





Network Working Group                                        T. Hardjono
Request for Comments: 3740                                      Verisign
Category: Informational                                          B. Weis
                                                                   Cisco
                                                              March 2004


               The Multicast Group Security Architecture

Status of this Memo

   This memo provides information for the Internet community.  It does
   not specify an Internet standard of any kind.  Distribution of this
   memo is unlimited.

Copyright Notice

   Copyright (C) The Internet Society (2004).  All Rights Reserved.

Abstract

   This document provides an overview and rationale of the multicast
   security architecture used to secure data packets of large multicast
   groups.  The document begins by introducing a Multicast Security
   Reference Framework, and proceeds to identify the security services
   that may be part of a secure multicast solution.

Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  2
       1.1.  Scope. . . . . . . . . . . . . . . . . . . . . . . . . .  2
       1.2.  Summary of Contents of Document. . . . . . . . . . . . .  3
       1.3.  Audience . . . . . . . . . . . . . . . . . . . . . . . .  4
       1.4.  Terminology. . . . . . . . . . . . . . . . . . . . . . .  4
   2.  Architectural Design: The Multicast Security Reference
       Framework. . . . . . . . . . . . . . . . . . . . . . . . . . .  4
       2.1.  The Reference Framework. . . . . . . . . . . . . . . . .  4
       2.2.  Elements of the Centralized Reference Framework. . . . .  5
             2.2.1.  Group Controller and Key Server. . . . . . . . .  6
             2.2.2.  Sender and Receiver. . . . . . . . . . . . . . .  7
             2.2.3.  Policy Server. . . . . . . . . . . . . . . . . .  7
       2.3.  Elements of the Distributed Reference Framework. . . . .  8
   3.  Functional Areas . . . . . . . . . . . . . . . . . . . . . . .  9
       3.1.  Multicast Data Handling. . . . . . . . . . . . . . . . .  9
       3.2.  Group Key Management . . . . . . . . . . . . . . . . . . 10
       3.3.  Multicast Security Policies. . . . . . . . . . . . . . . 11
   4.  Group Security Associations (GSA). . . . . . . . . . . . . . . 12
       4.1.  The Security Association . . . . . . . . . . . . . . . . 12



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       4.2.  Structure of a GSA: Introduction . . . . . . . . . . . . 13
       4.3.  Structure of a GSA: Reasoning. . . . . . . . . . . . . . 14
       4.4.  Definition of GSA. . . . . . . . . . . . . . . . . . . . 15
       4.5.  Typical Compositions of a GSA. . . . . . . . . . . . . . 17
   5.  Security Services. . . . . . . . . . . . . . . . . . . . . . . 17
       5.1.  Multicast Data Confidentiality . . . . . . . . . . . . . 18
       5.2.  Multicast Source Authentication and Data Integrity . . . 18
       5.3.  Multicast Group Authentication . . . . . . . . . . . . . 19
       5.4.  Multicast Group Membership Management. . . . . . . . . . 19
       5.5.  Multicast Key Management . . . . . . . . . . . . . . . . 20
       5.6.  Multicast Policy Management. . . . . . . . . . . . . . . 21
   6.  Security Considerations. . . . . . . . . . . . . . . . . . . . 22
       6.1.  Multicast Data Handling. . . . . . . . . . . . . . . . . 22
       6.2.  Group Key Management . . . . . . . . . . . . . . . . . . 22
       6.3.  Multicast Security Policies. . . . . . . . . . . . . . . 22
   7.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 23
   8.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 23
       8.1.  Normative References . . . . . . . . . . . . . . . . . . 23
       8.2.  Informative References . . . . . . . . . . . . . . . . . 23
   9.  Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 25
   10. Full Copyright Statement . . . . . . . . . . . . . . . . . . . 26

1.  Introduction

   Securing IP multicast group communication is a complex task that
   involves many aspects.  Consequently, a secure IP multicast protocol
   suite must have a number of functional areas that address different
   aspects of the solution.  This document describes those functional
   areas and how they are related.

1.1.  Scope

   This architecture is concerned with the securing of large multicast
   groups.  Whereas it can also be used for smaller groups, it is not
   necessarily the most efficient means.  Other architectures (e.g., the
   Cliques architecture [STW]) can be more efficient for small ad-hoc
   group communication.

   This architecture is "end to end", and does not require multicast
   routing protocols (e.g., PIM [RFC2362]) to participate in this
   architecture.  Inappropriate routing may cause denial of service to
   application layer groups conforming to this architecture.  However
   the routing cannot affect the authenticity or secrecy of group data
   or management packets.  The multicast routing protocols could
   themselves use this architecture to protect their own multicast and
   group packets.  However, this would be independent of any secure
   application layer group.




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   This architecture does not require IP multicast admission control
   protocols (e.g., IGMP [RFC3376], MLD [RFC3019]) to be a part of
   secure multicast groups.  As such, a "join" or "leave" operation for
   a secure group is independent of a "join" or "leave" of an IP
   multicast group.  For example, the process of joining a secure group
   requires being authenticated and authorized by a security device,
   while the process of joining an IP multicast group entails contacting
   a multicast-aware router.  Admission control protocols could
   themselves use this architecture to protect their own multicast
   packets.  However, this would be independent of any secure
   application layer group.

   This architecture does not explicitly describe how secure multicast
   groups deal with Network Address Translation (NAT) [RFC2663].
   Multicast routing protocols generally require the source and
   destination addresses and ports of an IP multicast packet to remain
   unchanged.  This allows consistent multicast distribution trees to be
   created throughout the network.  If NAT is used in a network, then
   the connectivity of senders and receivers may be adversely affected.
   This situation is neither improved or degraded as a result of
   deploying this architecture.

