STU-III History

The Secure Telephone Unit - Third Generation was produced from 19xx to 19xx but three different companies. The three companies were RCA, Motorola, and AT&T. Two of the companies have since changed their names through corporate acquisitions. RCA is now known as L-3 Communications and Motorola Secure Products has now become General Dynamics C4S.

A STU-III secure telephone; this model AT&T STU-III is a family of secure telephones introduced in 1987 by the NSA for use by the United States government, its contractors, and its allies. STU-III desk units look much like typical office telephones, plug into a standard telephone wall jack and can make calls to any ordinary phone user (such calls receiving, however, no special protection). However, when a call is placed to another STU-III unit that is properly set up, one caller can ask the other to initiate secure transmission (or, colloquially, to "go secure"). They then press a button on their telephones and, after a 15 second delay, their call is encrypted to prevent eavesdropping. There are portable and militarized versions and most STU-IIIs contain an internal modem and RS-232 port for data and fax transmission. Vendors were AT&T (later transferred to Lucent Technologies), RCA (Now L3-Communications Systems-East); and Motorola.

Different versions exist:

  • STU-III/Low Cost Terminal (LCT) designed for use in office environment by all types of users. (Motorola Sectel 1500, Lucent Technologies/GD 1100 and 1150)
  • STU-III/Cellular Telephone (CT) is interoperable with all STU-III versions. Works in all continental US mobile network and in most of the foreign cellular networks.
  • STU-III/Allied (A) specialized version of the STU-III/LCT that is compatible with the STU-II. It retains all basic STU-III functions and capabilities and incorporates STU-II BELLFIELD KDC, STU-II net, and STU-II multipoint modes of operation.
  • STU-III/Remote Control Interface (R or RCU)
  • STU-III/MultiMedia Terminal (MMT)
  • STU-III/Inter Working Function (IWF)
  • STU-III/Secure Data Device (SDD)
  • STU-III/CipherTAC 2000 (CTAC)

Most STU-III units were built for use with what NSA calls Type 1 encryption. This allows them to protect conversations at all security classification levels up to Top Secret, with the maximum level permitted on a call being the lower clearance level of the two persons talking. At the height of the Commercial COMSEC Endorsement Program, Type 2 encryption, Type 3 encryption, and Type 4 encryption STU-IIIs were manufactured, but they saw little commercial success.

Two major factors in the STU-III's success were the Electronic Key Management System(EKMS) and the use of a removable memory module in a plastic package in the shape of a house key, called a KSD-64A or Crypto Ignition Key (CIK). The EKMS is believed to be one of the first widespread applications of asymmetric cryptography. It greatly reduced the complex logistics and bookkeeping associated with ensuring each encryption device has the right keys and that all keying material is protected and accounted for.

The KSD-64A contains a 64kbit EEPROM chip that can be used to store various types of keying and other information. A new (or zeroized) STU-III must first have a "seed key" installed. This key is shipped from NSA by registered mail or Defense Courier Service. Once the STU-III has its seed key, the user calls an 800-number at NSA to have the seed key converted into an operational key. A list of compromised keys is downloaded to the STU-III at this time. The operational key is supposed to be renewed at least once a year.

The operational key is then split into two components, one of which replaces the information on the KSD-64A, at which point it becomes a Crypto Ignition Key or CIK. When the CIK is removed from the STU-III telephone neither unit is considered classified. Only when the CIK is inserted into the STU-III on which it was created can classified information be received and sent.

When a call "goes secure," the two STU-III's create a unique key that will be used to encrypt just this call. Each unit first makes sure that the other is not using a revoked key and if one has a more up-to-date key revocation list it transmits it to the other. Presumably the revocation lists are protected by a digital signature generated by NSA.

While there have been no reports of STU-III encryption being broken, there have been claims that foreign intelligence services can recognize the lines on which STU-IIIs are installed and that un-encrypted calls on these lines, particularly what was said while waiting for the "go secure" command to complete, have provided valuable information.

