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STU-II compatible secure phone - wanted item

STU-II/B, also known as TSEC/KY-71D, is a secure telephone unit, designed by the NSA, for use by NATO forces and governments of friendly nations. The design was based on third generation STU-III crypto phones. It was introduced in the early 1990's to replace the ageing and bulky STU-II. It was interoperable with the STU-II (KY-71), Spendex 40 and other compatible equipment [A]. Like the STU-II, it uses the SAVILLE cryptographic algorithm, developed by GCHQ and the NSA.

The STU-II/B terminal was manufactured by Motorola and is based on the Motorola SECTEL range of STU-III phones. At first glance, the phone is nearly identical to, say, a SECTEL 2500, but there are several significant differences.

It is also compatible with the Philips Spendex 40. The image on the right was taken from the user manual of the SECTEL STU-II/B [A]. It shows an extension to standard keypad of the Motorola range of SECTEL phones. The extra keys are for AUTOVON/IVSN operation only (see below).

First of all, the STU-II/B has 4 additional buttons on the keypad for priority override (see below). Secondly, the cryptographic KEYs are loaded into the STU-II/B by means of a NATO-standard fill device, such as the KYK-13 or KOI-18. For this purpose, the unit is fitted with a U-229 connector at the rear. All keys are loaded via de U-229 socket, rather than with the KSD-64 key storage device. With the STU-II/B (KY-71D) the KSD-64 device is used only as a Crypto Ignition Key (CIK).

Interoperable devices
Secure Telephone Unit - 2nd generation (KY-71)
Motorola STU-III/R
Philips Spendex-40 secure telephone for voice, fax and computer
Siemens/ANT/R&S Elcrovox 1/4
Siemens/ANT/R&S Elcrodat 5/4
STK (Thales) TCE-5000
The video below was posted on YouTube on 27 April 2023, and shows a STU-II/B on a desk at the operations center of the headquarters of the Greek Armed Forces. According to the title, the video was released in relation to the repatriation of Greek and foreign citizens from Sudan [3].

The video above starts at 0:25, just a few seconds before the STU-II/B becomes visible. If this video was taken recently, it would mean that the STU-II/B was still in operation in April 2023, but this is highly unlikely. It is more likely that the video was made from previously recorded footage.

Extra buttons
Compared to the SECTEL STU-III phones, the STU-II/B has four additional keys. They are located to the right of the existing numerical keypad and are present for backward compatibility with the non-secure AUTOVON and IVSN systems. AUTOVON (Automatic Voice Network) was a military phone system that was built in the US in 1963. It was designed to survive nuclear attacks [1] and allowed non-secure voice calls with precedence (piority override).

IVSN was the Initial Voice Switched Network developed by NATO in the mid-1970s for unclassified voice calls. Starting with 4 switches in Europe in 1980, the system grew to 24 switches at the peak of its use in the mid-1980s. When it was closed down on 30 November 2005 [2] it still consisted of 18 switches, some of which are still in use today.

  • FO - Flash override
  • F - Flash
  • I - Immediate
  • P - Priority
The four extra keys generate DTMF-signals in the rarely used 1633Hz column. On some later keyboards, these keys are sometimes called A, B, C and D. After a nuclear attack, it would be very difficult for government officials to obtain a free telephone line, as nearly everyone would try to make a phone call.

By pressing the letter P, the user would signal the switch to appoint a free line by priority. Higher ranking officials were allowed to press I (Immediate) to get an even higher priority. Military users were allowed to press F (Flash) in order to get a free line nearly instantly. It was thought that only the president and his circle were allowed to use FO (Flash Override) to give them the highest possible priority.

Key fill
On a normal STU-III phone, the plastic KSD-64 key is used as a CIK and to load key variables into the phone. On a STU-II/B, the KSD-64 is only used as a Crypto Ignition Key (CIK) (see below). The key variables are instead loaded into the phone by means of a standard military key fill device, such as the KYK-13 or the KOI-18, connected to the standard U-299 connector at the rear.

The drawing on the right shows the rear panel of the STU-II/B telephone. To the right of the power connector is a US Army/NATO-standard 5-pin U-229 socket (U-183/B) that allows the connection of a KYK-13 or similar.

Two FILL procedures are possible: NET FILL and UNIQUE FILL (KDC Mode). Up to 17 net variables and 1 unique key can be stored inside the STU-II/B. When using the unique key, a valid telephone number of a Key Distribution Center (KDC) have have been entered first.