   This architecture does not require the use of reliable mechanisms,
   for either data or management protocols.  The use of reliable
   multicast routing techniques (e.g., FEC [RFC3453]) enhance the
   availability of secure multicast groups.  However the authenticity or
   secrecy of group data or management packets is not affected by the
   omission of that capability from a deployment.

1.2.  Summary of Contents of Document

   This document provides an architectural overview that outlines the
   security services required to secure large multicast groups.  It
   provides a Reference Framework for organizing the various elements
   within the architecture, and explains the elements of the Reference
   Framework.

   The Reference Framework organizes the elements of the architecture
   along three Functional Areas pertaining to security.  These elements
   cover the treatment of data when it is to be sent to a group, the
   management of keying material used to protect the data, and the
   policies governing a group.

   Another important item in this document is the definition and
   explanation of Group Security Associations (GSA), which is the
   multicast counterpart of the unicast Security Association (SA).  The
   GSA is specific to multicast security, and is the foundation of the
   work on group key management.



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1.3.  Audience

   This document is addressed to the technical community, implementers
   of IP multicast security technology, and others interested in gaining
   a general background understanding of multicast security.  This
   document assumes that the reader is familiar with the Internet
   Protocol, the IPsec suite of protocols (e.g., [RFC2401]), related
   networking technology, and general security terms and concepts.

1.4.  Terminology

   The following key terms are used throughout this document.

   1-to-N

      A group which has one sender and many receivers.

   Group Security Association (GSA)

      A bundling of Security Associations (SAs) that together define how
      a group communicates securely.  The GSA may include a registration
      protocol SA, a rekey protocol SA, and one or more data security
      protocol SAs.

   M-to-N

      A group which has many senders and many receivers, where M and N
      are not necessarily the same value.

   Security Association (SA)

      A set of policy and cryptographic keys that provide security
      services to network traffic that matches that policy.

2.  Architectural Design: The Multicast Security Reference Framework

   This section considers the complex issues of multicast security in
   the context of a Reference Framework.  This Reference Framework is
   used to classify functional areas, functional elements, and
   interfaces.  Two designs of the Reference Framework are shown: a
   centralized design, and a distributed design that extends the
   centralized design for very large groups.

2.1.  The Reference Framework

   The Reference Framework is based on three broad functional areas (as
   shown in Figure 1).  The Reference Framework incorporates the main
   entities and functions relating to multicast security, and depicts



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   the inter-relations among them.  It also expresses multicast security
   from the perspective of multicast group types (1-to-N and M-to-N),
   and classes of protocols (the exchanged messages) needed to secure
   multicast packets.

   The aim of the Reference Framework is to provide some general context
   around the functional areas, and the relationships between the
   functional areas.  Note that some issues span more than one
   functional area.  In fact, the framework encourages the precise
   identification and formulation of issues that involve more than one
   functional area or those which are difficult to express in terms of a
   single functional area.  An example of such a case is the expression
   of policies concerning group keys, which involves both the functional
   areas of group key management and multicast policies.

   When considering the Reference Framework diagrams, it is important to
   realize that the singular "boxes" in the framework do not necessarily
   imply a corresponding singular entity implementing a given function.
   Rather, a box in the framework should be interpreted loosely as
   pertaining to a given function related to a functional area.  Whether
   that function is in reality implemented as one or more physical
   entities is dependent on the particular solution.  As an example, the
   box labeled "Key Server" must be interpreted in broad terms as
   referring to the functions of key management.

   Similarly, the Reference Framework acknowledges that some
   implementations may in fact merge a number of the "boxes" into a
   single physical entity.  This could be true even across functional
   areas.  For example, an entity in a group could act as both a Group
   Controller and a Sender to a group.

   The protocols to be standardized are depicted in the Reference
   Framework diagrams by the arrows that connect the various boxes.  See
   more details in Section 4, below.

2.2.  Elements of the Centralized Reference Framework

   The Reference Framework diagram of Figure 1 contains boxes and
   arrows.  The boxes are the functional entities and the arrows are the
   interfaces between them.  Standard protocols are needed for the
   interfaces, which support the multicast services between the
   functional entities.

   In some cases, a system implementing the multicast security
   architecture may not need to implement protocols to account for every
   interface.  Rather, those interfaces may be satisfied through the use
   of manual configuration, or even omitted if they are not necessary
   for the application.



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   There are three sets of functional entities.  Each is discussed
   below.

                 +--------------------------------------+
                 |                                      |
                 |                                      |
                 |  FUNCTIONAL                          |
                 |    AREAS                             |
                 |                                      |
                 |             +------+                 |
                 |  Multicast  |Policy|                 |
                 |  Security   |Server|                 |
                 |  Policies   +------+                 |
                 |                 ^                    |
                 |                 |                    |
                 |                 |                    |
                 |                 v                    |
                 |             +------+                 |
                 |  Group      |Group |                 |
                 |  Key        |Ctrl/ |<---------+      |
                 |  Management |Key   |          |      |
                 |             |Server|          V      |
                 |             +------+     +--------+  |
                 |                 ^        |        |  |
                 |                 |        |Receiver|  |
                 |                 |        |        |  |
                 |                 v        +--------+  |
                 |             +------+          ^      |
                 |             |      |          |      |
                 |  Multicast  |Sender|----------+      |
                 |  Data       |      |                 |
                 |  Handling   |      |                 |
                 |             +------+                 |
                 |                                      |
                 +--------------------------------------+

       Figure 1: Centralized Multicast Security Reference Framework

2.2.1.  Group Controller and Key Server

   The Group Controller and Key Server (GCKS) represent both the entity
   and functions relating to the issuance and management of
   cryptographic keys used by a multicast group.  The GCKS also conducts
   user-authentication and authorization checks on the candidate members
   of the multicast group.






