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RFC4046 - Multicast Security (MSEC) Group Key Management Architecture

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Network Working Group                                         M. Baugher

Request for Comments: 4046                                         Cisco

Category: Informational                                       R. Canetti


                                                              L. Dondeti


                                                             F. Lindholm


                                                              April 2005

      Multicast Security (MSEC) Group Key Management Architecture

Status of This Memo

   This memo provides information for the Internet community.  It does

   not specify an Internet standard of any kind.  Distribution of this

   memo is unlimited.

Copyright Notice

   Copyright (C) The Internet Society (2005).


   This document defines the common architecture for Multicast Security

   (MSEC) key management protocols to 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 permit 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.

Table of Contents

   1. Introduction: Purpose of this Document ..........................2

   2. Requirements of a Group Key Management Protocol .................4

   3. Overall Design of 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. 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 ..................................16

   5. Rekey Protocol .................................................16

      5.1. Goals of the Rekey Protocol ...............................17

      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 .................................................21

      5.6. Incorporating Group Key Management Algorithms .............22

      5.7. Stateless, Stateful, and Self-healing Rekeying

           Algorithms ................................................22

      5.8. Interoperability of a GKMA ................................23

   6. Group Security Association .....................................24

      6.1. Group Policy ..............................................24

      6.2. Contents of the Rekey SA ..................................25

           6.2.1. Rekey SA Policy ....................................26

           6.2.2. Group Identity .....................................27

           6.2.3. KEKs ...............................................27

           6.2.4. Authentication Key .................................27

           6.2.5. Replay Protection ..................................27

           6.2.6. Security Parameter Index (SPI) .....................27

      6.3. Contents of the Data SA ...................................27

           6.3.1. Group Identity .....................................28

           6.3.2. Source Identity ....................................28

           6.3.3. Traffic Protection Keys ............................28

           6.3.4. Data Authentication Keys ...........................28

           6.3.5. Sequence Numbers ...................................28

           6.3.6. Security Parameter Index (SPI) .....................28

           6.3.7. Data SA Policy .....................................28

   7. Scalability Considerations .....................................29

   8. Security Considerations ........................................31

   9. Acknowledgments ................................................32

   10. Informative References ........................................33

1.  Introduction: Purpose of this Document

   This document defines a common architecture for Multicast Security

   (MSEC) key management protocols to 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 permit 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.

   Group and multicast applications in IP networks have diverse security

   requirements [TAXONOMY].  Their key management requirements, briefly

   reviewed in Section 2.0, include support for internetwork-,

   transport- and application-layer security protocols.  Some

   applications achieve simpler operation by running key management

   messaging over a pre-established secure channel (e.g., TLS or IPsec).

   Other security protocols benefit from a key management protocol that

   can run over an already-deployed session initiation or management

   protocol (e.g., SIP or RTSP).  Finally, some benefit from a

   lightweight key management protocol that requires few round trips.

   For all these reasons, application-, transport-, and IP-layer data

   security protocols (e.g., SRTP [RFC3711] and IPsec [RFC2401]) benefit

   from different group key management systems.  This document defines a

   common architecture and design for all group key management (GKM)


   This common architecture for group key management is called the MSEC

   group key management architecture.  It 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] or broadcast key management schemes

   [FN93,Wool].  MSEC key management may in fact be complementary to

   other group key management designs, but the integration of MSEC group

   key management with Cliques, broadcast key management, or other group

   key systems is not considered in this document.

   Key management protocols are difficult to design and validate.  The

   common architecture described in this document eases this burden by

   defining common abstractions and an 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] defines

   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 (GSA).  Section 7 reviews the scalability issues

   for group key management protocols and Section 8 discusses security


2.  Requirements of a Group Key Management Protocol

   A group key management (GKM) 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.  Group membership may vary over time.  A group key management

   protocol helps to ensure that only members of a secure group can 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 secrecy and authentication.

