Re: [Sip] RFC 4474 and SBC traversal

[Dean Willis <dean.willis at softarmor.com>]

On Apr 7, 2008, at 5:43 PM, Eric Rescorla wrote:
> At Fri, 04 Apr 2008 11:50:53 +0300,
> Hannes Tschofenig wrote:
>>
>> This issue is totally independent from E.164
>>
>> I don't like the idea of requiring DTLS-SRTP to provide proof of
>> possession of the keying material.
>
> To be honest, I'm not even sure what this means. DTLS-SRTP inherently
> provides a proof that the peer you're encrypting to has keying
> material that matches whatever was in the fingerprint. That's
> a distinct question from whether the fingerprint is cryptographically
> bound to the message.

Right. The only dependency between DTLS-SRTP and RFC 4474 is integrity  
protection of the key's fingerprint, not the actual key itself. The  
fingerprint is shipped in the SDP, and the key is negotiated in the  
media channel itself using a partial-key combination approach (aka D-H).

The fingerprint is used to relate signaling to media. This works even  
without RFC 4474. What RFC 4474 does is reduce the opportunity for  
somebody on the signaling AND media paths to replace the key, and use  
this to tap the media flow without being noticed by endpoints. It is  
somewhat arguable as to how useful this integrity protection is.

Of course, given that the usual implementation of RFC 4474 is for the  
authentication service that puts in the RFC 4474 Identity header to be  
a proxy that's on the signaling path, this would make it fairly easy  
for that proxy to collude in tapping the call. But if RFC 4474 is used  
end-to-end with client keys, the it provides fairly comprehensive  
protection.

--
Dean
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