Hundreds of thousands of STU-III sets were produced and many are still in use as of 2004. STU-III replaced earlier voice encryption devices, including the KY-3 (1960s), the STU-I (1970) and the STU-II (1975). The STU-II had some 10,000 users. These, in turn, replaced less secure voice scramblers. Unlike earlier systems, the STU-III's encryption electronics are completely contained in the desk set. The STU-III is no longer in production, and is being replaced by the STE (Secure Terminal Equipment) or OMNI, more modern, all digital systems that overcome many of the STU-III's problems, including the 15 second delay.

Secure Terminal Equipment (STE) became the successor of STU-III in the 1990s. Similar to STU-III, an STE unit resembles physically to an ordinary telephone. Besides connecting to a regular wall phone jack (Public Switched Telephone Network), the STE was originally designed to be connected to Integrated Services Digital Network (ISDN) lines. As a result, in addition to having secured voice conversations, users can also use an STE unit for classified data and fax transmissions. Transfer rate of an STE is also considerably higher (STU-III: up to 9 kbit/s; STE: up to 128 kbps). Lastly, an STE unit is backward compatible with an STU-III unit when both units are connected to the PSTN.

The heart and soul of an STE unit lie within the Fortezza Plus (KOV-14) Crypto Card, which is a PCMCIA card. It contains both the cryptographic algorithms as well as the key(s) used for encryption. Cryptographic algorithms include BATON, FIREFLY, and SDNS signature algorithm. When the Crypto Card is removed from the STE unit, neither the phone or the card is considered classified. BATON is a block cipher developed by the NSA with a block size of 128 bits and key size of 320 bits. FIREFLY, on the other hand, is a key distribution protocol developed by the NSA. The FIREFLY protocol uses public key cryptography to exchange keys between two participants of a secured call.

Both STU-III and STE are built on technologies that are proprietary and detail of the cryptographic algorithms is classified (e.g. BATON, FIREFLY). Although the secrecy of the algorithms does not make the device less secure, it does limit the usage to within the U.S. military. The concept of secured voice application is nothing new to the commercial world. Synchronous transmission of confidential information is often necessary for the operation of a business. Many corporations have resorted to the more-available Voice Over IP (VOIP) technology. However, security of VOIP calls has been limited to compression to make eavesdropping difficult, security by obscurity, and encryption/cryptographic authentication which is not widely available. Within the Department of Defense, VOIP has slowly emerged as an alternative solution to STU-III and STE. The high bandwidth of IP networks makes VOIP attractive because it results in superior voice quality over STU-III and STE. To secure VOIP calls, VOIP phones are connected to classified IP networks (e.g. Secret Internet Protocol Router Network SIPRNET).

Both allies and adversaries of the United States are interested in STU-III, STE, and other secured voice technologies developed by the NSA. Espionage can be traced back to more than a thousand years as countries try to gain advantages over others by illegally obtaining sensitive information through various means. As of this moment, there has not been any reported cryptanalysis on the encryption algorithms used by the STU-III and STE. Any breaks in these algorithms could jeopardize national security and, potentially, threaten the lives of citizens both in the United States and allied countries..

Because of the sensitive nature of the subject, there are few relevant documents available on the Internet. The war on terrorism has caused many government agencies to remove any potentially-sensitive information from their websites in the public domain. During the course of research, the majority of the information originates from the manufacturers (e.g. L-3 Communications) of STU-III and STE. As mentioned earlier, the detail of the cryptographic algorithms is considered classified, and is therefore not available. Information about STU-III is very limited despite the fact that it is out of production.

