Note: The full circuit diagram is available in the electronics section. All names, labels and numbering of components and electrical contacts on this page are identical to those in the original repair and maintenance document.
The continuously rotating DC motor drives the AC generator, and through a 3 to 1 reduction gear the pulse generator and printer drum, both on the same axle. The pulse generator drives the timing unit through another reduction gear. The pulse generator has a magnetic armature that rotates inside two stators with a total of 37 coils, of which 26 coils for letters, 10 for figures and one for the space. These coils are arranged in a 360-degree pattern in two separate rings and produce the timing pulse for the Print Hammer.
Depressing a key will ground one of the pulse coils. The rotors encipher that signal on its way to the pulse generator. The encipherment is non-reciprocal and the sliding contact board must therefore switch the direction of the signal through the rotors to switch between enciphering and deciphering. When the armature of the pulse generator passes a grounded coil, it induces a pulse that is passed on to both the Gate vacuum tube and to the step-up transformer. The transformer sends the pulse to Sharpener tube, which cleans up the signal and sends it to the Print tube to activate the print hammer.
If the Shift tube is in FIG mode, it tells the Sharpener tube to only react on high pulses from figure coils and ignores any low pulse from letters. The single-shot Gate tube prevents double or stray pulses by disabling the Print tube after the desired single pulse is processed until the printing and rotor stepping is completed and ready for a new pulse.
To encipher and decipher, each key has its own three connections on the Contact Panel Assemble underneath the sliding contact board. From left to right, for instance for letter G, these are named GE (G key Encipher), GP (G key Plain to its own pulse coil) and GD (G key Decipher). The contacts on the left and rights side of the cipher unit that contains the rotors are named conversely. For contact G this is EG (Encipher G) on the left side and DG (Decipher G) on the right side, thus the signals enters at the E side to encipher and at D side to decipher.
In Encipher mode, the depressed G key is connected via the T-shape and its GE contact to the EG (Encipher letter G) contact on the left side of the cipher unit that holds the rotors, and exits at the DH (Decipher H) contact on the right side, passes the small connecting bar of the H key and its goes from its Print contact to the H pulse coil.
In Decipher mode, the depressed H key is connected via the T-shape and its HD contact to the DH (Decipher letter H) contact on the right side of the cipher unit and exits at the EG (Decipher G) contact on the left side, passes the connecting bar of the G key and its goes from its Print contact to the G pulse coil.
The use of two neighboring T-shapes for each key enables the O-P-E-D sequence from right to left. The above is a simplified example with 3-pin rotors. In reality, the KL-7 uses 36-pin rotors. Note that, to perform the piggyback functions, some E, P and D connections from J, V, X, Y, Z, SPACE, FIG and LET are swapped, and additional contacts on the sliding contact board switch some piggyback wires and other control functions.
Important note! The arrows in the above image are only used to easily visualize how to keys are connected to the pulse coils. In reality, the current travels in the opposite direction!
Each coil generates its own current when the armature of the pulse generator passes that coil. The positive side of the coil is connected through the key contacts and rotors to the common. However, this is not actually "grounding" the signal in the usual sense.
A pulse coil only uses the common to transport its signal to the step-up transformer, which inverts the signal before it is processed by the electronics. Within the circuit, each pulse coil can be considered as a separate power source with its own circuit.
When a key is depressed, and depending the Selector position, the positive side of the coil that represents its enciphered or deciphered version is connected to the general common. When energized, the coil sends its positive signal through the "common" and the step-up transformer (in opposite polarity) to end its route at the negative side of the same coil (see red arrows). See also the complete electronic circuit and timing signals.
How can depressing key G, connected to EG on the left of the rotor pack, ground the correct coil to print the encrypted letter H? For a given position and wiring of the rotors, when EG is connected to DH, then DH is also connected to EG. In fact, to encrypt the letter G, the KL-7 does not "encrypt" the letter G but grounds the letter G, and by doing so, the rotor pack grounds its counterpart DH, which is connected to the H coil that is energized by the pulse generator and sends the signal to print the letter H.
Since EG and DH are reciprocal, this works also for decryption. This however is only true for this particular position of the rotors, which changes after each letter of the message. This is what makes the KL-7's sliding contact board so clever. This is also why in the first example the black arrows are opposite to the actual direction of the current, because following the path, starting from the H coil, seems illogical when you must first depress key G.
The sliding contact board has more functions. There's a notched part in front of the printer mechanism. With the mode selector in Encipher position, this cam pushes a pin into the printer mechanism causing the KL-7 to print a space after each fifth character, the standard format for cryptograms.
The improper functioning of the sliding contact board or the rotor contacts affects the reliability of the KL-7. When a bad connection occurs inside the sliding contact board or between two of the rotors, the proper pulse coil will not be grounded and consequently the print hammer, the clutch and rotor stepping are not activated. This is called a dead-rove, usually caused by poor maintenance.
An optional TSEC/HL-1 Tape Reader was available for the KL-7. In NSA nomenclature, the H means ancillary device that helps processing, and L means letters output. The HL-1 reads five-bit punched tapes with plain or ciphertext and sends it to the KL-7. This enables fast processing of message tapes, received from or created on a standard teleprinter, avoiding the laborious manual typing of messages on the KL-7.
The use of the HL-1 tape reader requires the KLX-7/TSEC Keyboard Adapter, mounted between the keyboard and the Sliding Contact Board. The installation of the KLX-7 therefore requires the removal of the KL-7 keyboard cover.
To install the KLX-7, the Mode Selector must be in Decipher position. The five screws of the keyboard are removed, the keyboard lifted without moving the Sliding Contact Board and the standoffs removed.
The KLX-7 is placed on the contact panel while ensuring its Selector handle extension engages in the notch of the Sliding Contact Board. The KLX-7 is then fixed with screws in its five hex nuts. Finally, five standoffs are placed in the hex nuts and the keyboard is fixed back onto the chassis by tightening its screws into the standoffs.
Once the KLX-7 is installed, its connector is located at the center front, just underneath the KL-7 keyboard. The KLX-7 board has 30 pressure contacts with their caps through the upper part of the board. Each of these contacts is connected to the KLX-7 connector.
