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Datotek DH-26
Handheld encryption device

The DH-26 was an electronic handheld cipher machine, for the encryption and decryption of text-based messages, developed by Datotek in Dallas (Texas, USA) in 1977, and introduced in April 1978. It was one of the first electronic microprocessor-based pocket encryption devices.

The image on the right shows a nice example of a rare surviving DH-26 unit [1]. It is housed in a plastic case and is quite similar to the pocket calculators of the era, such as the ones from HP.

The device has a 5-character alphanumerica LED display and a keypad with 34 keys. Three slide-switches, immediately below the display, allow the mode of operation to be selected. The right­most one is the power switch, whilst the leftmost one is used to select between Decoding (D) and Encoding (E). For a detailed description of its operation, please consult the user manual [A].
Datotek DH-26

The device measures approx. 20 x 9.5 x 5 cm, weights just over 500 grams and was sold in 1978 for the firm price of US$ 1650 [2]. It is also known as DH-26 Electronic Code Book. Although it is not revealed which encryption algorithm is used, some of its characteristics can be found in the instruction manual [A]. The DH-26 allows 4.8 · 1052 basic key variables (175 bits) and 1.2 · 107 message keys (approx. 23 bits), and has a cycle length (cipher period) of 1.05 · 1065 (216 bits). The algorithm might be described in the US Patents 3,781,472 [C] and 3,781,473 [D] of 1971.

In November 1977, hot on the heels of the DH-26, Datotek also announced the DC-26, which should be seen as the DH-26's big brother [3]. The DC-26 is intended for desktop use and has a full size keyboard for faster data entry, plus a built-in thermal printer for making a hard copy of the text. The DC-26 is fully compatible with the DH-26 and was sold in 1978 for US$ 6800 [3].

Carrying pouch DH-26 with carrying pouch and instruction booklet DH-26 Datotek DH-26 Datotek DH-26 Datotek DH-26 front panel 5-character alphanumeric display Connections at the rear
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Carrying pouch
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DH-26 with carrying pouch and instruction booklet
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Datotek DH-26
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Datotek DH-26
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Datotek DH-26 front panel
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5-character alphanumeric display
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Connections at the rear

The diagram below shows the control panel of the DH-26. The exterior is very similar to the pocket calculators of the era. The main differences with a calculator however, are the presence of an alphanumeric keypad and a 5-character alphanumeric LED display. At the bottom is a battery pack that contains four 1.2V NiCd cells (removed here because of leakage). In addition, the unit can be powered by the battery charger that is connected at the rear. For expansion (and probably also for testing) a 21-pin socket is available at the rear. Its function is currently unknown.

After turning the device ON with the rightmost slide switch (just below the display) the display usually stays blank, although you may see five dots flashing briefly. Before doing anything else, press the yellow (MR) button to cause a Master Reset. Next press (MK) to enter the Message Key or (BK) to enter the Basic key, and enter the first 5 characters of the key. They should be visible on the display. For a full description of the operating procedure, please refer to the manual [A].

If the battery voltage drop below a certain point, five dots will become visible on the display. This is an indication that you should recharge the batteries. If the voltage is not too low, you may continue to use the DH-26. If you don't press a key within a certain period of time (typically 1 minute), the systems falls asleep in order to save batteries. This is visible by the appearance of a '/' in the leftmost display. If this happens, press (RC) to Resume Control.

Despite its calculator-style case, the DH-26 certainly wan't a cheap gadget. After removing the battery pack, the bottom panel can be taken away by removing the four screws in the corners. This reveals a stack of three PCBs, one of which (the control panel) is bolted to the front panel.

The other two PCBs can be removed after taking out four bolts and four plastic spacers. All three boards are connected together by means of long contact strips at the edges. They are separated carefully as shown in the image on the right.

The rightmost board (green) holds the keypad and the five alphanumeric displays. In the image its reverse side is shown, which holds the display driver. The leftmost board contains the power circuits. It is the smallest of the three and also holds the I/O expansion interface that ends in a military 21-pin miniature sub-D male socket.
Three boards

The board at the center contains the Central Processing Unit (CPU) and the actual cipher unit. It is built around an Intersil IM6100 processor running at 2 MHz, an IM6101 I/O expander and a custom-made cryptographic unit. The firmware is contained in two solderd-in masked ROMs. Furthermore, there are three 6561 memory chips, giving a total RAM capacity of just 384 bytes. As CMOS memory is used, the cryptographic keys are retained when the unit is switched OFF.