==External links==


The Secure Telephone Unit - Third Generation (STU-III) is a low-cost, user-friendly, secure telephone device. The terminals are designed to operate reliably, with high voice quality, as both ordinary telephones and secure instruments over the dial-up public switch telephone network. STU-III operates in full-duplex over a single telephone circuit using echo canceling modem technology. STU-IIIs come equipped with 2.4 and 4.8 kbps code-excited linear prediction (CELP) secure voice. Secure data can be transmitted at speeds of 2.4, 4.8, and 9.6 kbps. There are many manufacturers each having different maximum throughput rates. The data throughput between two STU-IIIs can only be as great as the slowest STU-III connected. After, 31 March 1998, General Dynamics (Formerly AT&T) became the only producer of STU-III terminals, and will continue to produce terminals until December 1999.

The STU-III Family consists of some of the following devices:

  • The STU-III/Low Cost Terminal (LCT) was designed for use in the office environment among a broad spectrum of military, civil, government, and selected private sector users. It is compatible with standard modular or multiline (key system) connectors and operates full-duplex over a single telephone circuit.
  • The STU-III/Cellular Telephone is interoperable with all other versions of the STU-III Family. It combines cellular mobile radiotelephone technology with advanced secure voice/data communications. The unit includes a message center that is integrated with the standard cellular handset; it can be conveniently mounted inside a vehicle and provides all STU-III functions, including authentication/classification display.
  • The STU-III/Allied (A) is a specialized version of the STU-III/LCT that is compatible with the STU-II. It retains all basic STU-III functions and capabilities and incorporates STU-II BELLFIELD Key Distribution Center (KDC), STU-II net, and STU-II multipoint modes of operation.
  • The STU-III/Remote Control Interface (RCU) provides RED enclave subscribers with STU-III compatible secure communications in a rack-mounted remotely controlled line encrypting unit. When used in conjunction with a RED switch or conferencing director, the STU-III/R allows STU-III users to confer with multiple STU-III users or others who have secure functions. It is capable of encrypting/decrypting voice or data over two-wire or four-wire telephone systems and incorporating a 2.4 kbps BLACK digital (external modem) interface.
  • The MultiMedia Terminal (MMT) 1500 is a diversified STU-III capable of clear or secure voice and data communications over both analog and digital mediums. The MMT interfaces to the commercial telephone system via a standard RJ-11 telephone jack and to digital systems through a Black Digital Interface (BDI). The BDI port will support both half-and full-duplex communications, precedence dialing, black digital network signaling, and multiple satellite hopsWhen unattended the MMT can automatically answer an inbound call without operator intervention and establish a secure link with any user on a preprogrammed Access Control List (ACL).
  • The Inter Working Function (IWF) is the shore gateway device that provides the digital to analog conversion between the MMT and the analog STU-III. The IWF supports half and full duplex voice and data communications with rates of 2.4, 4.8, and 9.6Kbps. The IWF improves secure voice and data synchronization over multiple satellite hops with programmable extended time-outs and pre-staging of STU-III call information. The IWF supports all necessary network-signaling functions to enable call setup and status messages including canned voice messaging to the analog user.
  • The STU-III Secure Data Device (SDD) is designed with the same capabilities as other members of the STU-III family including Secure Access Control System (SACS), remote authentication (RA), remote control, auto-answer secure data, and capable of operating in both attended and unattended environments. The SDD provides protection for facsimiles, e-mail, and computer communications.
  • The Motorola CipherTAC 2000 (CTAC) STU-III family compatible secure voice communications via cellular phone. CTAC without an inserted CipherTAC 2000 security module is unclassified and functions as a non-secure commercial off the shelf (COTS) telephone product. The CTAC CiphterTAC security module is certified for all levels of classified discussions up to and including SECRET in an adequate operating/security environment.

The Secure Terminal Equipment (STE)

The Secure Terminal Equipment (STE)/Office is the evolutionary successor to the STU-III. The STE program will improve shore secure voice communications as well as shipboard communications by changing out the analog STU-III products with digital-based STE products. The STE cryptographic engine is on a removable Fortezza Plus KRYPTON Personal Computer Memory Card International Association (PCMCIA) Card, which is provided separately. The STE Data Terminal provides a reliable, secure, high rate digital data modem for applications where only data transfer (FAX, PC files, Video Teleconferencing, etc.) is required. All STE products will be STU-III secure mode compatible with the following enhanced capabilities:

  • Voice-recognition quality secure voice communication.
  • High-speed secure data transfers (up to 38.4 Kbps for asynchronous or 128Kbps for synchronous).

