RE: [Ccrtp-devel] how to generate keystream segment for SRTP header

[Yogesh Devidas Patil]

I am implementing it for differant project. I have looked through the source 
files and
found working of encryption  algo as

16 bits
 * <----->
 * +------+------+------+------+------+------+------+------+
 * |           nonce           |    pakcet index    |  ctr |---+
 * +------+------+------+------+------+------+------+------+   |
 *                                                             |
 * +------+------+------+------+------+------+------+------+   v
 * |                      salt                      |000000|->(+)
 * +------+------+------+------+------+------+------+------+   |
 *                                                             |
 *                                                        +---------+
 *                                                        | encrypt |
 *                                                        +---------+
 *                                                             |
 * +------+------+------+------+------+------+------+------+   |
 * |                    keystream block                    |<--+
 * +------+------+------+------+------+------+------+------+

Doesn't this daviates from SRTP RFC specification ?
And how should i choose nonce value?



Regards
Yogesh Patil


________________________________________
From: Werner Dittmann address@hidden
Sent: Tuesday, July 28, 2009 11:15 PM
To: Yogesh Devidas Patil
Cc: address@hidden
Subject: Re: [Ccrtp-devel] how to generate keystream segment for SRTP header

Do you implement SRTP for this RTP stack? We already have SRTP
avaliable since some years. If you like to implement SRTP for some
other project maybe you can have a look in the source files to
see how this is implemented.

Best Regards,
Werner

Yogesh Devidas Patil schrieb:
> Hi
> I am implementing SRTP and have  little confusion about it.
>
> as per my understanding of SRTP RFC3711,
> Initialization vector(IV) is generated as
>
> IV = (salt_key << 16) XOR (SSRC << 64) XOR (packet_index << 16)
>
> and  keystream segment (required for AES ICM ) is generated as
>
> Keystream_seg = E(k,IV) where k=Session key
> i.e Keystream_seg = k XOR IV
>
> is my understanding correct wrt SRTP standard??
>
> Wherein AES ICM RFC3686, keystream segment consists
> keystream_seg = NONCE || IV || Counter where Counter advances from initial 
> val=0.
>
> I mean how to relate these keystream_seg forming units in AES 
> ICM(NONCE,IV,Counter) with SRTP header fields & fields computed from them??
>
> Second, how should i generate Master Key on sender side??And how to derive 
> session keys from Master Key??
>
> Thanks in advance!!
>
>
> Regards
> Yogesh Patil
>
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