The image above shows the component side of the CPU board which, for its age, is very well layed out, with the three main chips (the processor, the I/O expander and the cipher unit) placed side-by-side. The programmers had quite a challenge as they only had 384 bytes (!) of RAM at their disposal. Furthermore, the firmware had to be right first time, as there was no way of swapping the ROMs afterwards. The processor is equivalent to the Harris HM6100 and the DEC PDP-8.

Interior Interior - bottom view Sandwich of three boards Three boards Close-up of the displays Close-up of the key pad switches Tamper switch and crystal Expansion/test connector
Display area - reverse side Processor board Processor board - solder side Masked ROMs RAMs Crypto unit Display segment drivers Power board
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Interior - bottom view
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Sandwich of three boards
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Three boards
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Close-up of the displays
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Close-up of the key pad switches
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Tamper switch and crystal
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Expansion/test connector
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Display area - reverse side
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Processor board
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Processor board - solder side
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Masked ROMs
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Crypto unit
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Display segment drivers
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Power board

When we got the DH-26 shown here, it was not working, However, since it had been working well with the previous owner until just before we got it, we had a good chance of bringing it back to life again. It turned out that the original patent drawings and descriptions were a great help.

Unlike most other patents, which only give a rough description of the involved principle, the patent of the DH-26 presents the full circuit diagram and a flow-chart description of the sofware. Furthermore, all components, with the exception of the cipher unit, were standard off-the-shelf parts at the time, which means that their datasheets can be found on the internet.

After studying the circuit diagrams and the data­sheets of the individual components we got a good understanding of the circuit and were able to do some measurements on the three boards.
820K resistor replaced (value had increased to 960K)

It soon turned out that the CPU was running, but that the unit sometimes had trouble switching ON. Furthermore, the display was addressed, but was given so little time that it couldn't light up. After replacing the CD4016 switching IC (for the improved CD4066), we discovered that a 820K resistor, which is part of the display timing circuit, had increased to 960K as a result of which the timing circuit was disabled. The image above shows the replaced components, which are both located on the power board. After these two fixes, the DH-26 has been running well ever since.

820K resistor replaced (value had increased to 960K)
Similar devices
When microprocessors became widely available in the mid-1970s, Datotek was not alone in the development of electronic pocket encryptors. Similar devices, which also resembled the pocket calculators of the era, were made by several manufactuers, such as Mils Elektronik in Austria, who developed the PCCM-4000 around a single chip Harris microcontroller and Telesecurity Timmann (TST) in Germany who used an existing HP-19C calculator as the basis for its PPC-19 encryptor.

Another example is the HC-520 of the Swiss company Hagelin (now: Crypto AG). It is somewhat larger than the DH-26, but like the DH-26 is was part of a family of devices with which it was compatible. Although the core features of the DH-26 were patented [B], the same ideas were apparently used elsewhere in the world and did not infringe Datotek's patent rights.

Mils PCCM-4000 (late 1970s) Telesecurity Timmann (TST) PPC-19 (late 1970s) Hagelin HC-520 Cryptomatic
  1. Datotek Inc., DH-26 Operator's Manual
    1978. 53 pages. Retrieved May 2015. 1

  2. US Patent 4,229,817, Portable Electronic Cryptographic Device
    Filed 28 April 1978, Barry O. Morgan & Merlon B. Carter.

  3. US Patent 3,781,472, Digital Data Ciphering Technique
    Filed 15 April 1971, George E. Goode, Barry O. Morgan & Kenneth M. Branscome.

  4. US Patent 3,781,473, Random Digital Code Genrator
    Filed 15 April 1971, George E. Goode & Kenneth M. Branscome.
  1. Document kindly supplied by Karsten Hansky [1].

  1. Karsten Hansky, DH-26 pocket cipher machine - THANKS !
    Personal correspondence May/June 2015.

  2. Louis Kruh, DH-26 Handheld Encryption Machine
    Cryptologia, Volume 2, Issue 2, April 1978, pp. 172-177

  3. Computerworld, Datotek Designs Encryption Unit
    14 November 1977. p. 51.

  4. Jane's Military Communications 1981, 26 Series Portable, Off-line Message Security Units
    1981. pp. 404-405.
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Crypto Museum. Created: Thursday 28 May 2015. Last changed: Friday, 23 February 2018 - 22:13 CET.
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