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Full-duplex on-line cipher machine - not in collection

BID/610 was an fully transistorised on-line cipher machine, developed between 1962 and 1965 by Plessey in Liverpool (UK). The machine was introduced to the British Army in 1965 and was later also used by the Canadian forces and by NATO. BID/610 was approved for messages up to the level of TOP SECRET and was used until the mid-1980s. The machine is also known as ALVIS.
ALVIS could be used in two modes of operation: (A) asynchronous - this mode was suitable for fixed (telephone) lines and point-to-point radio links - and (B) synchronous, which provided full Traffic Flow Security (TFS) but required the extra BID/700 synchronizing unit to be installed too.

BID/610 was a rather large device that took up most of a full-height 19" rack. The image on the right shows a typical BID/610 system complete with power supplies and the optional BID/700 synchronizer (at the top). The image was taken by Henrik Teller at the Danish Signal Regiment Historical Collection in Fredericia (Denmark) [2]. The arrangement shown here is for full-duplex.

The machine was developed by Plessey Crypto in Liverpool (UK) for a NATO evaluation under the name TROL, which stands for Tapeless Rotorless On-Line. Development was started in 1962 and by 1965 the machine was ready for evaluation. NATO accepted the machine and turned down the competing TROL offer from Philips Usfa BV.

In many respects, BID/610 can be seen as a direct competitor of the US KW-7 (Orestes), although the latter did not offer Traffic Flow Security. Around 1988, NATO replaced both systems by the much smaller BID/950 (HORA), which was also made by Plessey Crypto.

 More about BID/700 (VENDOR)
Photograph by Henrik Teller [2], obtained via Jerry Proc [1].

BID/610 can be used in several configurations or arrangements, such as simplex, L.L.K. simplex and full-duplex. The device is suitable for teletype signals with a speed of 45.5, 50 or 75 baud and accepts an input data format of 7 or 7½ bit words. Only the 5 data bits are encrypted.
Foreign Office Canada
The image below was taken in a communications room of the Canadian Foreign Office, probably in the 1960s [4]. It shows ten full-height 19" racks that each contain a full-dubplex BID/610 setup. Only the first two racks have a BID/700 unit fitted at the top. It is believed that only these two units were used in MODE B (sync). The other racks have blanking panels in that position.

Modes of operation
  1. Non-synchronous operation
    This is a non-synchronous mode, suitable for operating on landlines. The key generator is filled from an internal noise source and the fill is transmitted to the recipient. There is no Traffic Flow Security (TFS) in this mode [A] which is also known as start/stop mode.

  2. Synchronous operation
    This is a synchronous mode, that allows operation over HF radio links and provides full Traffic Flow Control (TFC). It requires the use of an external synchronization unit, such as the BID/700 (Vendor) when working over HF radio, or the BAM/650 in case error detection and correction (EDC) units are involved. In this mode, the fill is not transmitted to the receipient, but is obtained from a one-time pad and is entered manually at either end [A].

  • BID/650/2
    Power Supply Unit
  • BID/610/22
    Power Supply Unit
  • BID/610/1
    Original cipher unit
  • BID/610/15
    BID/610/1 with modified plug field
  • BID/610/21
    Cipher unit (later version)
  • BID/610/3
    Receive Drive Control Unit
  • BID/610/23
    Receive Drive Control Unit (replacement for BID/610/3)
  • BID/610/6
    Test equipment for noise generator (test every 6 months)
  • BID/700
    Synchronizer (optional)
  • BID/720
    Line isolator (filtering and level conversion) (ITL)
  • BID/740
    Replacement for BID/720 based on fiber optics
  • BID/750
    Mains filter unit (TEMPEST)
Normal simplex arrangement
The block diagram below shows a standard simplex BID/610 setup. At the left are the inputs from the 7-unit input devices. At the right is the outgoing line the uses ±80V levels. All logic works at +8/+2V levels.

L.L.K. simplex arrangement
The block diagram below shows a similar setup with ±6V low-level logic, or Low Level Keying (LLK). Note that in this case, the Control Unit (CU), the PSU and the ITL are all different from the ones above. The BIC/740 converts the ±80V line levels to the required ±6V logic levels.

Main Cipher Unit   BID/610/1
The simplified block diagram below shows the operation of the Main Cipher Unit (MCU) in MODE A. At the bottom left is the internal noise generator that produces truely random bit pattern, which is derived from a noise diode. To ensure that the noise generator keeps producing purely random bit streams, the board has to be tested every 6 months with the BID/610/6 test unit.

The noise generator is used for the initial fill (when in mode A), and for the Alarm Test. Another crucial part of the device is the actual key generator which is also colour red in the diagram above. The key generator is of the Output Feedback (OFB) principle, and is based on two non-linear feedback shift registers (NLFSRs), known as P and Q. In MODE A this works as follows:

The cipher text output is fed into shift register P, which has several 'taps' all under control of the combining logic and the current key fill. The output of the combining logic is fed into shift register Q, which is also configured by the current key fill. The output of the key combining logic of register Q is fed into an integrator that produces the actual KEY, which is then XOR-ed with the plaintext in order to obtain the ciphertext. The combining logic is extremely complex and was modified several times during the life cycle of the BID/610, e.g. the BID/610/15 'alpha' mod.

In MODE B (synchronised operation), the configuration of the P and Q registers is altered and an extra register P' is used to feed register P. The extra register P' is also known as the Register P extension. The output of the P combining logic is XOR-ed with the ciphertext and fed into Q.
Alpha modification   BID/610/15
When the Alpha Modification was introduced (BID/610/15), the P Register Extension, that was previously only used in mode B, was used for mode A as well. At the same time, the length of the P' register was extended with one extra stage. This increases the length of the P-register from 30 to 47 stages, which no longer shares a common factor with the 24 stages of the Q register.

At the same time, the number of FILL characters was extended to 16, to ensure that the KEY generator got completely filled on initialisation. The above changes were not applied to mode B, which kept using the existing scheme.
  1. Technical notes on BID/610, BID/700 and BAM/650, Part 1
    Part of technical training course in 1975.
    Secret/Confidential notebook 1 . CM-301917-C [3].

  2. Technical notes on BID/610, BID/700 and BAM/650, Part 2
    Part of technical training course in 1975.
    Secret/Confidential notebook 1 . CM-301918-C [3].

  1. Declared unclassified by the author in 1993.

  1. Jerry Proc and contributors, BID/610 (Alvis)
    Retrieved May 2016.

  2. Henrik Teller, Image of rackmount BID/610 and BID/700
    Photographed at the Signals Regiment Historical Collection, Fredericia (Denmark).
    Retrieved May 2016 via [1]. Reproduced here by kind permission.

  3. Anonymous contributor, Technical training notes on BID/610 and BID/700
    Declassified in 1993. Obtained March 2014.

  4. Old Foreign Affairs Retired Technicians, Photograph of BID/610 units in Canada
    Website. Retrieved May 2016 from Ray Fortin, via [1].

Further information

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Crypto Museum. Created: Monday 30 May 2016. Last changed: Friday, 30 December 2016 - 09:09 CET.
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