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KzU-44
Electronic message encryptor

KzU-44 is a stand-alone digital handheld encryption device, developed around 1991 by Institut Mihaljo Pupin in Beograd (Belgrade, Serbia); at that time the capital of Yugoslavia. 1 Like its pre­decessor – KzU-42 – it was used by the Yugoslav National Army (JNA) during the Yugoslav Wars (1991-2001) for handling text messages (telegrams) which were sent by courier, morse code, telegraphy (teleprinter) or verbally via radio or telephone. Also known as 3215-1084-1511.

The device is housed in an extruded aluminium enclosure that measures 152 x 89 x 30 mm and weights 310 grams (without the batteries). It has an alphanumeric keyboard with 42 buttons, and a slightly raised 1 × 16-character (LCD) display. It is powered by four internal AA-size batteries.

Up to four cryptographic keys can be stored in the internal memory, each of which consists of 40 characters, using only the letters A to P. The first 5 characters of the key is a check group and is sent in clear at the beginning of an encrypted message. It allows the recipient to verify the key.
  
KzU-44 handheld encryptor

According to the original checklist that came with the device, it was supplied without operating instructions or a user manual. Instead the instructions were supplied directly to the user, as per secret order VP 4578-Beograd. This was probably done for security reasons. Fortunately, the device shown here was found with a brief instruction sheet that explains the basic operation. It has serial number 92 06 0378 and was issued on 26 June 1992. It was partly built in 1991 [1].

  1. Beograd (Belgrade) was the capital of Yugoslavia from its creation in 1918 to its dissolution in 2006. Yugoslavia itself had already collapsed in 1992. Belgrade is currently the capital of Serbia [2].

Carrying case
KzU-44 handheld encryptor
Display showing 'ENTER PASSWORD'
Keyboard
Front side with power socket
Complete kit
Enclosed documents
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Carrying case
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KzU-44 handheld encryptor
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Display showing 'ENTER PASSWORD'
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Keyboard
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Front side with power socket
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Complete kit
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Enclosed documents

Cryptographic key
The KzU-44 can hold up to four different keys, numbered 1 - 4. Each key consists of 40 letters, entered as 8 groups of 5 characters each. This first 5-letter group is the check group and is not actually part of the key. It is used by the recipient to verify whether the correct key is used. Note that only the letters A - P can be used for the key. Trying to enter other characters will raise an error. The table below gives some examples of the keys we have used when testing our KzU-44.

            #  Check ------------------ key ------------------
           001 AAAAA AAAAA AAAAA AAAAA AAAAA AAAAA AAAAA AAAAA
           002 AAAAB CCCCC CCCCC CCCCC CCCCC CCCCC CCCCC CCCCC
           003 AAAAB ABCDE FGHIJ KLMNO ABCDE FGHIJ KLMNO PPPPP
           004 AAAAC ABPOF KAEID PCFAH BBONA MCADM HGDEI LCDIA

Only the last 7 groups (35 letters) are used for the key. As only the letters A-P can be used for the key (16 possibilities), each letter represents 4 key bits, which gives a total of 140 key bits.




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Parts
KzU-44 portable text encryptor
Carrying case
List of items
Instruction sheet
Carrying case
When not in use, the KzU-44 can be protected against dust, moisture and dirt by stowing it in the supplied Tb-112 artificial leather storage case or wallet. Checklist and short instructions were usually included in the wallet.

The wallet does not offer space for cables and spare batteries.
  
Carrying case

Encryptor   KzU-44
The actual KzU-44 encryptor resembles a pocket calculator of the era. It measures 30 x 89 x 152 mm and weights 310 grams. The device has a full alphanumeric keyboard with 10 keys for the numbers (0-9), 26 keys for the letters (A-Z) and 6 function keys. Their functions are listed below.

The unit is powered by four 1.5V AA-size batteries that are installed internally. It has a serial port for connection to external (computer) equipment or a modem.
  
KzU-44 handheld encryptor

Checklist
The checklist shows the release date (25 June 1992 in this case) and lists the items that were delivered. The KzU-44 encryptor and the Tb-112 carrying case were part of the delivery, but the operating instructions were missing, as per secret order VP 4578-Beograd.

The checklist is printed at DIN A5 size and folded to A6 size, so that it fits inside the wallet. It also serves as a logbook or passport for any aftermarket modifications or repairs.

 Download checklist

  
Checklist

Instruction sheet
Although the operating instructions were not included with the device, the one shown here came with an instruction sheet that explains how to enter, encrypt and decrypt a message.

