Spy sets
Burst encoders
Logo (click for homepage)
Spendex-10 (UA 8301)
Tactical speech security terminal

Spendex-10 was the first wide-band voice encryption unit developed and built in the early 1970s by Philips Usfa in Eindhoven (Netherlands) for the Dutch Army. It uses Delta Modulation (DM) in combination with a single-bit self-synchronizing Cipher Feedback (CFB) stream cipher, known as an autoclave, and was designed to be used in combination with the Philips RT-3600 radio.
In 1960, Philips Usfa started with a range of experiments with voice cryptography for the Dutch Army, under the name Spendex-10. The design was changed several times, until a fully operational unit was ready in the early 1970s. The problems that were demonstrated by the first prototypes were subsequently solved in the final version: UA-8301/01 in 1973 [2].

The image on the right shows the final version of the Spendex-10. It is housed in a ruggedized green metal case, similar to that of the RT-3600 radio used by the Dutch Army at the time.
Spendex-10 with the door half-open

All controls are at the front. The cryptographic key is set by a series of lever-operated coding switches, hidden behind a rugged metal door. As only an officer was allowed to change the daily key, the door could be locked with a physical key. Also at the front are the audio-in and audio-out connectors. All other connectors are at the rear.

Although the Spendex-10 worked as expected and speech intelligibillity was excellent, the Army did not accept the fact that the addition of speech cryptography reduced the operational range of the radio by approx. 15%. The RT-3600 radio was specified for a 15 km range. In practice however, the unit would easily cover a distance of 20 km. With the addition of the Spendex-10, the range was reduced to approx. 17 km, which the Army did not find acceptable.

Development of the Spendex-10 was financed by the Dutch Department of Defense (DoD), but only a small quantity of them was ever built. The unit was never taken into full production as the Army did not give any further orders. In 1976, the project ended. The units that had been delivered to the Army would nevertheless be used for several years on special occasions.
Closed Spendex-10 unit Spendex-10 with the door half-open Clear view at the 20 thumb wheels used for setting the daily key Top view with door open Front view Front view with door open Spendex-10 seen from the rear Rear view

Setting the daily key
Spendex-10 contains a very powerful cryptographic unit, based on a stream cipher [9]. Data is modified in a non-linear cyclic manner. The way the data is modified is determined by the daily key. Once the receiver-clock is synchronized with the transmitter, incoming data is decrypted immediately whithout the need for further synchronization or framing (Late Entry Sync).
The classification of the Spendex-10 itself was no higher than restricted. The speech security was determinded exclusively by the key settings, entered by means of 20 lever-operated coding switches, with 8 positions each. This produces a total of 820 (1018 or 260) possible key-settings.

The 20 coding switches are hidden behind an enforced door at the front. As the key would only be changed by an authorized officer, the door can be locked (see the images below). The appropriate key settings would normally be distributed within the organisation on paper.
Clear view at the 20 thumb wheels used for setting the daily key

In the first design of the Spendex-10, ordinary thumbwheel coding switches were used for setting the key. In the final version however, these were replaced by the lever-operated coding switches that are clearly visible in the image above. The advantage of lever-operated switches is that, in case of emergency or compromise, they can easily be reset to zero in a single action.

For correct operation, the coding switches of all Spendex-10 units in a (radio) network, must be in the same position. In other words: they have to use the same key-settings. Operating the device was extremely simple. Being a simplex device, it would normally be in receive mode. When switched on, any transmission in progress would immediately be decrypted (late entry sync), provided that the correct key was used. Pressing the Push-To-Talk switch (PTT) on the handset, starts an encoded transmission.

Synchronization, key starting and operation are entirely automatic and instantaneous. A receiving Spendex-10 automatically differentiates between crypto speech, crypto data and clear speech [4]. With 260 possible key-settings (60-bit), it provided extremely good security for its time.
Front view Lock (covered) Uncovering the lock Opening the lock Unlocking the door Opening the door Clear view at the 20 thumb wheels used for setting the daily key Front view with door open

Special occasions
Although Spendex-10 was never built in large quantities, the units that were delivered to the Dutch Army were used on a number of special occasions. One example of its use is during a train hostage crises in The Netherlands in the mid-1970s by South Moluccan terrorists. As the terrorits were expected to use a radio-scanner, Spendex-10 was used as a counter measure.

