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INFORMATIONAL
Network Working Group                                         M. BaugherRequest for Comments: 4046                                         CiscoCategory: Informational                                       R. Canetti                                                                     IBM                                                              L. Dondeti                                                                Qualcomm                                                             F. Lindholm                                                                Ericsson                                                              April 2005Multicast Security (MSEC) Group Key Management ArchitectureStatus 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).Abstract   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 Contents1. Introduction: Purpose of this Document ..........................22. Requirements of a Group Key Management Protocol .................43. Overall Design of Group Key Management Architecture .............63.1. Overview ...................................................63.2. Detailed Description of the GKM Architecture ...............83.3. Properties of the Design ..................................113.4. Group Key Management Block Diagram ........................114. Registration Protocol ..........................................134.1. Registration Protocol via Piggybacking or Protocol Reuse ..134.2. Properties of Alternative Registration Exchange Types .....14Baugher, et al.              Informational                      [Page 1]

RFC 4046         MSEC Group Key Management Architecture       April 2005      4.3. Infrastructure for Alternative Registration           Exchange Types ............................................154.4. De-registration Exchange ..................................165. Rekey Protocol .................................................165.1. Goals of the Rekey Protocol ...............................175.2. Rekey Message Transport and Protection ....................175.3. Reliable Transport of Rekey Messages ......................185.4. State-of-the-art on Reliable Multicast Infrastructure .....205.5. Implosion .................................................215.6. Incorporating Group Key Management Algorithms .............22      5.7. Stateless, Stateful, and Self-healing Rekeying           Algorithms ................................................225.8. Interoperability of a GKMA ................................236. Group Security Association .....................................246.1. Group Policy ..............................................246.2. Contents of the Rekey SA ..................................256.2.1. Rekey SA Policy ....................................266.2.2. Group Identity .....................................276.2.3. KEKs ...............................................276.2.4. Authentication Key .................................276.2.5. Replay Protection ..................................276.2.6. Security Parameter Index (SPI) .....................276.3. Contents of the Data SA ...................................276.3.1. Group Identity .....................................286.3.2. Source Identity ....................................286.3.3. Traffic Protection Keys ............................286.3.4. Data Authentication Keys ...........................286.3.5. Sequence Numbers ...................................286.3.6. Security Parameter Index (SPI) .....................286.3.7. Data SA Policy .....................................287. Scalability Considerations .....................................298. Security Considerations ........................................319. Acknowledgments ................................................3210. Informative References ........................................331.  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.Baugher, et al.              Informational                      [Page 2]

RFC 4046         MSEC Group Key Management Architecture       April 2005   Group and multicast applications in IP networks have diverse security   requirements [TAXONOMY].  Their key management requirements, briefly   reviewed inSection 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)   protocols.   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, andSection 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 andSection 8 discusses security   considerations.Baugher, et al.              Informational                      [Page 3]

RFC 4046         MSEC Group Key Management Architecture       April 20052.  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      membership.   3. Keys will have a pre-determined lifetime and may be periodically      refreshed.   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      departure.Baugher, et al.              Informational                      [Page 4]

RFC 4046         MSEC Group Key Management Architecture       April 2005   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       material.   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 manyBaugher, et al.              Informational                      [Page 5]

RFC 4046         MSEC Group Key Management Architecture       April 2005   senders.  It may be that no single key management protocol can   satisfy the scalability requirements of all group-security   applications.   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:Baugher, et al.              Informational                      [Page 6]

RFC 4046         MSEC Group Key Management Architecture       April 2005      (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      options:      (1) Using source authentication [TAXONOMY], that is, enabling each          group member to verify that a rekey message originates with          the GCKS and none other.Baugher, et al.              Informational                      [Page 7]

RFC 4046         MSEC Group Key Management Architecture       April 2005      (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          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 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 byBaugher, et al.              Informational                      [Page 8]

RFC 4046         MSEC Group Key Management Architecture       April 2005   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.Baugher, et al.              Informational                      [Page 9]

RFC 4046         MSEC Group Key Management Architecture       April 2005      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   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.   The only optional SA is the Rekey SA.Baugher, et al.              Informational                     [Page 10]

RFC 4046         MSEC Group Key Management Architecture       April 2005   Figure 1 shows two blocks that are external to the group key   management protocol:  The policy and authorization infrastructures   are discussed inSection 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 ofSection 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 FigureBaugher, et al.              Informational                     [Page 11]

RFC 4046         MSEC Group Key Management Architecture       April 2005   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 sessionBaugher, et al.              Informational                     [Page 12]

RFC 4046         MSEC Group Key Management Architecture       April 2005   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 insideBaugher, et al.              Informational                     [Page 13]

RFC 4046         MSEC Group Key Management Architecture       April 2005   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-Baugher, et al.              Informational                     [Page 14]

RFC 4046         MSEC Group Key Management Architecture       April 2005   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   environment.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   required.Baugher, et al.              Informational                     [Page 15]

RFC 4046         MSEC Group Key Management Architecture       April 20054.4.  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.Baugher, et al.              Informational                     [Page 16]

RFC 4046         MSEC Group Key Management Architecture       April 20055.1.  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   used.   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.Baugher, et al.              Informational                     [Page 17]

RFC 4046         MSEC Group Key Management Architecture       April 2005   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   received.Baugher, et al.              Informational                     [Page 18]

RFC 4046         MSEC Group Key Management Architecture       April 2005   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)          [BatchRekey].   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 orBaugher, et al.              Informational                     [Page 19]

RFC 4046         MSEC Group Key Management Architecture       April 2005         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,Baugher, et al.              Informational                     [Page 20]

RFC 4046         MSEC Group Key Management Architecture       April 2005   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   implosion.   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.Baugher, et al.              Informational                     [Page 21]

