Editing NSA encryption systems (section)

=== Second generation: vacuum tubes ===

[[File:Kw-26.jpg|thumb|An array of [[KW-26]] cipher devices]]

Second generation systems (1970s) were all electronic designs based on [[vacuum tube]]s and transformer logic. Algorithms appear to be based on [[linear-feedback shift register]]s, perhaps with some non-linear elements thrown in to make them more difficult to cryptanalyze. Keys were loaded by placing a [[punched card]] in a locked reader on the front panel.<ref name="klein">Melville Klein, "Securing Record Communications: The TSEC/KW-26", 2003, NSA brochure, p. 4, [https://www.nsa.gov/about/cryptologic-heritage/historical-figures-publications/publications/misc/assets/files/tsec_kw26.pdf (PDF)]</ref> The cryptoperiod was still usually one day. These systems were introduced in the late 1960s and stayed in use until the mid-1980s. They required a great deal of care and maintenance, but were not vulnerable to EMP. The discovery of the [[Walker spy ring]] provided an impetus for their retirement, along with remaining first generation systems.