The instruction sheet is printed at DIN A4 size (double sided) and folded to A6 size so that it fits inside the wallet. The supervisor password (POT-986) is handwritten in the left margin.

 Download instruction sheet

  
Instruction sheet

Carrying case
KzU-44 handheld encryptor
Checklist
Instruction sheet
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Carrying case
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KzU-44 handheld encryptor
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Checklist
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Instruction sheet

Operation
Operating the KzU-44 is straightforward. The keyboard consists of 42 keys: 36 white ones for the letters (A-Z) and the number (0-9), and 6 black function keys. The upper function key (UK) is the power button. Pressing it turns the device on. Pressing it again turns the device off. The other black keys are explained in the table below. The middle four keys have a double function. They can be used by themselves (e.g. SPACE) or in combination with the shift key ↑ (e.g. ENTER).

After turning the device on with the (UK) button, it will ask for the password (UNESI LOZINKU !). Enter the password (POT-986) and press ENTER. The display now shows UNESI KOMMANDU ? (enter command). You may now enter any of the two-letter commands from the table below.

Key Normal function With shift Shift function
Power ON/OFF    
Space Enter
Cursor left Cursor to start
Insert character left of cursor Delete character at cursor
Cursor right Cursor to end
Shift    
Mnemonics   commands
MNE Description
BS Delete input and output buffers
DS Decipher numbers or words
GN Generate pseudorandom string
IK Change valid key at power cycle
IT Text correction
PL Change operator and supervisior password
PI Show contents of output buffer on display
PO Help (show mnemonics and function names)
SB Encrypt into numbers
SS Encrypt into text
TS Test device
UK Enter internal key
UZ Enter characters in the input buffer
Display
Display English Description
UNESI LOZINKU ! Enter password e.g.:
POT-986
UNESI KOMANDU ? Enter command Mnemonic, e.g.
UZ
OPER/PRETP (O/P) Operator/Supervisor e.g.:
O
ST/OT (S/O) Enter ciphertext or plaintext e.g.:
O
ST Šifrirani tekst (ciphertext)  
OT Otvoreni Tekst (plaintext)  
NEISPRAVAN KLJUC Faulty key  
PRAZAN BAFER Empty buffer  
Change operator password
Key(s) Description Display Keyboard
Power on UNESI LOZINKU ! (enter password)
Enter UNESI KOMANDU ?
    OPER/PRETP (O/P)
    UNESI LOSINKU ! (enter password)
Enter PONOVI LOZINKU (repeat password)
Enter PROMENA LOZ (password changed)  
  2 sec. UNESI KOMANDU ? (enter command) ..
Key entry
Key(s) Description Display Keyboard
Power on UNESI LOZINKU ! (operator password)
Enter UNESI KOMANDU ?
    UNESI LOZINKU ! (supervisor password)
Enter UPIS KLJUCA BR ? (enter key number)
   
001 _
   
002 AAAAA _
   
003 AAAAA _
 
    Enter 8 groups of 5 letters using only A-P  
   
008 AAAAA _
 
Confirm UPIS KLJUCA BR ? (enter key number)
  Repeat Repeat for keys 2 to 4  
Confirm UPIS KLJUCA BR ? (enter key number)  
Exit UNESI KOMANDU ?
    IZMENA KLJ. BR ? (select key)
Power off Key becomes active after power cycle  
Encryption
Key(s) Description Display Keyboard
Power on UNESI LOZINKU ! (enter password)
Enter UNESI KOMANDU ? (enter command)
    ST/OT (S/O) ? (ciphertext or plaintext)
   
xxxxxxxxxxxxxxxx
   
THIS IS A TESTxx
Exit UNESI KOMANDU ?
    BROJ KLJUCA - ? (select key)
    SIFR. SLOVCANO !  
  2 sec. UNESI KOMANDU ?
   
001 AAAAA FPIDB
 
Scroll
003 FLHBD GMJJI
 
Exit UNESI KOMANDU ?  
Decryption
Key(s) Description Display Keyboard
Power on UNESI LOZINKU ! (enter password)
Enter UNESI KOMANDU ? (enter command)
    ST/OT (S/O) ? (ciphertext or plaintext)
   
001 xxxxx xxxxx
...
Complete UNESI KOMANDU ? (enter command)
    DESIFROVANJE (decrypt)  
  2 sec. UNESI KOMANDU ?

