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Re: [MMUSIC] Hi, May I ask for your opinion on draft-zhou-mmusic-sdes-keymod-01?

"Dan Wing" <dwing@cisco.com> Thu, 19 April 2012 01:32 UTC

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From: Dan Wing <dwing@cisco.com>
To: zhou.sujing@zte.com.cn
References: <079c01cd1d68$05710670$10531350$@com> <OF24015959.EB99E9AE-ON482579E5.0003B462-482579E5.000465E0@zte.com.cn>
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Date: Wed, 18 Apr 2012 18:32:40 -0700
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Cc: mmusic@ietf.org
Subject: Re: [MMUSIC] Hi, May I ask for your opinion on draft-zhou-mmusic-sdes-keymod-01?
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> -----Original Message-----
> From: zhou.sujing@zte.com.cn [mailto:zhou.sujing@zte.com.cn]
> Sent: Wednesday, April 18, 2012 5:48 PM
> To: Dan Wing
> Cc: mmusic@ietf.org
> Subject: Re: RE: RE: RE: Hi, May I ask for your opinion on draft-zhou-
> mmusic-sdes-keymod-01?
> 
> 
> 
> > >
> > > Generaly it is preferable the session key  between two peers  be
> > > established with contribution from both peers,otherwise we will get
> > > into trouble
> > > as  SDES now in the scenarios of re-targetting and forking.
> > > Our 01 version actually suggests to change the unidirectional key
> > > transport in SDES into a key agreement(indicated by "keymod"):
> > > offerer provides: k1
> > > answer provides: keymod value
> > > the outgoing key from offerer to answerer is derived from k1 and
> keymod
> > > value no matter in which situation.
> > > Re-targeting and forking  happen to be the scenarios that
> especially
> > > benefit from the change.
> >
> > Which involves the same number of (signaling) round-trips, right?
> 
> In my opinion, the new method does not add extra round trips, it has
> the same round trips with
> the current SDES without re-INVITE or UPDATE.
> 
> offerer-->answerer:INVITE
>        a=crypto:1 AES_CM_128_HMAC_SHA1_80
>         inline:d0RmdmcmVCspeEc3QGZiNWpVLFJhQX1cfHAwJSoj|2^20|1:32 ---
> >k1
>         keymod:rand|xor|
> offerer<--answerer:Response
>        a=crypto:1 AES_CM_128_HMAC_SHA1_32
>        inline:NzB4d1BINUAvLEw6UzF3WSJ+PSdFcGdUJShpX1Zj|2^20|1:32; ---
> >k2
>        keymod:rand|xor|WVNfX19zZW1jdGwgKCkgew==         ->keymod value
> 
> after the single round,
>     k1 and keymod value-->k1' to protect session from offerer to
> answerer
>    k2 -->  to protect session from answerer   to offerer

I now understand what you're proposing, thanks for explaining it this way.

That avoids a signaling round trip, but does require the Offerer and
Answerer support keymod.  If either of them don't, the Offerer needs to
always do a re-Invite.  So this appears a reasonable optimization to avoid
always doing a re-Invite.

-d

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