[Dshield] DI-624 encryption - Run it if you have it!

Rick Klinge rick at jaray.net
Wed Feb 25 19:51:12 GMT 2004


> Rick,
> 
> re: your comment on using frequency hopping for additional security:
> 
> Although the 802.11 standard supports several physical layer 
> standards, 
> Direct Sequence Spread Spectrum (DSSS), Frequency Hopping Spread 
> Spectrum (FHSS), infrared, and Orthogonal Frequency Division 
> Multiplexing (OFDM - this method was added to the standard as 
> a part of 
> both 802.11a and 802.11g in ~2001 to support higher data 
> rates up to  54 
> mbps, see: http://www.wave-report.com/tutorials/OFDM.htm)  
> virtually all 
> of the commercial products of which I am aware use either  the DSSS 
> method (802.11 and 802.11b) or the OFDM method (802.11a) or both DHSS 
> and OFDM (802.11g). With the DHSS method the spreading 
> sequence (an 11 
> bit barker code) is standardized and is the same for all radios. Thus 
> spread spectrum is not used for message privacy but as a means for 
> maintaining satisfactory signal to noise (S/N) ratio  while 
> sharing the 
> channel in unlicensed radio bands where there may be other signals 
> (cordless telephones, microwave ovens, etc.) on specific 
> frequencies in 
> the same band. If different transmitter-receiver pairs were using 
> different and more complex spreading codes from those used in 
> the case 
> of DSSS, then such a multiplexing method would be referred to as Code 
> Division Multiple Access or CDMA, like we see in some of the mobile 
> wireless networks, especially in the US.
> 
> I would be interested to learn if there are commercial 
> products deployed 
> using FHSS. However I should point out that even with FHSS as defined 
> within the 802.11 standard, the hopping sequence is fixed by 
> region of 
> the world  and does not provide a means to assure any kind of message 
> privacy.
> 
> I suspect that with the introduction of 802.11b which increased the 
> channel data rate to 11 mbps versus the 802.11 FHSS maximum 
> data rate of 
> 4.5 mbps that, in the end, 802.11b and consequently DSSS won out 
> commercially over 802.11 FHSS. Better still, in terms of 
> throughput at 
> least, are the OFDM based standards, 802.11a and 802.11g, both of  
> which, as mentioned above, support the much higher 54 mbps 
> maximum data 
> rate.
> 

Thanks John for the very informative overview.  I have a T1 wireless link
that I use via my home office and I believe it does use DSSS/FHSS as a
secondary to data security.  The devices were made in Australia but I
believe they were special ordered by my provider, here in the us, to meet
there needs.  I have yet to effectively capture any data when this puppy is
switching frequencies, let alone random channels, what appears to be several
times a second.

The radio portion of it does rx/tx in the 2.4 GHz range and, for what I'm
told, it has never been hacked.  I do recall using devices from this same
company, while I was in the military, somewhere around the mid 1980's.. Of
course the technology has changed since then too.

Thanks again,

~Rick







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