Re: Cryptographic strength in Wireless Alarm Systems?

"Richard Herring" <junk@[127.0.0.1]> wrote in message
news:xIBilpKPI1TEFwkc@baesystems.com...
> In message <-eadnXVRn4acNtPZRVn-gA@comcast.com>, "Ed Weir (ComCast)"
> <Anon@Maus.duh> writes
>>"Richard Herring" <junk@[127.0.0.1]> wrote in message
>>news:K2+MaSJ3NLSEFwp7@baesystems.com...
>>| In message <444797cc@dnews.tpgi.com.au>, David Eather
>>| <eather@tpg.com.au> writes
>>| >Ed Weir (ComCast) wrote:
>>| >> "David Eather" <eather@tpg.com.au> wrote in message
>>| >>news:44462c4a@dnews.tpgi.com.au...
>>| >> | Ed Weir (ComCast) wrote:
>>| >> [snipped]
>>| >>
>>| >> | I don't understand your argument. It is possible to make a spread
>>| >> | spectrum wireless alarm secure or make it insecure, which seems to
>>be
>>| >> | the same situation as an alarm using a fixed frequency - so what
>>exactly
>>| >> | is your point with SS.
>>| >> The relative impracticality of jamming a spread spectrum vs. jamming
>>| >>a fixed frequency.
>>| >> - Ed.
>>| >You mean a nill difference don't you. It does not take much effort to
>>| >design a transmitter that put out noise all over a defined spectrum.
>>|
>>| That's only necessary in the linear regime. Unless the receiver can
>>| handle an infinite dynamic range, any sufficiently strong signal within
>>| its bandwidth can drive it out of linearity and effectively block it.
>>|
>
>>Yeah - right. http://spincom.ece.umn.edu/shengli/spie01.pdf
>
> Nothing there about nonlinearity of the physical layer. They're only
> considering additive noise and interference.
>
> Ideal linear radio receivers don't exist. Especially wideband ones. Ask
> any RF engineer.
>
>>(Some nuts are just plain numb...)
>>
>>If you really want to know more...
>
> Wrong layer. It doesn't matter how clever your coding is, if the receiver
> can't hear it.
>
>>http://www.tapr.org/ss_intro.html
>>
>>If you want to try jamming something for the fun of it (and test your
>>hypotheses)
>>http://www.dmartstores.com/ge27730ge2ad1.html (use a Faraday cage or don't
>>drop the soap)
>>
>>All goooogled with [jamming digital spread spectrum]
>>- all a great deal harder than you thought.
>>
>>Well, there you go.
>>Cheers,
>>- Ed.
>>
>
> --
> Richard Herring

Why all this discussion of jamming? Surely the sensible way of designing an
alarm is to use absence of a signal as the trigger.
I would expect each component to broadcast a heartbeat signal that stops
when the component wants to signal an alarm.
A jamming device would also trigger the alarm by preventing receipt of the
heartbeat.

        Mark
Received on Mon May 1 02:06:34 2006