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AT&T 1100
Secure Telephone Unit STU-III

The 1100 was a third generation Secure Telephone Unit (STU-III), developed in the US by the National Security Agency (NSA) around 1987 and introduced by AT&T (later: Lucent) in 1992. The 1100-series was interoperable with STU-III terminals from the other two manufacturers, Motorola and RCA (later: GE), and used a removable plastic KSD-64A key for key distribution and as CIK.
When the STU-III was developed by the NSA in 1987, it was decided that various manufactuers would be allowed to build it and that it would be available with different crypto-algorithms, so that the device could be used at various security levels, both inside and outside the government.

The image on the right shows a basic 1100 which was a true NSA Type 1 encryption device. It was interoperable with the STU-III terminals from other manufacturers and contained hardware-based (ASIC) encryption. A Crypto Ignition Key (CIK) was inserted at the right front.
AT&T STU-III with CIK installed

All AT&T 1100 STU-III phones have built-in LPC-10E, CELP and MRELP vocoders and can send voice and data at 2400, 4800 and 9600 bps (bits per second) in full duplex or half duplex, depending on the mode in which they are used. In Type 1 mode, all 1100 models supported 2400 bps (LPC-10E) and 4800 bps (CELP and HDLPC), but not 9600 bps like the Motorola SECTEL 1500. As a result, the Motorola 1500 offered a better voice quality. At the rear is a 25-way D-type socket with the serial (RS232) port that gives access to the built-in V.26 and V.32 modems. The units are AUTOVON compatible and are fully shielded against unwanted eminations (TEMPEST) [1].

The line of STU-III telephones from AT&T started in the late 1980s with a secure phone that was called the Security Plus or simply the STU-III [8]. It came in a Misty Cream case and, because of its large size, most users referred to it as the big white monster or the boat anchor. The KSD-64 key (CIK) was entered at the right upper side of the phone. It was eventually replaced by the more compact 1100-series, that has the size of a normal desktop telephone with a key at the right.

On 30 September 1996, the AT&T Technologies business unit of AT&T Corporation (including Western Electric and Bell Labs) was demerged and continued as Lucent Technologies [6]. On 1 October 1997, the products were rebatched again, after General Dynamics (GD) took over the Advanced Technologies Systems Unit from Lucent [7]. The 1100 became known as Surity 1100.

The AT&T/Lucent/GD 1100 was the last STU-III phone that remained in production after the other manufacturers had discontinued their STU-III products, following the announcement of its successors, the Secure Teminal Equipment (STE) and other SCIP compatible products. All STU-III products have now been phased out, with the last keys expiring on 31 December 2009.
AT&T STU-III with CIK installed Display AT&T logo, sometimes hidden under a Lucent label Keypad Inserting the CIK Zeroize button. Use a paperclip to press the recessed button. Removing the plastic cover Rear view of the 4100

The 1100-series consisted of a number of models, each with its own features, colour and security level. Globally speaking, the first digit of the model number determines the cryptographic level, as identified by the NSA. The 1100-series is therefore an NSA Type 1 product, the 2100 is Type 2, etc. Only the 1100 and the 2100 series are real STU-III terminals. Some examples:
  • 1100
    The 1100 is the basic STU-III with Type 1 encryption. It was used by the US Government, the Canadian Government and also by NATO, for conversations at the highest level (TOP SECRET). It uses a KSD-64 as a Crypto Ignition Key (CIK) and has built-in hardware-based encryption algorithms. For use by NATO the SAVILLE algorithm was used. All Type I phones supported LPC-10E (2400 bps) and CELP (4800 bps).

  • 1150
    This is the multi-line version of the 1100. It is also a Type 1 encryption product, but it can be connected to more than one line simultaneously.

  • 2100
    This is the Type 2 version of AT&T's STU-III terminal. The level of security is slightly less than that of the 1100 series, but still enough for many US Government services, such as the FBI and the CIA. Any Type 2 STU-III terminal can communicate securely with any other type or brand of Type 2 STU-III (e.g. Motorola SECTEL 2500), but also with Type 1 devices, such as the 1100. Like the 1100, the 2100 uses a KSD-64 as CIK. All Type II phones supported LPC-10E (2400 bps) and CELP (4800 bps).

