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Electronic message encryptor

KzU-42 is a stand-alone digital message encryption and decryption device, developed in the former Yugoslavia around 1985 and built by the Rudi Čajavec factory in Banja Luka (Bosnia and Herzegovina). It was intended for manually encoding and decoding textual messages (telegrams) which were send by means of courier, morse code, digital telegraphy (teleprinter) or verbally over radio or telephone. Used by the Yugoslav National Army (JNA). Also known as 3215-1083-1094.
The device is housed in a green metal enclosure that measures approx. 20 x 12 x 8 cm. The case is water-resistant and has a hinged top lid that is closed with two snap-locks at the sides. Inside the case is the encryptor that consists of a 27-button A-Z keyboard, an 8-digit alphanumeric display and a removable rechargeable battery.

The device was supplied in a small green leather carrying case, marked TB-101, complete with a spare battery, a battery charger, a piece of cloth (for cleaning) and a full set of documentation, including a maintenance book and key lists.
KzY-42 ready for operation

The carrying case measures 28 x 23 x 10.5 cm and is constructed in such a way that the KzU-42 can be operated without taking it out of the bag. The top lid of the bag can opened in two parts and both of the snap-locks of the device's top lid are reachable through cut-outs at the sides.
The rear section of the leather bag has one large bay for the battery charger, a 12V cable and a spare battery (optionally), plus a smaller bay for the spare fuses, a pencil, an insulated screw­driver and a flannel cloth for cleaning the unit.

The KzU-42 is fully self-contained, which means that once the battery is fitted and the keys are loaded, it can be used straight away. The keys are entered in two parts and requires a specific key loading procedure to be followed. The same keys are used for encoding and decoding, which means that a reciproke cipher algorithm is used.
KzU-42 operated from within the carrying case

According to secret order 101-10/85, dated 24 June 1985, the device was supplied without user manual, which was also the case with the KzU-63 voice encryptor [1]. According to its passport, the device shown here left the factory in December 1985 and was first used in February 1987 [2].
TB-101 green leather carrying case KzU-42 with closed top lid KzU-42 telegram encryptor KzU-42 packed inside the TB-101 leather carrying case KzU-42 operated from within the carrying case KzU-42 operated from within the carrying case Accessories in the rear section of the TB-101 carrying case PA-42 battery charger and 12V cable

The diagram below shows the various controls of the KzU-42, all of which are located on the control panel that is covered by a sealed top lid, in order to protect it against dust an moist. The 4.8V NiCd battery pack is fitted under the small green lid towards the rear. It is held in place by two bolts. The unit is turned ON by setting the power switch (just below the display) to the U-position (uključeno). Press the black B-button on the keyboard to initialise the internal counters.

The mode of operation is set with a 2-position MODE-switch, which is located next to the power switch. It is marked RAD (run) and UPIS (init). Set this switch to UPIS for entering the crypto­graphic keys. Once this is done, set it to RAD for encrypting or decrypting a message. Do not forget to reset the internal counters (pressing the black B-button) after changing the mode.

Example of an encoded letter group

The output is presented on a clear 8-character LED display that is mounted just behind the keyboard. Messages are entered in groups of 5 letters and the last group is always shown on the display, preceeded by the current group number. After entering a message, the counter shows the total number of groups that have been entered. This should be written on the telegram.
KzU-42 operated from within the carrying case Control panel Display and switches Zeroize button Keyboard Black key marked 'B' at the bottom right - counter reset (brojač resetiran) Physical key in VK position (outer key) KzU-42 with top lid open

The device has two keys that have to be set before encrypting or decrypting a message: the outer key (VK = vanjski ključ), and the inner key (UK = unutarnji ključ). The outer key is set by a crypto officer and requires the physical key at the top right to be set to VK. The inner key is altered by the operator every three or four days at 8:00 hours. It requires the physical key to be set to UK. Both keys can have an arbitrary length, but in practice the daily key was fixed at 50 characters.
Resetting the keys
When the device is first turned on, the contents of its internal memory, and hence the selected encryption key, are fully random. It is therefore necessary to reset the keys first. This is done by setting the MODE selector to Initialise (UPIS) and entering the letter 'X' at least 16 times. Then reset the counter (by pressing the black B-key) and enter a new inner and outer key.
Outer key
The outer key or master key is set by a crypto officer who has access to the unit's physical key. This key is unique for each device and was probably stored elsewhere (i.e. not with the device). The physical key is used to set the lock at the top right to the VK-position (outer key).

