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US rotor-based cipher machine

KL-7 was an electro-mechanical rotor-based off-line cipher machine, developed by the National Security Agency (NSA) in the US. It was introduced in 1952 and served for many years as the main cipher machine of the US and NATO. It is relatively light-weight (9.3 kg) and is basically a more advanced version of the German Enigma machine. It was a replacement for SIGABA (ECM Mk-II) and in some countries, such as the UK and Canada, also for the British Typex and CCM machines.
The initial name for the machine was AFSAM-7, but changed to TSEC/KL-7 in the early 1960s. It is also known by key-procedure codenames ADONIS (high-level) and POLLUX (low-level). KL-7 was withdrawn from service in 1983.

Unlike Enigma, the KL-7 has eight rotors, seven of which are moved in a complex irregular stepping pattern. The machines came in several variations and were used by the US Army, Navy and NATO for many years. They were also used for communication by Foreign Affairs.

Unfortunately, KL-7 is still a classified item and only few of them have survived. Most machines that are on public display today, have been 'sanitized', and all wiring has been removed.
KL-7 removed from the transit case

Despite the secrecy surrounding the KL-7 and its history, the mystery is gradually being unravelled as the NSA releases more and more historical documents and researchers manage to uncover the technical details of the machine. As a result, a very realistic computer simulation (for Windows) of the KL-7 has been created in 2011 by crypto-historian Dirk Rijmenants in Belgium, and in 2013 a JAVA-based KL-7 simulator by MIT (US), both of which can be downloaded below.
The KL-7 was introduced in the 1950s and remained in service well into the 1970s, when it was gradually phased out. In some countries, KL-7 machines were kept for special purposes and as backups for many years, until they finally were officially withdrawn from service in 1983. The last (unclassified) message was sent on 30 June 1983 by the Canadian Navy. KL-7 was replaced by a range of electronic machines, such as the KW-26, the KW-37, the KL-51 (RACE) and Aroflex.
KL-7 removed from the transit case KL-7 cipher machine Close-up of the keyboard and the rotor basket The drum base after the rotor basket is removed The plain drum basket with the rotors still in it, viewed from the bottom One side of the rotor basket removed and two wheels taken out Close-up of the fixed wheel (unfortunately, the spring-loaded contacts are missing) The thyratrons at the rear of the unit, used for the motor driver circuit

A complete KL-7 machine consists of the following basic components:
  • KLB-7
    This is the base unit that contains the motor, the generator and the electronics (valves or vacuum tubes).

  • KLK-7
    The eight rotors of the KL-7 are mounted together on a spindle inside a drum or rotor basket. The basket can be removed from the machine by releasing two levers; one at either side of the basket.

  • KLA-7
    The complex stepping unit, that senses the notches on the 7 movable rotors and controls the stepping motion of the other rotors.

  • KLX-7
    Input/output interface for 5-level (teleprinter) data. Consists of two units: one that is installed between the keyboard and the base unit, and one (the actual interface) that in mounted behind the rotor basket.

  • EZ-KL7
    Expansion unit, developed by the German Army (Bundeswehr), to allow two KL-7 units to run in tandem, in order to detect errors. The EX-KL7 is mounted in front of the keyboard. Requires the KLX-7 option to be present.

Block diagram
KL-7 was an electro-mechanical rotor-based cipher machine driven by electronic circuits with valves (vacuum tubes). The machine is powered by an external 24V DC source, such as a PSU or the battery of, say, a truck. Timing of the machine is provided by a complex mechanical unit with several rotating parts coupled by a common axle. The block diagram below shows how this is done. The main 24V motor runs at 6600 RPM. It drives the mechanical parts as well a an AC generator that provides the 400V to drive the valves.
Stepping unit (KLA-7) Rotor Basket (KLK-7) The keyboard of the KL-7 Mode Switch Base Unit KLB-7 Pulse Generator Printer TSEC/KL-7 (ADONIS) Block Diagram (version 0.90)

Pressing a key on the keyboard, grounds one of 26 lines that is routed via the mode-switch, through the coding wheels, to one of the 26 coils of the pulse generator. The pulses from the pulse generator are used to drive the printer. As all rotating parts (DC motor, AC generator, pulse generator, printer and stepping unit) are coupled, timing is guaranteed.
Base Unit   KLB-7
The KLB-7 is the actual chassis on which the machine is built, including the electronic circuits and the (mechanical) gearbox. The latter consists of the motor, the timing unit, the printer etc. Note that the KLB-7 is not (and never was) a classified item. Apparently, the electro-mechanical base unit was not considered to reveal any cryptographic secrets.
Rotors   KLK-7
The rotors of the KL-7 resemble those of the famous German Enigma machine and other wheel-based cipher machines. Each rotor has a series of flat-faced contacts on the right side, and the same number of spring-loaded contacts on the left. It also has an adjustable index ring with the letters of the alphabet on it, and an inner core which connects the contacts on one side with the contacts on the other side. There are some significant differences with the Enigma however.

