Internet Draft                                 Mark Baugher (Cisco)
        IETF MSEC WG                                      Ran Canetti (IBM)
        Expires: December 08, 2004             Lakshminath Dondeti (Nortel)
        Category:  Informational               Fredrik Lindholm (Ericsson)
                                                             June 09, 2004
                       MSEC Group Key Management Architecture
     Status of this Memo
        This document is an Internet-Draft and is in full conformance
        with all provisions of Section 10 of RFC2026.
        Internet-Drafts are working documents of the Internet Engineering
        Task Force (IETF), its areas, and its working groups.  Note that
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        at any time.  It is inappropriate to use Internet-Drafts as
        reference material or to cite them other than as work in progress.
        The list of current Internet-Drafts can be accessed at
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        This document defines the common architecture for Multicast Security
        (MSEC) key management protocols that support a variety of
        application, transport, and network layer security protocols.  It
        also defines the group security association (GSA), and describes the
        key management protocols that help establish a GSA.  The framework
        and guidelines described in this document allow for a modular and
        flexible design of group key management protocols for a variety of
        different settings that are specialized to applications needs.  MSEC
        key management protocols may be used to facilitate secure one-to-
        many, many-to-many, or one-to-one communication.
        Comments on this document should be sent to
        Baugher, Canetti, Dondeti, Lindholm                         June 2004
     Table of Contents
        Status of this Memo................................................1
        1.0 Introduction: Purpose of this Document.........................3
        2.0 Requirements of a Group Key Management Protocol................4
        3.0 Overall Design of the Group Key Management Architecture........6
        3.1 Overview.......................................................6
        3.2 Detailed Description of the GKM Architecture...................8
        3.3 Properties of the Design .....................................11
        3.4 Group Key Management Block Diagram............................11
        4.0 Registration protocol.........................................13
        4.1 Registration protocol via Piggybacking or Protocol Reuse......13
        4.2 Properties of Alternative registration Exchange Types.........14
        4.3 Infrastructure for Alternative registration Exchange Types....15
        4.4 De-registration Exchange......................................15
        5.0 Rekey protocol................................................16
        5.1 Goals of the rekey protocol...................................16
        5.2 Rekey message Transport and Protection........................17
        5.3 Reliable Transport of rekey messages..........................18
        5.4 State-of-the-art on Reliable Multicast Infrastructure.........20
        5.5 Implosion.....................................................20
        5.6 Issues in Incorporating Group Key Management Algorithms.......22
        5.7 Stateless, Stateful, and Self-healing Rekeying Algorithms.....22
        5.8 Interoperability of a GKMA....................................23
        6.0 Group Security Association....................................23
        6.1 Group policy..................................................24
        6.2 Contents of the Rekey SA......................................25
        6.2.1 Rekey SA Policy.............................................25
        6.2.2 Group Identity..............................................26
        6.2.3 KEKs........................................................26
        6.2.4 Authentication Key..........................................26
        6.2.5 Replay Protection...........................................26
        6.2.6 Security Parameter Index (SPI)..............................26
        6.3 Contents of the Data SA.......................................27
        6.3.1 Group Identity..............................................27
        6.3.2 Source Identity.............................................27
        6.3.3 Traffic Protection Keys.....................................27
        6.3.4 Data Authentication Keys....................................27
        6.3.5 Sequence Numbers............................................27
        6.3.6 Security Parameter Index (SPI)..............................27
        6.3.7 Data SA policy..............................................28
        7.0 Scalability Considerations....................................28
        8.0 Security Considerations.......................................30
        9.0 Acknowledgments...............................................31
        10.0 References and Bibliography..................................32
        11.0 Authors' Addresses...........................................36
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        Baugher, Canetti, Dondeti, Lindholm                         June 2004
     1.0 Introduction: Purpose of this Document
        Group and multicast applications have diverse requirements in IP
        networks [TAXONOMY].  Their key management requirements - briefly
        reviewed in Section 2.0 - include support for internetwork,
        transport, and application-layer protocols.  Some applications may
        achieve simpler operation by running key-management messaging over a
        pre-established secure channel (e.g., TLS, IPsec).  Other security
        protocols may benefit from a key management protocol that can run
        over already deployed session initiation or management protocol
        (e.g., SIP or RTSP).  Finally, some may benefit from a light-weight
        key management protocol that finishes in fewest round trips.  For
        these reasons, different application, transport, and internetwork-
        layer data security protocols (e.g., SRTP [RFC3711] and IPsec
        [RFC2401]) may benefit from using different group key management
        systems.  The purpose of this document is to define a common
        architecture and design for group key-management protocols for
        internet, transport, and application services.
        The common architecture for group key management is called the MSEC
        key management architecture and is based on the group control or key
        server model developed in GKMP [RFC2094] and assumed by group key
        management algorithms such as LKH [RFC2627], OFT [OFT], and MARKS
        [MARKS].  There are other approaches that are not considered in this
        architecture such as the highly distributed Cliques group key
        management protocol [CLIQUES] and broadcast key management schemes
        [FN93, Wool].  MSEC (Multicast Security) key management may in fact
        be complementary to other group key management designs, but these
        are not considered in this document.  The integration of MSEC group
        key management with Cliques, broadcast key management and other
        group key systems is not considered in this document.
        Indeed, key-management protocols are difficult to design and
        validate.  The common architecture described in this document eases
        this burden by defining common abstractions and overall design that
        can be specialized for different uses.
        This document builds on and extends the Group Key Management Building
        Block document of the IRTF SMuG research group [GKMBB] and is part of
        the MSEC document roadmap.  The MSEC architecture [MSEC-Arch] is a
        reference for a complete multicast or group security architecture, of
        which key management is a component.
        The rest of this document is organized as follows.  Section 2
        discusses the security, performance and architectural requirements
        for a group key management protocol. Section 3 presents the overall
        architectural design principles. Section 4 describes the registration
        protocol in detail and Section 5 does the same for rekey protocol.
        Section 6 considers the interface to the Group Security Association
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        Baugher, Canetti, Dondeti, Lindholm                         June 2004
        (GSA). Section 7 reviews the scalability issues for group key
        management protocols and Section 8 discusses security considerations.
     2.0 Requirements of a Group Key Management Protocol
        A group key management protocol supports protected communication
        between members of a secure group.  A secure group is a collection of
        principals, called members, who may be senders, receivers or both
        receivers and senders to other members of the group. (Note that group
        membership may vary over time.)  A group key management protocol
        helps to ensure that only members of a secure group gain access to
        group data (by gaining access to group keys) and can authenticate
        group data.  The goal of a group key management protocol is to
        provide legitimate group members with the up-to-date cryptographic
        state they need for their secrecy and authenticity requirements.
        Multicast applications, such as video broadcast and multicast file
        transfer, typically have the following key-management requirements
        (see also [TAXONOMY]).  Note that the list is neither applicable to
        all applications, nor exhaustive.
        1. The group members receive security associations including
           encryption keys, authentication/integrity keys, cryptographic
           policy that describes the keys, and attributes such as an index
           for referencing the security association (SA) or particular
           objects contained in the SA.
        2. In addition to the policy associated with group keys, the group
           owner or the Group Controller and Key Server (GCKS) may define
           and enforce group membership, key management, data security and
           other policies that may or may not be communicated to the
        3. Keys will have a predetermined lifetime and may be periodically
        4. Key material should be delivered securely to members of the
           group so that they are secret, integrity-protected and can be
           verified as coming from an authorized source.
        5. The key-management protocol should be secure against replay
           attacks and Denial of Service(DoS) attacks (see the Security
           Considerations section of this memo).
        6. The protocol should facilitate addition and removal of group
           members so that members who are added may optionally be denied
           access to the key material used before they joined the group,
           and that removed members lose access to the key material
           following their departure.
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        Baugher, Canetti, Dondeti, Lindholm                         June 2004
        7. The protocol should support a scalable group rekey operation
           without unicast exchanges between members and a group
           controller/key server, to avoid overwhelming a GCKS managing a
           large group.
        8. The protocol should be compatible with the infrastructure and
           performance needs of the data-security application, such as
           IPsec security protocols, AH and ESP, and/or application-layer
           security protocols, such as SRTP.
        9. The key management protocol should offer a framework for
           replacing or renewing transforms, authorization infrastructure
           and authentication systems.
        10. The key management protocol should be secure against collusion
            among excluded members and non-members.  Specifically,
            collusion must not result in attackers gaining any additional
            group secrets than each of them individually are privy to.  In
            other words, combining the knowledge of the colluding entities
            must not result in revealing additional group secrets.
        12. The key management protocol should provide a mechanism to
            securely recover from a compromise of some or all of the key
        13. Key management protocols may need to address real-world
            deployment issues such as NAT-traversal and may need to
            interface with legacy authentication mechanisms already
        In contrast to typical unicast key and SA negotiation protocols such
        as TLS and IKE, group key management protocols provide SA and key
        download capability.  This feature may be useful for point-to-point
        communication as well.  Thus, a group key management protocol may
        also be useful to unicast applications. In other words, group key
        management protocols may be used for protecting multicast
        communications, or unicast communications between members of a secure
        group.  Secure sub-group communication is also plausible using the
        group SA.