STE terminal products can use Integrated Services Digital Network (ISDN), analog PSTN, TRI-TAC, or direct connection to Radio Frequency (RF) assets via RS-530A/232E ports. Maximum STE performance may be attained only by those commands employing ISDN service with two Bearer Channels (2B+D ISDN Service). When connected to a PSTN (Analog Telephone) service, the STE/Office units will only support current STU-III voice and data capabilities.

A tactical version, STE/Tactical is a replacement for MMT 1500 with a Digital Non-secure Voice Terminal (DNVT) adapter. Though not a direct replacement for the KY-68, the STE/Tactical can serve as a DNVT replacement with secure voice communication capabilities in STU-III modes over TRI-TAC/Mobile Subscriber Equipment (MSE). STE/Tactical is not secure mode compatible with the Digital Secure Voice Terminal DSVT KY-68.

A STE Direct Dial capability, comprised of the STE/C2 Tactical terminal and/or associated STE/Interworking Function(s) will improve on the existing Navy "Direct Dial" secure voice ship to shore dial-up operations. STE Direct Dial improves secure mode connectivity, provides operational flexibility support for both plain text and cipher text voice modes, and provides a standardized secure ship digital telephone system solution and Joint CINC interoperability with forces at sea and ashore. ''' Individual STE Product Capabilities:'''

  • STE/Office provides enhanced STE capabilities over digital ISDN and STU-III over analog PSTN.
  • STE/Data provides STE and STU-III data capabilities only.
  • STE/Tactical with Wedge supports STU-III Black Digital Interface (BDI) over TRI-TAC/MSE or RF asset.
  • STE Direct Dial:
   o STE/C2 Tactical with Wedge supports STU-III BDI over ISDN or RF asset.
   o STE/IWF provides interface with PSTN (Analog) and ISDN (Digital). 

STE products without an inserted Fortezza Plus KRYPTON Card are unclassified and function as non-secure COTS telephone products. The Fortezza Plus KRYPTON Card is currently designated as an Accounting Legend Code 1 (ALC-1) item by the NSA. Even though STE's are unclassified items, they should still be treated as high-value Government property (e.g., such as an office computer). Certification of STE will provide security for all levels of traffic, up to and including TOP SECRET Special Compartmented Information (TS-SCI). When a Fortezza Plus KRYPTON Card is inserted into a STE, secure storage must be provided to the extent required by Operational Navy Instruction (OPNAVINST) 5510.1 (series) for the maximum classification level of the key used. Fortezza Plus KRYPTON Card is considered classified to the maximum level of key classification until it is associated with a STE terminal. Once associated with a STE terminal, the card is considered unclassified when not inserted in the associated STE terminal.


National Security Agency (NSA) has determined that the key order processes are under development for compliance with Year 2000; NSA memo of 12 Jun 98 applies. Equipment Height (in) Width (in) Depth (in) Weight (lb) STU-III/Low Cost Terminal (LCT) 4.0 10.0 9.0 8.0 STU-III/Cellular Terminal (SCT) 3.0 10.0 15.0 12.0 STU-III/Allied (A) 2.5 9.0 10.0 8.0 STU-III/Remote Control Interface (R) 3.2 10.5 12.75 12.5 MultiMedia Terminal (MMT) 1500 3.0 9.7 8.5 6.5 Inter Working Function (IWF) 7.0 19.0 22.0 6.5 STU-III Secure Data Device (SDD) 2.5 8.0 9.5 7.0 Motorola CipherTAC 2000 (CTAC)* 5.23 2.33 0.6 0.25 Motorola CipherTAC 2000 (CTAC)** 6.16 2.33 0.99 0.30 STE/Office w/Wedge 10 9.5 4.6 6.0 STE/Office w/o Wedge 10 9.5 2.57 6.0 STE/Data 10 9.5 2.4 6.0 STE/Tactical 10 9.5 4.6 6.0 STE Direct Dial (GW/IWF Module) 9.2 11.0 32.0 STE Direct Dial (VME Enclosure) 17.0 14.0 19.3

