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Motorola
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STU-III
SECTEL
  
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SECTEL 2500
Secure Telephone Unit · STU-III · Type 2

SECTEL 2500 is a STU-III secure telephone introduced around 1991 by Motorola in Seguin (Texas, USA). It is capable of utilizing two different encryption algorithms and was intended for use by US Government organisations and their contractors. When used as an NSA Type 2 product it uses a secret algorithm and is STU III compatible. When used as an NSA Type 3 product, it uses the FIPS Data Encryption Standard (DES). In this mode the SECTEL 2500 is not STU-III compatible.

The algorithm is selected by the information contained on the KSD-64 Key Storage Device (KSD) which is inserted at the right side. If the inserted KSD is a Crypto Ignition Key (CIK), the SECTEL 2500 use Type 2 encryption when going secure. If the inserted KSD-64 is a Terminal Activation Key (TAK) or a Security Activation Key (SAK), the Type 3 encryption will be selected.

The image on the right shows a typical SECTEL 2500 unit in a fully black enclosure. Its case is virtually identical to that of the other SECTEL models and so are the controls and connections.
  
Motorola STU-III with CIK

The Motorola SECTEL 2500 was primarily used by US Government agencies, such as the FBI, DEA, ATF, etc. It was also used by the US Military, Canadian defense agencies and some (approved) foreign government agencies. The SECTEL 2500 itself in an unclassified device, but when used in combination with a valid KSD, it is classified to the level of the key. The STU-III mode was phased out between 2005 and 2009, after which the units were replaced by Secure Telephone Equipment (STE). From 31 December 2009, the STU-III mode of he SECTEL 2500 was no longer approved.

Motorola STU-III with CIK
SECTEL 2500 (STU-III) - left angle view
Front view
Rear view
SECTEL PSU
ZEROIZE button
KSD-64 Crypti Ignition Key (CIK) inserted into the SECTEL 2500 (STU-III)
Manual, keys and various tags
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Motorola STU-III with CIK
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SECTEL 2500 (STU-III) - left angle view
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Front view
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Rear view
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SECTEL PSU
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ZEROIZE button
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KSD-64 Crypti Ignition Key (CIK) inserted into the SECTEL 2500 (STU-III)
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Manual, keys and various tags

Features
The image below provides an overview of the features of the SECTEL 2500. At the rear are the connection to the outside world: the analogue PSTN telephone line, the external power supply unit (PSU) and (optionally) a computer. Also at the rear are backup battery and ZEROIZE button.

Click to see more

All user controls are at the top surface. At the top is a large LCD with 16 function buttons below it, plus another 16 freely programmable speed dial buttons. Towards the front is a regular telephone keypad with the numbers 0-9 plus '*' and '#'. In rest, the handset is placed in the cradle to the left of the keypad. For half-duplex operation, a special handset with an integrated PTT button was seperately available. Handsfree operation is possible, but not in secure mode.

Modes of operation
SECTEL 2500 can be used in three different encryption modes, depending on the requirements and the local possibilities. When using the STU-III compatible mode, all key material is generated externally. The following modes are available:

  • STU-III
    SECTEL 2500 is used in Type 2 encryption mode. The required key material needs to be obtained from a COMSEC authority, using the NSA-endorsed FIREFLY protocol for EKMS.

  • Public Key
    All key material is generated locally by the crypto custodian, using the special Motorola KCA-3000 portable computer. In this mode the device uses Type 3 encryption.

  • Custom Mode
    All key material is generated locally by the user on the phone's keypad. It is then stored on a KSD in order to be transferred to other SECTEL 2500 phones. In this mode the device uses Type 3 encryption.
Compatible devices
Motorola SECTEL 1500 (Type 1 encryption)
AT&T/Lucent 1100 (STU-III) - later sold by General Dynamics
GE/RCA STU-III - later sold by General Electric
Motorola SECTEL 3500 (Type 3 encryption)
Operation
Key loading
Key material is generated externally and loaded into the SECTEL 2500 by means of a so-called Key Storage Device (KSD), such as the KSD-64A manufactured by Datakey Inc. It looks like a plastic toy key and works like starting a car.

The KSD can be used for a variety of purposes. such as: Crypto Ignition Key (CIK), Master CIK, FILL Key (FK), Terminal Activation Key (TAK), Security Activation Key (SAC) and Traffic Encryption Key (TEK).

 More about the KSD-64

  
KSD-64 Key Storage Device

Zeroizing
When security is compromised, the keys inside the STU-III have to be purged immediately. By convention this is always a two-step or double-action procedure, such as pressing two buttons simultaneously. On the SECTEL 2500 it involves opening a plastic door at the rear and pressing the red button – marked ZERO – behind it.