        There are other requirements for small group operation where there
        will be many senders or in which all members may potentially be
        senders.  In this case, the group setup time may need to be optimized
        to support a small, highly interactive group environment [RFC2627].
        The current key management architecture covers secure communication
        in large single-sender groups, such as source-specific multicast
        groups.  Scalable operation to a range of group sizes is also a
        desirable feature, and a better group key management protocol will
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        Baugher, Canetti, Dondeti, Lindholm                         June 2004
        support large, single-sender groups as well as groups that have many
        senders. It may be that no single key management protocol can satisfy
        the scalability requirements of all group-security applications.
        In addition to these requirements, it is useful to emphasize two non-
        requirements, namely, technical protection measures (TPM) [TPM] and
        broadcast key management.  TPM are used for such things as copy
        protection by preventing the user of a device to get easy access to
        the group keys.  There is no reason why a group key management
        protocol cannot be used in an environment where the keys are kept in
        a tamper-resistant store using various types of hardware or software
        to implement TPM.  However, for simplicity, the MSEC key management
        architecture described in this document considers design for
        technical protection measures out of scope.
        The second non-requirement is broadcast key management where there is
        no back channel [FN93, JKKV94] or where the device is not on a
        network, such as a digital videodisk player.  We assume IP network
        operation where there is two-way communication, however asymmetric,
        and that authenticated key-exchange procedures can be used for member
        registration.  It is possible that broadcast applications can make
        use of a one-way Internet group key management protocol message, and
        a one-way rekey message as described below.
     3.0 Overall Design of the Group Key Management Architecture
        This section describes the overall structure of a group key
        management protocol.  The design is based upon a group controller
        model [RFC2093, RFC2094, RFC2627, OFT, GSAKMP, and RFC3547] with a
        single group owner as the root-of-trust.  The group owner designates
        a group controller for member registration and GSA rekeying.
     3.1 Overview
        The main goal of a group key management protocol is to securely
        provide the group members with an up-to-date security association
        (SA), which contains the needed information for securing group
        communication (i.e., the group data).  We call this SA the Data SA.
        In order to obtain this goal, the Group Key Management Architecture
        defines the following protocols.
        (1) Registration protocol.
        This is a unicast protocol between the group controller/key server
        (GCKS) and a joining group member. In this protocol, the GCKS and
        joining member mutually authenticate each other. If the
        authentication succeeds and the GCKS finds that the joining member is
        authorized, then the GCKS supplies the joining member with the
        following information:
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        Baugher, Canetti, Dondeti, Lindholm                         June 2004
            (a) Sufficient information to initialize the Data SA within the
        joining member. This information is given only in the case that the
        group security policy calls for initializing the Data SA at
        registration, instead of or in addition to as part of the rekey
            (b) Sufficient information to initialize a Rekey SA within the
        joining member (see more details about this SA below). This
        information is given only in case that the group security policy
        calls for using a rekey protocol.
          The registration protocol must ensure that the transfer of
        information from GCKS to member is done in an authenticated and
        confidential manner over a security association.  We call this SA the
        Registration SA. A complementary de-registration protocol serves to
        explicitly remove Registration SA state.  Members may choose to
        delete Registration SA state on their own volition.
        (2) Rekey protocol.
          A GCKS may periodically update or change the Data SA, by sending
        rekey information to the group members.  Rekey messages may result
        from group membership changes, change in group security policy, the
        creation of new traffic-protection keys (TPKs, see next section) for
        the particular group, or from key expiration.  Rekey messages are
        protected by the Rekey SA, which is initialized in the registration
        protocol.  They contain information for updating the Rekey SA and/or
        the Data SA.  Rekey messages can be sent via multicast to group
        members or unicast from the GCKS to a particular group member.
        Note that there are other means for managing (e.g. expiring or
        refreshing) the Data SA without interaction between the GCKS and the
        members.  For example in MARKS [MARKS], the GCKS pre-determines TPKs
        for different periods in the lifetime of the secure group and
        distributes keys to members based on their membership periods.
        Alternative schemes such as the GCKS disbanding the secure group and
        starting a new group with a new Data SA are also possible, although
        this type of operation is typically limited to small groups.
        Rekey messages are authenticated using one of the two following
         o The first option is to use source authentication [TAXONOMY], that
           is to enable each group member to verify that a rekey message
           originates with the GCKS and none other.
         o The second option is to use only group-based authentication
           using a symmetric key.  Members can only be assured that the
           rekey messages originated within the group.  Therefore, this is
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        Baugher, Canetti, Dondeti, Lindholm                         June 2004
           applicable only when all members of the group are trusted not
           to impersonate the GCKS.  Group authentication for rekey
           messages is typically used when public-key cryptography is not
           suitable for the particular group.
        The rekey protocol ensures that all members receive the rekey
        information in a timely manner. In addition, the rekey protocol
        specifies mechanisms for the parties to contact the GCKS and re-synch
        in case that their keys expired and an updated key has not yet been
        received.  The rekey protocol for large-scale groups offers
        mechanisms to avoid implosion problems and ensure the needed
        reliability in its delivery of keying material.
        Rekey messages are protected by a Rekey SA, which is established by
        the registration protocol, and updated using rekey protocol.  When a
        member leaves the group, it destroys its local copy of the GSA. Use
        of a de-registration message may be an efficient mechanisms for a
        member to inform the GCKS that it has destroyed the SAs, or is about
        to destroy them.  Such a message may prompt the GCKS to
        cryptographically remove the member from the group (i.e., to prevent
        the member from having access to future group communication).  In
        large-scale multicast applications, however, de-registration has the
        potential to cause implosion at the GCKS.
     3.2 Detailed Description of the GKM Architecture
        Figure 1 depicts the overall design of a GKM protocol.  Each group
        member, sender or receiver, uses the registration protocol to get
        authorized, authenticated access to a particular Group, its policies,
        and its keys. The two types of group keys are the key encryption keys
        (KEKs) and the traffic encryption keys (TEKs).  For group
        authentication of rekey messages or data, key integrity keys or
        traffic integrity keys may be used as well.  We use the term
        protection keys to refer to both integrity keys and the encryption
        keys.  For example, the term traffic protection key (TPK) is used to
        denote the combination of a TEK and a traffic integrity key, or key
        material used to generate them.
        The KEK may be a single key that protects the rekey message,
        typically containing a new Rekey SA (containing a KEK) and/or Data SA
        (containing a TEK).  A Rekey SA may also contain a vector of keys
        that are part of a group key membership algorithm [RFC2627, OFT,
        TAXONOMY, SD1, SD2].  The TPKs are used by the data security protocol
        to protect streams, files, or other data sent and received by the
        data security protocol.  Thus the registration protocol and/or the
        rekey protocol establish the KEK(s) and/or the TPKs.
        Internet Draft    Group Key Management Architecture          [PAGE 8]

        Baugher, Canetti, Dondeti, Lindholm                         June 2004
        | +-----------------+                          +-----------------+ |
        | |     POLICY      |                          |  AUTHORIZATION  | |
        | | INFRASTRUCTURE  |                          | INFRASTRUCTURE  | |
        | +-----------------+                          +-----------------+ |
        |         ^                                            ^           |
        |         |                                            |           |
        |         v                                            v           |
        | +--------------------------------------------------------------+ |
        | |                                                              | |
        | |                    +--------------------+                    | |
        | |            +------>|        GCKS        |<------+            | |
        | |            |       +--------------------+       |            | |
        | |     REGISTRATION or          |            REGISTRATION or    | |
        | |     DE-REGISTRATION          |            DE-REGISTRATION    | |
        | |         PROTOCOL             |               PROTOCOL        | |
        | |            |                 |                  |            | |
        | |            v                REKEY               v            | |
        | |   +-----------------+     PROTOCOL     +-----------------+   | |
        | |   |                 |    (OPTIONAL)    |                 |   | |
        | |   |    SENDER(S)    |<-------+-------->|   RECEIVER(S)   |   | |
        | |   |                 |                  |                 |   | |
        | |   +-----------------+                  +-----------------+   | |
        | |            |                                    ^            | |
        | |            v                                    |            | |
        | |            +-------DATA SECURITY PROTOCOL-------+            | |
        | |                                                              | |
        | +--------------------------------------------------------------+ |
        |                                                                  |
                     FIGURE 1: Group Security Association Model
        There are a few, distinct outcomes to a successful registration
        Protocol exchange.
             o If the GCKS uses rekey messages, then the admitted member
               receives the Rekey SA.  The Rekey SA contains the groups
               rekey policy (note that not all of the policy need to be
               revealed to members), and at least a group KEK.  In
               addition, the GCKS may send a group key integrity key, and
               if the group uses a group key management algorithm, a set
               of KEKs (or key material used to derive the KEKs) according
               to the particular algorithm.
             o If rekey messages are not used for the Group, then the
               admitted member will receive TPKs (as part of the Data
               Security SAs) that are passed to the members Data Security
               Protocol (as IKE does for IPsec).
