RE: [AVT] SRTCP key derivation

["Guoqiang Lu" <[e-mail]>]

>This means that, e.g., the labels will not be in the same octet position for SRTP and SRTCP.

This seems to contradict the reference implementation by David McGrew found in srtp.sourceforge.net: The SRTCP key was derived as:

 srtp_kdf_generate(&kdf, (uint64_t) label_rtcp_encryption, 
		     tmp_key, cipher_get_key_length(srtp->rtcp_cipher));
While SRTCP key was derived as:

 srtp_kdf_generate(&kdf, (uint64_t) label_rtp_encryption, 
		     tmp_key, cipher_get_key_length(srtp->rtp_cipher));

And within the srtp_kdf_generate() function, the label is always assigned to nonce.octet[7].

Thanks!

Guoqiang Lu
ESN: 39-36277
Phone: [phone]
[e-mail]
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-----Original Message-----
From: Karl Norrman (KI/EAB) [mailto:[e-mail]] 
Sent: Thursday, July 21, 2005 5:47 AM
To: Lu, Guoqiang [CAR:9D40:EXCH]
Cc: [e-mail]; Mats Näslund (KI/EAB); [e-mail]
Subject: RE: [AVT] SRTCP key derivation


Hello!

The SRTCP index should not be padded with zeros to be 48 bits long.  This means that, e.g., the labels will not be in the same octet position for SRTP and SRTCP.

Regards,
Karl

> -----Original Message-----
> From: [e-mail] [mailto:[e-mail]]On Behalf Of 
> Guoqiang Lu
> Sent: den 20 juli 2005 18:40
> To: [e-mail]
> Cc: [e-mail]; Mats Näslund (KI/EAB)
> Subject: [AVT] SRTCP key derivation
> 
> 
> Hi,
> In RFC3711, section 4.3.2. SRTCP Key Derivation, it says:
> 
> "Replace the SRTP index by the 32-bit quantity: 0 || SRTCP index ..."
> 
> My question is that SRTP index is a 48-bit quantity, should
> the SRTCP 32-bit quantity "0 || SRTCP index" be patched with 
> 16 leading zeros?
> 
> Thanks!
> 
> Guoqiang Lu
> ESN: 39-36277
> Phone: [phone]
> [e-mail]
> --------------------------
> The contents of the this e-mail may be Nortel Confidential!
> 
> 
> _______________________________________________
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