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Secure voice/data terminal · DES · Type 3 - this page is a stub

SECTEL 3500 is a secure telephone, introduced in the late 1980s by Motorola in Seguin (Texas, USA). It was intended for use by approved US Government contractors. The device was suitable for the transmission of sensitive or company-proprietary information, and uses the FIPS Data Encryption Standard (DES) for protection of speech and data. It is interoperable with SECTEL 2500.

Speech is digitized by means of an internal LPC-10e vocoder which uses a very low bandwidth. The device offers a good quality audio at a data rate of just 2400 baud, as a result of which it can be used on virtually any POTS telephone network in the world. Data can be transferred at 2400, 4800 or 9600 baud with full encryption.

The SECTEL 3500 is an NSA Type 3 device and holds only one cryptographic algorithm: DES. Nevertheless it is interoperable with the SECTEL 2500, which has two built-in algorithms: a secret Type 2 STU-III algorithm and DES.
Image of a SECTEL 2500 which is externally identical to the SECTEL 1500

At present, Crypto Museum does not have a SECTEL 3500 in its collection, but its operation is similar to that of the SECTEL 2500. For further informatiom, please refer to this page page.

 SECTEL 2500

Key Storage Device
Key material can be transferred between SECTEL 3500 units by means of the KSD-64 Key Storage Device (KSD). Once the device is enabled, the KSD-64 is converted into a Crypto Ignition Key (CIK) which is paired with the device.

 More about the KSD-64

KSD-64 Key Storage Device

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.


Click any of the balloons above for additional information about the related SECTEL model. The SECTEL 3500 is interoperational with the SECTEL 2500 by using the latter in Type 3 mode.

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:

  1. +12V
  2. -12V
  3. +5V
  4. 0V (GND)
  5. 0V (GND)
  6. +5V
  7. 0V (GND)
  1. Robert Crotinger, Motorola STU-III Fact Sheet
Further information
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Crypto Museum. Created: Sunday 11 July 2010. Last changed: Monday, 16 January 2023 - 08:49 CET.
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