Once the device is zeroized, it can no longer be used in secure mode, until new keys are loaded.
  
ZEROIZING the STU-III

Vocoder
At 2400 baud, the SECTEL 2500 uses a 2.4 kbps Linear Predictive Coding algorithm known as Enhanced LPC-10 or LPC-10e. It is based on the FS-1015 or STANAG-4198 standard [A]. The same standard was used in other crypto phones from the same era, such as the Philips PNVX, the Philips Spendex 40, the AT&T 1100 (later: Lucent and General Dynamics), and the American CVAS-III. The LPC-10e algorithm is suitable for half-duplex as well as full-duplex.

At 4800 baud, Code-Exited Linear Predictive Coding (CELP) is used in full-duplex, providing better audio quality. At the highest speed, 9600 baud, an even better algorithm — Modified Residual Exited Linear Predictive Coding (MRELP) — is used in full duplex.

The SECTEL 2500 has a built-in V.24/V.32 modem that can also be used for computer data at baud rates ranging from 75 to 9600 baud. It is tested and compliant with virtually any telephone network in the world. If the quality of the (foreign) telephone line is below average, the system gracefully degrades to a lower speed, but maintains its Type 2 cryptographic security.

15 second delay
A STU-III telephone can be connected to any standard analog telephone line (POTS/PSTN). A call is always initiated in clear (non-secure) mode. In order to go secure, both parties have to insert and activate their unique Crypto Ignition Key (CIK). Then, one of the parties initiates the secure conversation by pressing the Secure button. After a 15-second delay, during which the message keys are exchanged and the phones are synchronised, a secure conversion is possible.

The 10 to 15 second delay is common for all STU-III phones and can be considered a nuisance to the user. Furthermore, valuable information is often given away in the clear voice conversation that takes place before secure mode is entered. This delay did not occur with the later STE phone.

Until today, there have been no reports of STU-III units being broken. That does not mean, however, that foreign intelligence services did not gather valuable information from intercepted lines, directly before and after the secure part of the conversation and from the metadata.

Presidential use
Due to its analogue interface, the STU-III could be used virtually anywhere in the world. When the US President (POTUS) went on a (foreign) trip, the Scret Service installed two STU-III phones at any location POTUS might visit that day, so that he could immediately contact his staff in case of an emergency.

The STU-III was used by no less than four US Presidents (POTUS): Ronald Reagan, George H.W. Bush (Sr), Bill Clinton and George W. Bush (Jr).

 Images of POTUS using a STU-III

  
Click to see more

Parts
Motorola SECTEL 2500 (STU-III) terminal
Power Supply Unit
PSU
Key Storage Device KSD-64 (CIK)
Operating Instructions
Terminal   SECTEL 2500
The heart of the system is the Motorola SECTEL 2500, which is STU-III compatible. It can be used in STU-III mode in which it is interoperable with other STU-III devices, such as the SECTEL 1500, both of which are then used in Type 2 mode.

It can also be used in non-STU-III mode in which it uses DES; a Type 3 algorithm. In this mode it is compatible with the SECTEL 3500.

 Look inside the terminal

  
Motorola STU-III with CIK

Power supply unit
The SECTEL 2500 is powered by an the external wide-range power supply unit (PSU) shown in the image on the right. It can be connected to virtually any AC power network in the world between 90 and 250V AC.

It provides +12V, -12V en +5V DC to the terminal via a 7-pin 270° bayonet DIN connector that can be fitted to the DIN socket at the rear of the terminal.

 Pinout of the DIN connector

  
SECTEL PSU

Key Storage Device   KSD-64
Key material is stored on a KSD-64 Key Storage Device manufactured by Datakey in Savage (Minesota, USA). It is a 64Kb EEPROM in the shape of a plastic toy key. The KSD-64 was later replaced by the compatible PK-64. Two keys were supplied with each SECTEL 2500 unit.

Once a valid key is loaded into the terminal, the KSD-64 is converted into a Crypto Ignition Key (CIK) and is paired with the terminal. From that moment on, the terminal can not be used in secure mode when the CIK is not present.

 More information

  
Click to see more

Operating instructions
Each SECTEL 2500 terminal was supplied with the Operating Instructions shown in the image on the right. It describes how the terminal should be used in a Type 2 (STU-III) or Type 3 (DES) environment. It also explains the various user menus that are accessible via the display.