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   The Key Server (KS) and the Group Controller (GC) have somewhat
   different functionality and may in principle be regarded as separate
   entities.  Currently the framework regards the two entities as one
   "box" in order to simplify the design, and in order not to mandate
   standardization of the protocol between the KS and the GC.  It is
   stressed that the KS and GC need not be co-located.  Furthermore,
   future designs may choose to standardize the protocol between the GC
   and the KS, without altering other components.

2.2.2.  Sender and Receiver

   The Sender is an entity that sends data to the multicast group.  In a
   1-to-N multicast group only a single sender is authorized to transmit
   data to the group.  In an M-to-N multicast group, two or more group
   members are authorized to be senders.  In some groups all members are
   authorized as senders.

   Both Sender and Receiver must interact with the GCKS entity for the
   purpose of key management.  This includes user and/or device
   authentication, user and/or device authorization, the obtaining of
   keying material in accordance with some key management policies for
   the group, obtaining new keys during key-updates, and obtaining other
   messages relating to the management of keying material and security
   parameters.

   Senders and Receivers may receive much of their policy from the GCKS
   entities.  The event of joining a multicast group is typically
   coupled with the Sender/Receiver obtaining keying material from a
   GCKS entity.  This does not preclude the direct interaction between
   the Sender/Receiver and the Policy Server.

2.2.3.  Policy Server

   The Policy Server represents both the entity and functions used to
   create and manage security policies specific to a multicast group.
   The Policy Server interacts with the GCKS entity in order to install
   and manage the security policies related to the membership of a given
   multicast group and those related to keying material for a multicast
   group.

   The interactions between the Policy Server and other entities in the
   Reference Framework is dependent to a large extent on the security
   circumstances being addressed by a given policy.








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2.3.  Elements of the Distributed Reference Framework

   The need for solutions to be scalable to large groups across wide
   geographic regions of the Internet requires the elements of the
   framework to also function as a distributed system.  Figure 2 shows
   how distributed designs supporting large group scalability fit into
   the Reference Framework.

    +-----------------------------------------------------------------+
    |                                                                 |
    |                                                                 |
    | FUNCTIONAL                                                      |
    |   AREAS                                                         |
    |            +------+                                  +------+   |
    | Multicast  |Policy|<-------------------------------->|Policy|   |
    | Security   |Server|                                  |Server|   |
    | Policies   +------+                                  +------+   |
    |                ^                                         ^      |
    |                |                                         |      |
    |                |                                         |      |
    |                v                                         v      |
    |            +------+                                  +------+   |
    | Group      |Group |<-------------------------------> |Group |   |
    | Key        |Ctrl/ |<---------+                       |Ctlr/ |   |
    | Management |Key   |          |                       |Key   |   |
    |            |Server|          V                       |Server|   |
    |            +------+     +--------+                   +------+   |
    |                ^        |        |                       ^      |
    |                |        |Receiver|                       |      |
    |                |        |        |                       |      |
    |                v        +--------+                       |      |
    |            +------+          ^                           V      |
    |            |      |          |                      +--------+  |
    | Multicast  |Sender|----------+                      |        |  |
    | Data       |      |-------------------------------->|Receiver|  |
    | Handling   |      |                                 |        |  |
    |            +------+                                 +--------+  |
    +-----------------------------------------------------------------+

       Figure 2: Distributed Multicast Security Reference Framework

   In a distributed design the GCKS entity interacts with other GCKS
   entities to achieve scalability in the key management related
   services.  GCKS entities will require a means of authenticating their
   peer GCKS entities, a means of authorization, and a means of
   interacting securely to pass keys and policy.





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   Similarly, Policy Servers must interact with each other securely to
   allow the communication and enforcement of policies across the
   Internet.

   Two Receiver boxes are displayed corresponding to the situation where
   both the Sender and Receiver employ the same GCKS entity (centralized
   architecture) and where the Sender and Receiver employ different GCKS
   entities (distributed architecture).  In the distributed design, all
   Receivers must obtain identical keys and policy.  Each member of a
   multicast group may interact with a primary GCKS entity (e.g., the
   "nearest" GCKS entity, measured in terms of a well-defined and
   consistent metric).  Similarly, a GCKS entity may interact with one
   or more Policy Servers, also arranged in a distributed architecture.

3.  Functional Areas

   The Reference Framework identifies three functional areas.  They are:

      -  Multicast data handling.  This area covers the security-related
         treatments of multicast data by the sender and the receiver.
         This functional area is further discussed in Section 3.1.

      -  Group Key Management.  This area is concerned with the secure
         distribution and refreshment of keying material.  This
         functional area is further discussed in Section 3.2.

      -  Multicast Security Policies.  This area covers aspects of
         policy in the context of multicast security, taking into
         consideration the fact that policies may be expressed in
         different ways: that they may exist at different levels in a
         given multicast security architecture, and that they may be
         interpreted differently according to the context in which they
         are specified and implemented.  This functional area is further
         discussed in Section 3.3.

3.1.  Multicast Data Handling

   In a secure multicast group, the data typically needs to be:

      1. Encrypted using the group key, mainly for access control and
         possibly also for confidentiality.
      2. Authenticated, for verifying the source and integrity of the
         data.  Authentication takes two flavors:
         a. Source authentication and data integrity.  This
            functionality guarantees that the data originated with the
            claimed source and was not modified en route (either by a
            group member or an external attacker).




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         b. Group authentication.  This type of authentication only
            guarantees that the data was generated (or last modified) by
            some group member.  It does not guarantee data integrity
            unless all group members are trusted.

   While multicast encryption and group authentication are fairly
   standard and similar to encrypting and authenticating a point-to-
   point communication, source authentication for multicast is
   considerably more involved.  Consequently, off-the-shelf solutions
   (e.g., taken from IPsec [RFC2406]) may be sufficient for encryption
   and group authentication.  For source authentication, however,
   special-purpose transformations are necessary.  See [CCPRRS] for
   further elaboration on the concerns regarding the data transforms.

