The SEED Cipher Algorithm and Its Use with the Secure Real-Time Transport Protocol (SRTP)
RFC 5669

The IESG requested that this draft be be withdrawn from the RFC Editor queue for evaluation for publication at Proposed Standard instead of Informational on 2010-06-17.

[Ballot comment]
In Section 5:
OLD:
  An RTP implementation that
  supports SEED, it MUST implement the modes listed in Section 5.
NEW:
  An RTP implementation that supports SEED MUST implement
  the modes listed in Section 5 of this document.

[Ballot comment]
While I understand what the section 5 is trying to say, I think it can be improved:

In Section 5:
OLD:
  "Mandatory-to-implement" means conformance to the specification, and
  that Table 1 does not supersede [RFC3711]. An RTP implementation that
  supports SEED, it MUST implement the modes listed in Section 5.
NEW:
  "Mandatory-to-implement" means conformance to the specification, and
  that Table 1 does not supersede a similar table in Section 5 of
  [RFC3711]. An RTP implementation that supports SEED MUST implement
  the modes listed in Section 5 of this document.

[Ballot discuss]
Updated as per version 12 of the document:

The new IANA Considerations section is an improvement.
However, as pointed out by Tim Polk, GCM variants are not registered.

[Ballot discuss]
This expands upon Pasi's discuss...

Unlike section 2.2 (CCM), section 2.3 does not discuss the parameters for SEED-GCM.
GCM requires specification of the authentication tag length, usually denoted t. 

Since 2.3 is silent, the assumption is that the 80 bit default specified in 2.1 for counter or 2.2 for
SEED-CCM would apply.  However, this violates the constraints specified in the GCM base spec.
From SP 800 38D:

    The bit length of the tag, denoted t, is a security parameter, as discussed in Appendix B.  In
    general, t may be any one of the following five values: 128, 120, 112, 104, or 96.  For certain
    applications, t may be 64 or 32; guidance for the use of these two tag lengths, including
    requirements on the length of the input data and the lifetime of the key in these cases, is given in
    Appendix C.

I would strongly suggest specifying 96 bits as the default for t (larger values do not appear to be
needed; selection of a smaller value would be insecure imho).

[Ballot comment]
I agree with the DISCUSS ballot position already provided by Alexey.
  In particular, Section 5 must be clarified.  The SEED algorithm is not
  mandatory-to-implement for all RTP implementations.  However, an RTP
  implementation that supports SEED, it must implement the modes listed
  in Section 5.

[Ballot discuss]
I have reviewed draft-ietf-avt-seed-srtp-11, and I have one concern
that I'd like to discuss before recommending approval of the document:

Section 2.3 does not actually say what the authentication tag length
is for GCM mode, but the example in Appendix A.3 suggests it would
be 80 bits. However, this length is not allowed by the GCM spec (800-38D).
The easiest way to fix this would be to change this to 96 bits.

[Ballot comment]
I would like to support Alexey's Discuss regarding IANA registration. It seems to me to be an unnecessary complexity to define the algorithm in one place and not register the necessary identifiers with the key-management. Sure SRTP has several keying mechanisms. However, this question should be well thought through as there may be issues down the road in publishing this as informational status.

From RFC 4568:

10.3.2.1.  SRTP Crypto Suite Registry and Registration

  The IANA has created a new subregistry for SRTP crypto suites under
  the SRTP transport of the SDP Security Descriptions.  An IANA SRTP
  crypto suite registration MUST indicate the crypto suite name in
  accordance with the grammar for srtp-crypto-suite-ext defined in
  Section 9.2.

  The semantics of the SRTP crypto suite MUST be described in an RFC in
  accordance with the RFC 2434 Standards Action, including the
  semantics of the "inline" key-method and any special semantics of
  parameters.

This may be a qualified downref for a future standards track document to register the crypto suit identifiers. But it should be considered now.

[Ballot discuss]
5. Default and mandatory-to-implement Transforms

  The default transforms also are mandatory-to-implement transforms in
  SRTP. Of course, "mandatory-to-implement" means conformance to the
  specification.

                        man.-to-impl.      optional        default

  encryption            SEED-CTR      SEED-CCM,SEED-GCM  SEED-CTR
  message integrity    HMAC-SHA1    SEED-CCM,SEED-GCM  HMAC-SHA1
  key derivation (PRF)  SEED-CTR              -          SEED-CTR


It is not clear if this section is supposed to replace Section 5
of RFC 3711, or whether it just extends it.
Please clarify.


  Table 1: Mandatory-to-implement, optional and default transforms in
  SRTP and SRTCP.

=========
I also need to discuss the following with the rest of IESG and/or authors before deciding whether I should clear this part of my DISCUSS or update it:

>7. IANA Considerations
>
>  This document has no actions for IANA.

This looks wrong.
I was trying to find in RFC 3711 where cipher/authentication mode identifiers are transported in SRTP and couldn't find anything apart from statements like:

>4.  Pre-Defined Cryptographic Transforms
>
>  While there are numerous encryption and message authentication
>  algorithms that can be used in SRTP, below we define default
>  algorithms in order to avoid the complexity of specifying the
>  encodings for the signaling of algorithm and parameter identifiers.

which made me wonder if the list of ciphers is hardcoded somewhere. Please help me find where cipher identifiers are described.

Document Shepherd Write-Up

    (1.a)  Who is the Document Shepherd for this document?  Has the
          Document Shepherd personally reviewed this version of the
          document and, in particular, does he or she believe this
          version is ready for forwarding to the IESG for publication?