The MODE-switch is then set to UPIS (INIT) and the black B-key is pressed to ensure that the internal position counter is reset. The officer then resets the existing key (see above) and enters a new one. Although at present we do not know how long the master key was and how often is was changed, we assume that it consisted of 50 characters and that it was valid for one month. Once the Master Key was set, the physical key was set to the UK-position (inner key) and removed.
Inner key
The inner key is set by the operator at 08:00 in the morning and is valid for three or for days. It consists of 50 characters (ten 5-letter groups) that were obtained via a complex procedure. First the operator has to consult the so-called A/B Key Table (tablica ključeva A/B) to obtain 10 pairs of A/B digits [3]. The table was valid for one year and had one page for each month. Each month was assigned 10 keys (each valid for 3 or 4 days). Let's examine the entry for 7 February 1987:
A 6 4 7 5 3 1 2 9 0 8row B 6 3 8 2 5 9 0 4 7 1column
These two lines define 10 digits for A and 10 for B. The operator then consults a secret Key Card, e.g.: Document S-42-S, number 059696 with serial number 147 [4]. This key card contains a table with 10 rows by 10 columns (i.e. 100 cells). Each cell contains one 5-letter group. The rows are marked A and the columns are marked B. The order of the rows and columns is scrambled.

Small Key Card in plastic cover. Click for a closer view.

Before each row and above each column, the operator has written the scrambling order, which was probably presented to him verbatim (e.g. during a briefing). It was also possible to write the scrambling order onto two small paper rulers that were supplied as part of the complete set.

The operator now translates the A/B number pairs into 5-letter groups from his Key Card. In the example above, this would produce the following groups:
The above is the 50-character inner key. The operator now sets the MODE-switch to UPIS (Init) and resets the existing key first (see above). He then resets the counter by pressing the black B-key and enters the 50 character inner key shown above. Whilst doing this, the display shows the total number of 5-letter groups that have been entered, plus the last 5-letter group.
(De)coding a message
Once the key has been entered, the MODE-switch is set back to RAD (Run). After resetting the internal counter (black B-key) the unit is ready for encrypting or decrypting a message. The encryption algorithm is reciproke, which means that the same settings are used for encryption and decription. The message is usually written in 5-letter groups without spaces, for example:
When entering the text, the display will first show the plaintext character (i.e. the key that is pressed). As soon as the key is released, it is translated into the ciphertext character. After each complete 5-letter group, the ciphertext is written onto a telegram sheet (of the supplied block).
Key Card and Key Table Documents supplied with KzU-42 Finding the right entry in the Key Table Key Card in plastic slip-cover Key Card Key Card removed from its slip-cover. Note the two plastic rulers. Telegram block

According to the checklist that was supplied with each KzU-42 [1], the following items were available. It is mentioned specifically, that the operator's manual is not supplied with the unit.
  1. 1 ×
    KzU-42 device with internal key
  2. 1 ×
    PA-42 battery charger
  3. 1 ×
    TB-101 carrying case
  4. 1 ×
    Battery pack
  5. 1 ×
    KE-204 charger cable
  6. 5 ×
    Fuse 0.315 A
  7. 1 ×
    Isolated screwdriver 0.8 x 4.5 - JUS K. G5.213
  8. 1 ×
    Message block for telegrams
  9. 1 ×
  10. 1 ×
    Flannel cloth 30 x 30 cm
  11. 1 ×
  12. 1 ×
    TS-100 Technical booklet (passport)
TB-101 green leather carrying case 4.8V NiCd battery pack Battery charger Piece of flannel cloth Spare 315 mA fuses, pencil and screwdriver 100-page telegram block Technical maintenance booklet (passport) Code Material (Key Tables and Key Chart)

Carrying Case   TB-101
The KzU-42 was usually delivered in a sturdy green leather carrying case, marked as TB-101. The case has a canvas carrying strap and a top lid that can be opened in two parts.

The case has three compartments: a large one with soft padding for the KzU-42, a smaller one for the battery charger and an even smaller one for the other accesories, such as the fuses, a pencil and a piece of cloth. The case has cut-outs that allow the snap-locks of the KzU-42 to be operated without removing it from the case.
TB-101 green leather carrying case

The KzU-42 is powered by a rechargable battery pack that is inserted under a removable green panel towards the rear. The battery pack contains four 1.2V NiCd cells that should be charged with the accompanying PA-42 battery charger. As the units are now (2015) some 30 years old, the batteries of any surviving units will be exhausted by now and will probably be leaking.
As leaking batteries may potentionally cause damage to the unit, it is important that they are removed from the unit as soon as possible. If possible, the NiCd cells should be removed from the battery pack and replaced by new ones. Alternatively, they could be replaced by a couple of so-called super-caps which are ideal for demonstrations and will never leak.   
Battery pack

Battery charger   PA-42
The KzU-42 was supplied with the purpose-built PA-42 battery charger shown in the image on the right. It is actually a large adapter that can be placed directly into a mains wall socket. The NiCd battery is inserted into a bay at the front.

The PA-42 is also suitable for charging the batteries from a 12V DC source, such as the battery of a car. A special cable was supplied to connect the car battery to the 5-pin socket at the front of the charger.
PA-42 battery charger and 12V cable

Flannel cloth
Each KzU-42 came with a 30 x 30 cm piece of flannel cloth, which was used to keep the unit, and especially its keyboard, dry and clean. Apparently, they used whatever piece of flannel was available, as nearly every set was supplied with a different one, ranging from plain white to the brightly coloured one shown here.