   Multicast data encrypted and/or authenticated by a sender should be
   handled the same way by both centralized and distributed receivers,
   (as shown in Figure 2).

   The "Multicast Encapsulating Security Payload" [BCCR] provides the
   definition for Multicast ESP for data traffic.  The "Multicast Source
   Authentication Transform Specification" [PCW] defines the use of the
   TESLA algorithm for source authentication in multicast.

3.2.  Group Key Management

   The term "keying material" refers to the cryptographic keys belonging
   to a group, the state associated with the keys, and the other
   security parameters related to the keys.  Hence, the management of
   the cryptographic keys belonging to a group necessarily requires the
   management of their associated state and parameters.  A number of
   solutions for specific issues must be addressed.  These may include
   the following:

   -  Methods for member identification and authentication.
   -  Methods to verify the membership to groups.
   -  Methods to establish a secure channel between a GCKS entity and
      the member, for the purpose of delivery of shorter-term keying
      material pertaining to a group.
   -  Methods to establish a long-term secure channel between one GCKS
      entity and another, for the purpose of distributing shorter-term
      keying material pertaining to a group.
   -  Methods to effect the changing of keys and keying material.
   -  Methods to detect and signal failures and perceived compromises to
      keys and keying material.

   The requirements related to the management of keying material must be
   seen in the context of the policies that prevail within the given
   circumstance.



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   Core to the area of key management is Security Association (SA)
   Management, which will be discussed further below.

   A "Group Key Management Architecture" document [BCDL] further defines
   the key management architecture for multicast security.  It builds on
   the Group Security Association (GSA) concept, and further defines the
   roles of the Key Server and Group Controller.

   "The Group Domain of Interpretation" [RFC3547], "GSAKMP" [GSAKMP],
   and "MIKEY" [ACLNM] are three instances of protocols implementing the
   group key management function.

3.3.  Multicast Security Policies

   Multicast Security Policies must provide the rules for operation for
   the other elements of the Reference Framework.  Security Policies may
   be distributed in an ad-hoc fashion in some instances.  However,
   better coordination and higher levels of assurance are achieved if a
   Policy Controller distributes Security Policies policy to the group.

   Multicast security policies must represent, or contain, more
   information than a traditional peer-to-peer policy.  In addition to
   representing the security mechanisms for the group communication, the
   policy must also represent the rules for the governance of the secure
   group.  For example, policy would specify the authorization level
   necessary in order for an entity to join a group.  More advanced
   operations would include the conditions when a group member must be
   forcibly removed from the group, and what to do if the group members
   need to resynchronize because of lost key management messages.

   The application of policy at the Group Controller element and the
   member (sender and receiver) elements must be described.  While there
   is already a basis for security policy management in the IETF,
   multicast security policy management extends the concepts developed
   for unicast communication in the areas of:

   -  Policy creation,
   -  High-level policy translation, and
   -  Policy representation.

   Examples of work in multicast security policies include the Dynamic
   Cryptographic Context Management project [Din], Group Key Management
   Protocol [Har1, Har2], and Antigone [McD].

   Policy creation for secure multicast has several more dimensions than
   the single administrator specified policy assumed in the existing
   unicast policy frameworks.  Secure multicast groups are usually large
   and by their very nature extend over several administrative domains,



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   if not spanning a different domain for each user.  There are several
   methods that need to be considered in the creation of a single,
   coherent group security policy.  They include a top-down
   specification of the group policy from the group initiator and
   negotiation of the policy between the group members (or prospective
   members).  Negotiation can be as simple as a strict intersection of
   the policies of the members or extremely complicated using weighted
   voting systems.

   The translation of policy rules from one data model to another is
   much more difficult in a multicast group environment.  This is
   especially true when group membership spans multiple administrative
   domains.  Policies specified at a high level with a Policy Management
   tool must be translated into more precise rules that the available
   security policy mechanisms can both understand and implement.  When
   dealing with multicast communication and its multiple participants,
   it is essential that the individual translation performed for each
   participant result in the use of a mechanism that is interoperable
   with the results of all of the other translations.  Typically, the
   translation from high-level policy to specific policy objects must
   result in the same objects in order to achieve communication between
   all of the group members.  The requirement that policy translation
   results in the same objects places constraints on the use and
   representations in the high-level policies.

   It is also important that policy negotiation and translation be
   performed as an integral part of joining a group.  Adding a member to
   a group is meaningless if they will not be able to participate in the
   group communications.

4.  Group Security Associations (GSA)

4.1.  The Security Association

   A security association is a commonly used term in cryptographic
   systems (e.g., [RFC2401, RFC2406bis, RFC2409]).  This document uses
   the term to mean any set of policy and cryptographic keys that
   provide security services for the network traffic matching that
   policy.  A Security Association usually contains the following
   attributes:

      -  selectors, such as source and destination transport addresses.
      -  properties, such as an security parameter index (SPI) or cookie
         pair, and identities.
      -  cryptographic policy, such as the algorithms, modes, key
         lifetimes, and key lengths used for authentication or
         confidentiality.
      -  keys, such as authentication, encryption and signing keys.



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   Group key management uses a different set of abstractions than
   point-to-point key management systems (such as IKE [RFC2409]).
   Notwithstanding, the abstractions used in the Group Key Management
   functional area may be built from the point-to-point key management
   abstractions.

4.2.  Structure of a GSA: Introduction

   Security associations (SAs) for group key management are more
   complex, and are usually more numerous, than for point-to-point key
   management algorithms.  The latter establishes a key management SA to
   protect application SAs (usually one or two, depending on the
   protocol).  However, group key management may require up to three or
   more SAs.  These SAs are described in later sections.

   A GSA contains all of the SA attributes identified in the previous
   section, as well some additional attributes pertaining to the group.
   As shown in Figure 3, the GSA builds on the SA in two distinct ways.