Also, two contacts connect the two sliding 24 Volts power contacts from the KL-7 to the KLX-7 connector to pass the 24 Volts to the HL-1. Moreover, depending the KL-7 Mode Selector position, a 24 Volts signal is sent to the HL-1 to either process letters, figures and spaces in Plain and Encipher mode, or only letters in Decipher mode, and ignore all other 5-bit characters on the tape.
The HL-1 Tape Reader senses five-bit punched tape and converts the bits, through a series of relays, into connecting the common to one of the 26 letters, space, LET or FIG wire, and passes that common through the adapter cable to the KLX-7 adapter underneath the keyboard. This has the same effect on the KL-7 as depressing the keyboard key and grounding its corresponding pulse coil(s). Note that in reality the connector on the HL-1 side is twice the size of the connector on the KLX-7 side. The connector also has an alligator clip for grounding. Before 3-wire sockets became standard, this clip was fixed onto some metal part near the socket. More on the HL-1 tape reader at Jerry Proc's Crypto Machines and Nick England's US Navy Crypto Equipment.
The enciphering in the KL-7 is performed by the rotors. These are set according to the key list. Each individual rotor performs a substitution cipher. The rotor core wiring is still classified and most surviving rotors are either stripped of their wirings or inaccessible.
Its wiring was also changed on a regular basis, in contrast to, for instance, the German Enigma where the wiring never changed during its whole service time (the U.S. cryptologists learned the lessons). The KL-7 rotors were easily rewired manually by simply plugging the contacts into other positions without the need of soldering.
The KL-7 had a set of 12 rotors to choose from, later expanded to 13, labelled "A" through "M". (see CORE on part of the key list at the right). Each rotor core has 36 flat contacts on the left side (as seen in cipher unit windows) that are wired in a scrambled fashion with 36 spring-loaded contacts on the right side. The wiring performs a substitution encipherment. An adjustable black alphabet ring is attached to the rotor core. These letters are visible through the little windows of the cipher unit. The drive-pawls from the rotor stepping unit use the notches on the alphabet ring to advance the rotors.
The side of the alphabet ring is also marked with the numbers 1 through 36. By depressing the alphabet ring, it can rotate relative to the rotor core wirings. This is done by aligning an alphabet ring number with the white arrow on the side on the rotor (ALPHA RING SET). The 36 positions on all alphabet rings are labelled as show in the table below. Note that 10 of the positions on the alphabet ring are left blank.
The detachable KLK-7 cipher unit or rotor cage holds the eight rotors that are selected from the key list. The KL-7 uses a complex re-entry system that can cause multiple enciphering of a single character. When the signal leaves the exit rotor there are two possible situations: the signal either is passed immediately to the pulse generator through one of the 26 wires, or it leaves the exit rotor on one of the 10 re-entry contacts. In the latter case, the signal is sent back to one of the 10 re-entry contacts at the entry rotor, to perform a new pass through the rotors. When the signal leaves the exit rotor again, the process is repeated.
Depending on the internal wiring and current position of the rotors, the signal performs one or more passes (theoretically up to 11 passes) through all rotors before leaving the exit rotor towards the pulse generator. This results in a most complex signal path that constantly changes in both number of passes and its way through the rotors. Note that the above drawing is a simplified example with three 6-pin rotors and 2 re-entry connections.
In reality, the cipher unit has eight 36-pins rotors and 10 re-entry wires. Also, the letters and re-entry wires are not connected straightforward to the end plates of the Cipher Unit but ordered in a scrambled fashion (see table base and end plate connections below). The combination of rotors and their positions creates a most complex signal path for each position of the rotors.
When a key is depressed, the pulse generator converts the signal that passed through the rotors and the keyboard into accurate timing signals for the electronics to control the printer and to activate the clutch of the timing unit which also provides mechanical power to the rotor stepping unit.
The print hammer must activate at the exact moment a given letter on the rotating printer drum passes the hammer. To achieve this, the pulse generator provides exact timing. He consists of a magnetic armature, rotating clockwise at 2200 rpm, and two stators with pulse coils. The print wheel with all letters and figures rotates on the same axle. When the electronics receive a pulse from a coil it will activate the print hammer at the appropriate moment. There are 37 coils for 26 letters, 10 figures and the space.
When the drive shaft has reached it maximum swing angle, the roller cams return forward and push any released drive linkages back to the forward position. The flat end of the drive pawl pushes its rotor one step further. The drive linkage spring eventually forces the stop pawl upward into a free alphabet ring notch. During a stepping cycle, these stop pawls prevent non-moving rotors from moving along by the force of moving neighboring rotors.
On the right of the drawing are the 7 actuator switches that sense the white notch rings on the rotors. In the Plain mode, the operator can use the 7 Set Key buttons to manually step the individual rotors and set them in the proper position (see procedures enciphering and deciphering). During enciphering and deciphering, each key stroke causes some of the rotors to step in a highly irregular fashion under control of the 7 actuator switches.
In the circuit diagram below, all actuator switches are shown inactive. Each switch is a pile-up of the two sections, Sa and Sb. Note that the order of the upper Sa section is as actually positioned in the rotor stepping unit. The order of the lower Sb section and the stepping electromagnets is mixed to make the circuit diagram more readable and these are actually placed from left to right according to their number. There are only 7 switches and stepping magnets because the stationary fourth rotor is skipped. The pins on all switches are numbered the same as on switch S301a and b.
The KL-7 can process 37 different characters: the letters A through Z, the figures 0 through 9 and the SPACE. The rotors, however, can only process 26 characters because 10 of the 36 connections, from and to the cipher unit, are hard-wired from output to input for the re-entry function (the 36 rotor contacts have no relation with the 36 letters and digits). Moreover, the enciphering reduces the 37 characters to a 26 letters ciphertext.
