U.S. KL-7 Crypto Machine - Last Details Unveiled

The ingenious sliding switch
Image © Dirk Rijmenants

We documented many technical details of the fascinating TSEC/KL7 crypto machine in 2011. Although we already knew how the KL-7 worked, the details on how the machine actually achieved this electronically and mechanically are only fully understood after the recent release of all technical documents, and these kept surprising us.

The KL-7 rotors, their flexible settings and complex irregular stepping, the use of electronics and the ingenious keyboard made it a cryptologic marvel when it was introduced by the National Security Agency (NSA) in 1953.

The compact printer
Image © Dirk Rijmenants

However, three important design features stretched the imagination of the ASA and AFSA engineers: using electronics to put their ideas into hardware, design a compact printer mechanism, find a flexible way to power the KL-7,  and all these properties in a small 12 by 12 inches (30 x 30 cm) machine, weighing a mere 20.5 lbs (9,3 Kg).

The circuitry and various timing signals to control its mechanics and operate the KL-7 required only four vacuum tubes. To power the KL-7, the machine uses 24 volts to run a DC motor that drives an AC generator, which in turn provides all voltages, from -70 to +220 volts, for the tubes.

The ingenious puls generator
alphanumeric double pulse

That same motor also drives a pulse generator that provides accurate timing to control a rotating drum printer, and the vacuum tubes ensure precise printing of alpha-numeric characters, all securely encrypted.

Only recently we had the chance to study the plans and schematics in detail to discover how all this was actually achieved, and it took some time to grasp some of the details. It seems as if they found completely new solutions, then encountered some problems with those solutions, and devised more complicated solutions to those problems.

The Pulse generator explained

A circuit with thyratron and double triode vacuum tubes, and the clever use of resistor networks that influence various voltages on the tube grids, requires perseverance to be understood.

As of today, these newly uncovered details are available on the website. You will find several new and updated drawings, photos and thorough detailed explanation about various parts of the machine. All you wanted to know about the KL-7 but were afraid to ask...

Examine the machine, its history of development, how it was used and the simulator at the Cipher Machines and cryptology.

More on the KL-7

• TSEC/KL-7 ADONIS & POLLUX all tech details and history.
  
• TSEC/KL-7 Simulator an accurate and realistic simulation.
• More related blog posts about the KL-7, its Cold War history, and the KL- simulator and its development.

Updates May-August 2022 - Added more images and details on processing and timing signals, expanded history and use of KL-7 in Vietnam, compromises and spy cases, additional documents Army Security Agency on Vietnam and use of the machine by FBI, CIA and asian countries.