[VOIPSEC] Key Negotiation for SRTP (Ahmar Ghaffar)

Mark Baugher mbaugher at cisco.com
Mon Aug 29 16:51:07 PDT 2005

hi Lakshminath,

On Aug 29, 2005, at 3:45 PM, Lakshminath Dondeti wrote
> MIKEY-NULL in my view is for debugging and initial deployment purposes.

Do you mean initial deployment inside a VPN of some sort?

> If a PSK has a lifetime, is that mode ok in your opinion?  I am 
> thinking of the option to use the RSA mode to establish a PSK to 
> amortize the cost of RSA ops (this is a very useful feature for known 
> callees).  Since the PSK is not based on human selected passwords or 
> passphrases, the common security issues with PSKs don't apply in that 
> case.

I don't think so, either.  I'm wondering about using PSK as the 
principal authenticator.  I don't understand exactly how that works in 
a telephony system.
> I don't disagree that we'd need public key authentication 
> infrastructure.

It's like Waiting for Godot.  Phil Zimmerman has an alternative 
approach that does not require one.  That's yet another way to key 
SRTP, however.

>>> Now, latency is an issue and so MIKEY, by design is a very efficient 
>>> key
>>> management protocol.
>> I think of MIKEY as a one or two-round IKE.  That's efficient in 
>> terms of message bandwidth, but it is another state machine to be 
>> maintained, another system to be tested and supported.  That's true 
>> of an S/MIME or PGP/MIME system as well, but we are going to need an 
>> S/MIME or PGP/MIME system anyway in order to protect the integrity of 
>> the signaling end-to-end.
> Yes, there is "implementation complexity" with MIKEY.  From what I 
> have seen on this list, e2e signaling protection seems quite far away.
> best regards,
> Lakshminath
>>> If number crunching is still an issue, there might be
>>> scope for system-level optimizations and perhaps for protocol 
>>> optimizations
>>> too.
>> I'd be interested to know if MIKEY run in NULL encryption mode needs 
>> further "optimization."
>> cheers, Mark
>>> cheers,
>>> Lakshminath
>>> At 10:51 AM 8/26/2005, Ahmar Ghaffar wrote:
>>>> Looking at the new MIKEY mode your mentioned helps me understand 
>>>> why things
>>>> never get done at IETF and why there are always loose ends. Mr. A 
>>>> proposes
>>>> something with ambiguities and then Mr. B proposes something 
>>>> claiming to be
>>>> better with little tweaks to A's proposal. This draft is more or 
>>>> less like
>>>> the public-key mode of MIKEY, and doesn't really solve the latency 
>>>> issue.
>>>> You still have to do number crunching here. So if number crunching 
>>>> is to be
>>>> done anyway then in my opinion its better to have secure SIP 
>>>> signaling via
>>>> TLS/SSL and send the encryption keys in clear text with other SRTP
>>>> parameters (key lifetime, MKI etc) in SDP. The SDP description 
>>>> draft for
>>>> SRTP from Cisco (draft-ietf-mmusic-sdescriptions-11.txt) looks 
>>>> really
>>>> promising in this regard and this is the direction in which 
>>>> everybody should
>>>> be moving.
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