To enable enciphering 37 different characters into letter-only code groups, the KL-7 uses a system, similar to the teletype code. Two signals, LET and FIG, switch the machine between letters and figures. Both character sets use the same signals and they are distinguished only by the FIG or LET mode on that particular moment. To show the current state, a neon bulb lights up when the KL-7 is in FIG mode. The characters QWERTYUIOP are processed as 1234567890 in FIG mode.
This still gives 26 alpha (-numeric) keys, the additional SPACE, LET and FIG. The KL-7 must encipher these three additional characters into a letters. Therefore, the KL-7 design permits the special functions to piggyback on some of the existing alphabet letters. The letters J, V, X, Y and Z were selected because they are some of the less frequently used letters.
This system of additional functions that piggyback on normal letters is the most practical method and also the least invasive for the readability of the text. Nonetheless, the design came with a cost. The KL-7 test phrase shows the small changes that occur. The first sentence is the text before enciphering and the second sentence is the same text after it is deciphered back into plain text. To show the effect of switching between LET and FIG mode more clearly, the spaces in the example below are replace by a dash. Notice that only the seldom used letters J and Z are affected by the piggyback system.
To accomplish the piggyback of the SPACE key on letter Z, the sliding contact board reroutes the X, Z and SPACE signals. The following drawings explain the rerouting of those letter in the different modes.
1. Below, any unspecified letter, key
or pulse coil (A through Z) is denoted by
(delta). Underneath each key are three contacts, named
e-p-d (encipher - plain - decipher. Thus, the contacts
underneath the A key are named Ae, Ap and Ad. The left
side contacts of the cipher unit that holds
the rotors are named EA through EZ (Encipher this letter)
and the right side contacts are named DA through DZ
(Decipher this letter). See sliding contact board
for routing through rotors.
In plain mode, the p-contact of each key is connected to its corresponding pulse coil (black route). Zp to the Z coil and Xp to the X coil. Both are high pulse coils (red). Depressing the letter Z or X will close the circuit of its corresponding coil. When the magnetic armature of the pulse generator passes that coil, the induced signal is stepped up by transformer T101, passed to the sharpener and print tube, and printed.
The SPACE key is connected to a junction on the V-SPACE coils pair. This special feature is also used in the separate FIG circuit. By using the junction, depressing the SPACE key will only close the SPACE coil circuit.
All other keys are also connected via their p-contact to the corresponding pulse coil. These coils can be high pulse letter coils (red) or low pulse alpha-numeric coils (green). The low pulse V coil from the V-SPACE pair is connected to the V key's Vp contact.
In plain mode, the signals do not pass
through the rotors in the cipher unit.
In encipher mode, the e-contact of each key is normally connected to the corresponding E-contact on the cipher unit left side. This however is not the case with the piggyback Z key.
The Z and X key e-contacts (red route) are now both switched to the EX-contact (Encipher X) on the cipher unit left side. This doesn't affect the X, but the letter Z also becomes X, because the SPACE needs the Z to piggyback.
Therefore, the SPACE key's e-contact (blue route) is connected to the cipher unit left side EZ-contact and actually enciphers the letter Z that will represent the SPACE.
The enciphered version of all letters exits the cipher unit right side at one of the D-contacts. This can be any of the 26 letters DA through DZ, even the depressed key itself, depending on the rotor positions and wiring.
The signal then travels to the key that
corresponds with the cipher unit's D-contact, enters that
key's d-contact, passes its small connecting bar and its
p-contact to the pulse coil and prints the enciphered
In decipher mode, the d-contact of each key is connected to the corresponding D-contact on the cipher unit right side (black route). The only exception is the SPACE key, which is not used, as only letters are to be deciphered.
The deciphered version of all letters exits the cipher unit left side at one of the E-contacts. This can be the any of the 26 contacts EA through AZ, even the depressed key itself, depending on the rotor positions and wiring.
The signal then travels from that E-contact to the key that corresponds with the deciphered letter, enters that key's e-contact, passes its connecting bar and p-contact to the pulse coil, and prints the deciphered letter.
When a depressed key represents the enciphered X, the signal exits the cipher unit as deciphered letter X on the left side EX-contact. The signal (red route) then travels to the Xe contact and via a connecting bar to the X coil, to print the X. Thus both originally plain X, and the Z (changed into X) are deciphered and printed as X.
When the depressed key represents the enciphered Z, the signal exits the cipher unit left side as deciphered Z at the EZ-contact. Since the deciphered Z actually represents a SPACE, the signal (blue route) does not travel to the Ze contact, but via the SPACE Key's e-contact and connecting bar to the SPACE coil to print a space.
The KL-7 can be powered in two ways. The Power Cable, connected to a 24 volts DC source (e.g. vehicle battery) or the AC Power Converter that converts 110V or 220V AC into 21 to 31 volts DC (24 volts is ideal). Both power cable and converter have an Amphenol AN 3101A Mil Spec connector (2 pins female) with AN 3057-4 cable clamp that mates with the KL-7's power cable with male connector.
To follow the KL-7 circuit diagram below, we need to know the type of vacuum tubes and how they function. These are crucial to process the timing signals for the printer. The KL-7 has one 12AX7 and three 2D21 tubes. These were produced by many manufacturers like RCA, General Electric, Brimar or Mullard. For military supply contracts the tubes were designated JAN 12AX7 and JAN 2D21 (Joint Army-Navy electronics standard nomenclature).
The pulse generator in the center left converts a key stroke into one or two negative pulses when that pulse coil is grounded and induced by a rotating magnetic armature. 15 letter keys have a single high pulse coil. 10 alpha-numeric keys have a low pulse and high pulse coil in series. For the piggyback system, the V key has a low pulse coil in series with the SPACE key high pulse coil and includes a junction.
When a key is depressed, a negative pulse triggers the V104 Gate tube, which acts as single-shot multivibrator that controls the Print tube screen grid bias, to limit when the Print tube is allowed to activate the printer. Next, when the armature passes the coil of the depressed key, a negative pulse is sent to the T101 step-up transformer in the center, which inverts the pulse (see also signal timing).
In the center of the diagram is the V102 Sharpener tube that receives the pulses from the step-up transformer. The Sharpener serves as a threshold to distinguish low and high pulses and its control grid bias is determined by the V103 Shift tube via the R119 - R122 - R123 resistor network.
