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.

Advertisement from 1987 for AT&T's Security-Plus Telephone

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:

Motorola's STU-III SECTEL 2500 secure telephone
(photo: Crypto Museum - click to enlarge)

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.

Part of another advertisement for AT&T's Security-Plus Communications Terminal
(published in the Airforce Magazine, June 1988 - click to enlarge)

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:

AT&T's Security-Plus Telephone on display at the National Cryptologic Museum
(photo: Flickr/Austin Mills (CC BY-SA 2.0) - click to enlarge)

AT&T's Security-Plus Telephone on display at the National Cryptologic Museum
(photo: Flickr/Austin Mills (CC BY-SA 2.0) - click to enlarge)

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.

AT&T's new secure telephone, here the 4100 version
(photo: Crypto Museum - click to enlarge)

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.

A common AT&T/Lucent/Avaya MLX office phone

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.

RCA's STU-III secure telephone (photo: Crypto Museum)

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

Preventing telephone manipulation: the TSG standards

(Updated: September 10, 2025)

Telephone conversations can be intercepted by tapping cables, but the telephone set itself can also be manipulated in order to secretly turn it into a listening device. To prevent the latter, the US Telephone Security Group (TSG) published several standards to enhance the security of landline/desktop phones.

The American secure phone of Canadian prime minister Trudeau

In my series about the phones of government leaders I will now look at Canada, where former prime minister Justin Trudeau had a rarely seen telephone on his desk: the vIPer Universal Secure Phone, which is manufactured by the American defense contractor General Dynamics.

Former Canadian prime minister Justin Trudeau with a vIPer secure phone, December 2020
(photo: Ottawa Catholic School Board - click to enlarge)

How US defense secretary Hegseth circumvents the official DoD communications equipment

(Updated: June 10, 2025)

US defense secretary Pete Hegseth appears to have a private computer in his office that is linked to the public internet. He wanted this computer to use the messaging app Signal, which is the preferred method of communication among Trump's government officials.

Here I will look at the secretary of defense's official communications equipment and the SecDef Cables communications center. There's also a photo in which Hegseth's private computer can be recognized.

US defense secretary Pete Hegseth in his office in the Pentagon, January 30, 2025
(Still from a video message on X, formerly Twitter)

Hegseth's government equipment

Like his predecessors, Trump's defense secretary Pete Hegseth has access to a range of secure and non-secure telephone and computer networks. The equipment is installed at a table behind his back, when sitting at his big writing desk in the Pentagon.

In the photo above we can see that equipment in a set-up that has basically been unchanged since Chuck Hagel, who was Obama's secretary of Defense from 2013 to 2015. In the photo of Pete Hegseth we see from left to right:

- On top of a wooden stand sits a Cisco IP Phone 8841 with a 14-key expansion module. This phone is part of the Crisis Management System (CMS), which connects the most senior government officials, including the President, the National Security Council, Cabinet members, the Joint Chiefs of Staff, and others. Its bright yellow bezel indicates that it can be used for conversations up to Top Secret/Sensitive Compartmented Information (TS/SCI).

- Below the CMS phone on the wooden stand is (hardly visible) an Integrated Services Telephone-2 (IST-2), which can be used for both secure and non-secure phone calls. This phone belongs to the Defense Red Switch Network (DRSN), also known as the Multilevel Secure Voice service. It's the main system for classified military conversations and connects the White House, all military command centers, intelligence agencies and NATO allies.

- Right in front of the IST-2 is another Cisco IP Phone 8841 with a 14-key expansion module, but this time with a green bezel, which indicates that it's for unclassified phone calls. This phone is part of the internal telephone network of the Pentagon. It replaced an Avaya Lucent 6424 executive phone, which can be seen in the following photo from 2021, along with a better view on the other phones:

Former secretary of defense Lloyd Austin in his Pentagon office in 2021,
with a Cisco IP phone with yellow bezel for the CMS and
an IST-2 phone with many red buttons for the DRSN.
(DoD photo - click to enlarge)

- Besides the telephones there are two computer screens, both with a bright green wallpaper, which again indicates that they are connected to an unclassified network, most likely NIPRNet. In the photo of Lloyd Austin's office we see that there's also a KVM switch which is used to switch securely to the SIPRNet (Secret) and JWICS (Top Secret/SCI) networks, using the same keyboard, video and mouse set.

- Finally, at the right side of the table there are two Cisco Webex DX80 videoteleconferencing screens. The one at the right has a yellow label, which indicates that it's approved for Top Secret/SCI and likely also belongs to the aforementioned Crisis Management System (CMS), more particularly as successor of the Secure Video Teleconferencing System (SVTS). The other screen might then be for videoconferences at a lower classification level.

> See also: US military and intelligence computer networks

Hegseth's personal computer

Despite the wide range of options for communicating via the proper and secure government channels, secretary Hegseth insisted on using Signal. Apparently it wasn't allowed or possible to install this app on one of the government computers, nor on a smartphone that is approved for classified conversations.

Therefore, Hegseth initially went to the back area of his office where he could access Wi-Fi to use Signal, according to AP News. It's not clear whether he used a private laptop or his personal smartphone, both of which would have been strictly forbidden to use in secure areas like this.

> See: The equipment that Trump's national security team should have used

Somewhat later, Hegseth requested an internet connection to his desk where he could use a computer of his own. This line connects directly to the public internet and bypassed the Pentagon's security protocols. Hegseth's new computer must be the one that can be seen in the photo below, as it wasn't there yet on February 21 and has no labels that indicate its classification level:

US defense secretary with a new desktop computer on his desk, March 20, 2025
(DoD photo, see also this video message on X)

Some other employees at the Pentagon also use direct lines to the public internet, for example when they don't want to be recognized by an IP address assigned to the Pentagon. That's risky because such a line is less well monitored than NIPRNet, which allows limited access to the outside internet.

At his new desktop computer, Hegseth had Signal installed, which means he effectively 'cloned' the Signal app that is on his personal smartphone. He also had interest in the installation of a program to send conventional text messages from this personal computer, according to some press sources.

The move was intended to circumvent a lack of cellphone service in much of the Pentagon and enable easier communication with the White House and other Trump officials who are using the Signal app.

Update: Ultimately on May 5, 2025, the new, unauthorized computer had apparently been removed, at least from secretary Hegseth's desk, as can be seen in this video that was published on X (formerly Twitter).

SecDef Cables

It is remarkable to what great lengths Hegseth went to use the Signal app, because as defense secretary he has his own communications center which is specialized in keeping him in contact with anyone he wants. This center is commonly called SecDef Cables and is part of Secretary of Defense Communications (SDC) unit.

SecDef Cables provides operational information management and functions as a command and control support center. It is staffed by 26 service members and 4 civilians. They provide "comprehensive voice, video, and data capabilities to the secretary and his immediate staff, regardless of their location, across multiple platforms and classifications."

Furthermore, SecDef Cables serves as a liaison to the National Military Command Center (NMCC), the White House Situation Room, the State Department Operations Center and similar communication centers. Finally, Cables manages the connections for the Defense Telephone Link (DTL), which is a lower-level hotline with military counterparts in about 25 countries, including Russia and China.

> See also: Bilateral Hotlines Worldwide

Secretary of Defense Communications recruitment video from 2023

Links and sources
- emptywheel: Whiskey Pete’s Dirty Desktop (April 25, 2025)
- AP News: Hegseth had an unsecured internet line set up in his office to connect to Signal, AP sources say (April 24, 2025)
- The Washington Post: Hegseth had Signal messaging app installed on an office computer (April 24, 2025)

See also the comments on Hacker News