NSA Clipper Chip
AT&T is the only manufacturer to produce a product with the Clipper chip, here is an example, the AT&T TSD 3600-E Telephone Security Device. AT&T made about 10,000 secure phones and sold about 2,000 to the US Government before the vulnerability was discovered. These phone sets were made in 1993 and the 8,000 unsold sets immediately lost their value. The original list price was around $2,000 but this pair of brand new TSD 3600s were purchased in 2010 for $50 each.
These devices are simple to operate and light and compact enough for use while traveling. They have 3 different modules which plug into the back of the machine for the different types of handsets supported. The handset is plugged into a jack on the side of the TSD and a short phone cable connects the TSD back to the now empty jack in the phone where the handset was located. Now you can make a phone call as usual and when you are ready to "go secure" both parties would agree to turn on their TSD units by pressing the "secure" button. The only other button on the TSD is the "clear" button to return to unenciphered speech. After pressing "secure", it takes about 10 seconds and the LCD screen will show that the two TSDs have successfully exchanged keys and are in a secure mode. They will also display a 4 digit code which can verify for both parties by verbal agreement that there is no man in the middle attack.
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