Another - still unconfirmed - use of the Spendex-10 was during the movement of nuclear warheads in The Netherlands and Germany in the 1970s and 1980s. Any additional information about these or other incidents where Spendex-10 was used, are most welcome.
RT-3600 radio set
Although Spendex-10 can be used with any suitable radio set, it was designed to be used in combination with the ruggedized RT-3600 radio, which was developed in the early 1970s by Philips Telecommunications Industry (PTI) in Hilversum (Netherlands). It is one of the best and robust analog military radio sets ever designed and many of them are still in use today.
A typical RT-3600 set consists of the RT-3600 radio itself, plus one or more additional units and/or accessories. It is often used with the IC-6320 intercom unit, the AF-3620 speaker unit and a variety of junction boxes.

The image on the right shows the Spendex-10 mounted on top of the IC-3620 intercom unit. The RT-3600 radio itself is at the bottom. The Spendex is connected to the handset-connector of the radio by means of the 5-pin cable at the right. The handset itself is connected directly to the Spendex-10 (to the left of the door).
Spendex-10 (with open door) mounted on top of an RT-3600 radio set.

In the configuration shown here, the Spendex-10 gets its power from the IC-3620 intercom by means of a bridge-connector at the rear, just like the radio. The IC-3620 is usually powered by a 24V source, such as the battery of a military vehicle, connected to the front right of the unit.
Spendex-10, or any other voice crypto device for that matter, requires more bandwidth than just analog speech. Because of all the filtering, both in the transmitter and in the receiver, the required bandwidth is normally not available.

That is why the mode-selector of the RT-3600 has an extra setting marked 'X'. Setting the knob (108) to the rightmost position bypasses all LF filters and opens the squelch. After that, noise suppression is taken over by the Spendex-10. Any handsets connected to the intercom need to be disconnected or silenced in this mode [12].
Setting the mode-switch of the RT-3600 to 'X'

For more information about the RT-3600 radio station, please refer to our special pages about this 'workhorse' of the 70s, 80s and 90s. In the early 2000s, a large batch of RT-3600 radios was given to the new Police Force in Afganistan. The radios are also popular with radio hams.
Spendex-10 (with open door) mounted on top of an RT-3600 radio set. Spendex-10 (with open door) mounted on top of an RT-3600 radio set. Spendex connected to the RT-3600 Close-up of keys and audio connection Front view of an RT-3600 radio with the Spendex-10 on top (with open door) Rear view Close-up of the bridges RT-3600 radio set with Spendex-10 mounted on top. Shown here with power cable connected.
Setting the mode-switch of the RT-3600 to 'X' Mode-switch of the RT-3600 should be set to 'X' Close-up of the mode-selector Audio cable between the RT-3600 and the Spendex-10 Complete setup, clearly showing the audio cable. Rear view Front view of the RT-3600 Front view of the IC-6320 intercom

Technical description
The Spendex-10 is a fully autonomous unit, which is inserted in the audio lines of both transmitter and receiver. In the description below, it is assumed that the Spendex-10 is used in combination with a Philips RT-3600 military radio, but it can, of course, also be applied to any other transceiver, provided that the signal levels match.

Delta Modulator
As only digital signals can be enciphered with any level of security, the analog speech must be digitized first. This can be done with various techniques, such as Vocoders (e.g. Formant, LPC-10, etc.), Pulse Code Modulation (PCM) or Delta Modulation (DM) [10]. Choosing the right method is always a trade-off between cost, size, speech quality and reliability in the field.

Sampling by means of Delta Modulation

As Delta Modulation was a Philips specialty at the time, it is used in the Spendex-10. The incoming audio signal (2) is sampled at approx. 10,000 times per second (1). Each sample (3) produces a value of either a '1' or '0' (4). A value of '1' indicates that the amplitude of the sample is higher than the previous one. A value of '0' indicates a relative decrease in amplitude. The bit rate is equal to the sampling rate and can be set to any value between 7,000 and 30,000 bits per second, by swapping a crystal. In the Spendex-10 it is set to 9,600 bits per second.

