypting the message. Note that he has to turn the trigger knob before encoding each new letter. The encrypted text is written down in groups of 5 letters each. The counter in the top right should be checked regularly to verify that the 5-letter groups are still 'in sync'.

Numbers are converted to letters by using a conversion table attached to the front of the machine. Two different letters can be used for each digit. A group of digits is preceeded by 'KK' and terminated by 'ZZ'. Each line is terminated by 'XX' (full stop) and the last 5-letter group of the message is padded with 'Z' characters. This best demonstrated with an example:

    To all units. The Operations will start at 15.30.
    Prepair for actions as ordered before.

First this message should be written down in 5-letter groups, without spaces between the words and with 'XX' inserted in place of full-stops. The time (15.30) is converted to letters (KK CLHB ZZ). For example:


Once enciphered, this might look like this :


The fist 5-letter group is the chosen message key. It was probably sent in an enrypted form, but the exact method for this is currently unknown.
How it works
Like the Hagelin M-209, the HC-9 has a number of mechanical cipher wheels, each of which has a different number of possible positions (i.e. the number of steps for a full revolution of that wheel). In order to achieve the maximum cipher period (i.e. the number of steps before the sequence repeats itself), the number of positions for the 5 wheels are relative prime numbers. From left to right, the number of positions on each wheel is:

29 31 33 34 35

As each wheel makes one step when a letter is enciphered, the total cipher period is 29 x 31 x 33 x 34 x 35 = 35,303,730 steps. Each wheel has a full alphabet around its circumfere. As the alphabet is used only for 26 of the wheel's positions, the remaining steps are not marked (i.e. are left blank). When setting the wheels to the daily key, only the marked positions are used.

When a letter is enciphered, each wheel produces an active or inactive code, comparable to the presence or absence of a pin in a Hagelin machine. In the HC-9 however, the pins have been replaced by a removable punched card (see below). The operation is nevertheless identical. The logic diagram below should help explaining the operation of the machine.

HC-9 Logical block diagram

When activating the trigger knob, each wheel makes a single step. For each wheel, the presence or absence of a hole in the punched card, then produces a logical '1' (hole present) or '0' (absence of a hole). The five bits from the wheels are then used in pairs to perform a (mechanical) XOR-operation. XOR means that the output is only '1' when both inputs are different. This results in a 4-bit binary number in the range 0000-1111. The binary value is then converted to a decimal number (angle of rotation) in the range 0-15. This is the number of steps counted back from 16.

The above principle is best explained with an example. The logical values for this example are shown in red in the above diagram. Assume that at a given position, the wheels produce the values 11101. When these 5 lines are reduced to 4 lines, using pair-wise XOR, this results in the binary value 0011. This represents the decimal value 3. When enciphering a letter, the trigger knob is first rotated clockwise. This moves the alphabet drum to beyond alphabet 16. When releasing the trigger knob, the drum stops at the count of three, i.e. at alphabet 13.
Alphabet drum
The HC-9 contains a drum with 16 reciprocal alphabets. When enciphering a letter, the trigger knob is rotated and one of the 16 alphabets rotate into place. The 16 alphabets are printed onto a small sheet of paper that is wound around a drum which is accessible through a small door at the right of the machine. Replacing the alphabet sheet is part of the cryptographic key.
In order to replace the alphabet sheet, open the door by releasing its lock at the right side of the machine. Then pull out the drum. The image on the right shows the drum partly removed.

Once the drum is taken out of the machine, the existing sheet can be removed by releasing the locking spring. When folding the new alphabet sheet around the drum, the two index lines aside alphabet 8 should line up with the white index lines on the circumfere of the drum. Once the alphabet is correctly positioned, the retaining pin should be locked in place again.
Removing the alphabet drum

The entire machine was constructed in such a way, that alphabet sheets (or strips) printed on a Hollerith machine could be used directly. It was anticipated however, that alphabets could also be made by means of other printing systems, such as a typewriter, with slight modification [6].