In LET mode, a low pulse is sufficient to overcome the minus 8 volt bias on the Sharpener tube control grid. In FIG mode, the voltage drop between terminal W5 and common makes the Sharpener control grid bias more negative, requiring a high pulse to fire the sharpener.
The V101 Print tube only fires when the triggered Gate tube makes the Print tube screen grid bias positive and the Sharpener tube pulse simultaneously makes the Print tube control grid bias positive.
When the Print tube has activated the timing unit, the Repeat Cam switch disconnects the positive voltage at the Gate tube A-plate anode, turning the Print tube screen grid bias negative. This prevents the Print tube from firing successive pulses or stray pulses from keyboard or from moving rotor pins.
Once the cycle and rotor stepping is completed, the Repeat Cam switch closes again and both Gate and Print tube are ready for the next cycle.
When the RPT key (repeat) is depressed, the negative bias on terminal X1 and on the Print tube screen grid is removed and the tube is ready to fire at any pulse it receives from the Sharpener tube. Depressing the RPT key together with a letter key will therefore continuously print the depressed plain or enciphered letter.
When the Print tube fires, the print and trip magnet are activated. Both magnets are energized in a quite special way. The Print capacitors C101 & C102 and Trip capacitors C106 & C107 are constantly charged though R109 and R110 to +200 Volts. When the Print tube fires, this short the circuit and discharges the caps though the tube-print-trip circuit, in series with the caps.
This activates the timing unit to perform one cycle, and switches on the cam shaft provide four timing signals. During this cycle, the Charge Cam switch shorts the 10K resistor R110, leaving only the 1K resistor R109 between the 220 Volts at W8 and the capacitors at W2, ensuring a fast recharge of the caps to be ready for the next machine cycle. The Charge Cam switch also disables the Print tube until the cycle is finished, preventing the Print tube from firing multiple times during a cycle.
At the bottom of the circuit diagram is the Shift tube section that memorizes the current shift mode, shows the current mode with a neon lamp and controls the threshold of the Sharpener to know what signals to accept in LET or FIG mode.
The switches with an orange dot in the above circuit diagram represent parts of the sliding contact board and shows how they influence the stepping of the rotors, the shift mode circuit and some of the keys. How those switches work is shown below.
The letters cam switch and figures cam switch from the timing unit control how the piggyback letters for LET and FIG keys are processed. Since we have the Plain, Encipher and Decipher mode and two shift modes, there are six different ways these keys are processed. To accomplish this, the sliding contact board reroutes the FIG and LET keys, and the figures and letters cam switches.
Note: Below, the unspecified key, letter or pulse coil (A through Z) is denoted by (delta). Also, the actual circuit uses a few more switches on the sliding contact board to avoid conflict with other key circuits in the different modes, but the route of the signals is as shown below.
The FIG key operation:
The LET key operation:
The reason for printing a space when deciphering the LET or FIG key instead of only changing the Shift mode is that activation of the Timing Unit is required for the timing signals and stepping of the rotors to correctly encipher or decipher the following letter.
The timing signals for the electronics are generated in two different ways. When depressing a key, the corresponding pulse generator coil is grounded, causing a first pulse. This pulse in not generated by the pulse generator itself, but by the key at the moment he closes the circuit! While the key is still depressed, the pulse generator armature passes the grounded coil and induces a second pulse (which could be single or double) that initiates the print and rotor stepping cycle. On the right the relevant parts of the circuit ( see also the full circuit diagram).
The Pulse Generator coils' common (test point V14) is connected through R131 and R114 to the +220 Volts. Therefore, depressing any key connects its pulse coil, which has low DC resistance, to the general commons of all electronics. This causes a large voltage drop across that pulse coil and the resulting counter-electromotive force (CEMF) makes the pulse coils' common negative to the general common of the electronics. This negative pulse is the so-called "transient pulse".
The voltage drop on key closure is passed through R131, terminal X3 and C105 to the Gate tube B section's cathode, causing A section to cut off and B section to conduct. The Gate tube single-shot multivibrator is now triggered, but the voltage drop also discharged C116, causing terminal X1 to stay 10 milliseconds longer negative. Terminal X1 is connected to the Print tube's screen grid, thus preventing a little longer the Print Tube from firing.
The negative 10 Volts transient pulse is also inverted by the transformer into a positive 30 Volts and sent to the Sharpener tube. However, the processed pulse will not initiate printing because the Print tube's screen grid bias is still negative due to the 10 milliseconds delay at terminal X1. The transient pulse thus only initiated the multivibrator.
When the 10 ms delays has passed, and the key is still pressed down, the rotating pulse generator's armature will eventually pass the grounded pulse coil. This creates again a negative 10 Volts pulse that enters the transformer (see direction i arrow ) which in turn inverts the pulse into a +30 Volts pulse and fires the Sharpener tube. This is the actual pulse of a letter or the initial pulse of an alpha-numeric double pulse.
Since the Print tube's screen grid bias is no longer negative, the Sharpener can now fire the Print tube. Note that after the pulse armature has passed the grounded coil, the decaying magnetic flux also induces a current in the opposite direction, but that pulse is blocked by the three diodes.
Firing the Print tube now energizes both the Printer Magnet to print the corresponding character and the Trip Magnet to rotate the Timing Unit that controls the cam switches. The Repeat Cam switch then cuts off the Gate tube A section's plate to prevent the multivibrator from completing a full cycle of pulse sensing, thus blocking stray pulses from the moving rotor's contacts in the Stepping Unit. More about the route of the signals through the rotors is found in the Sliding Contact Board section.
The motor-generator is a combination of a 24 Volts DC motor and a 400 cycle AC power generator, delivering 5 Watts and 150 Volts rms at 6600 rpm, together in one single housing. The motor exists in two versions. As of Serial No. 13149, the CE 87420 was replaced by the improved CE 88000. The difference is how the current in the field winding is managed.
