Subject: [AVT] SRTP changelist

From: Mats Näslund <[email-masked]>
Date: Thu, 09 May 2002 20:26:17 +0200

Hi

With reference to the recently posted I-D draft-ietf-avt-srtp-04.txt,
here is the list of changes made SRTP v03 to v04. Purely editorial
changes are not listed.

Best,

/Mats

------

* Implemented all comments and suggestions by AVT WG during last call,
  except:

     - We did not change the ROC update algorithm, only added 
       reference to RFC1889's appendix for an alternative method.

       Motivation: It's simply impossible to get it 100% robust
       in the absence of authentication, and mandating a fixed
       algorithm may make it more vulnerable to DoS attacks.

     - We do not create IANA registry for transforms, but we did 
       add a complete parameter listing (Sect 8.2).

       Motivation: The IANA registry is a key management issue.

     - We still allow the order between FEC and SRTP to be selected, 
       though a default is specified. 

       Motivation: there were comments both to remove and keep the 
       option. (The default value is the one for which most interest 
       was expressed during the last call).

* Made message authentication default also for RTP. 

  Motivation: Previously, auth. was mandatory and default "on" for 
  RTCP, but optional and default "off" for RTP. This increased the 
  complexity of the technical specification in several places. It is 
  also against best current practice to have security off by default: 
  an active choice should be needed in order to turn it off. Therefore, 
  both SRTP and SRTCP by default now applies message authentication. 
  For SRTCP it is still REQUIRED, but for SRTP it MAY be turned off.

* Removed the TMMH auth. transform specification as requested, but 
  kept the general framework (the definition of "keystream prefix", 
  Sect 4.1).

  Motivation: TMMH removed by request. The (optional) "prefix" remains 
  to allow smooth addition of TMMH at a later time as a companion RFC.

* Extended the set of allowable "key derivation rates" up to 2^24.

  Motivation: The old maximum, 2^16, would even at 50 packet/s mean a 
  key change at least every 20 min, which seems unnecessarily frequent.

* Added a list of all parameters that key mgmt might need to signal
  (Sect 8.2). This made the old Sect 6 superfluous so it was deleted.
 
  Motivation: see above.

* Added Section 2.1 for brief rationale of some (perhaps) non-obvious
  features. (Detailed rationale is still in Section 7.)

  Motivation: This was suggested to us, in order to make the draft 
  more easy to digest.

* Made clarifications to text based on M. Euchner's recent comments.

* Clarified "Scenarios", in particular when it comes to sharing of
  keys between streams and re-keying issues. 

* Also clarified general parameter sharing between SRTP and SRTCP.

* Security considerations: We now RECOMMEND that keys should not be 
  shared between hosts for more than a single SRTP stream, as it is
  difficult to "synchronize" SSRC choices between hosts. Sharing 
  among multiple streams coming from one and the same host is easier, 
  and can be done under the same considerations as before (SSRC 
  uniqueness). 

* Added test vectors for the key derivation function (PRF). While 
  doing that it became clear that the specification was slightly 
  ambiguous concerning bit-alignment, so we clarified that point 
  (see Sect 4.3).

---------------------------------------------------------
Mats Näslund, PhD

Mail adr:     Ericsson Research
              SE - 164 80 Stockholm
Visiting adr: Torshamnsgatan 23, Kista
Phone:        (+46) 8 585 337 39 (Mobile: +46 70 6616234)
Fax:          (+46) 8 508 780 90
email:        [email-masked]
---------------------------------------------------------

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