   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. Group members receive security associations that include

      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 entire


   3. Keys will have a pre-determined 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 verifiably

      obtained 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.  Members who are added may optionally be denied access to

      the key material used before they joined the group, and removed

      members should lose access to the key material following their


   7. The protocol should support a scalable group rekey operation

      without unicast exchanges between members and a Group Controller

      and Key Server (GCKS), 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 the

      IPsec security protocols AH and ESP, and/or application layer

      security protocols such as SRTP [RFC3711].

   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.

   11. The key management protocol should provide a mechanism to

       securely recover from a compromise of some or all of the key


   12. The key management protocol may need to address real-world

       deployment issues such as NAT-traversal and interfacing with

       legacy authentication mechanisms.

   In contrast to typical unicast key and SA negotiation protocols such

   as TLS and IKE, multicast group key management protocols provide SA

   and key download capability.  This feature may be useful for point-

   to-point as well as multicast communication, so that a group key

   management protocol may be useful for unicast applications.  Group

   key management protocols may be used for protecting multicast 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 with many all

   members as potential 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

   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


   It is useful to emphasize two non-requirements: technical protection

   measures (TPM) [TPM] and broadcast key management.  TPM are used for

   such things as copy protection by preventing the device user from

   getting 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.  For simplicity, however, the

   MSEC key management architecture described in this document does not

   consider design for technical protection.

   The second non-requirement is broadcast key management when there is

   no back channel [FN93,JKKV94] or for a non-networked device such as a

   digital videodisc player.  We assume IP network operation with two-

   way communication, however asymmetric, and authenticated key-exchange

   procedures that can be used for member registration.  Broadcast

   applications may use a one-way Internet group key management protocol

   message and a one-way rekey message, as described below.

3.  Overall Design of Group Key Management Architecture

   The overall group key management architecture is based upon a group

   controller model [RFC2093,RFC2094,RFC2627,OFT,GSAKMP,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 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 and 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:

      (a) Sufficient information to initialize the Data SA within the

          joining member.  This information is given only if the group

          security policy calls for initializing the Data SA at

          registration, instead of, or in addition to, as part of the

          rekey protocol.

      (b) Sufficient information to initialize a Rekey SA within the

          joining member (see more details about this SA below).  This

          information is given if the group security policy calls for 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.

   (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, from changes in group security

      policy, from 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 and can

      be sent via multicast to group members or via 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 is typically limited to

      small groups.

      Rekey messages are authenticated using one of the two following


      (1) Using source authentication [TAXONOMY], that is, enabling each

          group member to verify that a rekey message originates with

          the GCKS and none other.

      (2) Using only group-based authentication with a symmetric key.

          Members can only be assured that the rekey messages originated

          within the group.  Therefore, this is 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


      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 if their keys expired and an updated key has not been

      received.  The rekey protocol for large-scale groups offers

      mechanisms to avoid implosion problems and to ensure reliability

      in its delivery of keying material.

      Although the Rekey SA is established by the registration protocol,

      it is updated using a rekey protocol.  When a member leaves the

      group, it destroys its local copy of the GSA.  Using a de-

      registration message may be an efficient way for a member to

      inform the GCKS that it has destroyed, or is about to destroy, the

      SAs.  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 can potentially

      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 and 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 or

   traffic integrity keys may be used, as well.  We use the term

   protection keys to refer to both integrity and 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 the 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 TPK/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 data security protocol uses TPKs

   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.


   | +-----------------+                          +-----------------+ |

   | |     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 group's 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

         sends a group key integrity key for integrity protection of

         rekey messages.  If a group key management algorithm is used

         for efficient rekeying, the GCKS also sends one or more KEKs as

         specified by the key distribution policy of the group key

         management algorithm.

      o  If rekey messages are not used for the Group, then the admitted

         member receives TPKs (as part of the Data Security SAs) that

         are passed to the member's Data Security Protocol (as IKE does

         for IPsec).

      o  The GCKS may pass one or more TPKs to the member even if rekey

         messages are used, for efficiency reasons and 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 signed by the group owner

   to the group member, so that member can check the GCKS's

   authorization.  The GCKS, which may be co-located with a member or be

   physically separate, 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 KEK.

