[GreenKeys] Two-tape TTY for crypto

[Jim Haynes]

This is known as the Vernam cipher after Gilbert Vernam who worked for
AT&T during WW-I.  Later he worked for Postal Telegraph, and then for
Western Union after the two companies merged.

Teletype made a two-headed XD for the purpose.  Another purpose for the
two-headed XD was a simple two-channel time-division multiplex, but that's
another story.

One version of the Vernam cipher used something like three key tapes of
relatively-prime lengths, they and the message tape all being XORed or
added together.  This was breakable, so the unbreakable version used a
single key tape that was supposed to be totally random.  Believe this
was called SIGTOT by the crypto people.  The down side is that each
station needs as much key tape as all the messages it will send.  So
there is a problem of securely distributing that much tape to the
places that need it.  Today that would be trivial - a CD ROM can hold
millions of characters of key.

Western Union during WW-II had a similar system called Telekrypton.

Google for Gilbert Vernam

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