   -  First, the GSA is a superset of an SA (Figure 3(a)).  A GSA has
      group policy attributes.  For example, the kind of signed
      credentials needed for group membership, whether group members
      will be given new keys when a member is added (called "backward
      re-key" below), or whether group members will be given new keys
      when a member is removed from the group ("forward re-key").  A GSA
      also includes an SA as an attribute of itself.

   -  Second, the GSA is an aggregation of SAs (Figure 3(b)).  A GSA is
      comprised of multiple SAs, and these SAs may be used for several
      independent purposes.

            +---------------+              +-------------------+
            |     GSA       |              |        GSA        |
            |               |              | +-----+   +-----+ |
            |               |              | | SA1 |   | SA2 | |
            |    +----+     |              | +-----+   +-----+ |
            |    | SA |     |              |      +-----+      |
            |    +----+     |              |      | SA3 |      |
            |               |              |      +-----+      |
            +---------------+              +-------------------+

               (a) superset                  (b) aggregation

                   Figure 3: Relationship of GSA to SA







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4.3.  Structure of a GSA: Reasoning

   Figure 4 shows three categories of SAs that can be aggregated into a
   GSA.

      +------------------------------------------------------------+
      |                                                            |
      |                    +------------------+                    |
      |                    |       GCKS       |                    |
      |                    |                  |                    |
      |                    |   REG      REG   |                    |
      |                    |    /  REKEY \    |                    |
      |                    +---/-----|----\---+                    |
      |                       /      |     \                       |
      |                      /       |      \                      |
      |                     /        |       \                     |
      |                    /         |        \                    |
      |                   /          |         \                   |
      |       +----------/------+    |   +------\----------+       |
      |       |        REG      |    |   |      REG        |       |
      |       |            REKEY-----+----REKEY            |       |
      |       |     Sender      |        |      Receiver   |       |
      |       |             DATA----------DATA             |       |
      |       +-----------------+        +-----------------+       |
      |                                                            |
      |                                                            |
      +------------------------------------------------------------+

             Figure 4: GSA Structure and 3 categories of SAs

   The three categories of SAs are:

   -  Registration SA (REG): A separate unicast SA between the GCKS and
      each group member, regardless of whether the group member is a
      sender or a receiver or acting in both roles.

   -  Re-key SA (REKEY): A single multicast SA between the GCKS and all
      of the group members.

   -  Data Security SA (DATA): A multicast SA between each multicast
      source speaker and the group's receivers.  There may be as many
      data SAs as there are multicast sources allowed by the group's
      policy.

   Each of these SAs are defined in more detail in the next section.






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4.4.  Definition of GSA

   The three categories of SAs correspond to three different kinds of
   communications commonly required for group communications.  This
   section describes the SAs depicted in Figure 4 in detail.

   -  Registration SA (REG):
      An SA is required for (bi-directional) unicast communications
      between the GCKS and a group member (be it a Sender or Receiver).
      This SA is established only between the GCKS and a Member.  The
      GCKS entity is charged with access control to the group keys, with
      policy distribution to members (or prospective members), and with
      group key dissemination to Sender and Receiver members.  This use
      of a (unicast) SA as a starting point for key management is common
      in a number of group key management environments [RFC3547, GSAKMP,
      CCPRRS, RFC2627, BMS].

      The Registration SA is initiated by the member to pull GSA
      information from the GCKS.  This is how the member requests to
      join the secure group, or has its GSA keys re-initialized after
      being disconnected from the group (e.g., when its host computer
      has been turned off during re-key operations).  The GSA
      information pulled down from the GCKS is related to the other two
      SAs defined as part of the GSA.

      Note that this (unicast) SA is used to protect the other elements
      of the GSA.  As such, the Registration SA is crucial and is
      inseparable from the other two SAs in the definition of a GSA.

      However, the requirement of a registration SA does not imply the
      need of a registration protocol to create that Registration SA.
      The registration SA could instead be setup through some manual
      means, such as distributed on a smart card.  Thus, what is
      important is that a Registration SA exists, and is used to protect
      the other SAs.

      From the perspective of one given GCKS, there are as many unique
      registration SAs as there are members (Senders and/or Receivers)
      in the group.  This may constitute a scalability concern for some
      applications.  A registration SA may be established on-demand with
      a short lifetime, whereas re-key and data security SAs are
      established at least for the life of the sessions that they
      support.

      Conversely the registration SA could be left in place for the
      duration of the group lifetime, if scalability is not an issue.
      Such a long term registration SA would be useful for re-
      synchronization or deregistration purposes.



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   -  Re-key SA (REKEY):
      In some cases, a GCKS needs the ability to "push" new SAs as part
      of the GSA.  These new SAs must be sent to all group members.  In
      other cases, the GCKS needs the ability to quickly revoke access
      to one or more group members.  Both of these needs are satisfied
      with the Re-key SA.

      This Re-key SA is a unidirectional multicast transmission of key
      management messages from the GCKS to all group members.  As such,
      this SA is known by the GCKS and by all members of the group.

      This SA is not negotiated, since all the group members must share
      it.  Thus, the GCKS must be the authentic source and act as the
      sole point of contact for the group members to obtain this SA.

      A rekey SA is not absolutely required to be part of a GSA.  For
      example, the lifetime of some groups may be short enough such that
      a rekey is not necessary.  Conversely, the policy for the group
      could specify multiple rekey SAs of different types.  For example,
      if the GC and KS are separate entities, the GC may deliver rekey
      messages that adjust the group membership, and the KS may deliver
      rekey messages with new DATA SAs.

   -  Data Security SA (DATA):
      The Data Security SA protects data between member senders and
      member receivers.

      One or more SAs are required for the multicast transmission of
      data-messages from the Sender to other group members.  This SA is
      known by the GCKS and by all members of the group.