   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) via a rekey message or a registration protocol

   exchange, the member's group key management functional block will

   provide the new or updated security association (SA) to the data

   security protocol.  This protects the data sent from sender to


   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.

   The only optional SA 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].

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,SD1,SD2] 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 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 use this flexibility to

   obtain 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 in 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

   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 in a Host

   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 that is subject to group

   policy [GSPT] but its implementation is specific to the GCKS.  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 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 scalable to the

   particular application.  A GCKS application program will use the API

   to initiate the procedures for establishing 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 ("SECURITY PROTOCOL") of Figure

   2 may span internetwork and application layers or operate at the

   internetwork layer, such as AH and ESP.

4.  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 they are 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 callee's address.  In this case,

   it may be advantageous to tunnel the key exchange procedures inside

   call establishment [H.235,MIKEY], so that 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.

   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 a desirable requirement from RTSP.

   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 trade-offs.  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 retention of 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, but 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].

   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


4.3.  Infrastructure for Alternative Registration Exchange Types

   The registration protocol may need external infrastructures to handle

   authentication and authorization, replay protection, protocol-run

   integrity, and possibly 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) or may be handled using AAA

   infrastructure.  Replay protection using timestamps requires an

   external infrastructure or protocol for clock synchronization.

   However, external infrastructures may not always be needed; 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 be the end user who

   handles the authorization by manually accepting/rejecting the

   incoming calls.  In that case, infrastructure support may not be


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 [RFC3261].  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 unidirectional notification by one side that the GSA is to

   be destroyed.  For authentication of this notification, we may 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.  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  The rekey protocol ensures that all members receive the rekey

         information in a timely manner.

      o  The rekey protocol specifies mechanisms allowing the parties to

         contact the GCKS and re-sync when their keys expire and no

         updates have been received.

      o  The rekey protocol avoids implosion problems and ensures

         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,

      o  efficient rekeying after changes in group membership or when

         keys (KEKs) expire,

      o  reliable delivery of rekey messages,

      o  member recovery from an out-of-sync GSA,

      o  high throughput and low latency, and

      o  support 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, and

      6.  interoperability of GKMAs.

   Note that interoperation of rekey protocol implementations is

   insufficient for a GCKS to successfully rekey a group.  The GKMA must

   also interoperate, i.e., standard versions of the group key

   management algorithms such as LKH, OFT, or Subset Difference must be


   The rest of this section discusses 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.

   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 [SD1,SD2,BatchRekey] and

   may contain group key initialization messages [OFT].

5.3.  Reliable Transport of Rekey Messages

   The GCKS must 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 a

   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 necessary state

   in its rekey messages and does not need members to be always online

   or keeping state.  The Subset Difference algorithm does not require a

   back channel and can operate on a broadcast network.  If a rekey

   message is lost in transmission, the 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.  There are some special cases

   in which membership changes are processed as a batch, reducing the

   frequency of rekey messages but increasing 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.

   Three categories of solutions have been proposed:

      1.  Repeatedly transmit the rekey message.  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, upon

   detecting that it is using an out-of-date key, may notify the group

   key management module.  The action taken by the GCKS member is a

   matter of group policy.  The GCKS member should log the condition and

   may contact the GCKS to rerun the re-registration protocol to obtain

   a fresh group key.  The group policy needs to take into account

   boundary conditions, such as reordered 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 simultaneously contact the GCKS.

   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 (e.g., 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 multicast in the clear.  One may think that this opens the

         GCKS to DoS attacks by many bogus such requests.  This,

         however, does not seem to worsen the situation; in fact,

         bombarding the GCKS with bogus resynch requests would be much

         more problematic.

      o  The GCKS can post the rekey messages on some public site (e.g.,

         a web site) and the out-of-synch member can obtain the rekey

         messages from that site.