The components to adjust the field current, and thus motor speed, are not included in the general electronics because they are housed inside a cover at the back of the motor-generator (see rear view in image).
The CE 87420 motor is a shunt wound DC motor where the armature and shunt field are connected parallel, which typically produces a modest starting torque but good speed stability under varying load.
The motor speed is controlled by changing the current in the field. This is done by two resistors, the Negative Temperature Coefficient (NTC) resistor R201 and the adjustable resistor R202 to compensate any manufacturing variations of the field winding and NTC.
The newer CE 88000 motor is also a shunt wound motor but uses a centrifical governor switch to maintain a constant speed while operating between 21 and 31 Volts DC. The Governor speed adjustment is 6600 +/- 100 rpm.
The governor is open when the motor starts. Resistors R136 and R136 are then in series. This causes a lower current in the field winding. The resulting lower Counter-electromotive force (CEMF) speeds up the motor.
When the motor reaches its optimal speed, the governor closes and shorts R135. The resulting higher field current and higher CEMF then slow down the motor. C121 suppresses sparks on the governor and R136 limits current surge when the governor closes.
The AC generator, of which also two versions exist, is mounted in front of the motor on the same shaft. The initial version produced 2.4 amperes output current. The newer generators, above Serial No. 15412, produce 4.5 ampere. Although fully interchangeable, the newer version performs better under the new 88000 motor with governor and is also more efficient with the selenium rectifiers.
There are 2,821,109,907,456 ways to set 7 notch rings and 1 wide ring. There are 78,364,164,096 ways to set all core and notch combinations, relative to the machine. The alphabet ring, only a visual representation of the rotor positions, is disregarded. This gives the adversary a total of 1.9037 combinations, representing a 124 bit key.
Trying out all possible keys on a 124 bit key, a so-called brute-force attack, is considered infeasible with all present and future computer power. However, cryptanalysis is more than key size, theoretical security and brute-force attacks. Rotor cipher machines have proven vulnerable to certain types of cryptanalytic attacks, performed on fast computers. Therefore, the KL-7 is no longer considered secure. Nevertheless, even skilled cryptanalysts with current resources would today still face a huge task to mount a successful attack against the KL-7, especially when they only have a limited number of messages at their disposal.
Encrypting a large number of messages with the same key always poses a risk, as there is more chance of messages being in depth, giving the cryptanalyst more statistical clues about that specific key setting. During the KL-7s service time, the numbers of message encrypted with the same key settings was limited by compartmentalization. For a given day, instead of using one key list for all users, different key lists were distributed to different echelons and services, thus reducing the use of one specific key setting. An additional method to limit "in depth" messages is to encrypt each message with a unique random starting position of the rotors, called Message Rotor Alignment. More in the message procedures below.
There were two types of operation for the KL-7, called ADONIS for high-level communications and POLLUX for Army and Air Force. The difference between them is the set of rotor and the method to determine the random starting position of the rotors for each new message, called Message Rotor Alignment.
Each ADONIS rotor set consists of 12 (later 13) rotor cores, 11 notch rings and one stationary wide ring. Before using the KL-7, the internal settings must be set according to a key list that contains multiple keys, each valid for a period of 24 hours. Each daily key list determines the choice of rotor cores and its position in the cipher unit, the setting of its alphabet ring, and the type of notch ring and its setting on that rotor core. The rotor core that is selected for the fourth position must always be fitted with the stationary wide ring, which can also be set in any position on that rotor core. This wide ring prevents the forth rotor from moving.
Below, the ADONIS key list example, in the format as documented in the declassified KAO-41C/TSEC. Usually, a second cipher unit, with rotors arranged according the key list of the previous day was readily available. When they received a message from the previous day, the operator could quickly remove the current cipher unit from the rotor stepping unit and replace it with the old cipher unit.
Note: you can set the example key for day 31 on the KL-7 simulator and perform the encipher example shown below, to experience the encipher procedure as actually used by the operators. Be advised that the rotors of the simulator have their own core wirings because the secret wiring of the real rotors was never declassified and the wiring of surviving rotors has been removed.
After placing all prepared rotors in the cipher unit, the new settings are checked with the 36-45 letters check. With the mode selector in Plain mode, all rotors are set in the A position. Next, the machine is set to Encipher mode (some rotors will move one step). The counter is reset and the letter "L" is typed 45 times. The last two code groups should match the 36-45 letter check on the key list for that day.
The system indicator, shown at the bottom-right of the key for that day, consisted of five digits (ADONIS) or five letters (POLLUX) and was used before and after the enciphered message to identify the system and key.
For each new message, the operator must use a new unique and completely random start positions for the rotors. The method to convey these rotor positions to the receiver is called the Indicator System. The random start positions, called Message Rotor Alignment, is visible through the little windows of the cipher unit and crucial for the security of the message because using the same Message Rotor Alignment for multiple messages leads to patterns that can be exploited by codebreakers.
To produce a Message Rotor Alignment, the operator takes a previously prepared random five-letter group called Message Indicator (see note further down). For our example message, we use the random message indicator "ELXNO".
1. Set the KL-7 mode selector to Plain
Any incomplete final group should be completed by one space and, if required, followed by enough random letters to complete that last code group.
Below, the resulting ciphertext. Note that if you enciphered the message with the KL-7 sim, the result might partly differ, depending on whether you switched to FIG and LET mode before or after the space.
The complete message includes the system indicator, the Message Indicator spelled-out (NATO alphabet) , the enciphered text and repeated system indicator:
The standard format for enciphered messages was the so-called CODRESS, documented in the publication ACP 127 (unclassified). In such messages, the full originator, all addressees and security classification were included in the enciphered text. These messages were always unclassified, although the coded groups contained secret information.
Below our example message with the priority pro-sign (here routine), routing indicator(s) of the addressee(s) and serial number, priority again with date and time group, groups count, and finally the message between two BT breaks.
The receiver has a KL-7 with the same rotor arrangement, as provided for that particular day in his key list. The system indicator 28604 in the received message identifies the ADONIS crypto system and key for this particular message. He will proceed as follows:
1. He switches his KL-7 mode selector
to Plain mode.