First of all, a KL-7 rotor has 36 contacts, whereas an Enigma wheel has 26 contacts. Of the 36 contacts, 26 are used for the encryption of the 26 letters of the alphabet. The remaining 10 contacts are looped back to the input (see below). This results in a re-encipherment of part of the text. Each wheel has an index ring with 36 positions, each separated by a narrow gap. Only 26 positions are identified with one of the letters of the alphabet. The rest is empty. When unfolded, the index ring looks like this:

Another important difference, is the omission of the reflector (Umkehrwalze). In encoding mode, one side of the rotor basket is the input and the other side is the output. In decoding mode, all contacts are swapped, so that the output becomes input and vice versa. This has the advantage that, unlike on Enigma, a letter can become itself on a KL-7. Swapping all contacts however, does involve a rather complex multi-contact switch, which is integrated with the KL-7 keyboard.

The Drum or Rotor Basket of the KL-7 consists of a metal cage with 8 wheels on a spindle (KLK-7). The forth wheel from the left is fixed in position. It never rotates and hence does not have a window to show its setting. This wheel is sometimes referred to as the NSA rotor. For each of the other 7 wheels, a window is present in the cage. Through this window, three successive letters of the wheel are visible. The topmost letter visible through the window, represents the current setting. This position is indicated by a white line from left to right.

Each KL-7 machine was supplied with 12 rotors, marked A-L, in a metal box. The L-rotor was the so-called stationary NSA wheel, that was used in position 4. Of the remaining 11 rotors (A-K), 7 would be placed in the rotor basket on a given day, in a particular order, as per cipher instructions (codebook).

Exploded view of the rotor basket

The user would remove the rotor basket from the machine by releasing two levers; one at either side of the basket. Once removed, the rightmost end-plate of the basket can be removed by releasing a pawl that locks it on the spindle. After removing the rightmost end-plate, the rotors can be taken from the spindle. The spindle itself stays in the basket at it is fixed to the leftmost end-plate, that in turn is fixed to the cylindrical basket (see image above).
British Singlet   BID/60
The rotors of the KL-7 are identical to those used with the British Singlet (BID/60). It is quite possible that the cipher wheels were a joint US/UK development, or that the Americans allowed the British to use the KL-7 rotors in their own cipher machine. In any case where the Singlet wheels manufactured in the UK.

The British Singlet had 10 cipher wheels (rather than 8) but was reported to be interoperable with the KL-7 by using the same eight wheels plus two dummies (i.e. wired straight through).

 More about Singlet
Singlet (BID/60)

Rotor wiring
Each KL-7 wheel contains 36 wires which connect the flat-faced contacts from one side with the spring-loaded contacts at the other side, in a seemingly 'random' fashion. The wiring of the KL-7 rotors has always been kept secret, but whether or not this makes sense, remains to be seen.

For security reasons, it was forbidden to trace the wheel wiring of the KL-7. Even technical repair personnel was not allowed to trace each individual contact for a faulty connection. They were only allowed to place the spring-loaded contacts on a conducting (metal) surface and test each flat-faced contact for continuity only. This way, the wiring would not be revealed. Faulty rotors were never opened in the field, but had to be returned to the NSA for repair [11].

If you would happen to find a KL-7 now and trace the rotor wiring, it wouldn't be of much use, as the rotor wiring was different for many of its users. Furthermore, the wiring was changed frequently for safety reasons. Nevertheless, the Russians managed to read a significant part of the US Navy Submarine Command KL-7 traffic for many years (see below).
Gear box
At the heart of the KL-7 is a very compact, yet complex, mechanical unit. It consists of a DC motor, and AC high-voltage generator, a printer, a pulse generator and a timing unit. All components are driven by the DC motor, either directly, or through a 3:1 cog-wheel reduction.