        Internet Draft    Group Key Management Architecture          [PAGE 9]

        Baugher, Canetti, Dondeti, Lindholm                         June 2004
             o The GCKS may pass one or more TPKs to the member even if
               rekey messages are used, for efficiency reasons according
               to group policy.
        The GCKS creates the KEK and TPKs and downloads them to each member -
        as the KEK and TPKs are common to the entire group.  The GCKS is a
        separate, logical entity that performs member authentication and
        authorization according to the group policy that is set by the group
        owner.  The GCKS may present a credential to the group member that is
        signed by the group owner so the member can check the GCKSs
        authorization.  The GCKS, which may be co-located with a member or be
        a separate physical entity, runs the rekey protocol to push rekey
        messages containing refreshed KEKs, new TPKs, and/or refreshed TPKs
        to members.  Note that some group key management algorithms refresh
        any of the KEKs (potentially), whereas others only refresh the group
        Alternatively, the sender may forward rekey messages on behalf of the
        GCKS when it uses a credential mechanism that supports delegation.
        Thus, it is possible for the sender (or other members) to source
        keying material - TPKs encrypted in the Group KEK - as it sources
        multicast or unicast data.  As mentioned above, the rekey message can
        be sent using unicast or multicast delivery.  Upon receipt of a TPK
        (as part of a Data SA) from a rekey message or a registration
        protocol exchange, the members group key management functional block
        will provide the new or updated security association (SA) to the data
        security protocol to protect the data sent from sender to receiver.
        The Data SA protects the data sent on the arc labeled DATA SECURITY
        PROTOCOL shown in Figure 1.  A second SA, the Rekey SA, is optionally
        established by the key-management protocol for rekey messages as
        shown in Figure 1 by the arc labeled REKEY PROTOCOL.  The rekey
        message is optional because all keys, KEKs and TPKs, can be delivered
        by the registration protocol exchanges shown in Figure 1, and those
        keys may not need to be updated.  The registration protocol is
        protected by a third, unicast, SA between the GCKS and each member;
        this is called the Registration SA.  There may be no need for the
        Registration SA to remain in place after the completion of the
        registration protocol exchanges.  The de-registration protocol may be
        used when explicit teardown of the SA is desirable (such as when a
        phone call or conference terminates).  The three SAs compose the GSA.
        Only one SA is optional and that is the Rekey SA.
        Figure 1 shows two blocks that are external to the group key
        management protocol:  The policy and authorization infrastructures
        are discussed in Section 6.1.  The Multicast Security Architecture
        document further clarifies the SAs and their use as part of the
        complete architecture of a multicast security solution [MSEC-Arch].
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     3.3 Properties of the Design
        The design of Section 3.2 achieves scalable operation by (1) allowing
        the de-coupling of authenticated key exchange in a registration
        protocol from a rekey protocol, (2) allowing the rekey protocol to
        use unicast push or multicast distribution of group and data keys as
        an option, (3) allowing all keys to be obtained by the unicast
        registration protocol, and (4) delegating the functionality of the
        GCKS among multiple entities, i.e., to permit distributed operation
        of the GCKS.
        High-capacity operation is obtained by (1) amortizing
        computationally-expensive asymmetric cryptography over multiple data
        keys used by data security protocols, (2) supporting multicast
        distribution of symmetric group and data keys, and (3) supporting key
        revocation algorithms such as LKH [RFC2627, OFT, SDR] that allow
        members to be added or removed at logarithmic rather than linear
        space/time complexity.  The registration protocol may use asymmetric
        cryptography to authenticate joining members and optionally establish
        the group KEK.  Asymmetric cryptography such as Diffie-Hellman key
        agreement and/or digital signatures are amortized over the life of
        the group KEK: A Data SA can be established without the use of
        asymmetric cryptography - the TPKs are simply encrypted in the
        symmetric KEK and sent unicast or multicast in the rekey protocol.
        The design of the registration and rekey protocols is flexible. The
        registration protocol establishes either a Rekey SA or one or more
        Data SAs or both types of SAs.  At least one of the SAs is present
        (otherwise, there is no purpose to the Registration SA).  The Rekey
        SA may update the Rekey SA, or establish or update one or more Data
        SAs.  Individual protocols or configurations may take advantage of
        this flexibility for efficient operation.
     3.4 Group Key Management Block Diagram
        In the block diagram of Figure 2, group key management protocols run
        between a GCKS and member principal to establish a Group Security
        Association (GSA).  The GSA consists of a Data SA, an optional Rekey
        SA, and a Registration SA.  The GCKS may use a delegated principal,
        such as the sender, which has a delegation credential signed by the
        GCKS.  The Member of Figure 2 may be a sender or receiver of
        multicast or unicast data.  There are two functional blocks in Figure
        2 labeled GKM, and there are two arcs between them depicting the
        group key-management registration (reg) and rekey (rek) protocols.
        The message exchanges are the GSA establishment protocols, which are
        the registration protocol and the rekey protocol described above.
        Figure 2 shows that a complete group-key management functional
        specification includes much more than the message exchange.  Some of
        these functional blocks and the arcs between them are peculiar to an
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        Baugher, Canetti, Dondeti, Lindholm                         June 2004
        operating system (OS) or vendor product, such as vendor
        specifications for products that support updates to the IPsec
        Security Association Database (SAD) and Security Policy Database
        (SPD) [RFC2367].  Various vendors also define the functions and
        interface of credential stores, CRED in Figure 2.
           |                                                          |
           | +-------------+         +------------+                   |
           | |   CONTROL   |         |   CONTROL  |                   |
           | +------^------+         +------|-----+  +--------+       |
           |        |                       |  +-----| CRED   |       |
           |        |                       |  |     +--------+       |
           |   +----v----+             +----v--v-+   +--------+       |
           |   |         <-----Reg----->         |<->|  SAD   |       |
           |   |   GKM    -----Rek----->   GKM   |   +--------+       |
           |   |         |             |         |   +--------+       |
           |   |         ------+       |         |<->|  SPD   |       |
           |   +---------+     |       +-^-------+   +--------+       |
           |   +--------+      |         | |   |                      |
           |   | CRED   |----->+         | |   +-------------------+  |
           |   +--------+      |         | +--------------------+  |  |
           |   +--------+      |       +-V-------+   +--------+ |  |  |
           |   |  SAD   <----->+       |         |<->|  SAD   <-+  |  |
           |   +--------+      |       |SECURITY |   +--------+    |  |
           |   +--------+      |       |PROTOCOL |   +--------+    |  |
           |   |  SPD   <----->+       |         |<->|  SPD   <----+  |
           |   +--------+              +---------+   +--------+       |
           |                                                          |
           |     (A) GCKS                     (B) MEMBER              |
           Figure 2: Group key management block diagram for a host computer
        The CONTROL function directs the GCKS to establish a group, admit a
        member, or remove a member, or it directs a member to join or leave a
        group.  CONTROL includes authorization, which is subject to group
        policy [GSPT], but how this is done is specific to the GCKS
        implementation.  For large-scale multicast sessions, CONTROL could
        perform session announcement functions to inform a potential group
        member that it may join a group or receive group data (e.g. a stream
        of file transfer protected by a data security protocol).
        Announcements notify group members to establish multicast SAs in
        advance of secure multicast data transmission.  Session Description
        Protocol (SDP) is one form that the announcements might take
        [RFC2327].  The announcement function may be implemented in a
        session-directory tool, an electronic program guide (EPG), or by
        other means.  The Data Security or the announcement function directs
        group key management using an application-programming interface
        (API), which is peculiar to the host OS in its specifics.  A generic
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        Baugher, Canetti, Dondeti, Lindholm                         June 2004
        API for group key management is for further study, but this function
        is necessary to allow Group (KEK) and Data (TPKs) key establishment
        to be done in a way that is scalable to the particular application.
        A GCKS application program will use the API to initiate the
        procedures to establish SAs on behalf of a Security Protocol in which
        members join secure groups and receive keys for streams, files or
        other data.
        The goal of the exchanges is to establish a GSA through updates to
        the SAD of a key-management implementation and particular Security
        Protocol.  The data security protocol of Figure 2 may span
        internetwork and application layers or operate at the internetwork
        layer, such as AH and ESP.
     4.0 Registration protocol
        The design of the registration protocol is flexible, and can support
        different application scenarios.  The chosen registration protocol
        solution reflects the specific requirements of specific scenarios.
        In principle, it is possible to base a registration protocol on any
        secure-channel protocol, such as IPsec and TLS, which is the case in
        tunneled GSAKMP [tGSAKMP].  GDOI [RFC3547] reuses IKE Phase 1 as the
        secure channel to download Rekey and/or Data SAs.  Other protocols,
        such as MIKEY and GSAKMP, use authenticated Diffie-Hellman exchanges
        similar to IKE Phase 1, but specifically tailored for key download
        to achieve efficient operation.  We discuss the design of a
        registration protocol in detail in the rest of this section.
     4.1 Registration protocol via Piggybacking or Protocol Reuse
        Some registration protocols need to tunnel through a data-signaling
        protocol to take advantage of already existing security
        functionality, and/or to optimize the total session setup time.  For
        example, a telephone call has strict bounds for delay in setup time.
