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← Mils OTT
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Key Tape Perforator
- wanted item
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The machine consists of four major parts: an (optional) noise generator
(at the bottom left), a power supply (bottom right) an (optional) set of counters
at the centre to check the randomness of the keys, and a double paper tape
puncher at the top.
Two fresh reels of paper tape are fed in from the right and exit
the machine on the left.
The image on the right shows a typical A-6723 as it was used in many countries
well into the 1990s. It became obsolete when paper tape was no longer
used for the storage and transmission of messages and telex was gradually
phased out.
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Telex-based systems typically use 5-level paper tape for the storage of
information (e.g. text) and key material. This 5-level code is known as
ITA-2,
but is commonly called Baudot Code.
When creating One-Time Tapes (OTT)
it is important that extactly two
(no more and no less) paper tapes are punched with exactly the same
information. In the A-6723, this is solved by feeding two paper tapes
through a single puncher, so that they are punched simultaneously.
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Over the years, the design of the A-6723 has not changed much, and is
nearly identical to that of its predecessor. The image below identifies
the various components. The actual noise generator that is used for
producing the random key codes is at the bottom left. As it was also
possible to use the A-6723 in a cascaded setup,
it could also be used with an external noise generator.
It is of vital importance that the key stream generated by the
machine is truely random and evenly spread. With a 5-level code,
the number of possible characters is 32 (25), and each of
the (32) possible codes should appear on the key tape equally often
as the others. This is achieved by using five noise generators;
one for each hole (bit) of the 5-level code. Two sets of counters
are used to check the randomness of each of the 5 noise generators
(before and after punching).
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A separate counter shows how many random characters
have been generated in total.
Two blank paper tapes are fed into the machine from the right.
They both enter the tape puncher
at the right and are punched simultaneously so that they are
guaranteed to be identical.
After punching, the tapes are checked by the
optical tape rader
at the center. This reader was also built by Mils and replaced
the mechanical Siemens unit that had been used previously.
The reader checks whether the correct holes have indeed been
punched and drives the counters.
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From the reader, the two tapes are passed through an
index printer
that stamps insertion markers at 50, 100 or 200 character intervals,
which is useful when synchronizing the cipher machines at either end of
a communication link. Leaving the printer
at the left, the two paper tapes are separated
and wound onto the two empty reels at the left.
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The design of the A-6723 is based on the valve-based
5224 key generator
that was developed and built by
Reichert Elektronik in Trier (Germany)
in the mid-1950s [2]. In the early 1960s, Eberhard Scholz, head of development
at Reichert-Elektronik, redesigned the machine and replaced the valves (tubes)
by transistors that had just become available.
As Reichert-Elektronik was the predecessor of Mils Electronic, the design
was inherited when the company moved to Austria in 1967. The A-6723 appeared
in sales brochures for more than 30 years and probably had the longest
life-span of all Mils products. It was used by governments and armies
world-wide for the production of key tapes for their OTT cipher machines.
In some countries the machines were used well into the 1990s. The devices
were gradually phased out when teleprinter machines (telex) were more and
more replaced by computers.
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When using One-Time Tape systems (OTT) it is mandatory that only two
copies of a tape exist, that they are only used once, and that they
are both destroyed immediately after use. Only then can the confidentiality
of a message be guarateed permanently. As OTT is used by governments and
armies for messages that have to be kept secret indefinitely
(e.g. communication between embassies), a large supply, and hence an enormous
production capacity, might be needed.
One way of increasing production throughput is by using the synchyronized
system shown in the drawing above. This so-called electronic wave
was invented and patented by Eberhard Scholz of Mils Elektronic in 1980 [3]
and allows up to five A-6723 machines (WL1-WL5) to be connected to a single
noise source (ZG) via a central control unit (ST). In this case the first
machine (WL1) is used as a master to which the other machines are slaved.
The format of the output can be controlled via a programme tape reader (PL)
that is connected to the master machine (WL1).
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The random key tapes produced by the A-6723 are suitable for use with
virtually any type of telex-based mixing cipher machine (mixer), such as
the Siemens T-43,
the ETCRRM
(used on the America-Moscow Hotline),
the Hagelin TC-52,
the Siemens M-190,
the Philips Ecolex 4
and Mils' own mixers like the ME-640
and the ME-840.
For a complete overview, click here.
The above illustration explains how a mixer works.
Eacht letter from the Plain-text is added to a letter from
a key tape, using an exclusive-OR (XOR) operation
(sometimes called 'module-2 addition).
The advantage of this operation is that it is reversable:
adding the key stream to the cipher text, reveals the plain text again.
The A-6723 is used for generating the red (key) tapes.
For a detailed description of this principle,
read our page about the Vernam Cipher.
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© Crypto Museum. Created: Tuesday 11 June 2013. Last changed: Saturday, 24 February 2018 - 13:45 CET.
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