The motor and the generator are mounted on the same axle, rotating at 6600 RPM (rotations per minute). Through a 3:1 reduction, the pulse generator and printer are driven, making them rotate at 2200 RPM. Using a further cog-wheel reduction, the Timing Unit is driven.

Unlike the other components, the Timing Unit does not rotate continuously. Instead, a cluch, driven by the electronics, is used to couple it to the main axle, after which it will complete one full revolution. Whilst doing this, a set of 4 cam-controlled switches provide the timing signals for the electronics. The Timing Unit also drives the KLA-7 Stepping Unit (and hence the rotors), and the paper feed. On each revolution, the rotors can be advanced by one position.
The complex unit is housed in the left half of the KL-7, as shown in the image on the right. The motor is at the rear of the unit (at the right in the image). Imediately before the motor is the generator (sometimes called invertor), with two large bolts at the top.

The printer is the the other end of the unit (left in the picture). The black cap protects the print head and the ink ribbon against dust.

Gear-box Model plate of the gear-box, entitled 'Inverter' Left view of the KL-7, giving a good view of the Inverter, just behind the bracket View of the black cap shielding the printer. The print head is just visible at the bottom. Close-up of the print-head solenoid

Stepping Unit   KLA-7
The rotors are held in position by a locking lever (1). This is a spring-loaded arm that reaches under the wheel from the rear. At the end of the arm is a small sharp notch, that locks into a narrow rig (gap) between the index letters on the circumfere of the wheel. Further towards the front, at the bottom of the rotor basket, is the transport notch (2). These notches are driven by the main gear and lock into the same gaps on the index ring. They move forward to rotate the wheel to the next position. On each key-press a rotor can only make a single step.
Whether or not a rotor moves when a key is pressed, depends on the presence or absense of a notch on the stepping ring of one of the other rotors. The stepping ring of each rotor is sensed by a switch (3) towards the front of the basket.

Please note that the switches sense the stepping ring 10 positions further on the circumfere of the rotor. In other words: when the rotor is at A (visible in the window at the white line), the notch of position H is sensed. The switch in turn activates a solenoid (L1 thru L7) that allows the rotor to be moved by the main gear.
KL-7 Stepping mechanism after the rotor basket has been removed

When setting they daily key, the starting position of the rotors can be changed manually by pressing the keys (4) whilst in plain-text mode (P). When the key is pressed briefly, the rotor advances a single step. Holding the key down, makes the rotor step continuously.
KL-7 removed from the transit case KL-7 without the rotor basket KL-7 Stepping mechanism after the rotor basket has been removed Close-up of the manual stepping switches Close-up of the rotor movement and locking levers Close-up of the stepping sense switches Changing the setting of the wheels (here advancing wheel 3)

Rotor stepping
Rotor movement control is complex, but is fixed by the internal wiring. Although details about the rotor stepping mechanism have never been published, it is possible to deduce the wiring of the switches, simply from observing rotor movement. Two European researchers who wish to remain anonymous, recently sent us a table with rotor movements as they have observed them. From this table, we have been able to reconstruct the diagram below:

KLA-7/TSEC Circuit Diagram

Please note that the sensing switches at the top are in the proper order (1 thru 7), but that the order of the manual stepping switches and the solenoids is mixed here. This is done to make the circuit diagram less cluttered. The stepping unit as present here is implemented identically in Dirk Rijmenants' KL-7 Simulator and it has since been confirmed by people with access to a working KL-7, that it is indeed correct.
The keyboard of the KL-7 is part of the KLB-7 base unit. It consists of 29 green keys and a black space bar. It has the standard QWERTY layout divided over three rows. The numbers are shared with the top row. At the bottom right are 3 special keys marked LET, FIG and RPT.
Each key is in fact an electric switch, consisting of a contact and a spring, mounted below the key. Whenever a key is pressed, the contact is grounded (i.e. connected to the 0V rail), allowing the pulse-generator to issue a pulse. The keyboard interior is visible in the images below.