   The GCKS may always provide all three ways of resynching (i.e., re-

   registration, simple TCP, and public posting).  This way, the member

   may 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

   several types of reliable multicast protocols with different

   properties.  However, there are no standards track reliable multicast

   protocols published at this time, although IETF consensus has been

   reached on two protocols that are intended to go into the standards

   track [NORM,RFC3450].  Thus, this document does not recommend a

   particular reliable multicast protocol or set of protocols for the

   purpose of reliable group rekeying.  The suitability of NAK-based,

   ACK-based or other reliable multicast methods is determined by the

   application needs and operational 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 may be needed to signal the use of a

   reliable multicast protocol, which could 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 document.

   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 in the future, when

   standards-based methods become available.  It is unlikely that

   forward error correction (FEC) methods will benefit short rekey

   messages that 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

   same time, the GCKS might not be able to 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.  The proposed feedback suppression and

   aggregation solutions might be useful in the GKM context as well.

   Members may wait a random time before sending an out-of-sync or

   feedback message.  Meanwhile, members might receive the necessary key

   updates and therefore 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.  Aggregation raises authentication issues.  Local recovery

   is more complex because members need to establish SAs with the local

   repair server.  Any member of the group or a subordinate GCKS may

   serve as a repair server, which can 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 meant 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" message to one

   of the local repair servers.  The repair server can either 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 as

   well as other members of the group from DoS attacks.

5.6.  Incorporating Group Key Management Algorithms

   Group key management algorithms make rekeying scalable.  Large group

   rekeying without employing GKMAs is prohibitively expensive.

   Following are some considerations in selecting a GKMA:

      o  Protection against collusion.

         Members (or non-members) should not be able to collaborate to

         deduce keys for which they are not privileged (following the

         GKMA key distribution rules).

      o  Forward access control

         The GKMA should ensure that departing members cannot get access

         to future group data.

      o  Backward access control

         The GKMA should ensure that joining members cannot decrypt past


5.7.  Stateless, Stateful, and Self-healing Rekeying Algorithms

   We classify group key management algorithms into three categories:

   stateful, stateless, and self-healing.

   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 failed 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., that are not protected

   by KEKs therein), a member may go offline but continue to decipher

   future communications.  However, stateless GKMAs offer no mechanisms

   to recover past rekeying messages.  Stateless rekeying may be

   relatively inefficient, particularly for immediate (not 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 and some future

   rekey messages.

5.8.  Interoperability of a GKMA

   Most GKMA specifications do not specify packet formats, although many

   group key management algorithms need format specification for

   interoperability.  There are several alternative ways to manage key

   trees and to number nodes within key trees.  The following

   information is 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, and

         -  encryption key's ID (optional).

   Key IDs may change in some implementations in which case one needs to


         o List of  pairs.

6.  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 a rekey protocol for a large

   group could use IP Multicast with packet-level Forward Error

   Correction and source authentication.

   The group key management architecture provides an interface between

   the security protocols and the group SA (GSA).  The GSA consists of

   three SAs: 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.

   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, which may have more than one Data


   Case 2: Group key management is done WITH a Rekey SA to protect the

      rekey protocol.  The registration protocol initializes the one or

      more Rekey SAs as well as zero or more Data SAs, upon successful

      completion.  When a Data SA is not initialized in the registration

      protocol, initialization 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 the 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 establishing the

   key-management session, 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  the identity of the Group owner, the authentication method, and

         the delegation method for identifying a GCKS for the group;

      o  the group GCKS, authentication method, and delegation method

         for any subordinate GCKSs for the group;

      o  the group membership rules or list and authentication method.

   There are two additional policy-related requirements external to

   group key management.

      o  There is an authentication and authorization infrastructure

         such as X.509 [RFC3280], SPKI [RFC2693], or a pre-shared key

         scheme, in accordance with the group policy for a particular


      o  There is an announcement mechanism for secure groups and

         events, which 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 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


         This 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,SD1,SD2].  If the key

         revocation algorithm is NULL, the Rekey SA contains only one

         KEK, which serves as the group KEK.  The rekey messages

         initialize or update Data SAs as usual.  However, the Rekey SA

         itself can be updated (the 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



         This specifies a standard encryption algorithm such as 3DES or

         AES, and also the KEK KEY LENGTH.