        It is not feasible to run security exchanges in parallel with call
        setup since the latter often resolves the address: Call setup must
        complete before the caller knows the address of the callee.  In this
        case, it may be advantageous to tunnel the key exchange procedures
        inside call establishment [H.235, MIKEY] so both can complete (or
        fail, see below) at the same time.
        The registration protocol has different requirements depending on
        the particular integration/tunneling approach.  These requirements
        are not necessarily security requirements, but will have an impact
        on the chosen security solution.  For example, the security
        association will certainly fail if the call setup fails in the case
        of IP telephony.
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        Baugher, Canetti, Dondeti, Lindholm                         June 2004
        Conversely, the registration protocol imposes requirements on the
        protocol that tunnels it.  In the case of IP telephony, the call
        setup usually will fail when the security association is not
        successfully established.  In the case of video-on-demand, protocols
        such as RTSP that convey key management data will fail when a needed
        security association cannot be established.
        Both GDOI and MIKEY use this approach, but in different ways.  MIKEY
        can be tunneled in SIP and RTSP.  It takes advantage of the session
        information contained in these protocols and the possibility to
        optimize the setup time for the registration procedure.  SIP
        requires that a tunneled protocol must use at most one roundtrip
        (i.e. two messages).  This is also desirable requirement from RTSP
        as well.
        The GDOI approach takes advantage of the already defined ISAKMP
        phase 1 exchange [RFC2409], and extends the phase 2 exchange for the
        registration.  The advantage here is the reuse of a successfully
        deployed protocol and the code base, where the defined phase 2
        exchange is protected by the SA created by phase 1.  GDOI also
        inherits other functionality of the ISAKMP, and thus it is readily
        suitable for running IPsec protocols over IP multicast services.
     4.2 Properties of Alternative registration Exchange Types
        The required design properties of a registration protocol have
        different tradeoffs.  A protocol that provides perfect forward
        secrecy and identity protection trades performance or efficiency for
        better security, while a protocol that completes in one or two
        messages may trade security functionality (e.g. identity protection)
        for efficiency.
        Replay protection generally uses either a timestamp or a sequence
        number.  The first requires synchronized clocks, while the latter
        requires that it is possible to keep state. In a timestamp-based
        protocol, a replay cache is needed to store the authenticated
        messages (or the hashes of the messages) received within the
        allowable clock skew.  The size of the replay cache depends on the
        number of authenticated messages received during the allowable clock
        skew.  During a DoS attack, the replay cache might become
        overloaded. One solution is to over-provision the replay cache.
        However, this may lead to a large replay cache. Another solution is
        to let the allowable clock skew be changed dynamically during
        runtime. During a suspected DoS attack, the allowable clock skew is
        decreased so that the replay cache becomes manageable.
        A challenge-response mechanism (using Nonces) obviates the need for
        synchronized clocks for replay protection when the exchange uses
        three or more messages [MVV].
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        Baugher, Canetti, Dondeti, Lindholm                         June 2004
        Additional security functions become possible as the number of
        allowable messages in the registration protocol increase.  ISAKMP
        offers identity protection, for example, as part of a six-message
        exchange.  With additional security features, however, comes added
        complexity:  Identity protection, for example, not only requires
        additional messages, but may result in DoS vulnerabilities since
        authentication is performed in a late stage of the exchange after
        resources already have been devoted.
        In all cases, there are tradeoffs with the number of message
        exchanged, the desired security services, and the amount of
        infrastructure that is needed to support the group key management
        service.  Whereas protocols that use two or even one-message setup
        have low latency and computation requirements, they may require more
        infrastructure such as secure time or offer less security such as
        the absence of identity protection.  What tradeoffs are acceptable
        and what are not is very much dictated by the application and
        application environment.
     4.3 Infrastructure for Alternative registration Exchange Types
        The registration protocol may need external infrastructures to be
        able to handle authentication and authorization, replay protection,
        protocol-run integrity, and potentially other security services such
        as secure, synchronized clocks.  For example, authentication and
        authorization may need a PKI deployment (with either authorization-
        based certificates or a separate management for this) or may be
        handled by using AAA infrastructure.  Replay protection using
        timestamps requires an external infrastructure or protocol for clock
        However, external infrastructures may not always be needed,  if for
        example pre-shared keys are used for authentication and
        authorization; this may be the case if the subscription base is
        relatively small.  In a conversational multimedia scenario (e.g., a
        VoIP call between two or more people), it may very well be the end
        user who handles the authorization by manually accepting/rejecting
        the incoming calls.  Thus, infrastructure support may not be
        required in that case.
     4.4 De-registration Exchange
        The session-establishment protocol (e.g., SIP, RTSP) that conveys a
        registration exchange often has a session-disestablishment protocol
        such as RTSP TEARDOWN [RFC2326] or SIP BYE [RFC2543].  The session-
        disestablishment exchange between endpoints offers an opportunity to
        signal the end of the GSA state at the endpoints.  This exchange
        need only be a uni-directional notification by one side that the GSA
        is to be destroyed.  For authentication of this notification, we may
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        Baugher, Canetti, Dondeti, Lindholm                         June 2004
        use a proof-of-possession of the group key(s) by one side to the
        other.  Some applications benefit from acknowledgement in a mutual,
        two-message exchange signaling disestablishment of the GSA
        concomitant with disestablishment of the session, e.g., RTSP or SIP
        session.  In this case, a two-way proof-of-possession might serve
        for mutual acknowledgement of the GSA disestablishment.
     5.0 Rekey protocol
        The group rekey protocol is for transport of keys and SAs between a
        GCKS and the members of a secure communications group.  The GCKS
        sends rekey messages to update a Rekey SA, or initialize/update a
        Data SA or both.  Rekey messages are protected by a Rekey SA.  The
        GCKS may update the Rekey SA when group membership changes or when
        KEKs or TPKs expire.  Recall that KEKs correspond to a Rekey SA and
        TPKs correspond to a Data SA.
        The following are some desirable properties of the rekey protocol:
          o Rekey protocol ensures that all members receive the rekey
            information in a timely manner.
          o Rekey protocol specifies mechanisms for the parties
            involved, to contact the GCKS and re-sync when their keys expire
            and no updates have been received.
          o Rekey protocol avoids implosion problems and ensures the
            needed reliability in delivering Rekey information.
        We further note that the rekey protocol is primarily responsible for
        scalability of the group key management architecture.  Hence it is
        imperative that we provide the above listed properties in a scalable
        manner.  Note that solutions exist in the literature (both IETF
        standards and research articles) for parts of the problem.  For
        instance, the rekey protocol may use a scalable group key management
        algorithm (GKMA) to reduce the number of keys sent in a rekey
        message.  Examples of a GKMA include LKH, OFT, Subset difference
        based schemes etc.
     5.1 Goals of the rekey protocol
        The goals of the rekey protocol are:
          o to synchronize a GSA
          o to provide privacy and (symmetric or asymmetric)
            authentication, replay protection and DoS protection
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        Baugher, Canetti, Dondeti, Lindholm                         June 2004
          o efficient rekeying after changes in group membership, or when
            keys (KEKs) expire,
          o reliable delivery of rekey messages,
          o provide methods for members to recover from an out-of-sync
          o high throughput and low latency, and
          o to use IP Multicast or multi-unicast.
        We identify several major issues in the design of a rekey protocol:
          1. rekey message format
          2. reliable transport of rekey messages
          3. implosion
          4. recovery from out-of-sync GSA
          5. incorporating GKMAs in rekey messages
          6. interoperability of GKMAs
        Note that for a GCKS to successfully rekey a group, it is not
        sufficient that rekey protocol implementations interoperate.  We also
        need to ensure that the GKMA also interoperates, i.e., standards
        versions of group key management algorithms, such as LKH, OFT, subset
        difference and others need to be used.
        In the rest of this section we discuss these topics in detail.
     5.2 Rekey message Transport and Protection
        Rekey messages contain Rekey and/or Data SAs along with KEKs and
        TPKs.  These messages need to be confidential, authenticated, and
        protected against replay and DoS attacks.  They are sent via
        multicast or multi-unicast from the GCKS to the members.
        Rekey messages are encrypted with the Group KEK for confidentiality.
        When used in conjunction with a GKMA, portions of the rekey message
        are first encrypted with the appropriate KEKs as specified by the
        GKMA.  The GCKS authenticates rekey messages using either a MAC -
        computed using the group Authentication key - or a digital signature.
        In both cases, a sequence number is included in computation of the
        MAC or the signature to protect against replay attacks.
        Internet Draft    Group Key Management Architecture         [PAGE 17]

        Baugher, Canetti, Dondeti, Lindholm                         June 2004
        When group authentication is provided - with a symmetric key - rekey
        messages are vulnerable to attacks by other members of the group.
