Mark Preston mark at markpreston.co.uk
Sun Oct 19 14:36:19 UTC 2014

On 19/10/14 09:34, Christopher Hunter wrote:
> 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.
> Cheers
> Chris
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Hi Chris,
Thanks for your further reply which helps me a great deal to understand 
the benefits and flaws of sending faxes. The BT person I referred to was 
a very keen amateur radio enthusiast too, which may tie in with whatever 
he was doing in his job.

Mark Preston

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