Christopher Hunter cehunter at gb-x.org
Sun Oct 19 08:34:23 UTC 2014

On 19/10/14 09:21, Mark Preston wrote:
> Hi Chris,
> Thank you for your reply, and also thanks to Bernard, Richard and 
> James for their replies to my assertion which may well be wrong. I 
> have no great technical knowledge on this issue, but I am interested 
> to find out more about it. My comment was mainly based on some 
> conversations I had with a person who worked for BT many years ago. He 
> recently died so I can't ask him for clarification. Basically he told 
> me he worked for a special BT unit that installed the phone lines that 
> supplied fax machines and other telecommunications stuff in places 
> like security offices and foreign embassies in London. He claimed that 
> the encryption used for transmitting faxes was very secure.
>     In the same way that people like police can intercept phone calls 
> I imagine that people can intercept faxes, but in relation to plain 
> text email security this can still be considered very secure. I 
> imagine that your fax intercept machine worked by redirecting the 
> output of the fax, or copying it in some way similar to listening in 
> on a phone call.

Hi Mark

The encryption of basic faxes is non-existent.  The scanning data is 
compressed using a crude "modified Huffman" scheme.  There were several 
manufacturers of "secure" fax machines.  The most basic versions just 
changed the transmitted modem frequencies (often just transposing them) 
and were trivial to break.

There were some that used a method of messing with the compressed data 
stream, but all of them had a common flaw - the header data was sent 
twice - once in "clear" and once "encrypted".  It's safe to tell you 
this, because faxes are no longer used for secure communications.  
"Illicit" decryption was carried out in hardware! It was just a large 
board of CMOS logic with about 90 ICs.

In those days, the most interesting work was to develop voice scrambling 
systems and frequency-hopping radio equipment.  We had one 
single-channel method that just sounded like white noise if you tuned to 
it, and had such a low power density that you'd be unlikely to notice 
the signal as you tuned through it.  It really taxed all my electronic 
abilities to get these products from blue-sky thinking, through design 
to manufacture.



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