Subject: [MSEC] MIKEY key generation

From: Mark Baugher <[email-masked]>
Date: Thu, 15 Aug 2002 16:01:49 -0700

hi
   I gave 
http://search.ietf.org/internet-drafts/draft-ietf-msec-mikey-03.txt a first 
read.  I think it is a pretty complete specification.  I have a few 
comments and questions and plan to post a few notes this week and the week 
after next (when I return from vacation).

   I have questions about sections 4.1 and 4.1.4.  SRTP does not need the 
TEK-from-TGK generation method, does it?  MIKEY does not define any other 
security protocols beside SRTP.  So the key generation method won't be used 
until you define some other security protocol that needs it.  True?

   Regarding section 4.1.5, both public-key and pre-shared messages convey 
TGKs.  I don't understand the point of these derivations.

Mark

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From: "Fredrik Lindholm" <[email-masked]>
Date: Sat, 17 Aug 2002 17:40:19 +0200

Hi Mark,

see comments inline.

>until you define some other security protocol that needs it.  True?

I agree that SRTP might not need the TGK to TEK derivation in some
scenarios. However, we address scenarios where both parties will be
sending media. As stated in SRTP Section 9.1.
(http://www.ietf.org/internet-drafts/draft-ietf-avt-srtp-05.txt)
"It is RECOMMENDED that RTP senders on different hosts not use the
same master key to send.". One of our main applications is conversational
multimedia, where we can expect the RTP senders to be on different hosts.
For this and similar scenarios, I believe the TGK to TEK derivation is
useful.

>TGKs.  I don't understand the point of these derivations.

To encrypt and message authenticate the TGKs, one encryption and one
message authentication key are needed. This derivation specify how to do
this from one single pre-shared key (and analogous for an envelop key), so
that the output keys will be "independent" (and fresh from previous CSB in
the case of the pre-shared key).

Thanks (and have a nice vacation),

Fredrik