Re: [AVT] draft-zimmermann-avt-zrtp-00.txt

[Alan Johnston <alan at sipstation.com>]

Hi Roni,

Thanks for your comments - see mine below.

Thanks,
Alan

Even, Roni wrote:

> Hi,
> I read the draft and find it interesting I have some questions.
>
> 1. The key exchange is done after the RTP connection is established,
> this means that until the end of key exchange both sides will send
> non-encrypted information. 
Correct.

> I think that the document should address the
> issue. One way is to recommend using no-op packets until the system will
> start to encrypt and indicating this to the user.
>  
This is an excellent idea to use no-op packets.  The Zfone 
implementation uses a GUI and tones to indicate the status of the call 
to the user (insecure, securing, or secure).

> 2. It is not clear to me how does the receiver know which is the first
> packet that is encrypted.
>
>  
The first packet that is encrypted is the Confirm1 message.  It has a 
known plain text string in the SRTP payload so that the endpoint can 
verify that decryption has been successful.  The other side then sends a 
Confirm2 message to confirm the encryption on the other side.  After the 
Conf2ACK, the key agreement is over and normal SRTP packets with media 
can then be exchanged.

> 3. In section 10 you have "If both UAs support ZRTP, they will first try
> ZRTP before attempting SRTP". I assume that it means that even though
> the sides already have a shared secret they should not start SRTP and do
> ZRTP exchange first.
Correct.

> What is the reason for that. Based on my first
> comment it will make sense to start SRTP when starting to send media and
> maybe use re-key based on ZRTP.
>  
We were trying to avoid doing a double encryption, where the media was 
first encrypted as SRTP using a master key and salt from the signaling, 
then encrypting again with ZRTP using DH.  Instead, it would be better 
to exchange a master key and salt in the signaling, then mix this secret 
in with the DH shared secret.  If for some reason, ZRTP fails, then the 
two UAs should do SRTP using the exchanged master keys and salt.

> 4. In section 3.1 "If none of these are supported, an RTP packet
> containing
>   comfort noise can be generated to carry a ZRTP message." I understand
> that ZRTP is not only for audio so comfort noise is not a video or
> application payload. The common mode is no-op which was defined to all
> media types.
>
>  
Yes - we had forgotten about the no-op - it is the correct choice.

> Thank
> Roni Even 
>
> -----Original Message-----
> From: Alan Johnston [mailto:alan at sipstation.com] 
> Sent: Monday, February 27, 2006 3:57 PM
> To: avt at ietf.org
> Cc: Philip Zimmermann
> Subject: [AVT] draft-zimmermann-avt-zrtp-00.txt
>
> All,
>
> We have just submitted a new Internet-Draft 
> draft-zimmermann-avt-zrtp-00.txt.  Until it becomes available in the 
> archives, you can get a copy at:
>
>   http://www.philzimmermann.com/zfone/draft-zimmermann-avt-zrtp-00.txt
>   http://www.philzimmermann.com/zfone/draft-zimmermann-avt-zrtp-00.html
>
> We will request a slot in AVT in Dallas to discuss.  Discussion about 
> the RTP extensions and mechanism are best held on the AVT list while 
> discussions about key management for SRTP are best held on the MMUSIC 
> list, with some cross posting as needed.
>
> Thanks,
> Alan
>
> - - -
>
> ZRTP: Extensions to RTP for Diffie-Hellman Key Agreement for SRTP
>                   draft-zimmermann-avt-zrtp-00
>
> Abstract
>
> This document defines ZRTP, RTP (Real-time Transport Protocol) header
> extensions for a Diffie-Hellman exchange to agree on a session key
> and parameters for establishing Secure RTP (SRTP) sessions.  The ZRTP
> protocol is completely self-contained in RTP and does not require
> support in the signaling protocol or assume a Public Key
> Infrastructure (PKI) infrastructure.  For the media session, ZRTP
> provides confidentiality and, in cases where a secret is available
> from the signaling protocol, authentication.
>
>
> _______________________________________________
> Audio/Video Transport Working Group
> avt at ietf.org
> https://www1.ietf.org/mailman/listinfo/avt
>
>
>  


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