 Download the manual
 Quick reference card

  
Click to see more




Other models
To suit both the safety requirements of the customers and US Government regulations, different SECTEL models were manufactured, all based on the same basic hardware design and enclosure. The firmware and the cryptographic algorithms are different however. The diagram below shows how they are positioned. Only the SECTEL 1000 series and 2000 series are STU-III compatible.

SECTEL 9600 SECTEL 2500 SECTEL 1500 SECTEL 3500

Click any of the balloons above for additional information about the related SECTEL model. The SECTEL 2500 is interoperational with all SECTEL 1000, 2000 and 3000-series devices, including the SECTEL 1500 and 3500. When communicating with a 1000-series device, it is used in Type 2 mode. When communicating with a 3000-series device, the SECTEL 2500 is used in Type 3 mode.




Click to see more
Interior
The device is housed in a rectanglular enclosure that consists of two parts: a die-cast aluminium bottom part, with a plastic part on top. The interior can be accessed by removing four security torx screws from the corners of the bottom, after which the two case halfs can be separated.

Inside the device are three large printed circuit boards (PCBs) and two smaller daughter cards. The first large PCB is fitted inside the plastic upper case shell. It contains all parts for the user interface: keyboard, display, microcontroller and firmware. The upper part also contains a speaker — used in handsfree mode — a small buzzer (used as a ringer) and the cradle for the handset.

The top PCB is shown in the image on the right. It is connected to the bottom half by means of two flatcables and two flying leads, that must be disconnected when separating the case halfs.
  
Upper case shell with user controls and display

The bottom case half — i.e. the die-cast aluminium shell — contains the other two large PCB, One of these – the upper board – is visible in the image above this section. It contains the analogue telephone circuit, the main power circuits, and part of the LPC-10e / CELP / MRELP vocoder.

The power circuit is at the far right. It is fed by the +12V, -12V and +5V lines from the external power supply unit (PSU) and provides the correct voltages for the analogue and digital circuits.

The telephone circuit is located at the top left. It is galvanically isolated from the rest of the circuits by means of special ETAL transformers.

The vocoder consists of an Intel 8031 processor with RAM, EPROM – with firmware – and a Motorola 56001 Digital Signal Processor (DSP), which had just been introduced in 1986 [2].
  
Lower case shell - upper board

The board is connected to the third large PCB at the bottom of the case shell by means of a red interboard connector at the bottom right. It can be removed by removing 11 screws from the edge of the PCB, disconnecting the power wiring at the top right and lifting the board upwards.

This reveals the bottom PCB. It contains the other half of the LPC-10e / CELP / MRELP vocoder, which consists of two Hitachi 6303 processors with RAM, EPROMs and one Motorola 56000 DSP. It also holds a XILINX XC2018 Field Programmable Gate Array (FPGA) with firmware.

At the centre of the bottom PCB is a daughter card which is mounted upside down. It is the key generator or crypto heart in which the secret cryptographic algorithm is implemented in custom chips. The crypto heart is connected to the bottom PCB via a large 64-pin Dual-in-Line (DIL) socket, and is held in place by two screws.
  
Lower case shell - lower board (with key generator board))

The crypto heart has components at both side of the PCB The side that is visible in the image above (the bottom of the board) contains some glue logic. After removing the two screws, it can be lifted from the 64-pin socket. This has to be done carefully to avoid bending the contact pins.

After turning the board over, the top side becomes visible as shown on the right. A the right are the pins of the 64-pin DIL connector.

At the top left are three Motorola ON408219 custom chips which contain the secret Type 2 algorithm. One of these is used for the reception circuit, whilst the other two are for two identical transmission circuits. The outputs from the two transmission circuits are constantly compared and an alarm is raised as soon as a difference is detected. To the right of the three crypto chips is another Motorola custom chip: ON393570.
  
Key generator - component side

The rest of the board is taken by a XILINX XC2064 FPGA with firmware in EPROM, in which the device's second algorithm — DES — is implemented. This second algorithm is selected when the device is used in Type 3 mode. Note that the crypto heart is often missing from devices sold on on public auction sites such as eBay. This is done as part of the declassification procedure.