      Regardless of the number of instances of this third category of
      SA, this SA is not negotiated.  Rather, all group members obtain
      it from the GCKS.  The GCKS itself does not use this category of
      SA.

      From the perspective of the Receivers, there is at least one data
      security SA for the member sender (one or more) in the group.  If
      the group has more than one data security SA, the data security
      protocol must have a means of differentiating the SAs (e.g., with
      a SPI).










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      There are a number of possibilities with respect to the number of
      data security SAs:

      1. Each sender in the group could be assigned a unique data
         security SA, thereby resulting in each receiver having to
         maintain as many data security SAs as there are senders in the
         group.  In this case, each sender may be verified using source
         origin authentication techniques.

      2. The entire group deploys a single data security SA for all
         senders.  Receivers would then be able to maintain only one
         data security SA.

      3. A combination of 1. and 2.

4.5.  Typical Compositions of a GSA

   Depending on the multicast group policy, many compositions of a GSA
   are possible.  For illustrative purposes, this section describes a
   few possible compositions.

   -  A group of memory-constrained members may require only a REG SA,
      and a single DATA SA.
   -  A "pay-per-session" application, where all of the SA information
      needed for the session may be distributed over a REG SA.  Re-key
      and re-initialization of DATA SAs may not be necessary, so there
      is no REKEY SA.
   -  A subscription group, where keying material is changed as
      membership changes.  A REG SA is needed to distribute other SAs; a
      REKEY SA is needed to re-initialize a DATA SA at the time
      membership changes.

5.  Security Services

   This section identifies security services for designated interfaces
   of Figure 2.  Distinct security services are assigned to specific
   interfaces.  For example, multicast source authentication, data
   authentication, and confidentiality occur on the multicast data
   interface between Senders and Receivers in Figure 2.  Authentication
   and confidentiality services may also be needed between the Key
   Server and group members (i.e., the Senders and Receivers of Figure
   2), but the services that are needed for multicast key management may
   be unicast as well as multicast.  A security service in the Multicast
   Security Reference Framework therefore identifies a specific function
   along one or more Figure 2 interfaces.






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   This paper does not attempt to analyze the trust relationships,
   detailed functional requirements, performance requirements, suitable
   algorithms, and protocol specifications for IP multicast and
   application-layer multicast security.  Instead, that work will occur
   as the security services are further defined and realized in
   algorithms and protocols.

5.1.  Multicast Data Confidentiality

   This security service handles the encryption of multicast data at the
   Sender's end and the decryption at the Receiver's end.  This security
   service may also apply the keying material that is provided by
   Multicast Key Management in accordance with Multicast Policy
   Management, but it is independent of both.

   An important part of the Multicast Data Confidentiality security
   service is in the identification of and motivation for specific
   ciphers that should be used for multicast data.  Obviously, not all
   ciphers will be suitable for IP multicast and application-layer
   multicast traffic.  Since this traffic will usually be connectionless
   UDP flows, stream ciphers may be unsuitable, though hybrid
   stream/block ciphers may have advantages over some block ciphers.

   Regarding application-layer multicast, some consideration is needed
   to consider the effects of sending encrypted data in a multicast
   environment lacking admission-control, where practically any
   application program can join a multicast event independently of its
   participation in a multicast security protocol.  Thus, this security
   service is also concerned with the effects of multicast
   confidentiality services (intended and otherwise) on application
   programs.  Effects to both Senders and Receivers are considered.

   In Figure 2, the Multicast Data Confidentiality security service is
   placed in Multicast Data Handling Area along the interface between
   Senders and Receivers.  The algorithms and protocols that are
   realized from work on this security service may be applied to other
   interfaces and areas of Figure 2 when multicast data confidentiality
   is needed.

5.2.  Multicast Source Authentication and Data Integrity

   This security service handles source authentication and integrity
   verification of multicast data.  It includes the transforms to be
   made both at the Sender's end and at the Receiver's end.  It assumes
   that the appropriate signature and verification keys are provided via
   Multicast Key Management in accordance with Multicast Policy
   Management as described below.  This is one of the harder areas of
   multicast security due to the connectionless and real-time



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   requirements of many IP multicast applications.  There are classes of
   application-layer multicast security, however, where offline source
   and data authentication will suffice.  As discussed previously, not
   all multicast applications require real-time authentication and
   data-packet integrity.  A robust solution to multicast source and
   data authentication, however, is necessary for a complete solution to
   multicast security.

   In Figure 2, the Multicast Source and Data Authentication security
   service is placed in Multicast Data Handling Area along the interface
   between Senders and Receivers.  The algorithms and protocols that are
   produced for this functional area may have applicability to security
   services in other functional area that use multicast services such as
   Group Key Management.

5.3.  Multicast Group Authentication

   This security service provides a limited amount of authenticity of
   the transmitted data: It only guarantees that the data originated
   with (or was last modified by) one of the group members.  It does not
   guarantee authenticity of the data in case that other group members
   are not trusted.

   The advantage of group authentication is that it is guaranteed via
   relatively simple and efficient cryptographic transforms.  Therefore,
   when source authentication is not paramount, group authentication
   becomes useful.  In addition, performing group authentication is
   useful even when source authentication is later performed: it
   provides a simple-to-verify weak integrity check that is useful as a
   measure against denial-of-service attacks.

   The Multicast Group Authentication security service is placed in the
   Multicast Data Handling Area along the interface between Senders and
   Receivers.

5.4.  Multicast Group Membership Management

   This security service describes the functionality of registration of
   members with the Group Controller, and de-registration of members
   from the Group Controller.  These are security functions, which are
   independent from IP multicast group "join" and "leave" operations
   that the member may need to perform as a part of group admission
   control protocols (i.e., IGMP [RFC3376], MLD [RFC3019]).

