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Versions: 00 01                                                         
MSEC WG                                                       L. Dondeti
Internet-Draft                                                  J. Xiang
Expires: April 24, 2005                                  Nortel Networks
                                                        October 24, 2004

                 GKDP: Group Key Distribution Protocol

Status of this Memo

   This document is an Internet-Draft and is subject to all provisions
   of section 3 of RFC 3667.  By submitting this Internet-Draft, each
   author represents that any applicable patent or other IPR claims of
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   RFC 3668.

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Copyright Notice

   Copyright (C) The Internet Society (2004).


   This document specifies a group key distribution protocol (GKDP)
   based on IKEv2 [2]; the new protocol is similar to IKEv2 in message
   and payload formats, and message semantics to a large extent.  The
   protocol in conformance with MSEC key management architecture
   contains two components: member registration and group rekeying, and
   downloads a group security association from the GCKS to a member.
   This protocol is independent of IKEv2 except in its likeness.

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Conventions Used In This Document

   This document recommends, as policy, what specifications for Internet
   protocols -- and, in particular, IETF standards track protocol
   documents -- should include as normative language within them.  The
   capitalized keywords "SHOULD", "MUST", "REQUIRED", etc.  are used in
   the sense of how they would be used within other documents with the
   meanings as specified in BCP 14, RFC 2119 [1].

Table of Contents

   1.  Revision History . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Introduction and Overview  . . . . . . . . . . . . . . . . . .  3
     2.1   Why do we need another GSA management protocol?  . . . . .  3
     2.2   GKDP usage scenarios . . . . . . . . . . . . . . . . . . .  3
   3.  GKDP protocol  . . . . . . . . . . . . . . . . . . . . . . . .  4
     3.1   Member registration and secure channel establishment . . .  4
       3.1.1   Initial exchange:GSA_INIT_EXCH . . . . . . . . . . . .  4
       3.1.2   Authenticated exchange:GSA_AUTH_EXCH . . . . . . . . .  5
   4.  GSA maintenance channel  . . . . . . . . . . . . . . . . . . .  8
     4.1   GSA rekey protocol . . . . . . . . . . . . . . . . . . . .  8
   5.  GKDP protocol details  . . . . . . . . . . . . . . . . . . . .  9
   6.  Header and payload formats . . . . . . . . . . . . . . . . . .  9
   7.  Security considerations  . . . . . . . . . . . . . . . . . . .  9
   8.  Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . .  9
   9.  References . . . . . . . . . . . . . . . . . . . . . . . . . .  9
   9.1   Normative References . . . . . . . . . . . . . . . . . . . .  9
   9.2   Informative References . . . . . . . . . . . . . . . . . . .  9
       Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 10
       Intellectual Property and Copyright Statements . . . . . . . . 11

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1.  Revision History
   1.  The protocol has been renamed GKDP for Group Key Distribution
       Protocol as per discussions at the MSEC meeting at IETF-60 and
       mailing list discussions.  The name GDOIv2 will be used for a
       revision of GDOI which may retain the DOI concept and build upon
       RFC 3547.
   2.  After the IETF-61 meeting, we will resubmit as

2.  Introduction and Overview

   Security encapsulation protocols such as IPsec and SRTP provide
   confidentiality, message integrity, replay protection, and in some
   instances access control, and data origin authentication.  These
   security services require state establishment, maintenance, and
   teardown for correct operation.  While these security associations
   can be managed manually, automatic key management protocols are
   essential for efficient and scalable operation.  In case of
   point-to-point security associations, IKE and its successor IKEv2 are
   widely used for IPsec SAs, and MIKEY for SRTP associations.  For
   multi-point SAs or group SAs (GSA), GDOI, GSAKMP, and MIKEY have been
   specified by the MSEC WG.  GKDP is designed to be a counterpart - for
   GSA distribution and maintenance - to IKEv2 so we can reuse the work
   put in to its design and analysis, and of course implementation.