In the KL-7 simulator help file Appendix A you will find two fascinating enciphered training messages, related to the Cuban missile crisis, to practice the deciphering procedure. They use the KAO-41C/TSEC procedure and example key as described here.
The random five-letter Message Indicators are created beforehand. An unused KL-7 is loaded with a randomly chosen rotor arrangement that is never used in any existing key list. Next, all rotors are set in a random start position and the mode selector is set to Encipher mode. A large number of random words or letter is typed. The KL-7 prints a large number of random five-letter groups on the paper strip. The operator later uses these Message Indicators to produce a different Message Rotor Alignments for each new messages. To avoid re-use, each Message Indicator on the paper strip is used only once, then torn off the strip, and pasted on the corresponding message form.
The POLLUX rotor set consists of only 8 rotors, 7 notch rings and one stationary wide ring. The POLLUX procedure only uses letters as system indicator on the key list. Its 36-45 letters check is performed with the letter A instead of letter L for ADONIS. POLLUX also uses a random five-letter Message Indicator but prescribes setting the random Message Indicator directly as Message Rotor Alignment, instead of enciphering the Message Indicator. The documents adds the notice "do not encrypt using this system only. This system gives true indicator in clear and then refers to unspecified supplementary instructions. Various other methods to securely convey the Message Rotor Alignment are possible.
The roots of the KL-7 are found in the Second World War. In the 1940s, the electromechanical rotor cipher machine ECM (SIGABA) had set a new standard for secure high-level communications. At tactical level, the lightweight mechanical M-209 was widely used. By the end of the war, the M-209 was no longer considered secure and the Army expressed the need for a lightweight secure crypto machine that could replace the M-209 but that would have a cryptographic strength, comparable with cipher machines like the SIGABA. The Navy was also seeking a small cipher machine with the qualities of the ECM, with a focus on saving weight. In March 1945, the Army headquarter requested the Signal Security Service (SSS) to develop a machine that would fit their needs. Soon after, the SSS was renamed into the Army Security Agency (ASA), who initiated the research.
The project was designated MX-507 and ASA saw it as a long-range research project. The ASA researchers quickly decided to opt for a rotor-based machine. A design with 36-point rotors came on the forefront. They also had to design a completely new lightweight printing system, as the new machine was required to operate off-line and print out the messages on paper. Eventually, they were able to reduce a printer system to one quarter of its original size and weight.
ASA decided to apply a new cryptographic principle called re-entry or re-flexing. The idea was to take parts of the cipher output, re-enter the output back into the enciphering process and re-encipher it again. Cryptanalyst Albert W. Small conceived the system in 1940 and filed it for patent in 1944. However, his patent had been placed under Patent Office Secrecy Order. This would cause a patent conflict in 1957.
The Armed Forces Security Agency (AFSA) was created in 1949. It was the first American central cryptologic organization and one of its goals was to provide standardization of secure communications devices and to determine a general policy for crypto equipment. The research of the ASA was transferred to AFSA in December 1949.
Meanwhile, in April 1949, the United States and its allies had formed the North Atlantic Treaty Organization or NATO, and deteriorating relations with the Soviet Union resulted into a grim Cold War. Secure communications between the NATO members was an important part of making a front against the USSR.
An additional challenge that AFSA faced was to design a machine for themselves that could also be distributed to their NATO allies, without disclosing vital secret crypto technology that could come into Soviet hands, either directly or through infiltration of NATO members.
With such a large organization as NATO, it was more than likely that this machine or its specifications would sooner or later reach Russian soil. The design had to resist by far any possible cryptanalytic attack by Soviet codebreakers, even when the technical details of the machine were disclosed. The security of the machine had to depend solely on the secrecy of the key settings, thus obeying Kerckhoffs well known law on cryptography.
The MX-507 was renamed to AFSAM-7, which stands for Armed Forces Security Agency Machine No 7. In September 1950 AFSA demonstrated an engineering model. The final design used 8 rotors with 36 contacts, a re-entry of ten rotor signals, and a most complex irregular stepping, electrically controlled by notch rings on the rotors. The problems with the printer timing and the shift system were solved by a clever design with vacuum tubes, making the KL-7 the first tactical cipher machine ever to use electronics.
The AFSAM-7 was approved and the Army was allowed to build prototype models. By December 1950, the Army declared the AFSAM-7 ready for production. The machine would become the first standard crypto machine in the US Armed Forces. The cryptosystem was designated POLLUX. Contractors were selected and operational and maintenance manuals were composed. In February 1951 contracts were signed to produce 25,000 AFSAM-7s at a rate of 5,000 per year. The first repair and maintenance course for Army and Air Force personnel was scheduled in September 1951.
AFSA announced two types of operation in October 1951. The AFSAM-7 traffic for high-level communications was designated ADONIS and the traffic for the Army and Air Force was designated POLLUX. The differences between the two systems were the rotor sets and the message keying procedure. The final production contract was signed on February 9, 1952.
The KL-7 was initially only intended for use by the US Army, Air Force, Navy, CIA and FBI, but NSA proposed during the 1953 Communications Security Conference in London to share the ADONIS crypto-principle with their allies from the North Atlantic Treaty Organization (NATO). The goal was to improve their communications security and interoperability, and to replace the less secure Combined Cipher Machine (CCM) by the AFSAM-7.
Meanwhile, a new British crypto machine was developed with identical crypto principles as the AFSAM-7 to replace the outdated CCM (LUCIFER) which was the British CCM-Typex, interoperable with the American CCM/SIGABA. The new BID/60 SINGLET greatly resembled the AFSAM-7 and used rotors that were identical to those of the AFSAM-7, but was not expected to be in production before 1960. The U.S. made available 3,500 units of the AFSAM-7 to the United Kingdom and 3,000 units to other NATO countries. These machines would be in loan and remained property of NSA.