  • 3000
    The 3000-series was the US commercial variant of the above phone. It uses Public Key Encryption (PKE) algorithms, such as DES, Tripple DES (3DES) and Advanced Encryption Standard (AES-256). The 3000-series consists of many different commercial devices, such as the CSD-3600 and the ill-fated TSD-3600 with its Clipper Chip. The 3000-series are Type 3 encryption products and are not compatible with Type 1 and Type 2 devices.

  • 4100
    This version can been seen as the international variant of the phone. It does not use a CIK and does not contain hardware-based encryption. Instead it uses publicly available encryption standards, based on Public Key Encryption (PKE), such as DES. The 4100 series is classed as a Type 4 encryption product and can freely be exported from the US. Depending on the customer or country, it was supplied with a different algorithm. More...

The 1100-series was fully compatible with the STU-III terminals from other manufacturers, such as the Motorola SECTEL 1500. It was also (downwards) compatible with the 2100 series (Type 2 encryption). The AT&T range of STU-III telephones have appeared on the market under different names, inlcuding AT&T, Lucent and General Dynamics. These units are all interoperable.
Crypto Ignition Key
Within the STU family, the STU-III introduced the concept of the Crypto Ignition Key (CIK), which is used for protection of the cryptographic keys stored inside the phone. This concept is similar to that of the STU-II compatible Spendex 40 that was introduced by Philips in the early 1980s.
With the STU-III, the CIK takes the form of a physical plastic key that is inserted into a slot at the right hand side of the phone and rotated 90° clockwise. The image on the right shows the KSD-64 that was used with the STU-III family.

The key, manufactured by Datakey Electronics in the US, contains 64Kb of storage space, and can be used for a variety of purposes, including CIK functionality and key transport. In practice, the color of the label should give a hint to the key's functionality. A white label indicated that the key was used as CIK (see the image at the top).
Crypto Ignition Key (CIK)

When used as CIK, the key contains a random number (generated internally by the phone when the keys are loaded) that is used for encryption of the actual cryptographic keys stored inside the phone. This way, a phone without the CIK, or a lost CIK alone, has no meaning whatsoever and will not reveal any information about the cryptographic keys. Only when the CIK is entered into the phone it is paired to, can the original keys be recovered and can secure mode be enabled.

In the 1100, the KSD-64 is entered into the special keyhole, called keyceptical by Datakey, at the right front of the phone. This socket is not present on civil variants of the phone, such as the 4100. The KSD-64 is no longer in production and is now replaced by the PK-64 from the same manufacturer. Other versions of the key, also from the same manufacturer, are used with a variety of other crypto devices, such as the KIV-7; the US replacement for the KG-84.

 More about the KSD-64
AT&T STU-III with CIK installed CSD-4100 STU-III terminal with KSD-64 Crypto Ignition Key (CIK) Keyhole for the CIK at the front right of the phone CSD-4100 and KSD-64 key Inserting the CIK Activating the CIK CIK in the active position

The 1100-series secure phone is housed in a sturdy pre-shaped compartimented die-cast aluminium case. All connections to the outside world are at the rear (power, line and serial port), except for the handset which is connected at the left side. The unit needs an external PSU.
The case can easily be opened by removing four long crosshead screws at the bottom, after which the top half and the bottom half can be separated. The interior of the 1100 consists of three large PCBs: a main (digital) board at the center, a board with the controls (top) and an interface board which is mounted at the bottom.

The controls board is mounted inside the top half of the phone, just behind the control panel. It is connected to the bottom part by means of a double flat cable and a 2-wire cable for the hook switch. The top part also contains the display.
Close-up of some special components (DSP, V.32 interface and a large EPROM)

The bottom part contains the rest of the electronics and is covered by a metal shield in order to avoid the emission of compromising (radio) signals (TEMPEST). After removing the metal shield the solder side of the Main Board is revealed. At the top is the connector to the controls board.