Delta Modulation (DM) requires no synchronization bits at all and is far less sensitive to errors in the transmission path. Error rates of up to 5% still produce a reasonable level of intelligibility, whilst a 1% error rate is already enough to put PCM out of business. Improved versions of DM, such as CVSD and ΔΣ-Modulation are still widely used today, e.g. in Bluetooth headsets. CVSD was also used in the later Spendex-50 crypto phone.

Analog clear speech, e.g. from a handset, is filtered first, so that only signals between 300 and 3400 Hz are left. It is then amplified to the desired level and fed into the Delta Modulator, where the analog clear speech is translated into a digital bitstream of 9600 bits per second.

Spendex-10 Transmitter Block Diagram

The output of the Delta Modulator is then fed into the Crypto Logic, where it is modified (see below). The operation of the crypto logic is determined exclusively by the 20 coding switches. The digital bit-stream at the output of the crypto logic is then passed through a low-pass filter and amplified to the desired level for the transmitter.

Spendex-10 is equipped with a special data interface through which digital information can be transmitted at speeds up to 600 baud. Incoming data is detected by the Spendex automatically, and will override any voice mode.

The received signal is first filtered, amplified and shaped. A regenerator is used to eliminate the effect of disturbances and supplies a clean signal to the crypto logic. If the key-settings of the receiver are identical to those of the transmitter, the crypto logic produces the original raw bit stream from the transmitter's Delta Modulator.

Spendex-10 Receiver Block Diagram

Feeding the output of the crypto logic into a Delta Demodulator, produces an approximation of the original voice. After filtering and amplification, the signal is sent to the speaker. At the right is also the data interface. The Spendex-10 will automatically recognize data from speech by detecting specific characteristics in the data stream.

Crypto Logic
The actual Crypto Logic shown in the block diagrams above, consists of three functional parts that are further explained below. The first functional part is a Randomizer (RND). It uses a Linear Feedback Shift Register (LFSR) to modify the bit-stream from the Delta Modulator in such a way that repeated patterns of alternating 0s and 1s at the input of the crypto-units are avoided. Additional logic is used to make the behaviour of the RND non-linear.

If the operation of the RND could be kept secret, it would produce a powerful encryption on its own. At present, there are no known cryptanalytical attacs against it, other than exhaustive search [9]. For military use this is insufficient however, as it would be security by obscurity.

Spendex-10 Crypto Logic Block Diagram

The modified stream from the RND (A) is fed into the encryption unit which consists of two identical cascaded cryptographic units (B) and (C). Each bit from the scrambled bit-stream is modified twice (XOR) depending on the state of the previous crypto-bit. Each crypto-unit performs a series of complex non-linear feedback shift operations, the variables of which are determined by the settings of the coding switches. The algorithm used for this is still classified.

The above diagram shows the operation of the Crypto Logic in transmitting mode. The bitstream from the Delta Modulator is processed in the order ABC. When receiving, order of the blocks is reversed to CBA. The raw bitstream is first decrypted by the two crypto units and then de-randomized by the RND, before it is fed to the delta (de)modulator.

In order to add an extra layer of security, it was possible to insert an extra (external) encryption unit between the RND and the crypto units (E). Such an external cryptographic unit could be connected to socket 10 at the rear. In transmitting mode, the order would then be AEBC.
Like most other military equipment, the Spendex-10 is completely modular. The case is nearly identical to that of the RT-3600 radio and can be opened both from the front and the rear, by loosening 2 sets of 4 hex-bolts. The interior consists of two parts that can be extracted easily. They are connected together by means of a male/female 25-way sub-D connector (DB25).
The Rear Unit is extracted from the rear and contains the power supply unit (PSU) and the connectors to the outside world (expansion). The Main Unit is extracted from the front and contains all functional units and controls.