         This 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 to be



         This 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 which the control group address is protected is a

         matter of group policy.


         This address allows the registration server to be a different

         entity from the server used for rekeying, 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 server's

         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 simpler to restrict each registration

   invocation to a single group, but such a restriction is unnecessary.

   It is always necessary to identify the group when establishing a

   Rekey SA, either implicitly through an SPI or explicitly as an SA


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 pushing them

   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)

   The tuple  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 [RFC2401] and SRTP

   [RFC3711].  While the contents of each of these protocols are 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 the announcement protocol is only appropriate

   when there is no need to protect source or group identities.

6.3.3.  Traffic Protection Keys

   Regardless of the data security protocol used, the GCKS supplies the

   TPKs, or information to derive TPKs for traffic protection.

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-INFO,TESLA-SPEC]), 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.

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.  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.  However, video broadcast that

   uses multicast communications and media-on-demand that uses unicast

   are large-scale groups numbering hundreds to millions of


   As described in the Requirements section, Section 2, 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


   2. The registration and rekey protocols are separable to allow

      flexibility in how members receive group secrets.  A group may 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 systems.

   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 below.

   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



                   |       +-------+                        |

                   |       |  GCKS |                        |

                   |       +-------+                        |

                   |         |   ^                          |

                   |         |   |                          |

                   |         |   +---------------+          |

                   |         |       ^           ^          |

                   |         |       |    ...    |          |

                   |         |   +--------+  +--------+     |

                   |         |   | MEMBER |  | MEMBER |     |

                   |         |   +--------+  +--------+     |

                   |         v                              |

                   |         +-------------+                |

                   |         |             |                |

                   |         v      ...    v                |

                   |     +-------+   +-------+              |

                   |     |  GCKS |   |  GCKS |              |

                   |     +-------+   +-------+              |

                   |         |   ^                          |

                   |         |   |                          |

                   |         |   +---------------+          |

                   |         |       ^           ^          |

                   |         |       |    ...    |          |

                   |         |   +--------+  +--------+     |

                   |         |   | MEMBER |  | MEMBER |     |

                   |         |   +--------+  +--------+     |

                   |         v                              |

                   |        ...                             |


               Figure 3: Hierarchically Organized Key Distribution

   More analysis and work is needed 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 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 by group members who attempt to impersonate a

   GCKS or disrupt the operation of a GCKS, as well as by non-members.

   Thus, secure groups need to converge to a common group key when

   members are attacking the group, joining and leaving the group, or

   being evicted from the group.  Group key management protocols also

   need to be robust when DoS attacks or network partition leads to

   large numbers of synchronized requests.  An instantiation of group

   key management, therefore, needs to consider how GCKS operation might

   be distributed across multiple GCKSs 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.  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 on implementing

   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

   administrators must be aware of these inherent limitations of group

   key management.

   Another limitation of group key management is policy complexity.

   While 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) 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.  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


   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-

   articulated standard for security and risks of group key management.

9.  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 the MSEC

   WG and GSEC and SMuG research groups provided valuable comments that

   helped improve this document.  The authors appreciate their

   contributions to this document.

10.  Informative References

   [BatchRekey]    Yang, Y. R., et al., "Reliable Group Rekeying: Design

                   and Performance Analysis", Proc. ACM SIGCOMM, San

                   Diego, CA, August 2001.

   [CLIQUES]       Steiner, M., Tsudik, G., and M. Waidner, "CLIQUES: A

                   New Approach to Group Key Agreement", IEEE ICDCS 97,

                   May 1997

   [FN93]          Fiat, A. and M. Naor, "Broadcast Encryption, Advances

                   in Cryptology", CRYPTO 93 Proceedings, Lecture Notes

                   in Computer Science, Vol. 773, pp. 480-491, 1994.

   [GKMBB]         Harney, H., M. Baugher, and T. Hardjono, "GKM

                   Building Block: Group Security Association (GSA)

                   Definition," Work in Progress, September 2000.