SECTEL 2500 (STU-III) interior
Upper case shell with user controls and display
Display and keyboard controller
Lower case shell - upper board
Power circuitry
Analogue telephone circuit
Intel 8031 controller with RAM and Motorola DSP
Lower case shell - lower board (with key generator board))
XILINX FPGA and Motorola DSP
Red interface controller with firmware
Hitachi controller with firmware
Key generator board fitted onto lower board
Lower board with key generator removed
Key generator - bottom
Key generator - top
Motorola custom chips on the key generator board
Key generator - component side
Key generator board - solder side
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SECTEL 2500 (STU-III) interior
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Upper case shell with user controls and display
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Display and keyboard controller
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Lower case shell - upper board
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Power circuitry
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Analogue telephone circuit
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Intel 8031 controller with RAM and Motorola DSP
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Lower case shell - lower board (with key generator board))
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XILINX FPGA and Motorola DSP
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Red interface controller with firmware
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Hitachi controller with firmware
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Key generator board fitted onto lower board
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Lower board with key generator removed
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Key generator - bottom
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Key generator - top
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Motorola custom chips on the key generator board
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Key generator - component side
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Key generator board - solder side

Connections
Power supply unit
The SECTEL 2500 phone is powered by an external Power Supply Unit (PSU) that provides three different voltages: +5V (1.75 A), +12V (250 mA) and -12V (210 mA). The PSU is connected to the 7-pin (or 8-pin) 270° DIN socket at the rear of the unit. The socket has the following pinout:

  1. +12V
  2. -12V
  3. +5V
  4. 0V (GND)
  5. 0V (GND)
  6. +5V
  7. 0V (GND)
TELCO
At the rear of the device is a 6-pin RJ-25 (6P6C) modular socket for connection to a 2-wire or 4-wire subscriber line. When connecting to a 2-wire line, an RJ-11 connector can be used, as only the middle two contacts (3 and 4) are needed. The pinout is as follows:

  1. TIP
    Line (A)
  2. RING
    Line (B)
Data port   RS232
At the rear of the device is a 25-pin DB25 socket for connection to an external device such as a Personal Computer (PC). The socket is wired to the common RS232 standard and carries the common data and modem signal. The SECTEL unit support the AT-command set (like a modem).

  1. Protective Ground (shield)
  2. Transmitted Data (TXD)
  3. Received Data (RXD)
  4. Requesst to Send (RTS)
  5. Clear to Send (CTS)
  6. Data Set Ready (DSR)
  7. Signal Ground (GND)
  8. Data Carrier Detect (DTD)
  9. Data Terminal Ready (DTR)
Specifications
  • Device
    Secure voice terminal
  • Purpose
    STU-III voice communication
  • Manufacturer
    Motorola
  • Model
    SECTEL 2500
  • Years
    ~1991 - 2009
  • Users
    US Government, FBI, DEA, ATF, contractors
  • CIK
    KSD-64
  • Algorithms
    Secret NSA Type 2, DES (Type 3)
  • Price
    US$ 2145 (1991)
  • RJ25 (RJ11)
  • Voice
    2400, 4800, 9600 1 baud
  • Data
    75, 110, 330, 600, 1200, 2400, 4800, 9600 baud
  • Interface
    RS232
  • Vocoders
    LPC-10e (2400 baud), CELP (4800 baud), MRELP (9600 baud)
  • Power
    90 to 250V/AC, 47 to 63 Hz
  • Temperature
    0°C to +50°C (storage -10°C to +70°C)
  • Humidity
    90% (non-condensing)
  • TEMPEST
    MIL-STF-461B (RS03)
  • Dimensions
    229 × 254 mm × 64
  • Weight
    4 kg
Parts
  • SECTEL 2500 voice/data terminal
  • Handset
  • Coiled cable for handset
  • TELCO line cable (6-wire)
  • Power supply unit
  • Mains power cable
  • User manual
  • 2 × KSD-64 Crypto Ignition Key (or equivalent)
Options
  • RJ11/RJ45 adapter
  • Single-line surge protector
  • Cosmetic repair kit
  • Push-To_Talk handset
  • Carrying case
  • Wall mount bracket
  1. At 9600 baud, Motorola SECTEL terminals are not interoperable with the STU-III telephone sets from other manufactuers.

Documentation
  1. SECTEL 1000 & 2000 User's Guide 1
    Motorola Inc., 1987.

  2. STU-III / SECTEL 2500 User's Manual
    Motorola Inc., 1991.

  3. Quick Reference Guide
    Motorola Inc., 1991. Distributed with [1].
  1. Obtained from the Clinton Presidential Library, on 16 January 2023.

References
  1. Robert Crotinger, Motorola STU-III Fact Sheet
    Undated.

  2. Wikipedia, Motorola 56000
    Visited 15 January 2023.
Further information
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Crypto Museum. Created: Sunday 11 July 2010. Last changed: Tuesday, 17 January 2023 - 08:58 CET.
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