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AT&T TSD-3600-E
Telephone Encryptor

The TSD-3600-E was an advanced telephone security device, developed by AT&T (USA) in 1992. It was based on the controversial Clipper Chip, forcing users to escrow their cryptographic keys. The TSD-3600 was a small white box that was connected between the handset and the phone.

The image on the right shows a TSD-3600-E unit. It only has two buttons and an LCD at the front, two sockets on the right (for connecting the phone) and a power connector at the rear.

Also at the rear is a socket for a module, that adapts the TSD-3600 to the characteristics of the telephone's handset. A number of suitable modules was usually delivered with each phone and additional modules — for other handsets — could be ordered from the manufacturer. The TSD-3600-E box is inserted between the phone and its handset, by means of a short cable.
AT&T TSD-3600 phone encryptor

The TSD-3600-E was introduced in 1993 and the first large order was placed by the United States Government themselves. Eventually, the US Government appeared to be the only major customer for the device. It later turned out that many boxes remained unopened for over 10 years, catching nothing but dust. The cost of a single TSD-3600-E unit in 1993 was $1000 [3].

AT&T TSD-3600 phone encryptor LCD and controls The complete package Plug-in unit (module) Placing a module in the slot at the rear The TSD-3600 connected to a Panasonic phone Interior of the TSD-3600 The Clipper Chip inside the TSD-3600
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AT&T TSD-3600 phone encryptor
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LCD and controls
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The complete package
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Plug-in unit (module)
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Placing a module in the slot at the rear
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The TSD-3600 connected to a Panasonic phone
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Interior of the TSD-3600
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The Clipper Chip inside the TSD-3600

Between 1992 and 1996, a number of different TSD-3600 models were produced by AT&T. They all came in the same case, but contained different encryption algorithms. The information below was taken from a publication by Matt Blaze on Flickr [3].

  • TSD-3600-D
    This was the initial version of the TSD-3600, introduced in 1992. It used the DES algorithm with a 56-bit key. The units were recalled when the Clipper Chip initiative was announced.

  • TSD-3600-F
    This variant was developed for export purposes. It used a 'weak' cipher algorithm with a 40-bit key.

  • TSD-3600-P
    This model used a proprietary cipher algorithm, similar to DES, with a 56-bit key. It was, however, not compatible with the earlier TSD-3600-D.

  • TSD-3600-E
    This was the first model with the ill-fated Clipper Chip inside. It uses the Skipjack algorithm, implemented in the Mykotronx MYK-78T chip.

  • TSD-3600-S
    This was a later variant of the TSD-3600-E. It also contained the Clipper Chip, but was inter-operable with the F and P models.
All five models use a 768-bit Diffie-Hellman key exchange to establish the session key. A 4-digit hash code is derrived from this key and is displayed on the unit's Liquid Crystal Display (LCD). Users had to verify this has code verbally, to detect 'man-in-the-middle' attacks.

Clipper Chip
The Clipper Chip was a chipset developed and promoted by the US Government. It was intended for the implementation in secure voice equipment, such as crypto phones, and required users to give their cryptographic keys in escrow to the government. At the time is was a classified chip.

This enabled law enforcement agencies to decrypt any intercepted traffic for surveillance and intelligence purposes. The controversial Clipper Chip was announced in 1993 and was already defunct by 1996. During that period it was the subject of world-wide debate.

The image on the right shows the Mykotronx MYK78T Clipper Chip as it is present inside the TSD-3600-E.

 More about the Clipper Chip

Close-up of the Clipper Chip inside the TSD-3600

  1. Barry Wels, TSD-3600 - THANKS !
    Object kindly donated, February 2011.

  2. Granite Island Group, Secure Communications Systems
    Information about many different STU-III devices.

  3. Matt Blaze, AT&T TSD-3600E Telephone Security Device (Clipper Chip)
    Published on Flickr, 18 February 2008.
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Crypto Museum. Created: Wednesday 09 February 2011. Last changed: Friday, 23 February 2018 - 21:40 CET.
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