   Registration includes member authentication, notification and
   negotiation of security parameters, and logging of information
   according to the policies of the group controller and the would-be




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   member. (Typically, an out-of-band advertisement of group information
   would occur before the registration takes place.  The registration
   process will typically be invoked by the would-be member.)

   De-registration may occur either at the initiative of the member or
   at the initiative of the group controller.  It would result in
   logging of the de-registration event by the group controller and an
   invocation of the appropriate mechanism for terminating the
   membership of the de-registering member (see Section 5.5).

   This security service also describes the functionality of the
   communication related to group membership among different GCKS
   servers in a distributed group design.

   In Figure 2, the Multicast Group Membership security service is
   placed in the Group Key Management Area and has interfaces to Senders
   and Receivers.

5.5.  Multicast Key Management

   This security service describes the functionality of distributing and
   updating the cryptographic keying material throughout the life of the
   group.  Components of this security service may include:

      -  GCKS to group member (Sender or Receiver) notification
         regarding current keying material (e.g., group encryption and
         authentication keys, auxiliary keys used for group management,
         keys for source authentication, etc.).
      -  Updating of current keying material, depending on circumstances
         and policies.
      -  Termination of groups in a secure manner, including the secure
         group itself and the associated keying material.

   Among the responsibilities of this security service is the secure
   management of keys between Key Servers and group members, the
   addressing issues for the multicast distribution of keying material,
   and the scalability or other performance requirements for multicast
   key management [RFC2627, BMS].  Key Servers and group members may
   take advantage of a common Public Key Infrastructure (PKI) for
   increased scalability of authentication and authorization.

   To allow for an interoperable and secure IP multicast security
   protocol, this security service may need to specify host abstractions
   such as a group security association database (GSAD) and a group
   security policy database (GSPD) for IP multicast security.  The
   degree of overlap between IP multicast and application-layer
   multicast key management needs to be considered.  Thus, this security
   service takes into account the key management requirements for IP



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   multicast, the key management requirements for application-layer
   multicast, and to what degree specific realizations of a Multicast
   Key Management security service can satisfy both.  ISAKMP, moreover,
   has been designed to be extensible to multicast key management for
   both IP multicast and application-layer multicast security [RFC2408].
   Thus, multicast key management protocols may use the existing ISAKMP
   standard's Phase 1 and Phase 2 protocols, possibly with needed
   extensions (such as GDOI [RFC3547] or application-layer multicast
   security).

   This security service also describes the functionality of the
   communication related to key management among different GCKS servers
   in a distributed group design.

   Multicast Key Management appears in both the centralized and
   distributed designs as shown in Figure 2 and is placed in the Group
   Key Management Area.

5.6.  Multicast Policy Management

   This security service handles all matters related to multicast group
   policy including membership policy and multicast key management
   policy.  Indeed, one of the first tasks in further defining this
   security service is identifying the different areas of multicast
   policy.  Multicast Policy Management includes the design of the
   policy server for multicast security, the particular policy
   definitions that will be used for IP multicast and application-layer
   multicast security, and the communication protocols between the
   Policy Server and the Key Server.  This security service may be
   realized using a standard policy infrastructure such as a Policy
   Decision Point (PDP) and Policy Enforcement Point (PEP) architecture
   [RFC2748].  Thus, it may not be necessary to re-invent a separate
   architecture for multicast security policy.  At minimum, however,
   this security service will be realized in a set of policy
   definitions, such as multicast security conditions and actions.

   The Multicast Policy Management security service describes the
   functionality of the communication between an instance of a GCKS to
   an instance of the Policy Server.  The information transmitted may
   include policies concerning groups, memberships, keying material
   definition and their permissible uses, and other information.  This
   security service also describes communication between and among
   Policy Servers.  Group members are not expected to directly
   participate in this security service.  However, this option is not
   ruled out.






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6.  Security Considerations

   This document describes an architectural framework for protecting
   multicast and group traffic with cryptographic protocols.  Three
   functional areas are identified within the framework.  Each
   functional area has unique security considerations, and these are
   discussed below.

   This architectural framework is end-to-end, and does not rely upon
   the network that connects group controllers and group members.  It
   also does not attempt to resolve security issues in the unicast or
   multicast routing infrastructures, or in multicast admission control
   protocols.  As such, denial of service, message deletion, and other
   active attacks against the unicast or multicast routing
   infrastructures are not addressed by this framework.  Section 1.1
   describes the relationship of the network infrastructure to the
   multicast group security architecture.

6.1.  Multicast Data Handling

   Cryptographic protocols protecting multicast data are responsible for
   providing confidentiality and group authentication.  They should also
   be able to provide source authentication to uniquely identify senders
   to the group.  Replay protection of multicast data is also desirable,
   but may not always be possible.  This is due to the complexity of
   maintaining replay protection state for multiple senders.  Section
   3.1 elaborates on the security requirements for this area.

6.2.  Group Key Management

   Group key management protocols provide cryptographic keys and policy
   to group members.  They are responsible for authenticating and
   authorizing group members before revealing those keys, and for
   providing confidentiality and authentication of those keys during
   transit.  They are also responsible for providing a means for
   rekeying the group, in the case that the policy specifies a lifetime
   for the keys.  They also are responsible for revocation of group
   membership, once one or more group members have had their
   authorization to be a group member revoked.  Section 3.2 describes
   the security requirements of this area in more detail.

6.3.  Multicast Security Policies

   Cryptographic protocols providing multicast security policies are
   responsible for distributing that policy such that the integrity of
   the policy is maintained.  If the policy itself is confidential, they
   also are responsible for authenticating group controllers and group
   members, and providing confidentiality of the policy during transit.



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7.  Acknowledgements

   Much of the text in this document was derived from two research
   papers.  The framework for this document came from a paper co-
   authored by Thomas Hardjono, Ran Canetti, Mark Baugher, and Pete
   Dinsmore.  Description of the GSA came from a document co-authored by
   Hugh Harney, Mark Baugher, and Thomas Hardjono.  George Gross
   suggested a number of improvements that were included in later
   versions of this document.