2.1  Why do we need another GSA management protocol?

   Given the collection of key management protocols mentioned above,
   there is a question on the need for yet another group key management
   protocol.  First a look back at history: So far, we have two
   experimental RFCs, viz., RFC 1949 [3] and RFC 2093 [4], and a
   standards track RFC, RFC 3547 [5] specifying or describing group key
   management protocols.  Furthermore there is GSAKMP, currently a
   standards track MSEC I-D, which borrows quite a few concepts from
   IKEv2, but not quite similar to IKEv2.  The protocol we propose is
   mainly to reuse as much as the IKEv2 codebase, similar to GDOI
   reusing payload and message formats of IKE [7] and ISAKMP [6] .
   Consequently, GKDP requires fewer messages compared to GDOI,
   specifically 4 in most cases, compared to 10 in main mode and 7 in
   aggressive mode of GDOI.  We discuss the advantages of GKDP, the
   shortcomings and remedies to address those shortcomings.

2.2  GKDP usage scenarios

   GKDP is a key download protocol.  Key download as opposed to key
   negotiation has several interesting use cases.
   o  The first application is multicast security.  As with GDOI, the
      current version of the GKDP spec limits the scope to single sender

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      multicast applications.
   o  The second intended application is point to point data security
      associations facilitated by a centralized group key server.
   o  Others to be listed!

3.  GKDP protocol

3.1  Member registration and secure channel establishment

   The first of two components in GSA establishment and maintenance is
   member registration.

3.1.1  Initial exchange:GSA_INIT_EXCH

   The first step in the registration protocol is to establish a secure
   channel with the group controller and key server (GCKS).  This
   exchange is similar to IKE_SA_INIT exchange of IKEv2.  The
   registering member proposes various combinations of algorithms in
   SAi1 to constitute the secure channel, along with a nonce, Ni, and a
   DH exponent, KEi.  The GCKS has several options:
   o  In the first, it honors the member's request for registration and
      sends the necessary information to complete the DH exchange: it
      selects and specifies the parameters of the secure channel, and
      includes a nonce Nr, and a public DH value of its own, KEr.
   o  The second option is for the GCKS to consider if the request for
      secure channel establishment is spurious.  It has no way to tell
      except to throttle such requests by making the initiator do some
      work before it invests any computing resources.  This is known as
      the DoS protection mode in IKEv2 and is explained in detail in
      Section .
   o  Finally, if none of the proposals are acceptable to the GCKS, it
      may reject the initial exchange itself.

   GSA_INIT_EXCH message is as follows:

   Member->GCKS: M1:    HDR, SAi1, KEi, Ni
   GCKS->Member: M2:    HDR, SAr1, KEr, Nr, [CERTREQ]

                 Figure 1: Secure channel establishment

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   DoS protection exchange is as follows:

   Member->GCKS: IM:  HDR(A,0), SAi1, KEi, Ni
   GCKS->Member: CM:   HDR(A,0), N(COOKIE)

   Member->GCKS: M1:   HDR(A,0), N(COOKIE), SAi1, KEi, Ni
   GCKS->Member: M2:  HDR(A,B), SAr1, KEr, Nr, [CERTREQ]

   IM: Initial Message from the Member
   CM: Challenge Message from the GCKS

             Figure 2: DoS protection mode of GSA_INIT_EXCH

3.1.2  Authenticated exchange:GSA_AUTH_EXCH

   GSA_AUTH_EXCH message is as follows:

   Member->GCKS: M3: HDR, SK{ G-ID, IDi, [ID_CERT,] [ID_CERTREQ,] AUTH,
                  [IDr,] [GM_CERT,] [GM_CERTREQ,] [POP_I] }
   GCKS->Member: M4: HDR, SK{ IDr, [ID_CERT,] AUTH, GSA, KD [,SEQ]
                  [GCKS_CERT,] [,POP_R]}