At the center of the Main Board is a so-called tamper switch that causes the non-volatile RAM (containing the current cryptographic keys) to be cleared as soon as the metal shield is removed. This way the keys are protected against tampering. The Main Board is held in place by a single screw and can easily be removed. The component side of the board is partly covered by a metalized plastic EMC shield. Below the shield are the actual digital components of the 1100.

The main board is nicely compartimented, so that all sections of the circuit are easily identified. Two TMS320 Digital Signal Processors (DSPs) from Texas Instruments (TI) are used for the implementation of the vocoders (e.g. LPC-10). At the center of the board is the the hardware-based encryption chip, that contains NSA-developed algorithms. This chip is omitted (i.e. not mounted) in the civil 4100 models, which have a weaker software-based encryption algorithm.

Below the Main Board, in the bottom of the phone, is the Interface Board that contains all the electronics for connection to the outside world. Like the Main Board, it is nicely compartimented, so that its various functions are easily identified. In the left corner at the rear is the interface to the analogue (PSTN) telephone line. In the right corner at the rear is the RS-232 interface with its typical 25-way D-type socket. The section at the front contains the actual telephone electronics, consisting of a V.32 modem chip and AT&T's own DSP chip. At the center is a backup battery.
Opening the case Top and bottom part separated Bottom part Top part with controls board Close-up of the display (rear) Internal speaker Tamper switch Close-up of some special components (DSP, V.32 interface and a large EPROM)
Bottom part of the case containing the Interface Board Interface board Telephone line interface RS-232 interface Lithium back-up battery, used for retaining the keys when the phone is switched off. Modem chip (left) and DSP (bottom) Compartimented die-cast aluminum bottom case shell Close-up of the rear section of the bottom shell of the case

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ASIC   Application Specific Integrated Circuit
General term for a custom-built electronic chip.

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). (Wikipedia)

CELP   Codebook Excited Linear Prediction
Method for digitizing human speech, also known as a vocoder. CELP is a US Government standard that can be used at 4800 baud.

DSP   Digital Signal Processor

HDLPC   High-Definition Linear Predictive Coding
This is an improved variant of LPC that produces a better speech quality at 4800 baud. As the format is General Dynamics proprietary, it may not be supported by other STU-III manufacturers.

LPC   Linear Predictive Coding
A method for digitizing human speech by analyzing and storing specific characteristics of it, in such a way that an intelligible signal can be reconstructed later. LPC-10E was a US Government standard that was used at 2400 baud on all STU-III terminals.

PSU   Power Supply Unit

  1. Granite Island Group, Secure Communications Systems
    Details about AT&T STU-III systems. Retrieved March 2013.

  2. The Free Library, AT&T introduces new line of secure telephone equipment
    PR Newswire Association, 6 May 1992. Retrieved March 2013.

  3. Network World, AT&T encryption unit is NSA approved
    Telecommunications, 4 February 1991, p. 11. Retrieved March 2013.

  4. AOS Inc., Technical Communications Corporation, CSD 4100 Vice/Data Terminal
    4-page brochure. Date unknown. Retrieved March 2013.

  5. Navy INFOSEC website, Secure Terminal Unit Third Generation
    Date unknown. Retrieved March 2013.

  6. Wikipedia, Lucent
    Retrieved March 2013.

  7. General Dynamics, GD completes acquisition of Lucent Technologies
    Advanced Technologies Systems Unit

    General Dynamics website. News. Retrieved March 2013.

  8. Joseph Tag, Various bits of information about AT&T phones and STU-III in general
    Personal correspondence via e-mail. April 2013.

  9. ESTcorp, STU Products Page
    Website. Retrieved April 2013.

  10. General Dynamics, STU-III Voice/Data Terminal SCS Models 1100/1150
    Lucent Technologies. User's Manual ON-493106, Revision E, December 1997.

Further information

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