When entering numbers, the user first needs to press the FIG-key (figures). This acts like the shift-key on a modern computer. As long as the machine is in numbers-shift mode, a large neon lamp behind the keyboard is lit. When reverting to letters, the LET-key has to be pressed first.
Operating the keyboard of the KL-7

Some KL-7 machine have been upgraded with the KLX-7 input/output interface. This option consist of two parts: a contact unit that is mounted between the keyboard and the base, and the actual KLX-7 interface that is mounted behind the rotor basket. When the KLX-7 is fitted, the keyboard is removed and the contact unit is mounted in its place. The keyboard is then refitted on top of the contact unit. As a result, the keyboard will be positioned slightly higher than before.
Top view of the keyboard Perspective view of the keyboard Operating the keyboard of the KL-7 Interior of the keyboard Keyboard interior close-up Close-up of the numbers-shift lamp (FIG)

Mode Switch
A coded KL-7 message consists only of the 26 letters of the Latin alphabet. In order to allow the source text to contain letters, numbers and spaces, special tricks have to be used. This is done by surendering a couple of letters and using them for SPACE, Letter-shift (LET) and Figures-shift (FIG). The surendered letters are then no longer available and must be replaced by another one.
Furthermore, the operation has to be reversed when switching from encoding to decoding. All this is done with the MODE-switch that is hidden under the keyboard. The MODE-switch consists of a large pertinax board with contacts at either side, much like a PCB (but thicker).

It is controlled with a simple knob to the left of the keyboard. The image on the right shows the MODE-switch being operated. It has 4 settings: Off (O), paintext (P), encoding (E) and decoding (D). In the picture, it is set to encoding (E). The MODE-switch also acts as the power switch.
Operating the MODE-switch

The MODE-switch is in fact a big slide-switch. By rotating the knob, the large brown pertinax board is moved from right to left. It has 4 different positions. When pressing a key, a spring-loaded contact is pushed down onto one of the oval contact on the top side of the board. The oval contacts on the top side are connected to a different set of contacts at the bottom. The contacts at the bottom, are in turn connected with a set of fixed spring-loaded contacts in the base unit.
The MODE-switch in decipher mode (D) Operating the MODE-switch MODE-switch after removing the keyboard Close-up of the MODE-switch after removing the keyboard The MODE-switch removed, revealing the spring-loaded contacts underneath it. Other side of the MODE-switch Top-side of the MODE-switch Bottom-side of the MODE-switch

Pulse Generator
The timing for the printer is delivered by a pulse generator that is coupled to all other rotating parts by the main gear. The pulse generator consists of 36 coils divided over two rings, with a rotating magnet at the center. 26 coils are used for the 26 letters of the alphabet. The remaining 10 coils are for the numbers. They are each connected in series with one of the letters, but are mounted on the ring at a slightly different angle. This causes a short delay when in numbers-mode (FIG), just enough to select the next character on the printing wheel.
The KL-7 has a built-in printer with a continuously rotating print head. The output is printed on a narrow 9.5mm (3/8") paper strip, similar to the American M-209 and the Russian Fialka. The printer is part of the gear assembly on the left. The paper roll is located to the right of the printer.
The letters and numbers are all located on the circumference of the print head. When a letter is to be printed (i.e. when the pulse generator issues a pulse), the paper tape is advanced by one position and the print hammer is released.

This causes the selected character to be printed on the paper tape. Timing is guaranteed, as the pulse generator and the print head are driven by the same axle. Hidden under the black cap, is a small ink ribbon, that runs in between the print head and the paper tape. The hammer pushes the paper against the ribbon from below.
View of the black cap shielding the printer. The print head is just visible at the bottom.

The design of the printer is nearly identical to that of the SIGABA, the war-time predecessor of the KL-7. Like the KL-7 it featured a rotating print head with two ink ribbon reels in more or less the same arrangement. The paper strip also went under the print head.
View of the black cap shielding the printer. The print head is just visible at the bottom. The printer Removing the cap from the printer The printer after removing the cap Top view of the printer Close-up of the print hammer solenoid Paper transport Close-up of paper output

German tandem option   EZ-KL7
In operation, the KL-7 was not one of the most reliable machines. It fact, it was known for its many contact problems, some of which were, no doubt, related to bad or improper maintenance. Many former users recall their struggles with the KL-7 in order to properly (de)code a message.

The German Bundeswehr even developed an assembly known as the EZ-KL7, that allowed two machines to run in tandem (i.e. in parallel) so that errors could be detected. The EZ-KL7 unit was mounted in front of the keyboard of one of the two KL-7 machines and continuously compared their outputs. As soon as a difference was detected, an alarm was raised. Note that EZ-KL7 is pronounched 'Easy KL-7'. Image [13].