        Rekey messages are digitally signed when group members do not trust
        each other.   When asymmetric authentication is used, members
        receiving rekey messages are vulnerable to DoS attacks.  An external
        adversary may send a bogus rekey message, which a member cannot
        identify until after it performs an expensive digital signature
        operation.  To protect against such an attack, a MAC may be sent as
        part of the rekey message.  Members verify the signature only upon
        successful verification of the MAC.
        Rekey messages contain group key updates corresponding to a single
        [RFC2627, OFT] or multiple membership changes [SD, BatchRekey] and
        may contain group key initialization messages [OFT].
     5.3 Reliable Transport of rekey messages
        The GCKS needs to ensure that all members have the current Data
        Security and Rekey SAs.  Otherwise, authorized members may be
        inadvertently excluded from receiving group communications.  Thus,
        the GCKS needs to use a rekey algorithm that is inherently reliable
        or employ some reliable transport mechanism to send rekey messages.
        There are two dimensions to the problem:  Messages that update group
        keys may be lost in transit or may be missed by a host when it is
        offline.  LKH and OFT group key management algorithms rely on past
        history of updates being received by the host.  If the host goes
        offline, it will need to resynchronize its group-key state when it
        comes online; this may require a unicast exchange with the GCKS.
        The Subset Difference algorithm, however, conveys all the needed
        state in its rekey messages and does not need members to be always
        online, nor keeping state.  Subset difference algorithm does not
        require a backchannel and can operate on a broadcast network.  If a
        rekey message is lost in transmission, subset difference algorithm
        cannot decrypt messages encrypted with the TPK sent via the lost
        rekey message.  There are self-healing GKMAs proposed in the
        literature that allow a member to recover lost rekey messages, as
        long as rekey messages before and after the lost rekey message are
        Rekey messages are typically short (for single membership change as
        well as for small groups) which makes it easy to design a reliable
        delivery protocol.  On the other hand, the security requirements may
        add an additional dimension to address.  Also there are some special
        cases where membership changes are processed as a batch, which
        reduces the frequency of rekey messages, but increases their size.
        Furthermore, among all the KEKs sent in a rekey message, as many as
        half the members need only a single KEK.  We may take
        advantage of these properties in designing a rekey message(s) and a
        protocol for their reliable delivery.
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        Three categories of solutions have been proposed:
          1. Repeatedly transmit the rekey message:  Recall that in many
             cases rekey messages translate to only one or two IP packets.
          2. Use an existing reliable multicast protocol/infrastructure
          3. Use FEC for encoding rekey packets (with NACKs as feedback)
        Note that for small messages, category 3 is essentially the same as
        category 1.
        The group member might be out of synchrony with the GCKS if it
        receives a rekey message having a sequence number that is more than
        one greater than the last sequence number processed.  This is one
        means by which the GCKS member detects that it has missed a rekey
        message.  Alternatively, the data-security application might detect
        that it is using an out-of-date key and notifies the group key
        management module of this condition.   What action the GCKS member
        takes is a matter of group policy:  The GCKS member should log the
        condition and may contact the GCKS to re-run the re-registration
        protocol to obtain a fresh group key.  The group policy needs to
        take into account boundary conditions, such as re-ordered rekey
        messages when rekeying is so frequent that two messages might get
        reordered in an IP network.   The group key policy also needs to
        take into account the potential for denial of service attacks where
        an attacker delays or deletes a rekey message in order to force a
        subnetwork or subset of the members to synchronously contact the
        If a group member becomes out-of-synch with the GSA then it should
        re-register with the GCKS.  However, in many cases there are other,
        simpler methods for re-synching with the group:
          o The member can open a simple, unprotected connection (say, TCP)
             with the GCKS and obtain the current (or several recent) rekey
             messages.  Note that there is no need for authentication or
             encryption here, since the rekey message is already signed and
             is anyway multicasted in the clear.  One may think that this
             opens the GCKS to DoS attacks by many bogus such requests. But
             this does not seem to worsen the situation: in fact, bombarding
             the GCKS with bogus resynch requests would be much more
          o The GCKS can post the rekey messages on some public site (say,
             web site) and the out-of-synch memeber can obtain the rekey
             messages from that site.
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        It is suggested that the GCKS always provide all three ways of
        resynching (i.e., re-registration, simple TCP, and public posting).
        This way, it is up to the member to choose how to resynch; it also
        avoids adding yet another field to the policy token [GSPT].
        Alternatively, a policy token may contain a field specifying one or
        more methods supported for resynchronization of a GSA.
     5.4 State-of-the-art on Reliable Multicast Infrastructure
        The rekey message may be sent using reliable multicast.  There are
        multiple types of reliable multicast protocols and products, which
        have different properties.  However, there are no standard reliable
        multicast protocols at the present time.  Thus, this document makes
        no recommendation for use of a particular reliable multicast
        protocol or set of protocols for the purposes group key management.
        The suitability of NAK-based, ACK-based or other reliable multicast
        methods are determined by the particular needs of the group key
        management application and environment.  In the future, group key
        management protocols may choose to use particular standards-based
        approaches that meet the needs of the particular application.  A
        secure announcement facility is needed to signal the use of a
        reliable multicast protocol, which must be specified as part of
        group policy.  The reliable multicast announcement and policy
        specification, however, can only follow the establishment of
        reliable multicast standards and are not considered further in this
        Today, the several MSEC group key management protocols support
        sequencing of the rekey messages through a sequence number, which is
        authenticated along with the rekey message.  A sender of rekey
        messages may re-transmit multiple copies of the message provided
        that they have the same sequence number.  Thus, re-sending the
        message is a rudimentary means of overcoming loss along the network
        path.  A member who receives the rekey message will check the
        sequence number to detect duplicate and missing rekey messages.  The
        member receiver will discard duplicate messages that it receives.
        Large rekey messages, such as those that contain LKH or OFT tree
        structures, might benefit from transport-layer FEC when standard
        methods are available in the future.  It is unlikely that forward
        error correction (FEC) methods will benefit rekey messages that are
        short and fit within a single message.  In this case, FEC
        degenerates to simple retransmission of the message.
     5.5 Implosion
        Implosion may occur due to one of two reasons.  First, recall that
        one of the goals of the rekey protocol is to synchronize a GSA.  When
        a rekey or Data SA expires, members may contact the GCKS for an
        update.  If all or even many members contact the GCKS at about the
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        Baugher, Canetti, Dondeti, Lindholm                         June 2004
        same time, the GCKS cannot handle all those messages.  We refer to
        this as an out-of-sync implosion.
        The second case is in the reliable delivery of rekey messages.
        Reliable multicast protocols use feedback (NACK or ACK) to determine
        which packets must be retransmitted.  Packet losses may result in
        many members sending NACKs to the GCKS.  We refer to this as feedback
        The implosion problem has been studied extensively in the context of
        reliable multicasting.  Some of the proposed solutions viz., feedback
        suppression and aggregation, might be useful in the GKM context as
        Members may wait for a random time before sending an out-of-sync or
        feedback message.  Meanwhile, members might receive the key updates
        they need and therefore will not send a feedback message.
        An alternative solution is to have the members contact one of several
        registration servers when they are out-of-sync.  This requires GSA
        synchronization between the multiple registration servers.
        Feedback aggregation and local recovery employed by some reliable
        multicast protocols are not easily adaptable to transport of rekey
        messages.  There are authentication issues to address in aggregation.
        Local recovery is more complex in that members need to establish SAs
        with the local repair server (Any member of the group or a
        subordinate GCKS might serve as a repair server.  Repair servers may
        be responsible for resending rekey messages).
        Members may use the group SA, more specifically the Rekey SA, to
        authenticate requests sent to the repair server; however, replay
        protection requires maintaining state at members as well as repair
        servers.  Authentication of repair requests is to protect against DoS
        attacks.  Note also that an out-of-sync member may use an expired
        Rekey SA to authenticate repair requests, which requires repair
        servers to accept messages protected by old SAs.
        Alternatively, a simple mechanism may be employed to achieve local
        repair efficiently.  Each member receives a set of local repair
        server addresses as part of group operation policy information.  When
        a member does not receive a rekey message, it can send a "retransmit
        replay message(s) with sequence number n and higher" to one of the
        local repair servers.  The repair server can do one of two things:
        ignore the request if it is busy, or retransmit the requested rekey
        messages as received from the GCKS.  The repair server, which is also
        another member may choose to serve only m requests in a given time
        period (i.e., rate limits responses) or per a given rekey message.
        Rate limiting the requests and responses protects the repair servers
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        Baugher, Canetti, Dondeti, Lindholm                         June 2004
        as well other members of the group from being vulnerable to DoS
     5.6 Issues in Incorporating Group Key Management Algorithms
        Group key management algorithms make Rekeying scalable.  Large group
        Rekeying without employing GKMAs is prohibitively expensive.
        First we list some requirements to consider in selecting a GKMA:
          o Protection against Collusion:  Members (or non members) should
            not be able to collaborate to deduce keys that they are not
            privileged to (following the GKMA key distribution rules).
          o Forward access control: Ensure that departing members cannot get
            access to future group data.
          o Backward access control:  Ensure that joining members cannot
            decrypt past data.
