Subject: [MMUSIC] RE: MKI length and MKI format

From: Vavilapalli Srikanth-A19563 <[email-masked]>
Date: Wed, 26 Jan 2005 16:53:25 +0530

Hi Mark

Thanks for your explanation. Few more queries related to the same:

1. If MKI value is a byte string of specified length, then in the below
example, MKI is of 4 byte length. 

inline:d0RmdmcmVCspeEc3QGZiNWpVLFJhQX1cfHAwJSoj|2^20|1:4

But actual value has only one byte. Then should the master key be associated
with MKI of "0001" string or "1000" string?

Similarly in the following example also:
inline:PS1uQCVeeCFCanVmcjkpPywjNWhcYD0mXXtxaVBR|2^20|1:32

2. Grammatically are the below two crypto lines equivalent?

 .... inline:d0RmdmcmVCspeEc3QGZiNWpVLFJhQX1cfHAwJSoj|2^20|1:4
(.... inline:d0RmdmcmVCspeEc3QGZiNWpVLFJhQX1cfHAwJSoj|2^20|0001:4 or 
 .... inline:d0RmdmcmVCspeEc3QGZiNWpVLFJhQX1cfHAwJSoj|2^20|1000:4)

Regards
Srikanth

-----Original Message-----
From: Mark Baugher [mailto:[email-masked]] 
Sent: Wednesday, January 26, 2005 12:35 AM
To: Vavilapalli Srikanth-A19563
Cc: 'Karl Norrman (KI/EAB)'; [email-masked]'; [email-masked]'; 'Mats
Näslund'; 'David A. McGrew'; [email-masked]'
Subject: Re: MKI length and MKI format

hi Srikanth,

On Jan 25, 2005, at 10:42 AM, Vavilapalli Srikanth-A19563 wrote:

> Mark and MMUSIC community
>
> Ref: <draft-ietf-mmusic-sdescriptions-08.txt> Section:5.1 Page:14
>
> It was specified in the draft that the MKI length is the size of the 
> MKI
> field in the SRTP packet, specified in Bytes. And the upper limit for 
> that
> value is 128 Bytes. Is it (128) Bytes or (128) bits?

It's in bytes, that's clear.
> What is the intention
> of keeping such a big value for the MKI length value?

It has a high upper bound in order to help with possible future 
migration of digital TV services to IP networks.  I expect that two or 
four bytes would be the typical value.  We made the upper bound 
configurable in the SRTP specification owing to the practice found in 
TV conditional access systems that use a key indicator to encode a 
variety of information for a broadcast receiver.  I was told by people 
from NDS and Nagra that they would prefer the size to be configurable.  
We did not see any reason why it should not be, but we have not yet 
seen any applications materialize for this particular feature.  I'd be 
interested to know what the Motorola Digicipher folks think - when they 
think about video over IP.

I cannot imagine that the MKI will serve any telephony purpose 
whatsoever.  The default size is zero and this is what is mandatory to 
implement (and test).

>
> Also what is the data type of MKI? Is it an "integer" of length 
> specified by
> MKI-Length or a "byte string" of length specified by MKI-Length?

It is a byte string.
>
> According to the example "1026:4" given in the draft, it was said that 
> the
> example assigns to the key a master key identifier of 1026 that is 4 
> bytes
> long. In this case what is the value of MKI filed in each SRTP packet? 
> Is it
> "1026" or "0402" (where 1026 = 0x0402)?

Sorry, we're using decimal and not hexadecimal.  That should have been 
pointed out in the document.

Mark
>
> Please clarify me in this regard.
>
> Regards
> Srikanth
>

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From: Mark Baugher <[email-masked]>
Date: Wed, 26 Jan 2005 08:51:23 -0800

hi Srikanth,
    You're right.  I now see that the MKI is not well-specified.  It's  
clear that  
http://www.ietf.org/internet-drafts/draft-ietf-mmusic-sdescriptions 
-08.txt uses as the notation is a big-endian, integer with unused,  
high-order bytes set to zero.  This is illustrated in the examples (see  
the p. 14 inline example where the MKI has an initial value of 1066)  
provided that you assume that a number in an I-D has the usual, decimal  
notation by default.  This is not a good way to specify something.  I'd  
like to add some clarifying text and will check with the chairs and the  
ADs to see if I can do that while it's awaiting a second AD review.

   I think we should say that the MKI is expressed as a big-endian,  
positive integer with unused high-order bytes set to zero.  My original  
intention for the MKI was to allow it to be whatever the key  
establishment system wants it to be, but our syntax does not support  
that.  It's a bit late to change this, however, and no one has asked  
for us to support, say, hexadecimal notation.

thanks, Mark