This option required the KLX-7 input/output interface to be installed (see below).
KL-7 with KLX-7 and EZ-KL7 attachment [13]

Input/output interface   KLX-7
On its own, the KL-7 only accepts input from its built-in keyboard, and output is available only as printed text on a 9.5 mm (3/8") paper strip. In order to connect the KL-7 directly to teleprinter equipment, the KLX-7 input/output interface was available as an option. As far as we currently know, the KLX-7 consisted of two parts, one of which was mounted between the keyboard and the base. This required the keyboard to be removed, after which a contact unit was mounted in its place. The keyboard was then refitted on top of the contact unit. The actual input/output interface itself, was mounted at the rear of the machine, behind the rotor basket.

In order to read input directly from a 5-level teleprinter tape, the optional HL-1 tape reader was connected to the input of the KLX-7, as show in the diagram above. The KLX-7 interface was also needed if the German EZ-KL7 tandem option was installed, probably like this:

The only items that are classified are the rotor basket with the rotors (KLK-7/TSEC), the stepping mechanism (KLA-7/TSEC) and the circuit diagram. All other parts are unclassified. Given the age of the KL-7 and the fact that more and more of the operating principe is being discovered by researchers, it is assumed that the machine will be declassified before long. The NSA recently released a document that describes the history of the development of the KL-7 [3].

Some of the protographs on this page were taken at the Royal Dutch Signals Museum in 2009 shortly before the museum was closed. As becomes clear from these pictures, that machine is in beautiful condition. Unfortunately, however, the machine has been 'sanitized' and the rotors are empty. The full wiring is missing from the rotors and even the spring-loaded contacts have been removed. In April 2011, the NSA released the KL-7 Operating Instructions for both Pollux and Adonis, which were rediscovered in 2013 and are now available for download from the references section below [8][9][10]. This new information has led to an update of the KL-7 simulators.
During its lifetime, KL-7 was compromised on a number of occasions. It is believed that the Russians were able to read (break) messages encrypted with a number of high-level US cipher machines, including the KW-7, the KL-7 and the KL-47. The latter is a variant of the KL-7, used by the US Navy's Command Center for Atlantic submarine forces [5]. It is slightly bigger than the KL-7 and features a paper tape reader, a tape puncher and a different (teletype) keyboard.
Walker spy ring
The most famous story of cipher compromise is that of John Anthony Walker, born 1937, who worked for the US Navy and successfully spied for the Russians for nearly 17 years [4].
Walker joined the US Navy in 1955 and started spying for the Soviets in December 1967, when he had financial difficulties [6]. From that moment, until his retirement from the navy in 1983, he supplied the Russians with the key lists and other critical cipher material of the KL-47, the KW-7 and other cipher machines.

For his information he received several thousand dollars from the Soviets each month. In 1969 he began searching for assistance and befriended Jerry Whitworth, a student who would become a Navy Senior Petty Officer. In 1973, he was able to enlist Whitworth in his spy-ring.

In 1976, Walker left the Navy to become a Private Investigator (PI) but kept spying for the Russians. By 1984, he had enlisted his older brother Arthur and his son Michael, who kept the endless flow of classified documents going for another year. He also tried to recruit his youngest daughter who had just started to work for the US Army, but this attempt failed when she became pregnant and abandoned her military career.
Photograph showing John Anthony Walker during his trial. Taken from www.sodahead.com

Earlier, around 1976, Walker and his wife Barbara were divorced after a history of physical and alcohol abuse [6]. When Walker refused to pay alimony in 1985, she tipped-off the FBI, which eventually led to his arrest. After his arrest, Walker cooperated with the authorities and made a plea bargain in order to lower the sentence of his son Michael. Suffering from Diabetes Mellitus and throat cancer, John Walker died in prison on 28 August 2014. His son Michael was released on parole in February 2000.
Lost in Vietnam
Another example of compromised KL-7 security, is the loss of approx. 700 Adonis (i.e. KL-7) and NESTOR (KY-8) devices in Vietnam in February 1975 [14][15]. Earlier in the Vietnam War, around 1970, the Americans had decided to provide the Republic of Vietnam (RVN) with limited quantities of cryptographic equipment, such as the M-209 and KL-7 cipher machines, various One-Time Pad (OTP) systems, voice authentication codes and some NESTOR (KY-8) voice encryption units.