Message example
Below is an example of a simple text (THIS IS A TEST) that is encrypted with a key that consists of 40 x 'A'. In this case, the ciphertext consists of seven 5-letter groups, which is longer than the original plaintext. This is because the ciphertext also contains the check group (AAAAA) and a unique (machine-generated) message key. As a result, the ciphertext will be different each time, even when the plaintext and the key haven't changed. Furthermore, the message may contain letters and numbers, whilst the ciphertext consists of letters (or numbers) only. For example:

   Plaintext: THIS IS A TEST
Key 001: AAAAA AAAAA AAAAA AAAAA AAAAA AAAAA AAAAA AAAAA Encrypted: AAAAA FCOAK OGIEN OIPHK UJGFV BKYNK CPWRX

When decyrpting the text, the full ciphertext has to be entered, including the check group (AAAAA in this case). Once it is complete, press Enter followed by the DS command. This will decrypt the ciphertext. Then use the PI command to reveal the decrypted text.

   Plaintext: THIS IS A TEST

In the above example, the ciphertext consists of the letters (A-Z). This was achieved by using the SS command to encrypt the plaintext. Alternatively, the plaintext can be encrypted into a numeric ciphertext by using the command SB. The same message might now be something like this:

     Numeric: 00000 52602 51550 23112 56601 13304 08773 70499 95153



Interior
The interior of the KzU-44 can be accessed by depressing the knob at the right side with a sharp object, such as a pen, and sliding the case shell towards the rear until it locks. This reveals the battery compartment, which is covered by a hinged lid. It accepts four 1.5V AA-size batteries. The unit can now be closed again by depressing the knob at the side and sliding back the case.

Alternatively, the interior can be extracted by unlocking the case shell (with a pen) and sliding it further towards the rear until it comes off.

This reveals the electronic circuits as shown in the image on the right. At the bottom are the keyboard and the display, which are mounted onto a large printed circuit board (PCB). At the other side of the PCB is the battery compartment and a smaller PCB — the central processing unit or CPU. The latter is held in place by four 10 mm spacers at the corners, and is connected to the keyboard PCB by means of two headers (K1, K2).
  
Interior

After removing the screws from the spacers, the the CPU board can be extracted from the larger PCB. The CPU is built around a CDP1802 8-bit microprocessor with an external 8KB static RAM to hold the messages, and a 32KB EPROM that holds the firmware [a][3]. According to the date codes on the various components, the device shown here was built in 1991 or shortly thereafter.

Case shell halfway removed
Rear side with battery compartment
Opened battery compartment
Batteries installed (4 x AA-size)
Interior
Interior - left side
Interior - right side
The two PCBs separated
Display PCB
CPU board - component side
CPU board - solder side
Microcontroller and crystal
EPROM with firmware and static RAM
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Case shell halfway removed
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Rear side with battery compartment
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Opened battery compartment
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Batteries installed (4 x AA-size)
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Interior
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Interior - left side
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Interior - right side
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The two PCBs separated
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Display PCB
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CPU board - component side
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CPU board - solder side
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Microcontroller and crystal
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EPROM with firmware and static RAM

Specifications
  • Device
    Portable text encryptor
  • Model
    KzU-44
  • Designator
    3215-1084-1511
  • Manufacturer
    Institut Mihaljo Pupin (Belgrade, Serbia)
  • User
    Yugoslav National Army (JNA)
  • Year
    1991/92
  • Conflict
    Yugoslav Wars (1991-2001)
  • Keys
    4
  • Key length
    40 characters (8 groups of 5 letters) A-P only
  • Key strength
    2140 (140 bits)
  • Power
    6V
  • Batteries
    4 x 1.5V AA-size (penlight)
  • Password
    POT 986
  • Dimensions
    152 x 89 x 30 mm
  • Weight
    310 grams (without batteries)
Datasheets
  1. CDP1802A CMOS 8-bit Microprocessors, Datasheet
    Intersil, March 1997.
Documentation
  1. KzU-44 operating instructions
    Date unknown, but probably 1992.

  2. Original checklist
    26 June 1992.
References
  1. Anonymous donor, KzU-44 in wallet - THANKS !
    Received September 2021.

  2. Wikipedia, Belgrade
    Retrieved 23 September 2021.

  3. Wikipedia, RCA 1802
    Retreived December 2016.
Further information
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Crypto Museum. Created: Thursday 23 September 2021. Last changed: Friday, 13 May 2022 - 19:38 CET.
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