[VOIPSEC] Key Negotiation for SRTP (Ahmar Ghaffar)

[Robert Moskowitz]

At 11:42 AM 9/2/2005, Mark Baugher wrote:
>Robert,
>On Aug 31, 2005, at 11:03 AM, Robert Moskowitz wrote:
>...
>>I just shudder at the reaction from certain quarters when they notice
>>that sdescription passing of keys is an automatic CALEA tool.
>
>I think you are misunderstanding something.  sips is hop-by-hop by 
>definition, sdescriptions is not.  sdescriptions can use a 
>end-to-end public key techniques as well as any alternative key 
>establishment mechanism.  But without a public key solution, MIKEY 
>uses PSK and sdescriptions uses TLS, which is hop-by-hop in a sips 
>context.  Both have the issue that you mention above.

Ok, I was not clear.  It was the following messages that I was responding to:

"From: "Christian Stredicke" <Christian.Stredicke at snom.de>

The latest idea would look like this: Transfer the key in clear text via
a secure protocol in the SDP.

[SDP m-line]
A=crypto:1 F8_128_HMAC_SHA1_32
inline:MTIzNDU2Nzg5QUJDREUwMTIzNDU2Nzg5QUJjZGVm"


"Sdescriptions is an interesting alternative, but as has been pointed about,
it is not an end-to-end security protocol, and would need something like
S/MIME etc.  Hop-by-hop security (as with SIP/TLS) is not sufficient.

cheers,
Lakshminath"

"1) sdescriptions:  Amounts to just sending the key in the clear (base-64
encoded) via SDP, and so needs a secure channel.  There are two options
there again and one is hop-by-hop security using TLS and the other is
S/MIME.  The question in my mind is whether sdescriptions is a long-term
solution (more on that later).

best regards,
Lakshminat"

So we have been discussing key exchanges where:

1)  The SRTP session key is known to the SIP server(s).
2)  If and SIP-to-SIP hop is not encrypted, the key is exposed to 
eavesdropping.

Yes, a secure, end-to-end key exchange COULD be used.

We should select ONE that is relatively worth implementing and call 
that the interoperable mode.


Robert Moskowitz
Senior Technical Director
ICSA Labs, a division of Cybertrust, Inc.
W:      248-968-9809
F:      248-968-2824
VoIP:   248-291-0713
E:      rgm at icsalabs.com

There's no limit to what can be accomplished if it doesn't matter who 
gets the credit