[GreenKeys] Isochronous mode

[John Nagle]

> Date: Sun, 03 Jan 2010 12:44:18 -0500
> From: Bill Horne <ehorne at speakeasy.net>
> Subject: Re: [GreenKeys] SB-2244 TTY patch panel
> To: greenkeys at mailman.qth.net
> 
> I think it's just a throttling mechanism to slow the machines down while 
> the rotors align in the crypto gear.
> 
> Bill, W1AC
> 
> On 1/3/2010 11:52 AM, Brooke Clarke wrote:
>> Hi:
>>
>> It may be related to TEMPEST.  A mechanical TTY machine generates RF
>> signals at each on or off transition.  If a bunch of machines are
>> synchronized so they all have a transition at the same time then their
>> TEMPEST emanations may get mixed up.
>>
>> Have Fun,
>>
>> Brooke Clarke
>> http://www.PRC68.com
>>
>>
>> Jim Haynes wrote:
>>    
>>> Some implementations of Teletype with crypto equipment required that the
>>> stepping the Teletype keyboard or tape reader at a fixed rate.  In the
>>> Model 28 parts books you'll see mod kits to adapt things for "synchronous
>>> pulsed operation".  A large station might have a TD stepper facility
>>> consisting of a baud rate generator and then a bunch of boxes that
>>> send out TD stepping pulses to all the TDs.
>>>
>>> I don't know why it was this way.  I don't have any first-hand knowledge
>>> of crypto equipment, but I did once buy some TD stepping equipment at
>>> a surplus sale.
>>>
>>> Where this is done each TD or keyboard has to have its usual loop and
>>> then a loop for the stepping pulses.  So I'm guessing what you have
>>> provides for switching both the signal and the stepping pulses.
>>>
>>>
>>>
>>> jhhaynes at earthlink dot net

     That's called "isochronous mode".  The same bits are sent as if in normal
async mode, but the equipment runs in sync with a constant clock.  This is
normal cryptographic practice for fixed links.   The crypto gear (what NSA calls 
a "key generator", with a type beginning with "KG" generates psuedorandom bits 
at a constant clock rate, and those are XORed with the outgoing data.  When no
data is being sent, the process continues, so the encrypted data stream doesn't
indicate whether anything is being sent or not.  Bits go out even when the
sender is idle.  This prevents traffic analysis.

    For info on Teletype-era crypto, see

http://www.nsa.gov/public_info/_files/cryptologic_histories/history_comsec.pdf

Page 82 starts a clear discussion of TEMPEST problems in the Teletype era.
Teletypes radiated enough to be readable 50-200 feet away.  Friden
Flexowriters (especially the Justowriter, the first typewriter that
could right-justify automatically) could be read from over 600 feet away.
Teletypes could even be read by listening to the audio from a telephone.
The details of what was done about this are in that document.

    For the later generation of Teletype crypto (electronic, as opposed
to older rotor systems) see

	http://www.jproc.ca/crypto/kw7.html

The corresponding 60WPM Baudot rotor machine is the KW-2, but I can't
find any pictures of that on line.  Electromechanical on-line rotor
machines never worked very well - too many sliding contacts running
at too high a speed.

					John Nagle