8.  References

8.1.  Normative References

   [RFC2401]    Kent, S. and R. Atkinson, "Security Architecture for the
                Internet Protocol", RFC 2401, November 1998.

   [RFC2408]    Maughan, D., Shertler, M., Schneider, M. and J. Turner,
                "Internet Security Association and Key Management
                Protocol", RFC 2408, November 1998.

8.2.  Informative References

   [ACLNM]      J. Arkko, et. al., "MIKEY: Multimedia Internet KEYing",
                Work in Progress, December 2003.

   [BCCR]       M. Baugher, R. Canetti, P. Cheng, P. Rohatgi, "MESP: A
                Multicast Framework for the IPsec ESP", Work in
                Progress, October 2002.

   [BCDL]       M. Baugher, R. Canetti, L. Dondeti, F.  Lindholm, "Group
                Key Management Architecture", Work in Progress,
                September 2003.

   [BMS]        D. Balenson, D. McGrew, A. Sherman, Key Management for
                Large Dynamic Groups: One-Way Function Trees and
                Amortized Initialization,
                http://www.securemulticast.org/draft-balenson-
                groupkeymgmt-oft-00.txt, Work in Progress, February
                1999.

   [CCPRRS]     Canetti, R., Cheng P. C., Pendarakis D., Rao, J.,
                Rohatgi P., Saha D., "An IPSec-based Host Architecture
                for Secure Internet Multicast",
                http://www.isoc.org/isoc/conferences/ndss/2000/
                proceedings/028.pdf, NDSS 2000.





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   [Din]        Dinsmore, P., Balenson, D., Heyman, M., Kruus, P.,
                Scace, C., and Sherman, A., "Policy-Based Security
                Management for Large Dynamic Groups:  An Overview of the
                DCCM Project," DARPA Information Survivability
                Conference and Exposition,
                http://download.nai.com/products/media/nai/doc/discex-
                110199.doc.

   [GSAKMP]     H. Harney, et. al., "GSAKMP", Work in Progress, October
                2003.

   [Har1]       Harney, H. and C. Muckenhirn, "Group Key Management
                Protocol (GKMP) Specification", RFC 2093, July 1997.

   [Har2]       Harney, H. and C. Muckenhirn, "Group Key Management
                Protocol (GKMP) Architecture", RFC 2094, July 1997.

   [McD]        McDaniel, P., Honeyman, P., and Prakash, A., "Antigone:
                A Flexible Framework for Secure Group Communication,"
                Proceedings of the Eight USENIX Security Symposium, pp
                99-113, August, 1999.

   [PCW]        Perrig, A., Canetti, R. and B. Whillock, TESLA:
                Multicast Source Authentication Transform
                Specification", Work in Progress, October 2002.

   [RFC2362]    Estrin, D., Farinacci, D., Helmy, A., Thaler, D.,
                Deering, S., Handley, M., Jacobson, V., Liu, C., Sharma,
                P. and L. Wei, "Protocol Independent Multicast-Sparse
                Mode (PIM-SM): Protocol Specification",  RFC 2362, June
                1998.

   [RFC2406]    Kent, S. and R. Atkinson, "IP Encapsulating Security
                Payload (ESP)", RFC 2406, November 1998.

   [RFC2406bis] Kent, S., "IP Encapsulating Security Payload (ESP)",
                Work in Progress, March 2003.

   [RFC2409]    Harkins, D. and D. Carrel, "The Internet Key Exchange
                (IKE)", RFC 2409, November 1998.

   [RFC2627]    Wallner, D., Harder, E. and R. Agee, "Key Management for
                Multicast: Issues and Architectures", RFC 2627,
                September 1998.

   [RFC2663]    Srisuresh, P. and M. Holdrege, "IP Network Address
                Translator (NAT) Terminology and Considerations", RFC
                2663, August 1999.



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   [RFC2748]    Durham, D., Ed., Boyle, J., Cohen, R., Herzong, S.,
                Rajan, R. and A. Sastry, "COPS (Common Open Policy
                Service) Protocol", RFC 2748, January 2000.

   [RFC3019]    Haberman,  B. and R. Worzella, "IP Version 6 Management
                Information Base for The Multicast Listener Discovery
                Protocol", RFC 3019, January 2001.

   [RFC3376]    Cain, B., Deering, S., Kouvelas, I., Fenner, B. and A.
                Thyagarajan, "Internet Group Management Protocol,
                Version 3", RFC 3376, October 2002.

   [RFC3453]    Luby, M., Vicisano, L., Gemmell, J., Rizzo, M., Handley,
                M. and J. Crowcroft, "The Use of Forward Error
                Correction (FEC) in Reliable Multicast", RFC 3453,
                December 2002.

   [RFC3547]    Baugher, M., Weis, B., Hardjono, T. and H. Harney, "The
                Group Domain of Interpretation", RFC 3547, December
                2002.

   [STW]        M., Steiner, Tsudik, G., Waidner, M., CLIQUES: A New
                Approach to Group key Agreement, IEEE ICDCS'98 , May
                1998.

9.  Authors' Addresses

   Thomas Hardjono
   VeriSign
   487 E. Middlefield Rd.
   Mountain View, CA 94043, USA

   Phone:(650) 426-3204
   EMail: thardjono@verisign.com


   Brian Weis
   Cisco Systems
   170 W. Tasman Drive,
   San Jose, CA 95134-1706, USA

   Phone: (408) 526-4796
   EMail: bew@cisco.com








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10.  Full Copyright Statement

   Copyright (C) The Internet Society (2004).  This document is subject
   to the rights, licenses and restrictions contained in BCP 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
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   WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.

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Acknowledgement

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









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