RFC 4046         MSEC Group Key Management Architecture       April 2005   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         data.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 GCKSBaugher, et al.              Informational                     [Page 22]

RFC 4046         MSEC Group Key Management Architecture       April 2005   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).Baugher, et al.              Informational                     [Page 23]

RFC 4046         MSEC Group Key Management Architecture       April 2005   Key IDs may change in some implementations in which case one needs to   send:         o List of <old id, new id> 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      SA.   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.Baugher, et al.              Informational                     [Page 24]

RFC 4046         MSEC Group Key Management Architecture       April 2005   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         group.      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.Baugher, et al.              Informational                     [Page 25]

RFC 4046         MSEC Group Key Management Architecture       April 20056.2.1.  Rekey SA Policy      o  GROUP KEY MANAGEMENT ALGORITHM         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         protocol.      o  KEK ENCRYPTION ALGORITHM         This specifies a standard encryption algorithm such as 3DES or         AES, and also the KEK KEY LENGTH.      o  AUTHENTICATION ALGORITHM         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         specified.      o  CONTROL GROUP ADDRESS         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.      o  REKEY SERVER ADDRESS         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.Baugher, et al.              Informational                     [Page 26]

RFC 4046         MSEC Group Key Management Architecture       April 20056.2.2.  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   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 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 <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 [RFC2401] and SRTP   [RFC3711].  While the contents of each of these protocols are out ofBaugher, et al.              Informational                     [Page 27]

RFC 4046         MSEC Group Key Management Architecture       April 2005   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.Baugher, et al.              Informational                     [Page 28]

RFC 4046         MSEC Group Key Management Architecture       April 20057.  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   participants.   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      protocol.   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 contentBaugher, et al.              Informational                     [Page 29]

RFC 4046         MSEC Group Key Management Architecture       April 2005   distribution networks, however, the configuration shown in Figure 3   has additional security ramifications for physical security of a   GCKS.                   +----------------------------------------+                   |       +-------+                        |                   |       |  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.Baugher, et al.              Informational                     [Page 30]

RFC 4046         MSEC Group Key Management Architecture       April 2005   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   attacks.   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 policyBaugher, et al.              Informational                     [Page 31]

RFC 4046         MSEC Group Key Management Architecture       April 2005   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   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-   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.Baugher, et al.              Informational                     [Page 32]

RFC 4046         MSEC Group Key Management Architecture       April 200510.  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                   2003.   [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                   1994.   [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                   1999.   [MIKEY]         Arkko, J., Carrara, E., Lindholm, F., Naslund, M.,                   and K. Norrman, "MIKEY: Multimedia Internet KEYing",RFC 3830, August 2004.Baugher, et al.              Informational                     [Page 33]

RFC 4046         MSEC Group Key Management Architecture       April 2005   [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                   1998.   [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.Baugher, et al.              Informational                     [Page 34]

RFC 4046         MSEC Group Key Management Architecture       April 2005   [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",RFC3280, 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                   2003.   [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.Baugher, et al.              Informational                     [Page 35]

RFC 4046         MSEC Group Key Management Architecture       April 2005   [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,                   1999.   [TESLA-INFO]    Perrig, A., Canetti, R., Song, D., Tygar, D., and B.                   Briscoe, "TESLA: Multicast Source Authentication                   Transform Introduction", Work in Progress, December                   2004.   [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.Baugher, et al.              Informational                     [Page 36]

RFC 4046         MSEC Group Key Management Architecture       April 2005Authors' Addresses   Mark Baugher   Cisco Systems   5510 SW Orchid St.   Portland, OR  97219, USA   Phone: +1 408-853-4418   EMail: mbaugher@cisco.com   Ran Canetti   IBM Research   30 Saw Mill River Road   Hawthorne, NY 10532, USA   Phone: +1 914-784-7076   EMail: canetti@watson.ibm.com   Lakshminath R. Dondeti   Qualcomm   5775 Morehouse Drive   San Diego, CA 92121   Phone: +1 858 845 1267   EMail: ldondeti@qualcomm.com   Fredrik Lindholm   Ericsson Research   SE-16480 Stockholm, Sweden   Phone: +46 8 58531705   EMail: fredrik.lindholm@ericsson.comBaugher, et al.              Informational                     [Page 37]

RFC 4046         MSEC Group Key Management Architecture       April 2005Full Copyright Statement   Copyright (C) The Internet Society (2005).   This document is subject to the rights, licenses and restrictions   contained inBCP 78, and except as set forth therein, the authors   retain all their rights.   This document and the information contained herein are provided on an   "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS   OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET   ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED,   INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE   INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED   WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.Intellectual Property   The IETF takes no position regarding the validity or scope of any   Intellectual Property Rights or other rights that might be claimed to   pertain to the implementation or use of the technology described in   this document or the extent to which any license under such rights   might or might not be available; nor does it represent that it has   made any independent effort to identify any such rights.  Information   on the procedures with respect to rights in RFC documents can be   found inBCP 78 andBCP 79.   Copies of IPR disclosures made to the IETF Secretariat and any   assurances of licenses to be made available, or the result of an   attempt made to obtain a general license or permission for the use of   such proprietary rights by implementers or users of this   specification can be obtained from the IETF on-line IPR repository athttp://www.ietf.org/ipr.   The IETF invites any interested party to bring to its attention any   copyrights, patents or patent applications, or other proprietary   rights that may cover technology that may be required to implement   this standard.  Please address the information to the IETF at ietf-   ipr@ietf.org.Acknowledgement   Funding for the RFC Editor function is currently provided by the   Internet Society.Baugher, et al.              Informational                     [Page 38]

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