The U.S. Joint Chiefs of Staff approved the use of the AFSAM-7 by NATO in 1954. The plan was to introduce the machine to medium and high levels by mid-1956. NSA did recognized that the AFSAM-7, or reproductions with the same cryptographic principle, would eventually also find their way to non-military use in those countries, or might even end up in Soviet hands. NSA was confident the AFSAM-7 was secure against any attempt by the Soviets to decipher the messages, even when its cryptographic principles and specifications were compromised. The machine was therefore certified for Top Secret messages at the start of its career.
The Central Intelligence Agency (CIA) received its first four AFSAM-7 for local testing in 1954. Use of machines in the field only started the following year. From 1951 to 1954 the Army Security Agency (ASA) procured 6547 units. Once all delivered, they would replace the less secure M-209. ASA initially received 650 AFSAM-7 of which 120 were issued to the FBI. By 1954 all FBI offices, Quantico, Seat of Government, the White House Signal Detachment (WHSD) and president Dwight Eisenhower's Air Force One were equipped with the AFSAM-7.
In 1955 the U.S. Army Security Agency Europe assigned two military instructors to NATO. They assisted in the training of NATO personnel, designated to repair and maintain the AFSAM-7, by then renamed into TSEC/KL-7. Those who attended the maintenance school had to be qualified as teletypewriter mechanic and have the proper security clearances. Basic knowledge of electronics was also desirable. In 1956 NATO decided to order the HL-1 tape reader and KLX-7/TSEC keyboard adaptor to facilitate the increasing volume of encrypted traffic. The NATO members had to determine their required stock of KL-7 spare parts and could also order a kit with basic spare parts at the price of $150 per machine. The spare parts and kits were gradually delivered between 1957 and 1960.
In 1956 CIA's Operations and Training Divisions planned the AFSAM-7 for mobile message centers. Noise issues were solved with a sound proof container and a keyboard adaptor became available in 1957. That year, CIA O&T personnel also visited NSA to observe the HL-1 tape reader that could process perforated tapes. The HL-1 was later installed on loan at the CIA Signal Center.
In 1957 NATO agreed to adopt the KL-7 for second level NATO use and also for first level NATO use with ADONIS key lists, to replace the Typex (SIMPLEX) traffic that used the Typex II with SIMPLEX pads. This agreement also comprised each NATO member's Ministry of Defense and Foreign Affairs, their embassies in Paris and Washington, and their National Military Representative in Washington. In 1958 they extended the use of the KL-7 to minesweepers, fast patrol boats and long range maritime aircraft.
Besides the United States, the KL-7 was used by its NATO allies Australia, Belgium, Canada, Denmark, England, France, Federal Republic of Germany (former West Germany), Greece, Italy, Luxemburg, Netherlands, Norway, New Zealand, Portugal and Turkey. Outside NATO the KL-7 was also on loan to South Korea, South Vietnam and Nationalist China.
Some political background: South Vietnam, officially called Republic of Vietnam (RVN), existed from 1955 until the 1975 North Vietnamese victory and formation of the current Socialist Republic of Vietnam. South Korea, officially the Republic of Korea (ROK), was formed in 1948 with the division of the Korean peninsula in two political entities along the 38th parallel, the U.S.-backed South Korea and the Soviet-backed North Korea, officially the Democratic People's Republic of Korea (DPRK). The Republic of China, often called Nationalist China, was formed in 1912 by the Kuomintang. After the 1949 communist takeover the Kuomintang fled to Taiwan, since then called Republic of China (ROC) which is disputed by mainland People's Republic of China (PRC) to this day.
In 1958 the price per KL-7 totaled $1458. This comprised the KLB-7 base at $814, KLA-7 stepping unit $328, KLK-7 cipher unit $80, CE87054 Carrying Case $161, CE87066 AC Power Converter $75 and a set of rotors $100. A complete KL-7 would cost $14,313 when converted into present 2022.
The extended use of the KL-7 was discussed and NATO member Canada pointed out that the KL-7 had not reached 100% reliability yet due to problems with the pulse generator A book cipher as back-up would be essential. The KL-7 was also used on NATO submarines and in 1959 they approved the Basic Submarine Code as back-up system for the KL-7. The code, in itself not secure enough, was used in conjunction with letter one-time pads. By the end of 1959 NATO authorized the use of KL-7 ADONIS for first level military and diplomatic traffic. By 1966 some 25,000 KL-7 were produced for the U.S. and their allies.
When France left NATO's military structure in 1966, NATO needed a separate crypto system to exclude France from their most sensitive communications. Initially, two new KL-7 ADONIS key lists and a new set of rotors with other internal wiring was introduced. NATO did continue to distribute COSMIC TOP SECRET key lists to France but introduced separate key lists from which the French were excluded. The KL-7 key lists for General Small Ships, the Maritime Patrol Aircraft, Atlantic Channel North Sea and Baltic Area remained available to France.
Despite its extensive use in all armed forces, the KL-7 wasn't always the most popular machine. The KL-7 was notorious for its keyboard and rotor contact problems. The operator often had to push firmly on the keys to get the machine cycling, not allowing him to get any speed on the KL-7. To avoid contact problems, the rotors had to be cleaned regularly. On start-up, the vacuum tubes need 16 seconds to heat up before you can type on its keyboard, as the printer timing is controlled by the electronics.
The KL-7 also had a high acoustical signature. When the machine is turned on, the DC motor slowly takes speed and the reduction gears for the pulse generator and print drum produce a characteristic high pitched noise. The advancing rotors also produce their typical sound.
During its service time, the rotors of the KL-7 and KL-47 were rewired on a regular basis. Some rotors were rewired on a yearly basis on national or NATO level and some rotors, often referred to as the NSA rotors, were to be sent directly to NSA and were rewired by NSA personnel only. It was strictly forbidden to operators, even to the maintenance technicians with crypto clearance for KL-7, to check out the internal wiring of the rotors.
The technicians were not allowed to test the rotors pin-to-pin. They were instructed to place a defective rotor on a large conductive plate that made contact with all rotor pins at once, and then check out the connection on each pin at the other side with an Ohm meter. This way, the technician would see if a wire was broken, but didnt know to which pin it corresponded on the other side.