The STU-II/B supports the (older) DS-102 standard for key loading. The pinout of the U229 connector is specified in the table below. Before loading a key, the STU-II/B first has to be put in the appropriate mode. Then the key filler must be attached and activated. If a suitable key has not been sent by the key filler within 10 seconds, the operation has failed and all existing keys will be destroyed. This is done as an extra safety measure against tampering [A].

Pin DS-102 Description
A GND Ground (common wire)
B - Not used
C ACK FILL request acknowlegment
D DATA Fill data into STU-II/B
E CLK Fill clock into STU-II/B
F - -

Crypto Ignition Keys
For secure operation, a STU-II/B must be unlocked by inserting and activating a KSD-64A Key Storage Device. With the STU-II/B, the KSD-64A is only used as a Crypto Ignition Key (CIK) and not as a key fill device.

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.

 More about the KSD-64


The STU-II/B supports the following CIKs:

  • Master CIK
    Only one Master CIK can be created for each STU-II/B terminal. The Master CIK is unique to a particular terminal. This key is normally used and controlled by the Communications Security (COMSEC) custodian. It allows Interoperable CIKs (see below) to be added to the terminal after the fill procedure has been completed. It is also used to enable or disable the speakerphone feature.

  • Interoperable CIK
    Used by a typical user. 'Interoperable' means that the user can access up to 7 terminals with one CIK. The CIK data is stored in fields 2 thru 8 of the interoperable CIK.
Modes of operation
The STU-II/B has three modes of operation, each with a varying degree of security. The required mode of operation can be selected by pressing the MODE key until the display shows the correct mode. A responding STU-II/B will follow this setting automatically.

  • KDC mode
    KDC mode requires the availability of a so-called Key Distribution Center (KDC) and offers the highest level of security. In this mode, the device first places a call to the KDC to obtain a so-called KDC Message before completing a call to another STU-II/B. The KDC Message contains the per-call keying variable that allows the user to setup a secure call with one specific terminal. The STU-II/B can store KDC messages for up to 21 terminals. This mode supports full-duplex voice calls and data communication up to 9600 baud and half-duplex voice and data communication at 2400 baud.

  • Net mode (Net Point-to-Point)
    This mode allows secure communication with another STU-II, STU-II/B or compatible device, which has the same Net keying material. The SECTEL STU-II/B can store 17 Net variables. This mode supports full-duplex voice calls and data communication up to 9600 baud and half-duplex voice and data communication at 2400 baud.

  • Multipoint (Net Broadcast)
    This mode is used to broadcast secure voice and data to multiple users. It is one means of establishing a conference call. Multipoint communication is possible between multiple STU-II and STU-II/B terminals, provided they are loaded with the same Net keying material. In this mode, voice communication is possible in half-duplex only. Data can be transferred in synchronous half-duplex mode at 2400 baud only.
As far as we currently know, the STU-II/B is interoperable with the following devices:

Below, some specific expressions featured on this page are explaned. For more descriptions, please visit the special Crypto Glossary, or follow the links below.

AUTOVON   Automatic Voice Network
Military phone system that was built in the US in 1963. Designed to survive nuclear attacks, it allowed non-secure voice calls with precedence (piority override).  More  Wikipedia
CIK   Crypto Ignition Key
A physical token (usually an electronic device) used to store, transport and activate the cryptographic keys of electronic cipher machines.  Wikipedia
DTMF   Dual Tone Multy-Frequency
In-band tone system used for telecommunication signalling over analog telephone lines. The system uses 8 different tones, two of which are sent simultaneously. It is mainly used in push-button phones to produce the numbers 0-9 plus '*" and '#'. Some keypads are capable of sending A, B, C and D as well.  Wikipedia
IVSN   Initial Voice Switched Network
Analog switched telephone network developed by NATO in the mid-1970s. It was used for unclassified voice calls throughout NATO and was discontinued in 2005.  More
NATO   North Atlantic Treaty Organization
 More  Wikipedia   Website
  1. Motorola Inc., STU-II/B SECTEL, User's Manual
    October 1992.

  2. Motorola Inc., Quick Reference Guide
    Distributed with [1], 1992.
     Single-page version
  1. Wikipedia, AUTOVON
    Retrieved November 2012.

  2. NATO, Closure of Initial Voice Switched Network (IVSN)
    Retrieved November 2012.

  3. Operation for the Liberation of Greeks and Foreign Citizens from Sudan
    General Staff of the Greek National Defense (@hndgs) on YouTube, 27 April 2023.
Further information
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Crypto Museum. Created: Saturday 29 January 2011. Last changed: Monday, 15 April 2024 - 14:15 CET.
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