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KG-40 and KG-40A
Half-duplex Link 11 data encryptor - wanted item

KG-40 was a half-duplex cryptographic device that provided data protection for the US Navy's Link 11 system and other systems that met the Tactical Data Information Link A (TADIL-A) data standards. It was developed in the US in the mid-1970s and was used by the US Department of Defence (DoD) and by NATO. In 1991, the KG-40 was succeeded by the improved KG-40A.
 
The original KG-40 supported only 128-bit keys for which nearly every common DS-102 and DS-101 compatible FILL device, such as the popular KYK-13, could be used. In 1991, the KG-40 was followed by the KG-40A, which provided better cryptographic security by the implementation of an improved cryptographic algorithm.

At the same time, the existing KG-40 units were modified by installing the KG-40 MOD upgrade, involving the replacement of the motherboard and the front panel assembly. The new KG-40A units were manufactured by SPAWAR (US Navy).
  
KG-40A (right) and KGX-40 remote control unit (RCU).

After the upgrade, the KG-40A supported a variable key length that was larger than the initial 128-bits. As a result, the KYK-13 key transfer device and and the KYX-15 Net Control Device (NCD) could no longer be used 1 and the KOI-18 or CYZ-10 devices had to be used instead. When unkeyed, the KG-40 and KG-40A are an UNCLASSIFIED Controlled Cryptographic Items (CCI).

The KG-40A is approved by NATO SECAN for classified information up to the level of COSMIC TOP SECRET [2]. At present we have no better image of the KG-40 or the KG-40A. In 2006, the US Navy started looking for a replacement for the ageing KG-40A units and started a procurement procedure. Eventually in 2012, the order was given to Ultra Electronics in the UK, resulting in the development and delivery of the KG-40A Replenishment (KG-40AR) [1]. This is the first time in history that the US DoD has procured a Type 1 cryptographic device from an offshore company.
 
  1. It is sometimes erroneously suggested, even on renowned website such as [4] and [5], that the KG-40A introduced a key length of 128 bits and that after this, the KYK-13 fill device could no longer be used. Everyone seems to quote from the same source [4] however. This can not be correct, as the KYK-13 does support a maximum key length of 128 bits. This means that the original KG-40 probably used a 128-bit key, whilst the later KG-40A required a longer key.

Help required
If you have additional information or pictures of the KG-40 or the KG-40A, please contact us.
 
References
  1. FBO, KG-40A Replenishment (KG-40AR) COMSEC devices
    10 April 2012. Retrieved March 2015.

  2. NATO Information Assurance, KG-40A
    Retrieved March 2013.

  3. Federation of American Scientists (FAS), KG-40/KG-40A
    Website. Retrieved March 2015.

  4. Jerry Proc, KG-40/40A
    Retrieved March 2015.

  5. Thor A. Simensen, Link 11 communications (thesis)
    March 1992. Unclassified. Approved for public release. Retrieved March 2015.

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Crypto Museum. Last changed: Sunday, 01 March 2015 - 16:31 CET.
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