The cloth was also used to clean the red lens over the LED display without scratching it.
Flannel cloth (30 x 30 cm)

The leather carrying case has a compartment at the rear right that is used for storing the spare fuses, a pencil (for writing the telegrams), screwdriver, the flannel cloth and any further spares that were supplied with the set.

Click here to see the spares compartment which has small pockets for holding the spare fuses.

Telegram block
Encrypted (or decrypted) messages are usually written down in telegram-style in groups of five letters. A suitable A5-size block for writing down such telegrams was supplied with the set.

It consists of 100 pages, each of which can hold one telegram (both sides) with a maximum of 200 5-letter groups. The total number of groups should be written in a box at the top left. This should be equal to the number shown on the display after the last group has been entered.
Telegram block

Maintenance booklet   TS-100
Each device left the factory with its own 'personal' passport that shows when it was released and when it was first issued. Any repairs or other maintenance issues are also reported in this technical booklet.

According to the first page of the booklet, the KzU-42 unit shown here was manufactured in 1985 and was released to the Yugoslavian Army (JNA) in January 1987. It was first issued in the following month.
First page of the passport

Code material
The cryptographic keys for the KzU-42 came in two parts: an outer key that was set by a crypto officer and an inner key that was set every three or four days by the operator. The inner key was obtained via a procedure that involved a Key Table [3] and a Key Card [4] as shown here.

The Key Table contains one page for each month of a year. Each page contains ten indexes, which are used by the operator to find the required letter groups on the Key Card. This way, the 50-character inner key is recovered.
Key Card and Key Table

Storage area for spare fuses, pencil, screw driver and flannel cloth Cur-out in the leather case to allow the snap-locks of the top lid of the KzU-42 to be operated PA-42 battery charger and 12V cable Battery charger - mains plug 12V cable from (car) battery Leaking battery Battery pack Technical maintenance booklet of the KzU-42
First page of the passport Spares Telegram block Telegram block Key Card and Key Table Key Card in plastic slip-cover Key Card removed from its slip-cover. Note the two plastic rulers. Close-up of the Key Card

The KzU-42 is housed in a green die-cast aluminium enclosure that measures approx. 20 x 12 x 8 cm. It has a hinged top lid that is sealed with a rubber gasket. The actual device is mounted inside the die-cast enclosure and is held in place by two screws at the left and right center.
After releasing these two bolts, the device can be lifted out, but when doing so, the crypto­graphic keys are wiped by a small tamper switch that is mounted in a corner at the bottom.

After removing the green top cover (3 bolts at the top and 1 at the rear) a sturdy metal frame is revealed with 4 large PCBs fitted onto a back­plane. The PCBs are held together with a large retaining pin and a long piece of spring metal. After releasing the spring and the retaining pin, the PCBs can be taken from the backplane as shown with the lower two boards on the right.
PCBs partly removed from the frame

The lower board (marked MGP) is the encryption board that 'mixes' each plaintext character with a character produced by the pseudo random generator. As this uses binary adding (XOR), the same process of mixing can be used for decryption as well as it is fully reversible (reciproke).
The second board from the bottom (OUT) holds the actual cryptographic keys in a series of shift-registers. The keys are used as the start value (a.k.a. initial vector, or IV) of the Pseudo Random Number Generator, or PRNG. The complexity of this generator and its (non)linear behaviour determines the strength of the cipher and, hence, the cryptographic security of the system.

The remaining two boards are used for user interaction. The largest one holds the 27-button keyboard (shown on the right), the power switch (I / U) and the MODE-selector (RAD / UPIS).
Keyboard and PCB

The upper board is only half the size of the other ones and is mounted behind the keyboard. It holds the 8-digit alphanumeric LED display, which consists of two pre-assembled 4-digit units. The metal frame holds everything together and also holds the physical lock (at the top right) and a recessed ZEROIZE button (at the top left). A small PCB with two large copper pads, is mounted to the rear of the frame. It forms the base of the rear compartment and acts as battery contacts.
KzU-42 removed from the case KzU-42 removed from the case - seen from the rear Rear view of the interior, showing the battery contacts KzU-42 internal frame with 4 PCBs Frame seen from the front - 4 stacked PCBs Releasing the spring PCBs partly removed from the frame Some PCBs partly removed
Bottom board - marked MGP Second board - marked OUT Keyboard and PCB Display board Keyboard and PCB ZEROIZE switch 8-digit alphanumerical LED display Tamper switch

  1. Rudi Čajavec, KzU-42 Spisak Kompleta
    31 December 1985. Checklist. 2 pages, Croatian language.

  2. TS-100, Tehnička knjižica KzU-42 (VOJNA TAJNA)
    Technical book for electronic materials (military secret).
    JNA 3215-1083-1094. 27 February 1987.

  3. Tablica Ključeva A/B
    A/B Key Tables for 1987 (February-December). 11 pages.

  4. DOKUMENAT S-42-S, broj 059696, evid. br. 147
    Key Card. Document S-42-S, number 059696, serial number 147.
    STROGO POVERLJIVO (top secret). Issued 1987.

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Crypto Museum. Last changed: Friday, 25 September 2015 - 15:35 CET.
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