   [GSAKMP]        Harney, H., Colegrove, A., Harder, E., Meth, U., and

                   R.  Fleischer, "Group Secure Association Key

                   Management Protocol", Work in Progress, February


   [GSPT]          Hardjono, T., Harney, H., McDaniel, P., Colegrove,

                   A., and P.  Dinsmore, "The MSEC Group Security Policy

                   Token", Work in Progress, August 2003.

   [H.235]         International Telecommunications Union, "Security and

                   Encryption for H-Series (H.323 and other H.245-based)

                   Multimedia Terminals", ITU-T Recommendation H.235

                   Version 3, Work in progress, 2001.

   [JKKV94]        Just, M., Kranakis, E., Krizanc, D., and P. van

                   Oorschot, "On Key Distribution via True

                   Broadcasting", Proc. 2nd ACM Conference on Computer

                   and Communications Security, pp. 81-88, November


   [MARKS]         Briscoe, B., "MARKS: Zero Side Effect Multicast Key

                   Management Using Arbitrarily Revealed Key Sequences",

                   Proc.  First International Workshop on Networked

                   Group Communication (NGC), Pisa, Italy, November


   [MIKEY]         Arkko, J., Carrara, E., Lindholm, F., Naslund, M.,

                   and K. Norrman, "MIKEY: Multimedia Internet KEYing",

                   RFC 3830, August 2004.

   [MSEC-Arch]     Hardjono, T. and B. Weis, "The Multicast Group

                   Security Architecture", RFC 3740, March 2004.

   [MVV]           Menzes, A.J., van Oorschot, P.C., and S.A. Vanstone,

                   "Handbook of Applied Cryptography", CRC Press, 1996.

   [NORM]          Adamon, B., Bormann, C., Handley, M., and J. Macker,

                   "Negative-acknowledgment (NACK)-Oriented Reliable

                   Multicast (NORM) Protocol", RFC 3940, November 2004.

   [OFT]           Balenson, D., McGrew, P.C., and A. Sherman, "Key

                   Management for Large Dynamic Groups: One-Way Function

                   Trees and Amortized Initialization", IRTF Work in

                   Progress, August 2000.

   [RFC2093]       Harney, H. and C. Muckenhirn, "Group Key Management

                   Protocol (GKMP) Specification", RFC 2093, July 1997.

   [RFC2094]       Harney, H., and C. Muckenhirn, "Group Key Management

                   Protocol (GKMP) Architecture" RFC 2094, July 1997.

   [RFC2326]       Schulzrinne, H., Rao, A., and R. Lanphier, "Real Time

                   Streaming Protocol (RTSP)", RFC 2326, April 1998.

   [RFC2327]       Handley, M. and V. Jacobson, "SDP: Session

                   Description Protocol", RFC 2327, April 1998.

   [RFC2367]       McDonald, D., Metz, C., and B. Phan, "PF_KEY Key

                   Management API, Version 2", RFC 2367, July 1998.

   [RFC2401]       Kent, S. and R. Atkinson, "Security Architecture for

                   the Internet Protocol", RFC 2401, November 1998.

   [RFC2408]       Maughan, D., Schertler, M., Schneider, M., and J.

                   Turner, "Internet Security Association and Key

                   Management Protocol (ISAKMP)", RFC 2408, November


   [RFC2409]       Harkins, D. and D. Carrel, "The Internet Key Exchange

                   (IKE)", RFC 2409, November 1998.

   [RFC2412]       Orman, H., "The OAKLEY Key Determination Protocol",

                   RFC 2412, November 1998.

   [RFC2522]       Karn, P. and W. Simpson, "Photuris: Session-Key

                   Management Protocol", RFC 2522, March 1999.

   [RFC2693]       Ellison, C., Frantz, B., Lampson, B., Rivest, R.,

                   Thomas, B., and T. Ylonen, "SPKI Certificate Theory",

                   RFC 2693, September 1999.