The image on the right shows the complete interior with the Main Unit and the Rear Unit connected together. All the electronics are hidden inside a series of cassettes (modules or units) that are plugged-in to the bottom PCB. Each cassette contains two separate PCBs and it held in place by a bolt at the bottom.
Interior of the Spendex-10

In the image above, each module is given a number by means of a label on top of the cassette. The numbers are highlighted here in red. The modules have the following functions:
  1. Output amplifier and filter, Input circuit
  2. Time base, Regenerator, Squelch combiner
  3. TX/RX control, Randomizer, External Crypto
  4. Crypto units 1 and 2
  5. Delta Modulator, Mic amplifier, Phone amplifier, Data interface
  6. Power Supply Unit (PSU)
Interior of the Spendex-10 The two parts seperated - seen from the right Main unit bottom view Lifting one of the handles Taking a module out of the Spendex-10 Module outside the Spendex-10 Empty module slot Example of a module

After loosening the locking bolt at the bottom, a module can easily be removed by lifting its two handles. A metal stub at the bottom prevents the module from being inserted the wrong way around. The PCBs are removed from the silver-plated case, by removing the two bolts at the top.
Although the two PCBs inside each cassette are 'sandwiched', they are completely independant and have no connection between them. Each PCB has its own connection to the bottom plane.

The image on the right shows a close-up of the Delta Modulator PCB. It demonstrates the high level of engineering at Philips Usfa in those days (1973). The PCB contains first-class components from a variety of manufacturers. It also contains µA709 operational amplifiers and a collection of TTL ICs. Such components became available to the general public much later.
Close-up of the Delta Modulator

The images below show more details about the sandwich construction of the PCBs inside each cassette. The cassette is 'guided' by two metal pins on the bottom PCB. A locking bolt at the bottom prevents the cassette from 'vibrating' out of its slot. The construction is similar to that of the RT-3600 radio. The rightmost image shows the PSU module extracted from the Rear Unit.
Example of a module Module taken out of its case The two sandwiched boards of Unit 5; the Delta Modulator and some audio amplifiers. Module and case The two board of a sandwich separated. The lower board is the Delta Modulator. Close-up of the audio amplifiers Close-up of the Delta Modulator Power module separated from the Rear Unit

Rear connections
A number of connections is available at the rear of the Spendex-10. Below each connection is a number in the die-cast aluminium body. At the far left is the fuse (7). Immediately to the right of that is the power connector (8). Although this is a rather complex connector, only two lines are used (+) and (-). The reason for using this connector is to allow the Spendex-10 to take its power directly from the IC-3620 intercom with a short cable, just like the RT-3600 radio.
At the center are two identical connectors. The leftmost one (9) is for connection to a modem, in order to connect directly to a line-of-sight radio link, like the FM-200 transceiver.

The connector to the right of it (10) can be used to insert an extra encryption device between the randomizer and the two crypto units. This allowed an (optional) extra layer of security that could be added at a later date if necessary. Crypto and clear data is normally not available at this connector. By applying a voltage to a specific pin, external crypto can be inserted.
Rear view with the caps removed from the connectors. From left to right: power, modem, crypto and data.

The rightmost connector (11) is the data connector. The socket is of the same type as the other two (9 and 10), but only 3 pins are used: data-in, data-out and GND. This connector allowed digital data to be sent encrypted via the Spendex-10 at speeds up to 600 baud.
Lost key
The key-setting switches at the front of the Spendex-10 can be protected against accidental or unauthorized changes, by locking the metal door. Depending on the level at which the Spendex-10 was used, setting the daily key might have been a task exclusively for the crypto-officer.
However, if the crypto-officer got lost at the battle field, the radio operator would be unable to change the setting of the cryptographic key the next day. To overcome the problem of the lost key, a spare key was hidden inside a sealed compartment inside the Spendex-10.

The compartment was accessed by removing the Rear Unit from the case. Looking into the case from the rear reveals a sealed circular object to the right of the DB25 connector. The key is held in place by silicone foam. After breaking the seal, the operator could remove the key.
Removing the spare key from the hidden compartment

The image above shows the spare key being removed from the secret compartment. Once the key was removed, the operation would re-assemble the Spendex-10 and open the door to the key-setting switches, in order to change the daily key. The rightmost image below, shows the key storage compartment seen from the inside of the Spendex-10.
Clear view at the 20 thumb wheels used for setting the daily key Closing the metal door Locking the metal door Empty rear compartment Hidden compartment with spare key Removing the spare key from the hidden compartment The spare key half-way out Close-up of the key-holder (interior)

Development of the Spendex-10 started as early as 1960 with a range of voice encryption experiments. Several solutions and methods were tried and the design was changed frequently. This eventually led to the final version in the the early 1970s, the UA-8301/01 shown above.
Before the UA-8301/01 however, the prototype UA-8301/00 was used to test the usability of the set. Although the prototype was electrically more or less identical, there were a number of physical differences, such as the type of thumbwheels, the PSU, the connections at the rear and some physical aspects of the case and the interior.