                    Figure 3: Authenticated Exchange

   The various payloads in the GSA_AUTH_EXCH messages have the following
   o  G-ID:  The group identity payload constructed using the IKEv2
      Identification Payload specifies the secure group that M3 wants to
   o  ID_CERT: The optional ID_CERT payload contains a certificate(s)
      asserting the GCKS's or a member's claimed identity as in IDi or
      IDr payloads.
   o  GM_CERT: The optional GM_CERT payload contains a certificate
      asserting the group member's authorization to join the group G-ID
      as member.
   o  GCKS_CERT: The optional GCKS_CERT payload contains a certificate
      asserting the GCKS's authorization to serve the role of a group
      controller and key server for the group G-ID.
   o  AUTH: The AUTH payload constitues the "authenticated" portion of
      the 4 or 6 message AKE.  In other words, the member in M3 and the
      GCKS in M4 prove that they are indeed the entities that sent M1

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      and M2 respectively.  A pre-established shared secret or a
      certificate (optionally specified in the CERT payload) may be used
      for entity authentication.
   o  POP: Similar to the AUTH payload's use in providing host/entity
      authentication, the POP payload is for member/GCKS authorization
      to assume their claimed roles.  The GM_CERT or GCKS_CERT is used
      to sign a block of data, specified below, to constitute the POP
   o  GSA: The GSA payload contains the rekey and data security SA
      payloads.  Note that this SA is not negotiated; the GCKS simply
      sends this SA.
   o  KD: The KD payload contains the secret keys corresponding the
      rekey and the data security SAs included in the GSA payload.
   o  SEQ: The optional SEQ payload MUST be included if the GSA payload
      contains a rekey SA.  The SEQ payload contains a SEQ number for
      replay protection of the rekey messages.  Key material computation

   The key material computation and the AUTH payload are identical to
   that described in the IKEv2 specification.

   Key material and registration SA keys are computed as follows:

   SKEYSEED = prf(Ni | Nr, g^ir)

     {SK_d | SK_ai | SK_ar | SK_ei | SK_er | SK_pi | SK_pr }
                 = prf+ (SKEYSEED, Ni | Nr | SPIi | SPIr ), where

   prf+ is defined as follows:

      prf+ (K,S) = T1 | T2 | T3 | T4 | ...

      T1 = prf (K, S | 0x01)
      T2 = prf (K, T1 | S | 0x02)
      T3 = prf (K, T2 | S | 0x03)
      T4 = prf (K, T3 | S | 0x04)

           Figure 4: Registration SA key material computation  Member and GCKS authentication and authorization

   GKDP requires mutual authentication between each member and a GCKS,

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   as well as mutual authorization.  First the member and the GCKS
   authenticate to each other using pre-shared keys or certificates
   prior to establishing a secure channel.  M3 and M4 contain AUTH
   payloads that essentially protect against man-in-the-middle attacks
   against the DH exchange in M1 and M2.  The member and the GCKS
   construct AUTH payloads by computing an HMAC over or signing a block
   of data containing the message M1 or M2 they sent earlier, the other
   party's nonce payload, and a prf over own identity.  More formally,
   the block of data for HMAC or signature is as follows:

   Auth payload computation:

   Auth payload in M3 is computed over:

   auth-block-M3: M1 || Nr-Payload || prf(SK_pi, IDi-Payload)

   Auth payload in M4 is computed over:

   auth-block-M4: M2 || Ni-Payload || prf(SK_pr, IDr-Payload)

   For shared secret based host authentication AUTH payload is
   computed as follows:

   AUTH = prf(prf(Shared Secret,"KeyPad:GKDP-AUTH-MX"),

                   Figure 5: Auth payload computation  Use of asymmetric authentication methods

   GKDP also allows the member and the GCKS to use different
   authentication methods, similar to TLS and IKEv2.  More specifically,
   the GCKS uses a cert to authenticate itself and establish a secure
   channel, and the member uses EAP to send its authentication
   information via the secure channel.