     5.7 Stateless, Stateful, and Self-healing Rekeying Algorithms
        We classify group key management algorithms into three categories,
        viz., stateful, stateless, and self-healing algorithms.
        Stateful algorithms [RFC2627, OFT] use KEKs from past rekeying
        instances to encrypt (protect) KEKS corresponding to the current and
        future rekeying instances.  The main disadvantage in these schemes is
        that if a member were offline or otherwise fails to receive KEKs from
        a past rekeying instance, it may no longer be able to synchronize its
        GSA even though it can receive KEKs from all future rekeying
        instances.  The only solution is to contact the GCKS explicitly for
        resynchronization.  Note that the KEKs for the first rekeying
        instance are protected by the Registration SA.  Recall that
        communication in that phase is one to one, and therefore it is easy
        to ensure reliable delivery.
        Stateless GKMAs [SD1, SD2] encrypt rekey messages with KEKs sent
        during the registration protocol.  Since rekey messages are
        independent of any past rekey messages (i.e. not protected by KEKs
        therein), a member may go offline, but continue to be able to
        decipher future communications.  However, they offer no mechanisms to
        recover past rekeying messages.  Stateless rekeying may be relatively
        inefficient, particularly for immediate (in contrast to batch)
        rekeying in highly dynamic groups.
        In self-healing schemes [Self-healing], a member can reconstruct a
        lost rekey message, as long as it receives some past rekey messages
        and some future rekey messages.
        Internet Draft    Group Key Management Architecture         [PAGE 22]

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     5.8 Interoperability of a GKMA
        Most GKMA specifications do not specify packet formats although any
        group key management algorithms need to, for the purposes of
        interoperability.  In particular there are several alternative ways
        to managing key trees and numbering nodes within key trees.  The
        following information is generally needed during initialization of a
        Rekey SA or included with each GKMA packet.
          o GKMA name  (e.g. LKH, OFT, Subset difference)
          o GKMA version number (implementation specific).  Version may imply
            several things such as the degree of a key tree, proprietary
            enhancements, and qualify another field such as a key id.
          o Number of keys or Largest ID
          o Version specific data
          o Per key information
             - Key ID
             - Key lifetime (creation/expiration data)
             - Encrypted key
             - Encryption key's ID (optional)
        Key IDs may change in some implementations in which case we need to
          o List of <old id, new id>
     6.0 Group Security Association
        The GKM Architecture defines the interfaces between the registration,
        Rekey, and data security protocols in terms of the Security
        Associations (SAs) of those protocols.  By isolating these protocols
        behind a uniform interface, the architecture allows implementations
        to use protocols best suited to their needs.  For example, a rekey
        protocol for a small group could use multiple unicast transmissions
        with symmetric authentication, while that for a large group could use
        IP Multicast with packet-level Forward Error Correction and source
        The Group Key Management Architecture provides an interface between
        the security protocols and the group SA (GSA), which consists of
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        Baugher, Canetti, Dondeti, Lindholm                         June 2004
        three SAs, viz., Registration SA, Rekey SA and Data SA.  The Rekey SA
        is optional.  There are two cases in defining the relationships
        between the three SAs.  In both cases, the Registration SA protects
        the registration protocol.
        In Case 1, group key management is done WITHOUT using a Rekey SA. The
        registration protocol initializes and updates one or more Data SAs
        (having TPKs to protect files or streams).  Each Data SA corresponds
        to a single group - and a group may have more than one Data SA.
        In Case 2, group key management is done WITH a Rekey SA to protect
        the rekey protocol. The registration protocol initializes the Rekey
        SAs (one or more) as well as zero or more Data SAs upon successful
        completion.  When a Data SA is not initialized in the registration
        protocol, this is done in the rekey protocol.  The rekey protocol
        updates Rekey SA(s) AND establishes Data SA(s).
     6.1 Group policy
        Group policy is described in detail in the Group Security Policy
        Token document [GSPT].  Group policy can be distributed through group
        announcements, key management protocols, and other out-of-band means
        (e.g., via a web page).  The group key management protocol carries
        cryptographic policies of the SAs and keys it establishes as well as
        additional policies for the secure operation of the group.
        The acceptable cryptographic policies for the registration protocol,
        which may run over TLS [TLS], IPsec, or IKE, are not conveyed in the
        group key-management protocol since they precede any of the key
        management exchanges.  Thus, a security policy repository having some
        access protocol may need to be queried prior to key-management
        session establishment to determine the initial cryptographic policies
        for that establishment.  This document assumes the existence of such
        a repository and protocol for GCKS and member policy queries.
        Thus group security policy will be represented in a policy repository
        and accessible using a policy protocol.  Policy distribution may be a
        push or a pull operation.
        The group key management architecture assumes that the following
        group-policy information may be externally managed, e.g., by the
        content owner, group conference administrator or group owner.
          o Identity of the Group owner, and authentication method, and
            delegation method for identifying a GCKS for the group
          o Group GCKS, authentication method, and delegation method for any
            subordinate GCKSs for the group
          o Group membership rules or list and authentication method
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        There are also two additional policy-related requirements external to
        group key management.
          o There is an authentication and authorization infrastructure such
            as X.509 [RFC 2459], SPKI [RFC 2693], or a pre-shared key scheme
            in accordance with the group policy for a particular group.
          o There is an announcement mechanism for secure groups and events
            that operates according to group policy for a particular group.
        Group policy determines how the registration and rekey protocols
        initialize or update Rekey and Data SAs.  The following sections
        describe potential information sent by the GCKS for the Rekey and
        Data SAs.  A member needs to have the information specified in the
        next sections to establish Rekey and Data SAs.
     6.2 Contents of the Rekey SA
        The Rekey SA protects the rekey protocol.  It contains cryptographic
        policy, Group Identity and Security Parameter Index (SPI) [RFC2401]
        to uniquely identify an SA, replay protection information, and key
        protection keys.
     6.2.1 Rekey SA Policy
        The GROUP KEY MANAGEMENT ALGORITHM represents the group key
        revocation algorithm that enforces forward and backward access
        control.  Examples of key revocation algorithms include LKH, LKH+,
        OFT, OFC and Subset Difference [RFC2627, OFT, TAXONOMY, SDR].  The
        key revocation algorithm could also be NULL.  In that case, the Rekey
        SA contains only one KEK, which serves as the group KEK.  The rekey
        messages initialize or update Data SAs as usual.  But, the Rekey SA
        itself can be updated (group KEK can be Rekeyed) when members join or
        the KEK is about to expire.  Leave Rekeying is done by re-
        initializing the Rekey SA through the rekey protocol.
        The KEK ENCRYPTION ALGORITHM uses a standard encryption algorithm
        such as 3DES or AES.  The KEK KEY LENGTH is also specified.
        The AUTHENTICATION ALGORITHM uses digital signatures for GCKS
        authentication (since all shared secrets are known to some or all
        members of the group), or some symmetric secret in computing MACs for
        group authentication.  Symmetric authentication provides weaker
        authentication in that any group member can impersonate a particular
        source.  The AUTHENTICATION KEY LENGTH is also be specified.
        The CONTROL GROUP ADDRESS is used for multicast transmission of rekey
        messages.  This information is sent over the control channel such as
        in an ANNOUNCEMENT protocol or call setup message. The degree to
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        Baugher, Canetti, Dondeti, Lindholm                         June 2004
        which the control group address is protected is a matter of group
        The REKEY SERVER ADDRESS allows the registration server to be a
        different entity from the server used for Rekey, such as for future
        invocations of the registration and rekey protocols.  If the
        registration server and the Rekey server are two different entities,
        the registration server sends the Rekey servers address as part of
        the Rekey SA.
     6.2.2 Group Identity
        The Group identity accompanies the SA (payload) information as an
        identifier if the specific group key management protocol allows
        multiple groups to be initialized in a single invocation of the
        registration protocol or multiple groups to be updated in a single
        rekey message.  It is often much simpler to restrict each
        registration invocation to a single group; no such restriction is
        necessary.  There is always a need to identify the group when
        establishing a Rekey SA either implicitly through an SPI or
        explicitly as an SA parameter.
     6.2.3 KEKs
        Corresponding to the key management algorithm, the Rekey SA contains
        one or more KEKs.  The GCKS holds the key encrypting keys of the
        group, while the members receive keys following the specification of
        the key-management algorithm.  When there are multiple KEKs for a
        group (as in an LKH tree), each KEK needs to be associated with a Key
        ID, which is used to identify the key needed to decrypt it.  Each KEK
        has a LIFETIME associated with it, after which the KEK expires.
     6.2.4 Authentication Key
        The GCKS provides a symmetric or public key for authentication of its
        rekey messages.  Symmetric-key authentication is appropriate only
        when all group members can be trusted not to impersonate the GCKS.
        The architecture does not rule out methods for deriving symmetric
        authentication keys at the member [RFC2409] rather than being pushed
        from the GCKS.
     6.2.5 Replay Protection
        Rekey messages need to be protected from replay/reflection attacks.
        Sequence numbers are used for this purpose and the Rekey SA (or
        protocol) contains this information.
