Subject: [MMUSIC] SDES implies two (in/out) SRTCP crypto contexts?

From: "John W. Floroiu" <[email-masked]>
Date: Mon, 21 Jan 2008 19:46:43 +0100

Hello,

There are a couple of statements in RFC4568, like for instance the one 
from section 5.1.1:

    The key(s), however,
    apply to data packets (e.g., SRTP and SRTCP packets) that will be
    sent by the same party that generated the SDP.  That is, each
    endpoint determines its own transmission keys and sends those keys,
    in SDP, to the other endpoint.

that seem to suggest that different (master) keys will be used for the 
SRTCP traffic associated to a SRTP stream, in each direction.

This further hints at having separate "in" and "out" SRTCP crypto 
contexts per SRTP stream (or one crypto context with one "in" and one 
"out" master key).

However, SRTP appears to define only one set of <k_e, k_a, k_s> keys for 
the SRTCP stream, which are derived from the same master key as the SRTP 
stream.

So how will be the two SRTCP key sets mapped into SRTCP crypto contexts?

Regards,
John

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---

From: Mark Baugher <[email-masked]>
Date: Mon, 21 Jan 2008 16:38:43 -0800

John,

On Jan 21, 2008, at 10:46 AM, John W. Floroiu wrote:

>
> Hello,
>
> There are a couple of statements in RFC4568, like for instance the  
> one from section 5.1.1:
>
>    The key(s), however,
> that seem to suggest that different (master) keys will be used for  
> the SRTCP traffic associated to a SRTP stream, in each direction.

I don't think so.  RFC 3711 describes how to derive SRTCP and SRTP  
keys from a given (single) master key.
> contexts per SRTP stream (or one crypto context with one "in" and  
> one "out" master key).
>
> However, SRTP appears to define only one set of <k_e, k_a, k_s>  
> keys for the SRTCP stream, which are derived from the same master  
> key as the SRTP stream.
>

There is only one.

Mark

---

From: "John W. Floroiu" <[email-masked]>
Date: Tue, 22 Jan 2008 11:04:27 +0100

Hello Mark,

Mark Baugher schrieb:
>>     [...]
> I don't think so.  RFC 3711 describes how to derive SRTCP and SRTP keys 
> from a given (single) master key.

Then, how should I interpret the following statement in RFC4568:

7.2.  SRTP-Specific Use Outside Offer/Answer

    Use of SRTP security descriptions outside the offer/answer model is
    not defined.

       Use of SRTP security descriptions outside the offer/answer model
       could have been defined for sendonly media streams; however, there
       would not be a way to indicate the key to use for SRTCP by the
       receiver of said media stream.

in particular the very last sentence (according to SRTP, the SRTCP key 
would be derived from the master key specified in the offer, so why 
"there would not be a way to indicate the key ...").

Regards,
John

>> However, SRTP appears to define only one set of <k_e, k_a, k_s> keys 
>> for the SRTCP stream, which are derived from the same master key as 
>> the SRTP stream.
>>

---

From: Mark Baugher <[email-masked]>
Date: Tue, 22 Jan 2008 08:13:38 -0800

hi John,

>       receiver of said media stream.

Even in a sendonly RTP session, there may be RTCP feedback sent by  
the receiver.  So even in a sendonly SRTP session, the receiver needs  
to provide a key to the sender to authenticate any SRTCP receiver  
reports being sent by the receiver back to the sender.

Mark
>
> in particular the very last sentence (according to SRTP, the SRTCP  
> key would be derived from the master key specified in the offer, so  
> why "there would not be a way to indicate the key ...").
>
> Regards,
> John
>