There were some incidents where a KL-7 or KL-47 was compromised. One well known incident is the seizure of USS Pueblo by North Korea in 1968. Officially the ship was an AGER-2 (Auxiliary General Environmental Research). In reality, the ship was stuffed with SIGINT (Signals Intelligence) and ELINT (Electronic Intelligence) equipment to eavesdrop on North Korean and Soviet communications. When the ship wat attacked and boarded, the U.S. Navy immediately stopped all communications with the KL-47 until NSA had distributed new key lists. The machine was designed to resist cryptanalysis, even when the technical specifications were known to the adversary. What they didnt know was that the KL-7 keys were already compromised.
Although the KL-7 was only meant to be used by the U.S. military, its NATO allies and some state departments, there are some rare cases where civilians operated the KL-7. One such case was during the 1982 Falklands War. Within a few days, the British Navy had to sail a huge naval task force across the South Atlantic. They quickly chartered merchant ships to support the operations. One of them was the Eburna tanker that carried fuel oil, diesel and aviation fuel to transfer fuel at sea. The civil radio officer had no experience with naval communications nor crypto systems and had learn the basics of cryptography and operating the KL-7 within very short time. Read the full story in KL-7 on Merchant Ships during the Falklands War
Advances in technology and the introduction of miniature electronic components increased the computational power to support cryptanalysis tremendously in the next decades. As a result, the KL-7 had become operationally insecure by the mid 1960s and vital message traffic was often superenciphered on other systems after being enciphered with the KL-7. From the 1970s on, the KW-26 and KW-37 online cipher equipment largely replaced the outdated KL-7. Some KL-7s stayed in service, mostly as back-up, and retired in the mid 1980s.
The last known recorded message, enciphered with a KL-7, was sent by the Canadian armed forces in June 1983. The fully electronic KL-51 RACE off-line cipher machine could be regarded as its successor. The KL-7 machine itself was unclassified but the cipher unit wiring, the entry plates and the stepping circuitry were confidential. After its service time, most KL-7s and KL-47s and their rotors were recalled. The surviving KL-7s were carefully stripped from their stepping mechanism wiring and rotor entry wiring, commonly denoted as sanitized.
The KL-7 is a unique machine in many ways. It was the first machine developed under one centralized cryptologic organization and introduced as a standard crypto device in all parts of the armed forces. At that time, the KL-7 used the latest cryptologic techniques and was the first ever cipher machine with electronics, yet its rotor based design would soon lose the battle against miniaturization of electronics and computational power. It proved to be the last of a breed of true cipher machines.
Many operators cursed the machine for its quirky keyboard and regular contact problems. They welcomed its electronic successors, but today they speak with sentiment about that wonderful machine and even vividly remember the typical sound of its stepping rotors. Maybe it is because of the era in which the KL-7 and the operators gave their best. Maybe because the KL-7 served all over the world, collecting secrets and memories about the Cold War, companionship, and even exciting stories about treason and espionage.
Because this was not the end of the KL-7 story...
In 1974 the FBI started counterintelligence operation "Hookshot" to identify a person who was in contact with Soviet intelligence services. A highly sensitive well-placed source provided information that the Soviet military intelligence GRU had an agent with the codename "Greenwood", who was an American from the U.S. military that had been posted in France and Vietnam. INSCOM narrowed down the search to former U.S. Army Signals Warrant Officer Joseph Helmich (1937-2002) who served in 1963 as crypto custodian in France, in 1964-65 in Vietnam with crypto clearance in a supply unit, and later at Fort Bragg, North Carolina. FBI started an extensive investigation with surveillance.
Being faced with financial problems, Helmich had contacted in 1963 the Soviet Embassy in Paris, France. He received $131,000 in return for technical information on the KL-7, at that moment the most widely used crypto machines in U.S. military. After returning to the United States Helmich continued to provide KL-7 key lists to the Soviets until 1966, enabling them to decrypt KL-7 messages from U.S. troops and Intelligence in Vietnam. Although already under suspicion in 1964 because his wealth did not match his pay grade, it was only during FBI surveillance in early 1980 that they observed him visiting the Soviet embassy in Canada to contact the KGB. After extensive interrogations Helmich eventually confessed in 1981 and was sentenced to life imprisonment.
In 1985 the FBI received a tip from the ex-wife of John Anthony Walker (1937-2014), a retired US Navy communications specialist. Later on, he was observed by the FBI while dropping a grocery bag alongside a road north of Washington D.C. The bag contained 129 copies of stolen secret U.S. Navy documents. At the same moment and a few miles further, a Soviet KGB agent left a grocery bag with $200,000. It was clearly a dead drop exchange to covertly exchange documents and money without meeting face-to-face. The following night, John Walker was arrested by the FBI in a motel.
The investigation shook up the military intelligence community. As later turned out, already in 1967, Chief Warrant Officer John Walker simply walked into the Soviet Embassy in Washington DC with a KL-47 key list and offered the Soviets to sell secret Navy documents for cash. It was the beginning of a spying career of no less than 18 years. During a search of his house after his arrest, the FBI discovered a special device, provide by the KGB, to read the internal wiring of the KL-47 rotors. During interrogations, Walker admitted providing the Soviets with complete technical maintenance manuals which enabled the reconstruction of a fully operational KL-47, cryptographically identical to the KL-7. He was also sentenced to life imprisonment.
The importance Soviet Intelligence gave to the key lists, despite possessing all technical details of the KL-47, shows that they were probably unable to break the KL-47 or KL-7 message traffic purely by cryptanalysis, or that they had no sufficient computer power to decipher them within reasonable time for practical use, at least in the early 1960s.
You can download a realistic simulation of the TSEC/KL-7 on this website. This simulator is based on the most recent information and developed in collaboration with Crypto Museum. The simulator operates in exactly the same way as the real machine. With most surviving KL-7's sanitized, this simulator is the only remaining way to actually work with this beautiful machine, and serves to keep this machine and its history alive. The simulator has an extensive 20 page manual that includes the use of the simulator, the technical and historical details on the KL-7 and some example messages. Please visit the KL-7 Simulator page to download the software,