In July 2012, an original prototype of the UA-8301/00 turned up completely unexpected. It is shown in the image on the right, complete with a suitable power supply unit mounted below it. It has serial number 002 and was built in 1969.
Spendex-10 prototype with PSU (key door open)

After evaluating the UA-8301/00 prototypes, a number of design changes were carried out. The connections at the rear were changed in such a way that they matched with the RT-3600 radio set that was developed simultaneously at Philips Telecommunications in Hilversum (Netherlands).

The standard thumbwheels were replaced by lever-operated types in order to allow easy zeroizing, and the physical design of the locked door in front of the thumbwheels was replaced by a more practical design. The door was given a better lock and a spare key was to be hidden inside the rear compartment of the unit. It could be used in case of an emergency (see above).
The most important changes however, were made in the interior of the Spendex 10. In the prototype the autoclave (crypto logic) was built around a series of early ICs (e.g. the FCH132) that required negative voltages, whereas the final version contained standard TTL logic ICs.

Similar changes were made to the other plug-in units. The new crypto logic would no longer integrated with the thumbwheels but became separate plug-in units. All electronics, except for the power supply, were moved from the rear compartment to the front compartment.
Spendex-10 prototype interior (front part)

As a result, the front compartment became larger and the 'wall' between the front and rear section had to be moved. In the final design, the rear compartment would only contain the power supply (voeding) and the connections to the RT-3600 radio set and the matching intercom.

In October 1972, the revised UA8301/01 design was ready, but the user manual [4] had already been finalized. As a result, the manual still contained a large number of photographs of the UA8301/00 prototype version and only a few that were taken from the new design.
Spendex-10 prototype (top) with PSU (bottom) Spendex-10 prototype with PSU (key door open) Spendex-10 prototype (door closed) Spendex-10 (door closed) Spendex-10 prototype interior (front part) Spendex-10 prototype interior (rear part) Spendex-10 prototype interior (front part) Close-up of the logic boards
Spendex-10 prototype with all cables installed Spendex-10 prototype with all cables installed Rear view of Spendex-10 prototype and PSU (with cable) Philips Usfa headset Mains cable and connectors Mains output Power cable (between PSU and Spendex-10) ID tag

  1. Interview with a former Philips employee
    Eindhoven, July 2011.

  2. Philips Usfa, Internal Memo L/5636/AvdP/JG
    23 August 1982, page 5.

  3. Philips Usfa, Spendex 10 leaflet
    Document number 13806/E, January 1973.

  4. Philips Usfa, Spendex 10, Tactical Speech Security Terminal
    Spendex 10 user manual / short description.
    13916/E. December 1972.

  5. Philips Usfa, Spendex 10 Technical Description (2)
    UA-8301/00. 13902/N. December 1969. 3/5-TH11-.../2.

  6. Philips Usfa, Spendex 10 Technical Description (3)
    UA-8301/00. 13902/N. December 1969. 3/5-TH11-.../3.

  7. Philips Usfa, Spendex 10 Technical Description (Draft)
    UA-8301/01. 21 November 1972.

  8. Wikipedia, Linear Feedback Shift Register

  9. Wikipedia, Stream cipher
    Retrieved December 2011.

  10. Wikipedia, Delta modulation (DM)
    Retrieved December 2011.

  11. Stukken betreffende het project Spendex 10. 1974-1976 (Dutch)
    Nationaal Archief, Den Haag, Ministerie van Defensie: Generale Staf; Staf van de bevelhebber der landstrijdkrachten (Landmachtstaf), (1969) 1973-1979 (1980),
    nummer toegang 2.13.110, inventarisnummer 1632.
    NL-HaNA, Generale Staf, 2.13.110, inv. nr. 1632.

  12. Konklijke Landmacht, Technische Handleiding, RT-3600 (Dutch)
    Bediening en 1e echelons onderhoud. 28 October 1974.

Further information

Any links shown in red are currently unavailable. If you like this website, why not make a donation?
Copyright 2009-2013, Paul Reuvers & Marc Simons. Last changed: Sunday, 03 March 2013 - 16:59 CET
Click for homepage