Re: [LinkSec] Teleconf 12/17 notes
allyn romanow wrote:
>
> Security is a system design, 802 is at the MACs only
> look at DOCSIS - similar problem?
> yes, same concerns as EPON
> prevent theft of service and protect customers from each other
>
> Dolors - DOCSIS encaps the entire frame
> put fields in front of the frame
> 802.16 arch is very similar to DOCSIS
> no fields in clear outside?
> single key upstream, multiple keys downstream, 1 SA upstream
>
> [SA= Security Association]
> Mick - each receeiver on LAN only has to decrypt under 1 key
> The problem is which key should you use to decrypt? if receive frame
> from upstream
> headend receives all frames
> several SAs, headend belongs to all
> multiple multicast SAs on downstream
> SA sent in clear, the rest is protected
> protects one listener from another
> down stream cable modems, headend uses different keys for different
> cable modems
> one key for me, one for each group, get the SAID in clear which says
> which key to use
> encrypt after the first 12 bytes of the frame, first 12 Bytes not encrypted
> CRC encrypted, so if change source address or destination address,
> need to change CRC
> CRC is a guessing game, not a hamming distance
> would cause trouble in 802.3
> some people think still need hamming distance of 4
I've never been clear on what "upstream" and "downstream" mean in cable plants, so
bear with me. If the stream from the headend to the subscriber is protected
by a single key shared among all subscribers, then it is based on an assumption
about "average" use of the service--namely web surfing. The data coming from
the headend is either cleartext dancing bears on port 80, in which case, having
your neighbour be able to see that is no big deal. If it isn't on port 80,
it's on port 443, which means your neighbour can't see it due to the encryption
happening at higher layers.
There are two problems with this:
o Security model based on "average use" case. Since they have to do key
management anyway (to arrange for keys between the subscriber and the headend),
it seems that they already have the infrastructure necessary to "do it right".
o Ability for me to inject packets that appear to be coming from the headend
is restricted purely by the fact that off-the-shelf DOCSIS modems don't
allow users to inject traffic onto the cable on the stream that the headend
sends on. This is assuming a threat model where the bad guy is lazy.
He already has the key, all he needs is some special hardware, and the DOCSIS
specification, and he can inject traffic onto the cable. Granted the bad
guy needs motivation and a little bit of cash to do this, but security models
based on assumptions of laziness and poor finances are generally very bad.
--
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Marcus Leech Mail: Dept 8M70, MS 012, FITZ
Advisor Phone: (ESN) 393-9145 +1 613 763 9145
Security Architecture and Planning Fax: (ESN) 393-9435 +1 613 763 9435
Nortel Networks mleech@nortelnetworks.com
-----------------Expressed opinions are my own, not my employer's------