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.
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 They have since been replaced by the
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
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
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
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
If the quality of the (foreign) telephone line is below average, the
system gracefully degrades to a lower speed, but maintains its Type 1
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
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:
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© Crypto Museum. Created: Sunday 11 July 2010. Last changed: Saturday, 24 February 2018 - 15:00 CET.