In later years, Transvertex suggested the use of the IBM 1401 computer for the generation of randomized alphabets, and the complementary 1403 printer for producing a suitable output directly on paper [8]. This computer could also be used to generate the randomized schemes for the punched cards.
Opening the alphabet drum door Alphabet drum door open Removing the alphabet drum Removing the old alphabet Aligning the new alphabet Fitting the new alphabet Placing the drum with the new alphabet Adjusting the new alphabet

Alphabet sheets are 107 x 87 mm with the alphabets centered on the page. The alphabets themselves are typeset in a 12pt monospaced font, such as Courier, OCRB or the original Hollerith font. Creating such alphabets on a standard typewriter is rather straight forward as the default letter spacing should match the HC-9. Vertical spacing may need some experimentation.

Click here to download the reproduction alphabet in PDF format

The image above shows a suitable alphabet sheet. It is a reproduction of an original alphabet sheet from the Crypto Museum collection. The top line contains the identifier (BUMNP). The first alphabet is marked with a '*' at either side. Also not the horizontal index lines at either side of alphabet 8. These lines should line up with the white index lines on the drum.

Click the image to download the sheet as a printable PDF file. The alphabets are presented in two different typefaces: a new and an old (used) version of Courier. Next print the file onto faded (yellow-ish) paper, cut it out and fold it around the drum. Once inserted in the machine, the alphabet may be aligned somewhat by using the black knob on the front of the machine.
Punched cards
Like the early Hagelin machines, the HC-9 belongs to the class of pin-and-lug cipher machines. The presence or the absence of a pin at a certain position on a cipher wheel, determines the state of that wheel as part of the overall complex mechanical encryption algorithm.
On Hagelin machines, such as the M-209, the user needs to set the pins of each wheel to a desired state by using tweezers. This is a time consuming process with much room for error. On the HC-9, a punched paper card is used instead of the pins. The card is placed behind a door at the back of the machine (shown here).

The card compartment is opened by releasing a slide switch on top of the machine. The image on the right shows a typical HC-9 machine with the card compartment open. To avoid mistakes, the card can only be inserted in one way.
HC-9 with open punched card compartment

Three guide holes should each mate with a fixed guide pin. When a letter is enciphered, each wheel makes one step. For each position of a wheel, it senses the presence or absence of a hole in the punched card. The punched card is therefore divided into 5 segments, as shown here:

Pin layout of the punched card [8]

The card is shown when looking at the device from the rear. This means that wheel 1 is at the right and wheel 5 is at the left. The letters A-Z correspond with the alphabet on the circumfere of the wheel. The varying number of blanks are here replaced by numbers (starting with '1').

The original cards were made from 70 mm wide paper rolls supplied by IBM. The rolls had a diameter of 400 mm and contained 3000 pre-cut cards. Transvertex supplied a small hand-operated card puncher, that allowed 120 cards to be punched per minute. The puncher also contained a built-in printer that could be used to print date, time, purpose and serial number directly onto the card. Each puncher came with two inserts, in order to reduce down-time whilst setting up a new punch scheme [8].
Opening the card compartment

Reproduction cards
A card is made from rather thick paper of, say, 250 grams and is exactly 100 x 70 mm in size. A cut-off corner marks the top left. Three guide holes (two at the bottom and one near the top between segment 4 and 5) are used to ensure correct positioning of the card. Each sensing pin has a diameter of 1.5 mm, but the holes are all 3 mm to allow for tolerances.

The image below shows the layout of a fully punched card. It can be used as a mask when creating your own cards for the HC-9. Click the image to download the mask as a 100% PDF file. Print the page on 200 or 250 gram paper and punch the holes as required.

Click to download this mask as a PDF file.

Below is an example of a card that was actually issued. The cards were punched by a machine that also printed the identity and the serial number on it. The stars have no meaning and were probably pre-printed on the paper. They might have been used for alignment of the machine.

