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A-6723
Key Tape Perforator - wanted item

The A-6723 is arguably the most well-known machine for producing truely random 5-level (Baudot) punched paper tapes, also known as One-Time Tapes (OTT), built by Mils Elektronik in Mils (Austria) from 1967 onwards as the successor to the earlier valve-based Reichert Elektronik 5224 key generator. The tapes are intended for used with the so-called mixer machines.

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.
  
The 6723 tape duplicator

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.

The 6723 tape duplicator Tape puncher Close-up of the 2134 optical tape reader Optical tape reader (open) Index printer Paper leaving the index printer Tape separation mechanism Connections at the left panel

Controls
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).

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.
  
Tape puncher

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.

Development
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.

Synchronous operation
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).

Mixer machines
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.

<i>Mixing of the <b>plain text</b> and the <b>key</b></i>

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.

References
  1. Mils Elektronik, A-6723 Sales Leaflet
    Date unknown, but probably late 1960s. 1

  2. Werner Liebknecht, German Patent DE958933
    C. Lorenz AG, Stuttgart (Germany). 8 June 1952.

  3. Eberhard Scholz, Austrian Patent AT370931
    System, bestehend aus mehreren, jeweils zur herstellung eines
    Schlüssellochstreifenpaares eingerichteten Streifenlochern (German).
    Mils Elektronik GmbH. 23 January 1980.

  4. Reichert Elektronik, Fotoleser 2134
    Date unknown. Retrieved July 2013.
  1. Document kindly supplied by Mils Electronic.

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