From: Erik Baigar
very interesting reading
If you want to see a great example of why it was important, check out the
so-called 'Berlin Tunnel':
https://en.wikipedia.org/wiki/Operation_Gold
http://www.fas.org/irp/cia/product/tunnel-200702.pdf
Some of the traffic that was intercepted was teletype traffic - which had
been encrypted. However, the equipment that connected the gear to the line
allowed a tiny electronic whisper of the original plain-text onto the line,
along with the encrypted form, and it was possible to read the plaintext off
the line with suitable gear.
Noel