  • Without security module
    • With security module

Data Rates:

STU-III/Low Cost Terminal (LCT) Supports voice/data rates of 2.4, 4.8, 9.6, and 14.4 Kbps STU-III/Cellular Terminal (SCT) Synchronous (sync): 2.4, 4.8, 9.6 Kbps Asynchronous (async): 300, 1.2, 2.4 Kbps STU-III/Allied (A) Data: 300, 1200, 2400 bps async & 2400 bps sync STU-III/Remote Control Interface (R) Voice: 2.4, 4.8, 9.6 Kbps full-duplex synch/async 2.4 Kbps half-duplex

Data: 2.4, 4.8, 9.6 Kbps full-duplex sync/async 75, 110, 300, 600, 2400, 4800, 9600 bps full-duplex async 2.4 Kbps (or slower rates) half-duplex async

MultiMedia Terminal (MMT) 1500 BDI Voice: 2.4, 4.8, 9.6 Kbps secure full duplex, 2.4 Kbps half-duplex Analog Voice: 2.4, 4.8, 9.6 Kbps secure full/half-duplex

Data: 4.8, 9.6 Kbps secure full-duplex sync & async

Simultaneous Voice/Data: 2.4 Kbps secure full-duplex (voice/data) Inter Working Function (IWF) Data: 2.4, 4.8, 9.6 Kbps full & half-duplex STU-III Secure Data Device (SDD) 2.4, 4.8, 9.6 and 14.4 Kbps full-duplex sync & async w/2.4 Kbps half-duplex sync secure data communications link Motorola CipherTAC 2000 (CTAC) Voice: up to 4.8 Kbps STE/Office Synchronous Secure: 2.4 to 128 Kbps Asynchronous Secure: 2.4 to 38.4 Kbps STE/Data Synchronous Secure: 2.4 to 128 Kbps Asynchronous Secure: 2.4 to 38.4 Kbps STE/Tactical Synchronous Secure: 2.4 to 128 Kbps Asynchronous Secure: 2.4 to 38.4 Kbps SSTE Direct Dial (GW/IWF Module) Voice and Data: Narrowband: 2.4, 4.8, 9.6 Kbps

Wideband: 64, 128 Kbps

STU-III/Low Cost Terminal (LCT) 20 Watts (nominal), 115 or 230 VAC, 50-60 Hz STU-III/Cellular Terminal (SCT) Volts: +10 to +14 Vdc negative ground, Amps: 3.5 maximum STU-III/Allied (A) 90 Vac to 265 Vac, 50 Hz to 60Hz STU-III/Remote Control Interface (R) 90-265 Vac, 117V. 305 nominal MultiMedia Terminal (MMT) 1500 90-270 Vac, 47-63 Hz (Auto-ranging) Motorola CipherTAC 2000 (CTAC) Less than 2.0 watts in secure with 1.75 hours talk time on a 3-hour battery in secure mode. Less than 0.8 watts in clear with 2.75 hours talk time on a 3-hour battery in clear mode. Less than 0.2 watts in power down with 24 hours standby time on 3-hour battery STE/Office External Power Supply 90-253 Vac, 47-63 Hz, Autoranging 20 watts maximum STE/Data External Power Supply 90-253 Vac, 47-63 Hz, Autoranging 20 watts maximum STE/Tactical External Power Supply 90-253 Vac, 47-63 Hz, Autoranging 20 watts maximum Standard EOC Connector Portable Uninterruptible Power Supply (PUP) SSTE Direct Dial (GW/IWF Module) External Power Supply 90-253 Vac, 47-63 Hz, Autoranging


