Handheld encryption device
The DH-26 was an electronic handheld
cipher machine, for the
encryption and decryption of text-based messages, developed by
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 . 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
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
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
[D] of 1971.
In November 1977, hot on the heels of the DH-26,
also announced the DC-26,
which should be seen as the
DH-26's big brother . 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 .
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
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.
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
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.
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.
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.
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
in Austria, who developed the
around a single chip Harris microcontroller and
Telesecurity Timmann (TST)
in Germany who used an
existing HP-19C calculator as the basis for its
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.
- Datotek Inc., DH-26 Operator's Manual
1978. 53 pages. Retrieved May 2015. 1
- US Patent 4,229,817, Portable Electronic Cryptographic Device
Filed 28 April 1978, Barry O. Morgan & Merlon B. Carter.
- US Patent 3,781,472, Digital Data Ciphering Technique
Filed 15 April 1971, George E. Goode, Barry O. Morgan & Kenneth M. Branscome.
- US Patent 3,781,473, Random Digital Code Genrator
Filed 15 April 1971, George E. Goode & Kenneth M. Branscome.
Document kindly supplied by Karsten Hansky .
- Karsten Hansky, DH-26 pocket cipher machine - THANKS !
Personal correspondence May/June 2015.
- Louis Kruh, DH-26 Handheld Encryption Machine
Cryptologia, Volume 2, Issue 2, April 1978, pp. 172-177
- Computerworld, Datotek Designs Encryption Unit
14 November 1977. p. 51.
- 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 August 2019 - 06:42 CET.