The document shepherd is Roni Even. I have reviewed the document, and
believe it is ready for publication.

    (1.b)  Has the document had adequate review both from key WG members
          and from key non-WG members?  Does the Document Shepherd have
          any concerns about the depth or breadth of the reviews that
          have been performed?

This document describes the use of the SEED block cipher algorithm in the
Secure Real-time Transport Protocol (SRTP) for providing confidentiality for
the Real-time Transport Protocol (RTP) traffic and for the control traffic
for RTP, the Real-). The document went through a long WGLC. Earlier version
of the draft was reviewed and there were comments that were addressed. The
latest revision was given sufficient time for review. The document shepherd
has no concerns about the review process.


    (1.c)  Does the Document Shepherd have concerns that the document
          needs more review from a particular or broader perspective,
          e.g., security, operational complexity, someone familiar with
          AAA, internationalization or XML?

I suggest that the document will be reviewed by security experts. In the
past it was reviewed by David McGrew and Eric Rescorla. The latest version
had comments from Alfred Hײ¾nes all were addressed


    (1.d)  Does the Document Shepherd have any specific concerns or
          issues with this document that the Responsible Area Director
          and/or the IESG should be aware of?  For example, perhaps he
          or she is uncomfortable with certain parts of the  document, or
has concerns whether                there really is a need for it.  In any
          event, if the WG has discussed those issues and has indicated
          that it still wishes to advance the document, detail those
          concerns here.  Has an IPR disclosure related to this document
been filed?  If so, please include a reference to the disclosure and
summarize the WG discussion and conclusion on this issue.



No concerns. There is no disclosed IPR on this document.



    (1.e)  How solid is the WG consensus behind this document?  Does it
          represent the strong concurrence of a few individuals, with
          others being silent, or does the WG as a whole understand and
          agree with it?

The document has good consensus from the WG.



    (1.f)  Has anyone threatened an appeal or otherwise indicated 
          extreme discontent?  If so, please summarize the areas of
conflict in
          separate email messages to the Responsible Area Director. 
        (It  should be in a separate email because this questionnaire is
          entered into the ID Tracker.)


No.



    (1.g)  Has the Document Shepherd personally verified that the
          document satisfies all ID nits?  (See
          http://www.ietf.org/ID-Checklist.html and
          http://tools.ietf.org/tools/idnits/).  Boilerplate checks are
          not enough; this check needs to be thorough.  Has the 
          document  met all formal review criteria it needs to, such as
          the MIB Doctor, media type and URI type reviews?

The document has normative downward references, see 1.h

No other nits.



    (1.h)  Has the document split its references into normative and
          informative?  Are there normative references to documents 
          that  are not ready for advancement or are otherwise in an
unclear
          state?  If such normative references exist, what is the
          strategy for their completion?  Are there normative 
          references that are downward references, as described in
[RFC3967]?  If
          so, list these downward references to support the Area
          Director in the Last Call procedure for them [RFC3967].


References have been split. There are no normative references to 
internet-drafts, there are downward references to RFC3610 and RFC4269.


    (1.i)  Has the Document Shepherd verified that the document IANA
          consideration section exists and is consistent with the body
          of the document?  If the document specifies protocol
          extensions, are reservations requested in appropriate IANA
          registries?  Are the IANA registries clearly identified?  If
          the document creates a new registry, does it define the
          proposed initial contents of the registry and an allocation
          procedure for future registrations?  Does it suggest a
          reasonable name for the new registry?  See [RFC2434].  If the
          document describes an Expert Review process has Shepherd
          conferred with the Responsible Area Director so that the IESG
          can appoint the needed Expert during the IESG Evaluation?


The IANA considerations section exists. There are no IANA actions required.



    (1.j)  Has the Document Shepherd verified that sections of the
          document that are written in a formal language, such as XML
          code, BNF rules, MIB definitions, etc., validate correctly in
          an automated checker?


No such sections.


    (1.k)  The IESG approval announcement includes a Document
          Announcement Write-Up.  Please provide such a Document
          Announcement Write-Up?  Recent examples can be found in the
          "Action" announcements for approved documents.  The approval
          announcement contains the following sections:


          Technical Summary

              Relevant content can frequently be found in the abstract
              and/or introduction of the document.  If not, this may be
              an indication that there are deficiencies in the abstract
              or introduction.


  This document describes the use of the SEED [RFC4269] block cipher
algorithm in the Secure Real-time Transport Protocol (SRTP) [RFC3711] for
providing confidentiality for the Real-time Transport Protocol  (RTP)
[RFC3550] traffic and for the control traffic for RTP, the Real-time
Transport Control Protocol (RTCP) [RFC3550].


Working Group Summary

              Was there anything in WG process that is worth noting? 
              For example, was there controversy about particular points or
              were there decisions where the consensus was particularly
              rough?


The document has been reviewed by the AVT working group to ensure
consistency with SRTP.


          Document Quality

              Are there existing implementations of the protocol? 
              Have a significant number of vendors indicated their plan to
              implement the specification?  Are there any reviewers that
              merit special mention as having done a thorough review,
              e.g., one that resulted in important changes or a
              conclusion that the document had no substantive 
              issues?  If  there was a MIB Doctor, Media Type or other
expert review,
              what was its course (briefly)?  In the case of a Media 
              Type review, on what date was the request posted?

There are implementations of SEED and this draft specifies how to use it for
SRTP

          Personnel

              Who is the Document Shepherd for this document?  Who is 
              the Responsible Area Director?


Roni Even is the document shepherd.

Cullen Jennings is the Responsible AD.