   Members may also use EAP to prove their authorization to join a
   secure group.  For instance, consider a use case where a member may
   use a SIM card for authentication, or a pre-paid SIM card to pay for
   content distributed to a secure group.  In these cases, the
   authentication or authorization information can be sent via EAP.  Proof of possession

   Proof of possession payload (POP) provides a mechanism so that

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   members and/or GCKS can prove to the other party that they are indeed
   authorized to be a member or the GCKS, respectively.  For POP payload
   derivation in GKDP, the member or the GCKS first constructs a message
   block, POP-HASH, containing the two nonces exchanged in GSA_INIT_EXCH
   and the prf over the ID payload as defined in the AUTH payload
   construction.  Next, the member or the GCKS signs the POP-HASH value.

   POP-HASH construction is as follows:

   POP payload :

   POP payload in M3 is constructed over the following message block:

   POP-HASH-M3: "KeyPad:GKDP-POP-M3" ||
                 Ni-Payload || Nr-Payload || prf(SK_pi, IDi-Payload)

   POP payload in M4 is computed over:

   POP-HASH-M4: "KeyPad:GKDP-POP-M4" ||
                Ni-Payload || Nr-Payload || prf(SK_pr, IDr-Payload)

                Figure 6: POP payload computation block

4.  GSA maintenance channel

4.1  GSA rekey protocol

   GSA rekey protocol is optional to implement, but it plays a crucial
   role for large and dynamic groups.

   The GCKS is responsible for rekeying of the secure group as per the
   group policy.  The GCKS uses multicast or multi-unicast to transport
   the rekey message.  When multi-unicast is used, it may be appropriate
   in some scenarios to have a reply message from the member(s) to the
   GCKS.  The reply message is optional.

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   Rekey message is as follows:

   GCKS->Member:    HDR, SK {[N], SEQ, GSA, KD, [GCKS_CERT,] SIG}

   GCKS->Member:    HDR, SK {N, SEQ, GSA, KD, [GCKS_CERT,] SIG}
   [Member->GCKS]:    [HDR, SK {N, SEQ, AUTH}]

                        Figure 7: Rekey message

5.  GKDP protocol details

6.  Header and payload formats

   To be copied from IKEv2 and GDOI specifications.  We do anticipate
   some minor changes however.

7.  Security considerations

8.  Acknowledgments

   GKDP is based on IKEv2 and GDOI.  Several sections of this document
   are quite identical to IKEv2 and GDOI specifications.  We included
   the text for completeness of this specification.  We appreciate the
   efforts of the contributors and editors of those protocols.

9.  References

9.1  Normative References

   [1]  Bradner, S., "Key words for use in RFCs to Indicate Requirement
        Levels", BCP 14, RFC 2119, March 1997.

   [2]  Kaufman, C., "Internet Key Exchange (IKEv2) Protocol",
        Internet-Draft: draft-ietf-ipsec-ikev2-14.txt (Work in
        progress), May 2004.

9.2  Informative References

   [3]  Ballardie, T., "Scalable Multicast Key Distribution", RFC 1949,
        May 1996.

   [4]  Harney, H. and C. Muckenhirn, "Group Key Management Protocol
        (GKMP) Specification", RFC 2093, July 1997.

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   [5]  Baugher, M., Weis, B., Hardjono, T. and H. Harney, "The Group
        Domain of Interpretation", RFC 3547, July 2003.

   [6]  Maughan, D., Schneider, M. and M. Schertler, "Internet Security
        Association and Key Management Protocol (ISAKMP)", RFC 2408,
        November 1998.

   [7]  Harkins, D. and D. Carrel, "The Internet Key Exchange (IKE)",
        RFC 2409, November 1998.

Authors' Addresses

   Lakshminath Dondeti
   Nortel Networks
   600 Technology Park drive
   Billerica, MA  01821

   Phone: +1 978 288 6406
   EMail: ldondeti@nortelnetworks.com

   Jing Xiang
   Nortel Networks
   600 Technology Park drive
   Billerica, MA  01821

   Phone: +1 978 288 8985
   EMail: jxiang@nortelnetworks.com

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   except as set forth therein, the authors retain all their rights.

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   Funding for the RFC Editor function is currently provided by the
   Internet Society.

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