Subject: [AVT] SRTCP Key Derivation

From: "Ofer Idan" <[email-masked]>
Date: Sun, 27 Nov 2005 15:29:54 +0200

Hi,

I have a question regarding SRTCP Key Derivation.

The SRTP FAQ states the following:

Q39. In SRTCP key derivation, does "Replace the SRTP index by the 32-bit
quantity: 0 || SRTCP index ..." mean that quantity should be padded on
the left with 16 leading zeros?

A. Yes; the specification means to treat the SRTP index as an unsigned
integer, in which case the shorter value would be implicitly padded on
the left with zeros, so that its length matches that of the SRTP index. 

Awhile ago there was a discussion about the location of the "label" in
the SRTCP key derivation. It was stated that the SRTCP index should not
be padded with zeros to be 48 bits long (and therefore the labels will
not be in the same octet position for SRTP and SRTCP).

Isn't there a contradiction between the two options?

Option 1:

Byte:   9    10    11    12    13    14    15
    -+-+-+-+-+-+-+-+-+-+-+-++-+-+-+-+-+-+-+-+-+-+-
...          |label|   srtcp index    |     |     |
    -+-+-+-+-+-+-+-+-+-+-+-++-+-+-+-+-+-+-+-+-+-+-
 
option 2:
 
Byte:   9    10    11    12    13    14    15
    -+-+-+-+-+-+-+-+-+-+-+-++-+-+-+-+-+-+-+-+-+-+-
...                |label| srtcp index|     |     |
    -+-+-+-+-+-+-+-+-+-+-+-++-+-+-+-+-+-+-+-+-+-+-

Which is correct? 

Thanks,

Ofer Idan

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