After the withdrawal of American troops from Vietnam, the cryptographic depot, known as Don Vi' 600, stayed in place, with American personnel accounting for the crypographic items. In late 1974 and early 1975, the situation worsened and the Americans began to withdraw some of the equipment, with the intent of sending it back to CONUS 1 or Hawaii.

When in January/Februari the situation became critical, priority was given to removal of the equipment to Don Vi' 600 so that it could be moved to CONUS immediately. In the last three weeks of the existence of the RVN, some 700 pieces of ADONIS and NESTOR equipment had been gathered and prepared for shipment to CONUS, but unfortunately none of this equipment was shipped or destroyed in time. Eventually the equipment fell into North Vietnamese hands.

Whilst the M-209 was considered obsolete by the Americans, the KL-7 and the NESTOR equipment certainly was not. It is quite likely that the North Vietnamese eventually sold some of the machines to the Russians and possibly also to the Chinese, along with 12 months worth of key material and one-time pads that were also found in Don Vi' 600.
  1. CONUS = Continental US.
Rotor reader
The information passed by John Walker and his spy ring, allowed the Russians to build an analog of the KL-7 and to find ways to mount a cryptanalytical attack on the machine [5]. This allowed the Russians to decrypt at least one million sensitive classified (TOP SECRET) messages [7].
The Russians even supplied him with a small device, called a rotor reader, that allowed him to easily trace the internal wiring of each rotor [2].

The image on the right shows the device, as it was confiscated by the FBI. It was small enough to be carried inconspiciously, and could easily be hidden in a pocket. When folded it measures approx. 7.5 x 10 cm (about a pack of cigarettes).

The device consists of two halves that are kept together by springs and hinges. Once opened, 36 flat-faced contacts become visible. They mate with the 36 spring-loaded contacts of a KL-7 rotor (photograph supplied by Keith Melton) [2].
Photograph published here with kind permission from the author.
Rotor Reader courtesy H. Keith Melton [2]

A hand-operated slide contact, hidden inside a storage compartment at the top left, was then inserted through the center hole of the rotor. It kept the rotor in place, provided the correct pressure for the spring-loaded contacts, and allowed the slide contact to 'brush' over each individual rotor contact at the other side. The rotor would be placed with index arrow opposite the position 0 index of the reader. The slide contact was then moved over the individual contacts of the rotor, and each time one of 36 lamps on the lamp panel (at the left) would be lit.

Below is a 3-D drawing of the rotor reader. It gives a good idea of how it was used. The manually operated slide contact is here taken out of its storage compartment. It has a rectangular 'key' at the bottom (left in the drawing) that is inserted in the rectangluar hole at the center of the reader.

3D view of the rotor reader. Copyright Paul Reuvers 2011.

It is assumed that the rotor reader was not one-of-a-kind, but that at least a modest quantity of them was built. The Soviets supplied Walker with the device only three weeks after he started spying for them in 1967. Furthermore, Walker was not the only person who compromised the KL-7 and similar machines. When Army Sergeant Joseph Helmich was caught spying in the mid-1970s, an identical rotor reader was found on him [5].
KL-7 Simulator for Windows
In 2009, Dirk Rijmenants managed to crack part of the KL-7 mystery. From the information that he received from researchers and former KL-7 users all over the world, he managed to put together a good-looking KL-7 Simulator for Windows™. Since then, he received numerous e-mails with new information about the working principle of the machine.

In May 2013, new information was found in the Operating Instructions for the TSEC/KL-7 ADONIS and POLLUX, that have recently been declassified by the NSA in 2011 [8][9][10]. Based on this new information, both KL-7 simulators below have been updated (version 5.0 or later).
Version 5.0.1 - 27 May 2013

In february 2011, after we uncovered the secrets of the stepping unit of the KL-7 (KLA-7/TSEC), Dirk released a major update of his simulator, that includes the new stepping unit plus a number of corrections to the operation of the mode-switch switch under the keyboard. It also includes realistic sounds, sampled from a real KL-7 in operation. Although the KL-7 has not yet been declassified, we are about 99% certain that this simulation is accurate.

The image on the right shows a screenshot of the KL-7 Simulator running on Windows XP.

