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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.
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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. The module was needed to allow the
TSD-3600 to cope with the large variety of handsets used with modern telephone
sets, each with its own characteristics. A number of suitable modules was
usually delivered with each phone and additional modules could be ordered
from the manufacturer.
The TSD-3600-E is connected in
between the telephone and the handset
using a short cable.
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The TSD-3600-E was introduced in 1993 and the first large order was placed by the
US Government themselves. In the end, the US Government appeared to be the
only major customer for the device and even then, 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]
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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].
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- 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.
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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.
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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.
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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
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© Copyright 2009-2013, Paul Reuvers & Marc Simons. Last changed: Thursday, 18 April 2013 - 13:04 CET
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