   [RFC3261]       Rosenberg, J., Schulzrinne, H., Camarillo, G.,

                   Johnston, A., Peterson, J., Sparks, R., Handley, M.,

                   and E. Schooler, "SIP: Session Initiation Protocol",

                   RFC 3261, June 2002.

   [RFC3280]       Housley, R., Polk, W., Ford, W., and D. Solo,

                   "Internet X.509 Public Key Infrastructure Certificate

                   and Certificate Revocation List (CRL) Profile", RFC

                   3280, April 2002.

   [RFC2627]       Wallner, D., Harder, E., and R. Agee, "Key Management

                   for Multicast: Issues and Architectures", RFC 2627,

                   June 1999.

   [RFC3450]       Luby, M., Gemmell, J., Vicisano, L., Rizzo, L., and

                   J.  Crowcroft, "Asynchronous Layered Coding (ALC)

                   Protocol Instantiation", RFC 3450, December 2002.

   [RFC3547]       Baugher, M., Weis, B., Hardjono, T., and H. Harney,

                   "The Group Domain of Interpretation", RFC 3547, July


   [RFC3550]       Schulzrinne, H., Casner, S., Frederick, R., and V.

                   Jacobson, "RTP: A Transport Protocol for Real-Time

                   Applications", STD 64, RFC 3550, July 2003.

   [RFC3711]       Baugher, M., McGrew, D., Naslund, M., Carrara, E.,

                   and K.  Norrman, "The Secure Real-time Transport

                   Protocol (SRTP)", RFC 3711, March 2004.

   [SD1]           Naor, D., Naor, M., and J. Lotspiech, "Revocation and

                   Tracing Schemes for Stateless Receiver", Advances in

                   Cryptology - CRYPTO, Santa Barbara, CA: Springer-

                   Verlag Inc., LNCS 2139, August 2001.

   [SD2]           Naor, M. and B. Pinkas, "Efficient Trace and Revoke

                   Schemes", Proceedings of Financial Cryptography 2000,

                   Anguilla, British West Indies, February 2000.

   [Self-Healing]  Staddon, J., et. al., "Self-healing Key Distribution

                   with Revocation", Proc. 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., and J. Wiener,

                   "Authentication and Authenticated Key Exchanges",

                   Designs, Codes and Cryptography, 2, 107-125 (1992),

                   Kluwer Academic Publishers.

   [TAXONOMY]      Canetti, R., et. al., "Multicast Security: A Taxonomy

                   and some Efficient Constructions", IEEE INFOCOM,


   [TESLA-INFO]    Perrig, A., Canetti, R., Song, D., Tygar, D., and B.

                   Briscoe, "TESLA: Multicast Source Authentication

                   Transform Introduction", Work in Progress, December


   [TESLA-SPEC]    Perrig, A., R. Canetti, and Whillock, "TESLA:

                   Multicast Source Authentication Transform

                   Specification", Work in Progress, April 2002.

   [tGSAKMP]       Harney, H., et. al., "Tunneled Group Secure

                   Association Key Management Protocol", Work in

                   Progress, May 2003.

   [TLS]           Dierks, T. and C. Allen, "The TLS Protocol Version

                   1.0," RFC 2246, January 1999.

   [TPM]           Marks, D. and B. 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.

   [Wool]          Wool, A., "Key Management for Encrypted broadcast",

                   5th ACM Conference on Computer and Communications

                   Security, San Francisco, CA, Nov. 1998.

Authors' Addresses

   Mark Baugher

   Cisco Systems

   5510 SW Orchid St.

   Portland, OR  97219, USA

   Phone: +1 408-853-4418


   Ran Canetti

   IBM Research

   30 Saw Mill River Road

   Hawthorne, NY 10532, USA

   Phone: +1 914-784-7076


   Lakshminath R. Dondeti


   5775 Morehouse Drive

   San Diego, CA 92121

   Phone: +1 858 845 1267


   Fredrik Lindholm

   Ericsson Research

   SE-16480 Stockholm, Sweden

   Phone: +46 8 58531705


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