AT&T's very rare Security-Plus Telephone

Since 1987, AT&T advertised a new secure telephone that operated according to the STU-III standard of the NSA. This was the Security-Plus Telephone, which is probably one of the rarest devices from the STU-III family. Already in 1992, AT&T replaced the Security-Plus Telephone by a completely different model that became more commonly used.

The STU-III standard

STU-III stands for Secure Telephone Unit - Third Generation. This was a standard from the NSA for secure telephone equipment capable of encrypting voice calls (and data) up to the highest classification level.

The STU-III standard was developed from 1985 to 1986 by NSA in cooperation with the Government Electronics Group (GEG) of Motorola. In 1986, three companies were selected for the production of telephone sets based upon this standard: Motorola, RCA and AT&T.

Under the new standard, they could manufacture devices not much larger than a conventional desktop telephone set, where previous voice encryption systems, like the STU-I, required equipment as large as a small fridge.

See also: President Reagan using a STU-II telephone

Probably the best known and most widely used STU-III telephone set was the version manufactured by Motorola and sold under the brand name SECTEL:

AT&T's Security-Plus Telephone

The first STU-III phone made by AT&T was called the "Security-Plus Telephone". There's very little information about this particular phone, but from the advertisement from 1987 we can learn that:

• It's was "the only STU-III with a 4.8 Kb/s transmission rate";
• It accomodated up to 32 crypto-ignition keys per terminal;
• The same crypto-ignition key could be used in more than one terminal;
• It had four independent key sets to handle multiple programs and security levels;
• It could handle clear as well as secure data;
• It provided a remote interface to data processing equipment.

In an advertisement from 1988 the phone is called "Security-Plus Communications Terminal" and it's emphasized that the 4.8 Kb/s transmission rate offered a better voice quality compared to STU-III devices that only had a 2.4 Kb/s data rate. However, Motorola's Sectel 1500 was also able to provide 4.8 Kb/s transmission, and even 9.6 Kb/s, ensuring a the best voice quality of all available STU-III phones.

The AT&T Security-Plus phones shown in the advertisements appear to be fully black, but their actual color might have been burgundy, as suggested in an anonymous comment on this weblog from January 2013:

"There was one "version/flavor" of the US STU-III phone from AT&T that was Burgundy Red with an "R" type handset. It was the same size as the Boat Anchor / Big White Monster AT&T Security Plus STU-III with "K" handset (in Misty Cream). [...] The later AT&T/LucentTech/General Dynamic phones were white."

The AT&T Security-Plus Telephone in "Misty Cream" was/is on display in the NSA's National Cryptologic Museum (NCM), which is open for public. Some photos made by visitors provide a closer look at this very rare encryption device, which in the museum seems not to have been identified by name:

Another photo of the AT&T Security-Plus Telephone was posted on the Twitter-account of the National Cryptologic Museum on October 16, 2024. This photo gives a good impression of how large this phone from the STU-III family actually was:

Finally, a small drawing from an unknown source shows the basic parts and functions of the AT&T Security-Plus Telephone:

AT&T's new secure phone

In 1992, AT&T replaced its Security-Plus Telephone by a completly new STU-III phone. This telephone set had no specific name, but only a numerical designator indicating its encryption level. For example, model 1100 for Type 1 encryption, model 2100 for Type 2 encryption and model 4100 for Type 4 encryption. This made the device available for a wide range of users, ranging from US intelligence agencies to foreign customers.

This new design was less futuristic and almost similar to AT&T's series of common desktop phones for the commercial market. These became known as the MLX-series and were used in offices all over the world (see photo below). Later, these phones were sold under the newer brand names Lucent and Avaya.

While AT&T's new STU-III phone was significantly smaller than the Security-Plus Telephone, it was still larger and much heavier (ca. 3.5 kg) than the conventional office phones from the MLX series. This because the bottom part of the secure phone was made of die-cast aluminium in order to shield most of the electronic components.

Some context

There's some irony in the fact that AT&T manufactured these STU-III phones for securing voice and data communications up to the highest level. Because at the same time, this company was a very close and even willing partner of the NSA when it came to intercepting (foreign) telephone and internet traffic.

See also: FAIRVIEW: Collecting foreign intelligence inside the US

The same applies of course to the NSA itself, which on one hand develops sophisticated encryption methods and standards (like STU-III) for protecting American secrets, while on the other hand takes all efforts necessary to collect foreign communications that are of interest for US foreign policy and military operations.

The STU-III made by RCA

Somewhat surprisingly, the phone in the AT&T advertisement from 1987 looks much more like the STU-III phone that was manufactured by RCA, an American electronics company that was founded in 1919 as Radio Corporation of America. RCA's STU-III unit was the largest of the three versions (measuring ca. 34 x 31 x 13 cm), but also the one that was least commonly used.

A close look shows that the arrangement of the buttons on RCA's STU-III phone is a bit different from those in the AT&T advertisement, but the general design and button layout is much closer than the eventual unit sold by AT&T.

Links and sources

• Crypto Museum: STU III Third generation secure telephone unit
• Web page by Jerry Proc: STU III (Secure Telephone and KSD-64)
• Granite Island Group: Secure Communications Systems
• Wikimedia Commons: Voice encryption devices in the National Cryptologic Museum