     6.2.6 Security Parameter Index (SPI)
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        The tuple <Group identity, SPI > uniquely identifies a Rekey SA.  The
        SPI changes each time the KEKs change.
     6.3 Contents of the Data SA
        The GCKS specifies the data security protocol used for secure
        transmission of data from sender(s) to receiving members.  Examples
        of data security protocols include IPsec ESP [RFC 2401] and SRTP [RFC
        3711].  While the content of each of these protocols is out of the
        scope of this document, we list the information sent by the
        registration protocol (or the rekey protocol) to initialize or update
        the Data SA.
     6.3.1 Group Identity
        The Group identity accompanies SA information when Data SAs are
        initialized or Rekeyed for multiple groups in a single invocation of
        the registration protocol or in a single rekey message.
     6.3.2 Source Identity
        The SA includes source identity information when the group owner
        chooses to reveal Source identity to authorized members only.  A
        public channel such as announcement protocol is only appropriate when
        there is no need to protect source or group identities.
     6.3.3 Traffic Protection Keys
        Irrespective of the data security protocol used, the GCKS supplies
        the TEKs or information to derive TEKs, used for data encryption.
     6.3.4 Data Authentication Keys
        Depending on the data-authentication method used by the data security
        protocol, group key management may pass one or more keys, functions
        (e.g., TESLA [TESLA]), or other parameters used for authenticating
        streams or files.
     6.3.5 Sequence Numbers
        The GCKS passes sequence numbers when needed by the data security
        protocol, for SA synchronization and replay protection.
     6.3.6 Security Parameter Index (SPI)
        The GCKS may provide an identifier as part of the Data SA contents
        for data security protocols that use an SPI or similar mechanism to
        identify an SA or keys within an SA.
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     6.3.7 Data SA policy
        The Data SA parameters are specific to the data security protocol but
        generally include encryption algorithm and parameters, the source
        authentication algorithm and parameters, the group authentication
        algorithm and parameters, and/or replay protection information.
     7.0 Scalability Considerations
        The area of group communications is quite diverse.  In
        teleconferencing, a multipoint control unit (MCU) may be used to
        aggregate a number of teleconferencing members into a single session;
        MCUs may be hierarchically organized as well.  A loosely coupled
        teleconferencing session [RFC3550] has no central controller but is
        fully distributed and end-to-end.  Teleconferencing sessions tend to
        have at most dozens of participants whereas video broadcast, which
        uses multicast communications, and media on demand, which uses
        unicast, are large-scale groups numbering hundreds to millions of
        As described in the Requirements section above, the group key
        management architecture supports multicast applications with a single
        sender.  The architecture described in this paper supports large-
        scale operation through the following features.
        1. There is no need for a unicast exchange to provide data keys to a
        security protocol for members who have previously-registered in the
        particular group; data keys can be pushed in the rekey protocol.
        2. The registration and rekey protocols are separable to allow
        flexibility in how members receive group secrets.  A group can use a
        smart-card based system in place of the registration protocol, for
        example, to allow the rekey protocol to be used with no back channel
        for broadcast applications such as television conditional access
        3. The registration and rekey protocols support new keys, algorithms,
        authentication mechanisms and authorization infrastructures in the
        architecture.  When the authorization infrastructure supports
        delegation, as in X.509 and SPKI, the GCKS function can be
        distributed as shown in Figure 3.
        Internet Draft    Group Key Management Architecture         [PAGE 28]

        Baugher, Canetti, Dondeti, Lindholm                         June 2004
                     |       +-------+                        |
                     |       |  GCKS |                        |
                     |       +-------+                        |
                     |         |   ^                          |
                     |         |   |                          |
                     |         |   +---------------+          |
                     |         |       ^           ^          |
                     |         |       |    ...    |          |
                     |         |   +--------+  +--------+     |
                     |         |   | MEMBER |  | MEMBER |     |
                     |         |   +--------+  +--------+     |
                     |         v                              |
                     |         +-------------+                |
                     |         |             |                |
                     |         v      ...    v                |
                     |     +-------+   +-------+              |
                     |     |  GCKS |   |  GCKS |              |
                     |     +-------+   +-------+              |
                     |         |   ^                          |
                     |         |   |                          |
                     |         |   +---------------+          |
                     |         |       ^           ^          |
                     |         |       |    ...    |          |
                     |         |   +--------+  +--------+     |
                     |         |   | MEMBER |  | MEMBER |     |
                     |         |   +--------+  +--------+     |
                     |         v                              |
                     |        ...                             |
                 Figure 3: Hierarchically-organized Key Distribution
        The first feature in the list allows fast keying of data security
        protocols when the member already belongs to the group.  While this
        is realistic for subscriber groups and customers of service providers
        who offer content events, it may be too restrictive for applications
        that allow member enrollment at the time of the event.  The MSEC
        group key management architecture suggests hierarchically organized
        key distribution to handle potential mass simultaneous registration
        requests.  The Figure 3 configuration may be needed when conventional
        clustering and load-balancing solutions of a central GCKS site cannot
        meet customer requirements.  Unlike conventional caching and content-
        distribution networks, however, the configuration shown in Figure 3
        has additional security ramifications for physical security of a
        More analysis and work needs to be done on the protocol
        instantiations of the group key management architecture to determine
        how effectively and securely the architecture can support large-
        scale multicast applications.  In addition to being as secure as
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        Baugher, Canetti, Dondeti, Lindholm                         June 2004
        pairwise key management against man-in-the-middle, replay, and
        reflection attacks, group key management protocols have additional
        security needs.  Unlike pairwise key management, group key
        management needs to be secure against attacks not only by non-
        members but by members who may attempt to impersonate a GCKS or
        disrupt the operation of a GCKS.  Thus, secure groups need to
        converge to a common group key under the conditions of members
        attacking the group, joining and leaving the group, and being
        evicted from the group.  Group key management protocols also need to
        be robust when denial of service attacks or network partitions lead
        to large numbers of synchronized requests.  An instantiation of
        group key management, therefore, needs to consider how GCKS
        operation might be distributed across multiple GCKS as designated by
        the group owner to serve keys on behalf of a designated GCKS.
        GSAKMP [GSAKMP] protocol uses the policy token and allows
        designating some of the members as subordinate GCKSs to address this
        scalability issue.
     8.0 Security Considerations
        This memo describes MSEC key management architecture.  This
        architecture will be instantiated in one or more group key management
        protocols, which must be protected against man-in-the-middle,
        connection hijacking, replay or reflection of past messages, and
        denial of service attacks.
        Authenticated key exchange [STS, SKEME, RFC2408, RFC2412, RFC2409]
        techniques limit the effects of man-in-the-middle and connection-
        hijacking attacks.  Sequence numbers and low-computation message
        authentication techniques can be effective against replay and
        reflection attacks. Cookies [RFC2522], when properly implemented,
        provide an efficient means to reduce the effects of denial of service
        This memo does not address attacks against key management or security
        protocol implementations such as so-called type attacks that aim to
        disrupt an implementation by such means as buffer overflow.  The
        focus of this memo is on securing the protocol, not an implementation
        of the protocol.
        While classical techniques of authenticated key exchange can be
        applied to group key management, new problems arise with the sharing
        of secrets among a group of members:  Group secrets may be disclosed
        by a member of the group and group senders may be impersonated by
        other members of the group.  Key management messages from the GCKS
        should not be authenticated using shared symmetric secrets unless all
        members of the group can be trusted not to impersonate the GCKS or
        each other.  Similarly, members who disclose group secrets undermine
        the security of the entire group.  group owners and GCKS
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        Baugher, Canetti, Dondeti, Lindholm                         June 2004
        administrators must be aware of these inherent limitations of group
        key management.
        Another limitation of group key management is policy complexity:
        Whereas peer-to-peer security policy is an intersection of the policy
        of the individual peers, a group owner sets group security policy
        externally in secure groups.  This document assumes there is no
        negotiation of cryptographic or other security parameters in group
        key management.  Group security policy, therefore, poses new risks to
        members who send and receive data from secure groups.  Security
        administrators, GCKS operators, and users need to determine minimal
        acceptable levels of security (e.g., authentication and admission
        policy of the group, key lengths, cryptographic algorithms and
        protocols used etc.) when joining secure groups.
        Given the limitations and risks of group security, the security of
        the group key management registration protocol should be as good as
        the base protocols on which it is developed such as IKE, IPsec, TLS,
        or SSL.  The particular instantiations of this Group Key Management
        architecture must ensure that the high standards for authenticated
        key exchange are preserved in their protocol specifications, which
        will be Internet standards-track documents that are subject to
        review, analysis and testing.
        The second protocol, the group key management rekey protocol, is new
        and has unknown risks associated with it.  The source-authentication
        risks described above are obviated by the use of public-key
        cryptography.  The use of multicast delivery may raise additional
        security issues such as reliability, implosion, and denial of service
        attacks based upon the use of multicast.  The rekey protocol
        specification needs to offer secure solutions to these problems.
        Each instantiation of the rekey protocol, such as the GSAKMP Rekey or
        the GDOI Groupkey-push operations, need to validate the security of
        their Rekey specifications.