Click here to download these cards as a PDF file

The identity was printed between segments 2 and 3. In this case the identity is TOTAL-UTB NR 2. The serial number is printed between segments 3 and 4. The card on the left has serial number 5882, whilst the one on the right bears the number 5883. They are otherwise identical. Click on the image to download the cards as a PDF file.
The HC-9 is a true marvel of mechanical engineering. Although much of its principles are based on the knowledge of Hagelin machines of the same era, the mechanical construction of the HC-9 is completely different. Apparently, the designer had to circumvent the existing Hagelin patents, whilst at the same time trying to make the operation simpler, by avoiding the use of pin-wheels.
The image on the right shows the interior of the HC-9 after its green case has been removed. The five large black cipher wheels are clearly visible towards the rear of the machine. The card reader is placed directly behind the cipher wheels.

The most ingenious part of the machine is located at the front, just below the alphabet drum (which has been removed here). The construction at the front is a series of clever couplings that perform the action of the four XOR mechanisms as well as the conversion into a rotation angle (check the logic diagram above).
HC-9 interior

Don't be fooled by the seemingly simple construction of this machine. Especially the coupling mechanism at the front (called the 'adder') demonstrates the ingenuity of the designer Vigo Waldemar Lindstein. More detailed images of the interior of the HC-9 can be found below. Click any of the thumbnails for a closer look.
HC-9 interior HC-9 interior Cipher wheels and setting mechanism Cipher wheel axle adjustment Counter assembly The binary added that performs the XOR-functions as well as decimal conversion Close-up of the adder mechanism Bottom assembly

HC-9 Simulator
7 December 2012
Russian programmer Vyacheslav Chernov (Чернов) has made a beautiful graphical HC-9 Simulator for Windows TM. The image on the right shows the main window of the program.

All items can be clicked, and the wheels can be moved forward and backward. Both the alphabet ruler and the punched card can be fully edited.

 Download HC-9 Simulator
Click the image to download the simulator (packed as a RAR-file).

  1. AB Transvertex Stockholm, Hand-driven cipher machine type HC-9
    Brief description of the HC-9 (English). Publishing date unknown.
    Transcribed by Crypto Museum, 6 January 2012.

  2. AB Transvertex, Ciphering Machine HC-9 - Operation
    Operating instructions and spare parts list. 25 August 1967.
    Transcribed by Crypto Museum, 6 January 2012.

  3. Crypto Museum, Punched card mask (blank)
    Printable version of a card with all holes present. 7 January 2012.

  4. Crypto Museum, Examples of punched cards 5882 and 5883
    Reproduction of two existing cards. 7 January 2012.

  5. Crypto Museum, Example of alphabet sheet BUMNP
    Reproduction of an existing alphabet sheet. 8 January 2012.

  1. Torbjörn Andersson, Transvertex, Swedish Manufacturer of Ciphering Machines
    Information about Transvertex reproduced here with permission from the author.

  2. Ralph Simpson, Transvertex HC-9 Cipher
    Good description of the working principle and operating procedure.

  3. Deavours and Kruth, The Swedish HC-9 Ciphering Machine
    Cryptologia, Volume XIII, Number 3, July 1989, pp. 251-265.

  4. Deavours and Kruth, The Swedish HC-9 Ciphering Machine Challenge
    Cryptologia, Volume XIV, Number 2, April 1990, pp. 139-144.

  5. H.P. Greenough, Cryptanalysis of the Swedish HC-9: A Known-Plaintext Approach.
    Cryptologia, Volume XXI, Number 4, November 1997, pp. 353-367.

  6. AB Transvertex Stockholm, Hand-driven cipher machine type HC-9
    Brief description of the HC-9 (English). Publishing date unknown.
    Transcribed by Crypto Museum, 6 January 2012.

  7. AB Transvertex, Ciphering Machine HC-9 - Operation
    Operating instructions and spare parts list. 25 August 1967.
    Transcribed by Crypto Museum, 6 January 2012.

  8. AB Transvertex, HC-9, Handgetriebenes Chiffriergerät
    8-page brochure (German). Publishing date unknown, but later than [6].

  9. AB Transvertex, Operation Manual for Ciphering Machine HC-9
    10-page user manual (English). Publishing date unknown, but later than [7].

Further information

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