Click for homepage
← UK
BID/2510 →
KY-57 →
BID/250   Lamberton
Wideband secure voice encryption - wanted item

BID/250 was a wide-band voice encryption/decryption module developed by Communications Electronics Security Group (CESG) in Cheltenham (UK) in the late 1960s or the early 1970s. It was built by CESG and by QINETIC in Malvern (UK). BID/250 was used in combination with the Digital Master Unit (DMU) of a UK Clansman VRC-353 radio and implemented the highly secret SAVILLE encryption algorithm, developed by the British GCHQ and the American NSA [1]. BID-250 is also known as Lamberton and, amoung others, by its stock number NSN 5810-999-645-0301.

The DMU allowed speech and data to be sent at 16 Kb/s over wide-band (8 kHz) VHF and UHF links. Secure conversations were only possible if the BID/250 crypto module was present inside the DMU. The crypto module replaced the blue cover plate at the center of the DMU front panel.

Cryptographic keys were loaded into the BID/250 by means of a standard key-fill device, such as the KYK-13 or the KOI-18. Due to the fact that the BID/250 used the SAVILLE crypto algorithm, it was crypto compatible with the American KY-57 (and with the later KY-99).

The units had an extremely long life span. Although BID/250 was introduced in the early 1970s, some versions of it were still in use in 2009. From 2009 onwards [2], existing BID/250 units were gradually phased-out when Clansman was replaced by the new Bowman communications system. In December 2014, BID/250/11 was replaced by the compatible software-defined BID/2510/16.

DSU with BID/250 simulator installed [3]
Operating the BID/250 simulator [3]
Close-up of the BID/250 simulator [3]
DSU without BID/250, replaced by blue blanking plate. (source unknown)
BID/250 removed from the DSU (source unknown)
1 / 5
DSU with BID/250 simulator installed [3]
2 / 5
Operating the BID/250 simulator [3]
3 / 5
Close-up of the BID/250 simulator [3]
4 / 5
DSU without BID/250, replaced by blue blanking plate. (source unknown)
5 / 5
BID/250 removed from the DSU (source unknown)

A complete Clansman radio station, for example as it was built inside an FFR Land Rover, consists of a VRC-353 VHF 50 Watt radio plus a Digital Master Unit (DSU). The latter is actually a switch box that allows speech and data to be routed through the (optional) BID/250. The images below show how the rather small BID/250 is installed in the rectangular slot at the centre of the front panel. It is locked in place by a black hinged metal frame with a locking knob at the right.

Under normal conditions, the BID/250 unit was not installed and the blue blanking plate that replaced it, showed that secure operation was not possible. This was done to avoid the risk of losing the precious encryption module. Only in actual battle missions, a crypto officer would install the BID/250 unit and load it with the appropriate keys. Once the mission was over, the encryption unit was removed and replaced by the blue blanking plate again.

During training missions, the BID/250 was often replaced by a simulator. It had the same size as the BID/250/1 and BID/250/11 units and allowed the use of digital communications without providing the encryption/decryption capabilities. The simulator is further explained below.

Block diagram
The diagram below shows how the BID/250 was used. At the right is a typical VRC-353 Clansman transceiver, consisting of a transmitter (TX) and a receiver (RX). The unit at the left is the Digital Master Unit (DMU), which is in fact a switch box that can pass the analogue audio signals straight through to the transceiver, or route them through the (optional) BID/250 at the heart of the DMU.

Inside the BID/250, the analogue microphone signal is first converted to a digital signal using Continuous Variable Slope Delta modulation (CVSD). The digital signal is then encrypted with a Cipher Text Auto Key (CTAK stream cipher), after which the signal is fed into the wide-band 'X' input of the transmitter. In receiving mode, this works in reverse direction. Due to the nature of the CTAK stream cipher, also known as CFB, 1 the crypto-unit is fully self-synchronising.

The BID/250/1&11 Simulator consists of a metal enclosure with the same form factor as the real BID/250. It can be installed in the DMU instead of the BID/250 for training purposes. In this case, the digital CVSD signal is fed directly into the wide-band input of the transceiver. Although the result is similar as with the encryptor in place (the user will just hear noise) the signal is not actually encrypted. During training missions this prevented the risk of losing real BID/250 units.

  1. CFB : Ciphertext Feedback. Also known as 'Autoclave'.

Model NSN Description
BID/250/1 5810-99-645-0301 Encryption device
BID/250/11 7050-99-500-1818 Encryption device (later version of the above)
BID/250/41 6130-99-776-9599 Power Supply Unit
BID/250/42 ? Probably another type of Power Supply Unit
BID/250/1&11 6625-99-633-6358 Simulator Assembly
BID/250/11 POUCH 5810-99-651-7544 Carrying bag (radio)
BID/2510/16 ? Replacement for BID/250/11
Wanted item
At present Crypto Museum has no BID/250 Lamberton unit in its collection, which is why we are currently unable to show an image of it. We would very much like to find a complete unit and/or any kind of information about it. If you think you can help, please contact us.

  1. Crypto Museum, The SAVILLE Encryption Algorithm
    Some characteristics of SAVILLE as described by a former cryptographer.
    Interview at Crypto Museum, December 2011.

  2. DIN Digest for March 2009, Declaration of Obsolescence of BID/250/41 and 42

  3. Ian Moffat (G0OZS), Photographs of BID/250/1&11 simulator
    Retrieved February 2015. 1

  4. CESG, BID/2510/16
    Retrieved Febrary 2015.
  1. Website no longer available (2024).

Further information
Any links shown in red are currently unavailable. If you like the information on this website, why not make a donation?
Crypto Museum. Created: Friday 27 February 2015. Last changed: Thursday, 04 January 2024 - 09:10 CET.
Click for homepage