        Novelty and complexity are the biggest risks to group key management
        protocols.  Much more analysis and experience are needed to ensure
        that the architecture described in this document can provide a well-
        articulate standard for security and risks of group key management.
     9.0 Acknowledgments
        The GKM Building Block [GKMBB) I-D by SMuG was a precursor to this
        document; thanks to Thomas Hardjono and Hugh Harney for their
        efforts.  During the course of preparing this document, Andrea
        Colegrove, Brian Weis, George Gross and several others in MSEC WG
        and GSEC and SMuG research groups provided valuable comments that
        helped improve this document.  The authors appreciate their
        contributions to this document.
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     10.0 References and Bibliography
        [BatchRekey] Yang, Y. R., et al., Reliable Group Rekeying: Design and
        Performance Analysis, in Proc. of ACM SIGCOMM, San Diego, CA, August
        [CLIQUES] M. Steiner, G. Tsudik and M. Waidner, CLIQUES: A New
        Approach to Group Key Agreement, IEEE ICDCS 97, May 1997
        [FN93] A. Fiat, M. Naor, Broadcast Encryption, Advances in
        Cryptology - CRYPTO 93 Proceedings, Lecture Notes in Computer
        Science, Vol. 773, 1994, pp. 480 -- 491.
        [GSAKMP] H.Harney, A.Colegrove, E.Harder, U.Meth, R.Fleischer, Group
        Secure Association Key Management Protocol,
        01.txt, February 2003, Work in Progress.
        [GSPT] Hardjono, T., H. Harney, P. McDaniel, A. Colegrove, and P.
        Dinsmore, The MSEC Group Security Policy Token, draft-ietf-msec-gspt-
        02.txt, August 2003, Work in Progress.
        [H.235] ITU, Security and encryption for H-Series (H.323 and other
        H.245-based) multimedia terminals, ITU-T Recommendation H.235 Version
        3, 2001, Work in progress.
        [JKKV94] M. Just, E. Kranakis, D. Krizanc, P. van Oorschot, On Key
        Distribution via True Broadcasting. In Proceedings of 2nd ACM
        Conference on Computer and Communications Security, November 1994,
        pp. 81--88.
        [MARKS] Briscoe, B., MARKS: Zero Side Effect Multicast Key
        Management Using Arbitrarily Revealed Key Sequences, in Proc. of
        First International Workshop on Networked Group Communication (NGC),
        Pisa, Italy, November 1999.
        [MIKEY] J. Arkko, E. Carrara, F. Lindholm, M. Naslund, and K.
        Norrman, MIKEY: Multimedia Internet KEYing, Internet Draft,
        06.txt, February 2003, Work in progress.
        [MSEC-Arch] Hardjono, T., and B. Weis, The Multicast Group Security
        Architecture, RFC 3740 (Informational), March 2004.
        [MVV] A.J.Menzes, P.C.van Oorschot, S.A. Vanstone, Handbook of
        Applied Cryptography, CRC Press, 1996.
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        [OFT] Balenson, D., D. McGrew, and A. Sherman, Key Management for
        Large Dynamic Groups: One-Way Function Trees and Amortized
        Initialization, draft-irtf-smug-groupkeymgmt-oft-00.txt, IRTF, August
        2000, Work in progress.
        [RFC2093] Harney, H., and C. Muckenhirn, Group Key Management
        Protocol (GKMP) Specification, RFC 2093 (experimental), July 1997.
        [RFC2094] Harney, H., and C. Muckenhirn, Group Key Management
        Protocol (GKMP) Architecture, RFC 2094 (experimental), July 1997.
        [RFC2326] Schulzrinee, H., A. Rao, and R. Lanphier, Real Time
        Streaming Protocol (RTSP), RFC 2326 (Proposed Standard), April 1998.
        [RFC2327] Handley, M., and V. Jacobson, SDP: Session Description
        Protocol, RFC 2327 (Proposed Standard), April 1998.
        [RFC2367] McDonald, D., C. Metz, and B. Phan, PF_KEY Key Management
        API, Version 2, RFC 2367 (Informational), July 1998.
        [RFC2401] Kent, S., and R. Atkinson, Security Architecture for the
        Internet Protocol, RFC 2401 (proposed standard), November 1998.
        [RFC2406] Kent, S., and R. Atkinson, IP Encapsulating Security
        Payload (ESP), RFC 2406 (proposed standard), November 1998.
        [RFC2408] Maughan, D., et al., Internet Security Association and Key
        Management Protocol (ISAKMP), RFC 2408 (proposed standard), November
        [RFC2409]  Harkins, D., and D. Carrel, The Internet Key Exchange
        (IKE), RFC 2409 (proposed standard), November 1998.
        [RFC2412] H. Orman, The OAKLEY Key Determination Protocol, RFC 2412
        (Informational), November 1998.
        [RFC2522] Karn, P., and W. Simpson, Photuris: Session-Key Management
        Protocol, RFC 2522 (Informational), March 1999.
        [RFC2543] Handley, M., et. al., SIP: Session Initiation Protocol,
        RFC 2543 (Proposed Standard), March 1999.
        [RFC2627] Wallner, D., E. Harder, and R. Agee, Key Management for
        Multicast: Issues and Architectures, RFC 2627(informational), IETF,
        June 1999.
        [RFC3547] M. Baugher, T. Hardjono, H. Harney, B. Weis, The Group
        Domain of Interpretation, RFC 3547 (Proposed Standard), July 2003.
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        [RFC3550] H. Schulzrinne, S. Casner, R. Frederick, V. Jacobson, RTP:
        A Transport Protocol for Real-Time Applications, RFC 3550 (Proposed
        Standard), July 2003.
        [RFC3711] Baugher, M., et. al., The Secure Real Time Transport
        Protocol, RFC 3711 (Proposed Standard), March 2004.
        [SD1] Naor, D., M. Naor, and J. Lotspiech, Revocation and Tracing
        Schemes for Stateless Receivers, in Advances in Cryptology - CRYPTO,
        Santa Barbara, CA: Springer-Verlag Inc., LNCS 2139, August 2001.
        [SD2] Moni Naor and Benny Pinkas, Efficient Trace and Revoke
        Schemes, In Proceedings of Financial Cryptography 2000, Anguilla,
        British West Indies, February 2000.
        [Self-Healing] Staddon, J., et. al., Self-healing Key Distribution
        with Revocation, In proceedings of the 2002 IEEE Symposium on
        Security and Privacy, Oakland, CA, May 2002.
        [SKEME] H. Krawczyk, SKEME: A Versatile Secure Key Exchange
        Mechanism for Internet, ISOC Secure Networks and Distributed Systems
        Symposium, San Diego, 1996.
        [STS] Diffie, P. van Oorschot, M. J. Wiener, Authentication and
        Authenticated Key Exchanges, Designs, Codes and Cryptography, 2,
        107-125 (1992), Kluwer Academic Publishers.
        [TAXONOMY] R. Canetti et al, Multicast Security: A taxonomy and some
        Efficient Constructions, IEEE INFOCOM, 1999.
        [TESLA-INFO] Perrig, A., R. Canetti, D. Song, D. Tygar, and B.
        Briscoe, TESLA: Multicast Source Authentication Transform
        msec-tesla-intro-01.txt, October 2002,  Work in Progress.
        [TESLA-SPEC] Perrig, A., R. Canetti, and Whillock, TESLA: Multicast
        Source Authentication Transform Specification,
        00.txt, April 2002,  Work in Progress.
        [tGSAKMP] Harney, H., et. al., Tunneled Group Secure Association Key
        Management Protocol,
        msec-tgsakmp-00.txt, May 2003, Work in Progress.
        [TPM] D.S. Marks, B.H. Turnbull, Technical protection measures:  The
        intersection of technology, law, and commercial licenses, Workshop
        on Implementation Issues of the WIPO Copyright Treaty (WCT) and the
        WIPO Performances and Phonograms Treaty (WPPT), World Intellectual
        Property Organization, Geneva, December 6 and 7, 1999
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        [Wool] Wool. A., Key Management for Encrypted broadcast, 5th ACM
        Conference on Computer and Communications Security, San Francisco,
        CA, Nov. 1998.
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     11.0 Authors' Addresses
        Mark Baugher
        Cisco Systems
        5510 SW Orchid St.
        Portland, OR  97219, USA
        +1 408-853-4418
        Ran Canetti
        IBM Research
        30 Saw Mill River Road
        Hawthorne, NY 10532, USA
        +1 914-784-7076
        Lakshminath R. Dondeti
        Nortel Networks
        600 Technology Park Drive
        Billerica, MA 01821, USA
        +1 978-288-6406
        Fredrik Lindholm
        Ericsson Research
        SE-16480 Stockholm, Sweden
        +46 8 58531705
     Full Copyright Statement
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        Internet Draft    Group Key Management Architecture         [PAGE 36]

        Baugher, Canetti, Dondeti, Lindholm                         June 2004
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     Additional Acknowledgements
        Funding for the RFC Editor function is currently provided by the
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        Internet Draft    Group Key Management Architecture         [PAGE 38]