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It was developed in the 1970s by a Norwegian company, Standard Telefon og Kabelfabrik (STK). It used digital electronics for encryption instead of rotors, and it may have been the first machine to use software based crypto algorithms. KL-51 is a very robust machine made to military specifications.
U.S. National Security Agency bought it in the 1980s to replace the earlier KL-7. As of 2006, the U.S. Navy was developing plans to replace KL-51 units still in use with a unit based on a more modern Universal Crypto Device.
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