STU-III/Low Cost Terminal (LCT) STU-III/Cellular Terminal (SCT) STU-III/Allied (A) Storage Temperature: -40C to 70C Operating Temperature: 0C to 50C Motorola CipherTAC 2000 (CTAC) Operating Temperature: 0C to 50C Storage Temperature: -40C to 70C STE/Office Operating Temperature: 0C to 40C Operating Rel Humidty: 10% to 90% Noncondensing Storage Temperature: -20C to 60C STE/Data Operating Temperature: 0C to 40C Operating Rel Humidty: 10% to 90% Noncondensing Storage Temperature: -20C to 60C STE/Tactical Operating Temperature: 0C to 40C Operating Rel Humidty: 10% to 90% Noncondensing Storage Temperature: -20C to 60C STE Direct Dial (GW/IWF Module) Operating Temperature: 0C to 50C Operating Rel Humidty: 10% to 90% Noncondensing Storage Temperature: -20C to 60C


  • Software Support Activity (SSA): Not applicable
  • Primary Inventory Control Activity (PICA): Commercial Supply Support / U. S. Army
  • Depot: Cryptographic Repair Facility (CRF) San Diego, CA / Motorola Inc.
  • In-Service Engineering Agent (ISEA): SPAWARSYSCEN Charleston
  • Material Support Date (MSD): Not applicable


  • Operational Requirements Document (ORD): 063-094-87, 10 Jan 86
  • Test and Evaluation Master Plan (TEMP): Aug 89
  • Navy Training System Plan (NTSP): Not required
  • Integrated Logistics Support Plan (ILSP): National Security Agency (NSA) document, 31 Aug 92
  • Technical Manuals (TMs): Manufacturer User's Manuals and Handbooks


  • Contact Director COMSEC Material Systems (DCMS) in accordance with CMS 5A "The policies and Procedures for issuing accounting, handling, safeguarding, and disposing of COMSEC Communications Security Manual: artical 410 for repair/disposition instructions.

STU-III LCT-I: Motorola SECTEL 1500 User's Manual 68-P35279D002 Security-Plus AT&T STU-III Communications Terminal User's Manual ON416007-1 Operators Guide for the RCA/STU-III Terminal (Type 1) F001-R1 GE Operating Guide for the GE 9600 STU-III Terminal (Type 1) 10038631 STU-III Secure Voice/Data Terminal Model 1100/1150 User's Manual ON-49306 Operators Guide for the GE 9600 STU-III Terminal (Type 1) 10038631 Rev. A STU-III Celluar: STU-III DYNASEC Secure Cellular Telephone User's Manual 68-P333623C001 STU-III DYNASEC Secure Cellular Telephone Installation and Service Manual 68-P33624C DYNA T-A-C 6000XL Cellular Mobile Telephone User's Manual 68-P81116E58-B Motorola Ins. Cellular Mobile Telephone Installation and Programming 69-P81119E99-A Installation Instructions for Mobile Antennas None Installation Instructions for DYNA T-A-C Cellular Mobile Telephone None Programming Guide for Cellular telephone None STU-III/A: STU-III/A SECTEL User's manual 68-P33236C001 STU-III/R: STU-III/R Remote Controllable Secure Voice/Data Terminal 68-P39024D001 STU-III/R Interface Control Document 5983-B0004-001 STU-III/SDD: AT&T STU-III Secure Data Device Model 1900 User' Manual None STU-III/MMT: SECTEL MMT 1500 Multimedia Terminal User's Manual 6844083D02 P/N 5DGA14X BDI Black Digital Interface User's Manual 6844085D02 User Logistics Support Summary (ULSS): SPAWARSYSCOM, Sep 95 (P4110.969) (Draft) Computer Resources Life Cycle Management Plan (CRLCMP): Not required

johnsherer?15 December 2008, 09:39

Just to remind anyone (or inform if they're new to this)...The STU III was a decent device, but is obsolete, does not comply with SCIP, and is logistically unsuportable and so needs to be replaced.