Subject: [AVT] RFC3711: Srtp salt_key

From: "milo diurn" <[email-masked]>
Date: Fri, 19 Nov 2004 15:13:20 +0000

Karl, and Srtp community,

In RFC3711, Chap 11.1 shows how the generated keys order is :
SRTP_encr_key, SRTP_auth_key, SRTCP_encr_key, and SRTCP_auth key.

How about the generated SRTP_salt_key, SRTCP_salt_key ?
Where are they located in the list ?

Could you, please, clarify this point ?

Thanks

Regards

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From: "David A. McGrew" <[email-masked]>
Date: Fri, 19 Nov 2004 07:56:51 -0800

Milo,

> Where are they located in the list ?

the salt is an input to the key derivation process, not an output from 
it.

David

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From: Mats Näslund <[email-masked]>
Date: Fri, 19 Nov 2004 19:52:16 +0100

Hi Milo,

I think you have received answers to all the questions,
I would just to add to David's reply on the salt:

> the salt is an input to the key derivation process, not an output from

To be precise, there's both "input salt" and "output salt". Please refer
to Fig 5 of rfc3711:
                          packet index ---+
                                          |
                                          v
                +-----------+ master  +--------+ session encr_key
                | ext       | key     |        |---------->
                | key mgmt  |-------->|  key   | session auth_key
                | (optional |         | deriv  |---------->
                | rekey)    |-------->|        | session salt_key
                |           | master  |        |---------->
                +-----------+ salt    +--------+

    Figure 5: SRTP key derivation.

The master salt (and master key) are among parameters signalled
out-of-band by a key management protocol. From this "session
salts" are derived (see above). I think this pictures nicely shows
how the external key mgmt comes in here ("ext" means "external").

Cheers

/Mats