RE: Early arriving media before 200 OK

["Dan Wing" <dwing@xxxxxxxxx>]

> > ...
> > > I agree that this is a concern--though not as serious a one as you
> > > seem to believe (see S 8.4 of draft-fischl-sipping-media-dtls-01)
> > > but it can be ameliorated in a number of ways.
> > > 
> > > 1. If ICE is being used it's not an issue at all.
> > > 2. We can have the SDP offer include a randomly-generated 
> key which
> > >    is used to key TLS PSK mode.
> > 
> > #2 would essentially re-invent Security Descriptions, wouldn't it?
> 
> Not really, no. 
> 
> 1. You would be secure from passive attack on unencrypted signalling
>    channels.

I read through RFC4279 ("PSK Ciphersuites for TLS") again, and I 
admit I still don't understand what you meant in your original 
reply when you wrote "key which is used to key TLS PSK mode".  You
meant something different from what I understood, because what I
understood was that you were going to send the PSK in SDP, which
would share most of the features and drawbacks of Security 
Descriptions.  You meant something else, but I can't tease apart
what you meant.

-d

> 2. You would have one-sided authentication immediately and two-sided
>    as soon as the 200 arrives, even with unencrypted signalling
>    channels.
> 3. The PSK would be used to add entropy to a media level key exchange
>    (RSA, DH, whatever) so you would have secure forking.
> 4. You wouldn't be carrying algorithm negotiation information 
> in the SDP,
>    just a random value.
> 
> 
> > Another technique is to stick the DTLS cookie into SDP 
> > (http://tools.ietf.org/html/rfc4347#section-4.2.1).  Or, if the
> > DTLS cookie is sacrosanct for DoS, an additional cookie could
> > be defined.
> 
> Why would this be superior?
> 
> -Ekr