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Motorola SECTEL 1500
Secure Telephone Unit STU-III

The SECTEL 1500 was a secure telephone manufactured by Motorola in Seguin (Texas, USA) around 1990. It was intended for national (US) use at the highest level and uses dedicated, NSA-developed encryption algorithms for secure communication. It is a STU-III compatible, NSA Type 1 cryptographic product. The SECTEL 1500 was also used in Canada and some other countries.

The SECTEL 1500 was designed under the NSA STU-III endorsement program, which implies that it supports at least the LPC-10E (at 2400 baud) and CELP vocoders (4800 baud) in order to be interoperable with the STU-III terminals from other manufacturers, such as the AT&T 1100.

In addition, the SECTEL 1500 offers a far better speech quality by implementing MRELP encoding at 9600 baud, something that the AT&T 1100 doesn't. As a result, the SECTEL range of STU-III terminals became arguably the most popular one amoung the various services of the Government.
  
Motorola SECTEL 1500

The image above shows a typical SECTEL 1500 unit as it was used by the FBI. The units were taken out of service and sold on Ebay in 2011. Unfortunately though, the cryptographic heart of each unit had been removed before they were sold, so that they are no longer functional.

The exterior of the SECTEL 1500 is identical to the other Motorola SECTEL units. The case is either black (as shown here) or cream. The controls are always black. The crypto part of the phone is activated with a valid KSD-64A key or similar (see below). When in use, a similar KSD-64 is used as a Crypto Ignition Key (CIK). The SECTEL 1500 was gruadually phased out in the mid-2000s, with the last keys expiring on 31 December 2009. They have since been replaced by the STE.

Key Storage Device
All key material is usually generated externally and loaded into the SECTEL 2500 by means of a so-called Key Storage Device (KSD), such as the KSD-64A or the later PK-64KC manufactured by Datakey Inc. These KSDs look like a plastic toy key, and act like the ignition key of a car.

The KSD is entered into a so-called keyceptacle at the right of the SECTEL unit, just below the display. Once inserted, it needs to be rotated 1/4 clockwise, in order to unlock the secure features of the phone.

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

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 [1]. 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 alogorithm is suitable for full-duplex as well as half-duplex.

At 4800 baud, Code-Exited Linear Predictive Coding or CELP is used in full-duplex, providing better audio quality. At the highest speed, 9600 baud, an even better algorithm, called Modified Residual Exited Linear Predictive Coding or MRELP, is used in full duplex. The Motorola SECTEL 1500 is the only STU-III phone that could use a Type 1 algorithm at 9600 baud. Speech quality in this mode is better than width, say, the AT&T 1100.

The SECTEL 1500 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 most telephone systems around the world. The label at the bottom indicates that it is approved for use in The Netherlands. If the quality of the (foreign) telephone line is below average, the system gracefully degrades to a lower speed, but maintains its Type 1 security.

Other models
To suit the various customers and their safety requirements, different SECTEL models were manufactured. They all use the same enclosure and operation is more or less identical. The diagram below shows the positioning of the various SECTEL models.

SECTEL 9600 SECTEL 2500 SECTEL 1500
Connections
The SECTEL 1500 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:

Power cable pinout when looking into the socket on the SECTEL phone.


References
  1. Wikipedia, LPC-10 Vocoder
    FS-1015 standard. Retrieved July 2011.

  2. Motorola Inc. SECTEL 1500 User's Manual
    Motorola, 1992.
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