 Download KL-7 for Windows (off-site)
Click here to download the KL-7 Simulator for Windows

KL-7 Simulator in JAVA
In September 2012, we teamed up with some researchers of the Cyber Systems and Technology Department of the Lincoln Laboratory of the Massachusetts Institute of Technology (MIT) in Lexington (MA, USA), to produce a JAVA version of Dirk Rijmenants' KL-7 Simulator for Windows (see above). The main advantage of using the JAVA language is that the application can run on virtually any platform, including Windows, Apple (Macintosh), Unix and Linux.
Version 5.0.2 - 22 December 2013

In February 2013, Uri Blumenthal of MIT, released the first version of the JAVA KL-7 Simulator. As it uses the graphics from Dirk Rijmenants' KL-7 Simulator for Windows (above) and the sounds and other information from this website, the two simulators show great resemblance.

The software comes as a JAR archive and works 'out of the box' on most platforms, including the Apple Macintosh. An extensive 30-page manual is included with the software. Simply click the question mark (?) at the top bar to read it. It even has a built-in codebook generator.

 Download JAVA KL-7 Simulator
Click here to download the Java KL-7 Simulator

Please note that the above KL-7 simulator requires the latest version of JAVA (8) to be installed on your computer. For security reasons it is always recommended to use the latest version of JAVA. To check your current version and download the latest version of JAVA, click here. If you are still using JAVA version 6 or 7 and do not want to upgrade yet, you may download the JAVA-6 or 7 compatible version of the KL-7 simulator below.

 Download JAVA-7 compatible version of KL-7 Simulator
 Download JAVA-6 compatible version of KL-7 Simulator
Audio sample
The audio file below illustrates the use of the KL-7. First, the machine is turned on. Then 10 keys are typed in plain text mode. The unit is then switched to encryption, after which 7 letters and 5 spaces are typed. It is then switched to decryption, after which 8 letters are typed. We then switch to encryption and then to plain text. Next, the rotor positions are changed. Finally, the KL-7 is switched off again and you can hear the motor fading out.
Known locations

  • KAO-83/TSEC
    This is the official operator's manual for the KL-7, which is still classified.

  1. H. Keith Melton, Ultimate Spy
    ISBN: 0-7513-4791-4. 1996-2002. p. 54.

  2. H. Keith Melton, The Ultimate Spy Book
    ISBN: 07894074435. 2009.

  3. NSA, Cryptologic Almanac 50th Anniversary Series, AFSAM-7

  4. TruTV, Family of Spies: The John Walker Jr. Spy Case

  5. Laura H. Heath, Analysis of Systematic Security Weaknesses of the US Navy...
    M.S., Georgia Institute of Technology, 2001. Fort Leavensworth, Kansas (USA), 2005. Thesis of Major Laura Heath, detailing how John Walker exploited weaknesses in the US Navy Broadcasting System between 1967 and 1974.

  6. Wikipedia, John Anthony Walker
    Retrieved November 2010.

  7. FBI, The Year of the Spy
    Famous Cases and Criminals. John Anthony Walter Jr.
    Retrieved November 2010.

  8. NSA, TSEC/KL-7 Canadian User Report After First Year of Operation
    National Security Agency. CSEC 115. 1 May 1959, 15 pages. SECRET. 1

  9. NSA, Operating Instructions for TSEC/KL-7 ADONIS Operation
    Department of Defense. National Security Agency. Washington, DC 20305.
    KAO-41C/TSEC. September 1966, 28 pages, Confidential - Crypto. 1

  10. NSA, Interim Operating Instructions for Pollux Cryptosystems-Joint
    Department of Defense. Armed Forces Security Agency. Washington 25, DC.
    AFSAG 1236. January 1953, 45 pages, Confidential Security Information. 1

  11. Chuck Aston, Personal correspondence
    Former KL-7 maintenance engineer at USAF. Received Febrary 2015.

  12. CSP-6620A, TSEC/HL-1 and TSEC/HL-1B system block diagrams
    4 June 1962. Unclassified.

  13. Klaus Schmeh, Die Welt der geheimen Zeichen
    2004. ISBN 3-937137-90-4.

  14. Joachim Beckh, Blitz und Anker - Band II
    2005. ISBN 3-8334-2997-6. p. 282.

  15. James Bamford, Body of Screts
    2008. ISBN 1407009206. pp. 352-353.

  1. NSA information declassified and approved for release on 21 April 2011. FIOA Case # 64246. CSEC information declassified and approved for release on 28 April 2011. CSEC ATIP Case # A-2010-00015.
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