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HELL
The actual key generator (German: Impulsgenerator) consists of a mechanism
with five looped perforated tapes of different length, that have been created
externally with a true random noise source. Copies of these tapes are
distributed to all parties of the communication network.
The current key list dictates the order in which the tapes are installed, plus
their starting position. It also determines which channel of each tape will
be used. In addition, five code plugs (from a set of 32) must be installed behind
a panel at the front to define the order of the bits.
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When the machine is started, the short key described above is used to generate
a pseudo-random sequence — the key stream — with a very long period.
To ensure a maximum length key stream, the lengths of the looped tapes must be
different and must be coprimes. If all machines in the network are set up
identically, they will generate the same key stream.
This stream is then punched into a paper tape, which is fed directly to
the key tape reader of an existing one-time tape cipher machine
(OTT), such as the Lorenz Mi544 mixer.
As the tape puncher is faster than the tape reader on the mixer,
a key tape buffer will be created. When the buffer is full, the HL-502
stops temporarily. A sensing mechanism ensures that the buffer
will never be empty.
Development of the HL-502 was started in 1961, and the first machines became
available in 1962 and 1963. It was used by the German Army (Bundeswehr) and
the German Navy (Marine) until at the least the late 1960s or early 1970s,
after which they were replaced by more modern machines, such as the
ELCROTEL family of machines, and in the early 1980s
by the Aroflex.
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NOTE —
This page currently acts as a placeholder for future information about
this device.
Development of the HL-502 started in 1961, at a time when the German Army and
NATO were looking for alternatives to the existing OTT machines. Although
OTT machines are absolutely secure (when used correctly), the distribution
of the bulky key tapes was a logistic nightmare. Within NATO, the search
for alternative solutions was known as TROL, which stood for
Tapeless, Rotorless, On-Line. In this context, Tapeless means
that it does not require and OTT key tape.
The HL-502 allowed a smooth transition from OTT cipher machines (mixers) to
key generator-based cipher machines. Since it came as an addition to existing
OTT cipher machines, rather than a full replacement, it gave the existing
machines an extended lease of life.
A similar solution was developed by Philips
in the Netherlands, where the Tarolex key generator
replaced the key tape reader of an existing Ecolex IV machine.
However, it required the existing Ecolex IV machines to be
reworked, whereas the HL-502 could be used without modifying the existing
equipment.
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Device Key tape generator Purpose Real-time key tape generation, replacement of OTT keytape Family TROL Model HL-502 Manufacturer HELL Year 1962 Country Germany Users Germany Army (Bundeswehr), German Navy Power 110/220 V (selectable), AC 50/60 Hz Consumption 145-160 W (without heater) Temperature -10°C to +50°C Storage -40°C to +70°C Dimensions ? Weight 30 kg (withough base plate)
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50210035 Hell HL-502 impulsgenerator 50210038 Siemens T.loch.17 tape puncher (modified) ? Set of 32 code plugs ? Interconnection cable, 30 pin
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- Key tape generator
- Impulsgenerator
- Schlüsselstreifengeber
- Tarnstreifengerät
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- Schlüsselstreifengeber HL 502 - Geratebeschreibung und Bedieningsanleitung
Device descriptions and operating instructions (German).
Dr.-Ing. Rudolf Hell, Kiel (Germany),November 1963.
- Schlüsselstreifengeber HL 502 - Betriebsanleitung
Operating instructions (German).
Dr.-Ing. Rudolf Hell, Kiel (Germany), March 1962.
- Schlüsselstreifengeber HL 502 - Betriebsanleitung und Wartungsvorschift
Operating instructions and maintenance instructions (German).
Dr.-Ing. Rudolf Hell, Kiel (Germany), May 1963.
- HL-502 circuit diagrams
Hell, 30 October 1961, updated 14 March 1962.
- Herstellung der Lochstreifenschleifen für HL 502
Creation of perforated tape loops (German).
Hell. No date, but probably 1962.
- Schlüsselstreifengeber HL-502 (Gerätesatz) - Teil 1, 2, 3
Technical Description - Part 1, 2 and 3 (German).
TDv 5810/005-13. 27 November 1963, updated August 1968.
- Schlüsselstreifengeber HL-502 (Gerätesatz) - Teil 4
Technical Description - Part 4 (German).
TDv 5810/005-40. 27 November 1963, updated January 1969.
- Schlüsselstreifengeber HL-502 (Gerätesatz) - Teil 5 - Ersatzteilliste
Technical Description - Part 4 - Spare parts list (German).
TDv 5810/005-40. 27 November 1963, updated January 1969.
- T.Loch.17 circuit diagrams
Siemens, 11 February 1958.
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- Klaus Kopacz, HL-502 history and documentation - THANKS !
Crypto Museum, October 2026.
- HL-502 in Bundeswehr Study Collection
Feldafing (Germany), Date?.
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© Crypto Museum. Last changed: Monday, 26 October 2026 - 10:19 CET.
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